Marshall DSL40c 40w out of 2 EL34's?

[JustMike]

 And it's nowhere near as loud as a 50w JMP or Plexi. So what's going on? Is it a biasing thing?

[HotBluePlates]

Power output is largely determined by supply voltage and output transformer primary impedance. Then of course, you have to have tubes that can swing the power required by the design combination.

It probably has nothing to do with bias (alone). Marshall only claims 40w, and it has a single 12" speaker. A tweed Champ played through a 4x12 cab sounds much, much louder than it does through its stock 8" speaker.

[stratele52]

Power output ; Circuit desing , voltage .

Nothing to do with Output tubes bias 

[Ed_Chambley]

There is also speaker efficiency to consider as well.  Usually on a speaker spec sheet.

It is true as well that multiple speakers seem much louder.  I have read, one the internet so may not be factual, that it takes 2 x the present wattage to double the loudness all other factors being the same.

IOW, a 5 watt amp played through a 4, 12 cab will be half as loud as a 50 watt amp.  We have a load of watt info brought to our attention these days with the half wattage switch and even less.  The little Vox ac4 has a 4 watt, 1 watt and 1/4th watt.  All that happens is the amp begins to distort easier it has less headroon and as guitar players most like this early breakup/distortion.

I normally practice with a 5 watt amp and a 15 watt amp played together both with  their own Celestion Blue 12 inch speaker and it can get really loud, but the tone of the 5c1 champ mixed makes it seem like I am playing a huge setup because the 15 watt will deliver fairly loud clean and the 5 watt champ gets nice breakup easily.  I sometimes use a 50 watt bassman instead of the 15 watt and there is not much difference in the loudness.  I prefer mainly clean with just a little hair and have found this this to work well.

It has gotten so crazy that now Wampler has made a pedal with a switch that supposedly increases and decreases headroom on a pedal.  It works and is a cool pedal, but I have already heard some guys talking about how it changes the headroom in your amp.  Not possible.

So to wrap up, you can actually change a speaker and double your loudness so wattage alone really does not determine how loud an amp is.  It is only 1 factor and to complicate matters even worse, wattage is determined in different ways.  Also, most people readily think a Marshall JTM 45 is 45 watts, but it is not, it is the name of the amp.  And some Marshall considered to be 50 watts running 2 EL34's, but I have read PRR (one of the moderators here) state easily getting 70 watts from them.

This is a tad over my head in understanding because the EL34 tube is a 25 watt tube and 2 are 50 watts, but a couple of others here can explain it.  I guess if I had a reason to understand it I would, but now that I have mentioned it I will probably have to figure it out.  I have a brain that works like that.

I hope I have not confused you, but feel free to ask about what you do not understand.  That is what is so cool about this forum.  The guys with knowledge normally teach and maintain respect for guys like us that are new to this.

[HotBluePlates]

... the EL34 tube is a 25 watt tube and 2 are 50 watts ...

Tubes themselves do not make power. They allow a small control voltage (sent by the preamp or phase inverter) to control the flow of a relatively big current.

One form of the equation for power is Power = Current² * Resistance. Replace "resistance with impedance" and note that the output transformer has a primary impedance. The output tubes control the current through that primary impedance, and the power in the prrimary is transferred to the secondary and on to the speaker.

You mentioned the EL34 is a 25w tube, but keep in mind that is its plate dissipation. The first push-pull condition on page 2 of Mullard's EL34 data sheet shows an output power of 40w. Other conditions show more and less power.

You could have a 1000w plate dissipation-rated tube and still have a power supply and output transformer that only allowed 5w to the speaker. It would be an expensive and wasteful way to make a 5w amp, which also explains why you don't see any 15w output power conditions on the EL34 data sheet (because smaller, cheaper tubes can fill the output tube role in those amps).

[Jack1962]

Simple answer a single 12 inch speaker at 40 watts will not move as much air as  2 or 4 12 inch speakers at 40 watts . Air movement is the key the more movement the louder it is .

[JustMike]

 It's not just the speaker complement. I've had 50w 1-12 JCM-800 combos that were much louder than this DSL40c. Anyone have a schematic we can look at and see what's really going on?

[SILVERGUN]

It's not just the speaker complement. I've had 50w 1-12 JCM-800 combos that were much louder than this DSL40c. Anyone have a schematic we can look at and see what's really going on?

You're right, it's not just the speaker compliment.....
Do you understand what HBP said here?

Power output is largely determined by supply voltage and output transformer primary impedance.

OR

One form of the equation for power is Power = Current² * Resistance. Replace "resistance with impedance" and note that the output transformer has a primary impedance. The output tubes control the current through that primary impedance, and the power in the prrimary is transferred to the secondary and on to the speaker.

You mentioned the EL34 is a 25w tube, but keep in mind that is its plate dissipation. The first push-pull condition on page 2 of Mullard's EL34 data sheet shows an output power of 40w. Other conditions show more and less power.

You could have a 1000w plate dissipation-rated tube and still have a power supply and output transformer that only allowed 5w to the speaker. It would be an expensive and wasteful way to make a 5w amp, which also explains why you don't see any 15w output power conditions on the EL34 data sheet (because smaller, cheaper tubes can fill the output tube role in those amps).

Re-read that until it registers.....that is the answer to your question

So your JCM 800 probably had higher supply voltages, and maybe a better matched load, and maybe a more efficient speaker......it really can be viewed that simply

The closest schematic that I saw didn't give voltages, and there are no specs for "perceived loudness" 

[SILVERGUN]

Merlin's site has some great info, and I think this page might help expand on what HBP stated above:
http://www.valvewizard.co.uk/pp.html

[HotBluePlates]

It's not just the speaker complement. I've had 50w 1-12 JCM-800 combos that were much louder than this DSL40c.

I could build a 2x EL34 amp that only outputs 10w.

You're right that it's not just the speaker. But it's not just the number of output tubes either; it's the design. Looking at the tube compliment, you can only estimate a range of power output. Normally, we assume the maker will stay towards the top end of that range (which extends down to 0w), because it seems silly to have 12 cylinders and use 4. But not every EL34 amp is the same.

[JustMike]

 Yes, I get it. I'd just like to find out how and where they're doing it. Is it just the power tubes? Are the preamp voltages at the tubes the same as a 50w JCM?
 And things we can only speculate about like why did they decide to go with 40w? Cheaper components? More easily divisible voltages for the op amp stuff? Just curious... this amp really sounds good and has the classic Marshall "Bite" as I call it. The digital reverb is kind of weak, but usable.

[HotBluePlates]

I don't know myself.

But I have noticed a general downward trend in amp power since the 90's. The first amp company that was obviously going lower was Dr Z when they first came out: a ~35w amp was their high-power model, with others at 18w and others still lower. When asked in a magazine article why the low powers, they said the ~35w amp was just the right size for club gigs and most players were wanted something smaller for output tube distortion.

[Ed_Chambley]

... the EL34 tube is a 25 watt tube and 2 are 50 watts ...

Tubes themselves do not make power. They allow a small control voltage (sent by the preamp or phase inverter) to control the flow of a relatively big current.

One form of the equation for power is Power = Current² * Resistance. Replace "resistance with impedance" and note that the output transformer has a primary impedance. The output tubes control the current through that primary impedance, and the power in the prrimary is transferred to the secondary and on to the speaker.

You mentioned the EL34 is a 25w tube, but keep in mind that is its plate dissipation. The first push-pull condition on page 2 of Mullard's EL34 data sheet shows an output power of 40w. Other conditions show more and less power.

You could have a 1000w plate dissipation-rated tube and still have a power supply and output transformer that only allowed 5w to the speaker. It would be an expensive and wasteful way to make a 5w amp, which also explains why you don't see any 15w output power conditions on the EL34 data sheet (because smaller, cheaper tubes can fill the output tube role in those amps).

I do understand what you are stating.  Of course, I have read the Pout on the Mullard sheet and you know I have built a few EL34 amps.  What I don't understand is why the tube is referred to as a 25 watt tube and why are others referred to by their dissipation wattage?  Like the 6L6, 6L6G, 6L6GB and 6L6GC.  Each is referred to as dissipation and I did not understand it until PRR wrote to me dissipation is not wattage.  It did make me begin to look at datasheets and the rest, as they say, is history. 

I was did not get into explaining as I am not as fluent as you and others.  Why all the focus on dissipation and what is the importance?

[SILVERGUN]

Why all the focus on dissipation and what is the importance?

Ed,,,,sometimes I just post to read myself be right  , and other times it's to put my thoughts out there to be corrected....this post will share both qualities

To me, dissipation is much more of an absolute limiting guideline for a tube than any other rating.
Max. voltage ratings get exceeded by hundreds of volts everyday,,, max current is voltage dependant, and most times we just follow the rule for 70% bias settings and all is well...
i.e you could run an EL34 with 200 volts on the plate and bias for 90mA of cathode current (at idle) and wind up with approx. 18 watts plate dissipation
OR
you could run that same EL34 at 500 volts on the plate and bias for 37mA of cathode current (at idle) and wind up with approx. 18 watts plate dissipation

It's a simple equation of Volts X Amps = Watts

Run that same tube with 500 volts on the plate and try to push 90mA current, and the tube WILL fail because you will exceed max. dis. (500 X .09 = 45 watts dis.)

When you start to play around with those numbers, you need a "maximum operating value", and that is max. dissipation.....as long as you keep max. dis. in mind you shouldn't wind up with a glowing orange glass ball, and no sound coming from your amp..

EL34s are probably commercially advertised as 25 watt tubes because that's how most guitarist consumers see them.....2 tubes = 50 watts and so-on......a dumbed-down generalization of a description based on the most common usage.
And, traditionally that's how they have been used,,,,as tubes in designs that are meant to produce approx. 25 watts of output power per tube.

We've been brainwashed  

Respectfully, and still learning right by your side,
LC
 

[SILVERGUN]

I sat and typed this whole big dumb post about dissipation and then went and read your last post on lego's AC15 thread where you so eloquently demonstrated your complete understanding of max. dissipation......
So I just figured I'd stop back and apologize (in advance) for explaining something that you already have a grasp on....

I'll leave the post up just in case Justmike will find it informational...

And I'll stop back and read it from time to time, so I feel better about all the stuff I'm wrong about 

[HotBluePlates]

... What I don't understand is why the tube is referred to as a 25 watt tube and why are others referred to by their dissipation wattage?  Like the 6L6, 6L6G, 6L6GB and 6L6GC.  ...

The EL34's plate dissipation rating is 25w. That you are used to seeing a pair in an amp rated for 50w power output is, to some degree, a coincidence. Consider the 12w (or 14w under some rating systems) 6V6: in a single-6V6 Champ, you know the power output is ~4w. Why is it not 12w if dissipation equals power output?

It's easier to see what's happening in tubes you have no preconceptions about, like the 2A3 power triode. Got an RCA tube manual? If not, you can download them from Pete Millett's site. If you look at p. 19 of RC-19 (other editions' page numbers will probably be similar), you'll see a design example for push-pull class A 2A3's. This is a tube with a 15w dissipation rating, and being class A the example seeks to design for maximum output power by idling the tubes at max, so 30w of dissipation total. Cut to the chase, this setup yielded 10w of output power.

But that's class A, where idle dissipation is at the tube's maximum, and the tube actually runs cooler at full power output, because power is transferred to the (speaker) load. What does that mean?

For class A push-pull 2A3's, RCA specified a B+ of 250v, a peak plate current of 0.2A and a plate-to-plate load impedance of 2kΩ. Page 23 (the class AB design description) mentions the maximum-signal average plate current is 2*Imax/п, and the power input to the tubes' plate is average current times supply voltage. For our 2A3's, that's (2*Imax/п) * Eb = (2*0.2A/п) * 250v = 31.8w. This is how much power the output stage is sucking from the power supply, hence it is the input to the power tube plates. That 2*0.2A/п = 127mA, and would be how much the PT would need to be rated to supply (plus whatever preamp B+ draw there is).

Now follow me... We found there is 31.8w of input to the 2A3 plates for the class A push-pull output stage, and we also said it has 10w of audio output. 31.8w input - 10w output = 21.8w 2A3 dissipation divided among the pair of tubes, or 10.9w per 2A3. They idled at 15w, and at full output, the heating of the plate drops to 10.9w.

Therein lies the difference: dissipation is waste heating of the tube plate, while power output is power actually transferred through the OT to the speaker.

How did RCA figure power output (mathematically and not from experiment)? They related the peak current (allowed by the available supply voltage and OT primary impedance) and the OT primary impedance, in the form Current²*Resistance = Power. Because we (they) found a peak plate current, that is converted to RMS current, and the portion of the OT impedance is that seen by one tube. So, Power = (Imax/√2)² * Primary Impedance/4 = (0.2A/√2)² * 2000/4 = 10w.

So you'll see power output is dependent on the peak current the tube reaches (as determined by supply voltage and OT primary impedance), but otherwise is calculated with numbers exclusively related to the power supply or OT.

The way that Class AB and Class B stages provide more output power with the same tubes is to make the OT primary impedance smaller (allowing a higher peak current), maybe increasing supply voltage (which works with the OT impedance to flow that peak current), and bias the tube cooler so it shuts off during part of the signal cycle. The period of cut-off reduces the average current to the one tube and therefore reduces the power input, averaged over time. If the tube didn't cut off, Power Input - Power Output would exceed the plate dissipation rating.

It is in this way that a pair of "25w tubes" can be run with high voltage, low OT impedance for high peak currents and deliver 50-70w output to a speaker, while dissipating 25w or less per tube (because they're shut off part of the time).

[jazbo8]

From RDH4, Class A triode output power & plate dissipation. I recall seeing a similar chart for the pentode but can't find it now...

[Ed_Chambley]

I sat and typed this whole big dumb post about dissipation and then went and read your last post on lego's AC15 thread where you so eloquently demonstrated your complete understanding of max. dissipation......
So I just figured I'd stop back and apologize (in advance) for explaining something that you already have a grasp on....

I'll leave the post up just in case Justmike will find it informational...

And I'll stop back and read it from time to time, so I feel better about all the stuff I'm wrong about  

SG, man you are way too hard on yourself.  I set aside ego when posting and try to stay ignorant since I can glean so much information from everyone.  I love your posts.  My question was one I had in in my early days of amp repair, which was not that long ago.

I try to remember those things that were hard for me to understand and ask questions rather than blab my ignorant understanding.  The OP is trying to understand why one amp is not as loud and mentioned wattage.  I remember HBP explaining the wattage deal to me and breaking through all the myths and PRR explaining why tube watts seem louder then SS.  They are the same except for compression of the tube.

I just feel there is much more qualified folks.  I ask the questions because I know I am dumb and do not care if you know it.

I also try to be helpful as well.  Logo4040 wan not attempting to understand anything other than how to bias, or really check bias in a cathode bias design.  Not really much different than a preamp tube except for those pesky screen grids they jammed in there to confuse us and is the key to getting the correct Pout for the build he is doing.  Since he asked about the pentode/triode mode I knew he was looking at the data sheet and understood that, he just did not know how the AC15 was designed.  Now he can look at the schematic and see the difference, or should be able to.

I also love it when the knowledgeable folks like HBP and Jazbo8 take the time to explain in-depth.  I cannot yet understand, but remember me saying what kind of brain I have.  I will copy this information and I do have a pair of 2A3 tubes as spares for a radio I own, but have considered a build using them.  I have read where they make a very nice Hifi, so they need to be made Lofi and see what the distortion sounds like.

[SILVERGUN]

SG, man you are way too hard on yourself.  I set aside ego when posting and try to stay ignorant since I can glean so much information from everyone.  I love your posts.  

Thanks man.....I know you know I know you know I'm always just trying to be helpful.

Even just re-stating HBP's posts earlier in the thread.....I feel bad for him sometimes because he gets asked to explain the same things over and over and over and over.......I guess that's how you earn all those moderator perks  
And I agree,,,it's always good to see jazbo hangin around ( I just wish he'd update his blog more often  )

I will now switch back to geetar picker mentality and talk more about what JustMike might be experiencing....

I've been "secretly" working on an amp designed specifically for heavy metal distortion, and after laying it out on the board and tweaking, I thought I had it right where it needed to be........so I decided to do a little research,,and I went into Guitar Center.
I asked my 3rd favorite sales person to show me his favorite heavy distortion sound in the store, and I was shocked when he sat me down in front of a DSL100H....(I had given up on Marshal distortion after the JCM900 series)...but I went with it, and it sounded good...
And then I pushed the "Ultra Gain" mode and scooped out some mids and twisted the "contour" knob and was really shocked by how quiet, fat, tight, harmonic laden, and just flat out bad-ass this thing sounded, with the Master on 2 (so I wasn't scaring any women or children out of the store).
I said "holy crap, that thing is viscious!",,and he said "yup,,that's it's bread and butter".....and I understood

This is where I'm going with this.....
the JCM 800 series was notoriously "dry" sounding (to me) and most guys would really crank them up to get "the goods" out of em...including some output tube distortion.
With this new DSL series, it's a lot easier to get a bigger, badder (not necessarily better) sound at a lower volumes, and therefore not really have to crank it to get "the goods"..
It sounds like everything is being generated in the preamp (probably some ss clipping),,,,and the output tubes are just there to act as a fairly "clean" form of amplification....
The JCM800s (with no built in diode clipping) had to be cranked to get some kind of distortion going......plus, back then, you wanted the most output power for the buck, and no-one would've bought an amp advertised as 2-EL34 @ 40 watts

SO, besides the (probable) lower operating point of the output tubes I think there might be something to the idea that we just don't have to turn them up as far as we used to...

I don't think there is that much difference in perceived volume between a 40 watt and 50 watt amp if the preamps are set up exactly the same,,,,but there is a big perceived difference between 40 watts of a heavily distorted tone and 50 watts of a cleaner "crunch" type tone
 (more compression in the heavily distorted tone?)

JustMike,,,do you use a lot of the built in distortion on the DSL?

[Ritchie200]

SG, since you are in the apology mode, I expect an email or text some time soon........

Getting back to JustMike's question, I think HBP hit the nail on the head and JustMike added the rest.  The trend has been to lower wattage amps as a fairly small amp can easily cover a club and anything larger, a mic is planted by your speaker.  There is really no need for stage volume any more - just enough to give the player the sonic dynamics he or she needs, and be heard over the maniacal drummer.

To JustMike's comment, yes, yes, and YES.  As has been discussed, yes, the output is dictated by the design and operation of the circuit controlling the OP tubes.  Yes, preamp voltages are probably the same.  And YES, it is all completely driven by cost.  A large portion of the cost is in the iron.  You are running at 20% less than the 50 watt counterpart.  Parts can be designed accordingly. That can have a HUGE impact on margin.  Plus, they can still advertise it as an EL34 amp - woohoo!  I'm sure the designer got a bonus that year. 

Being somewhat old school, and just old, I still like the overall dynamics of a pushed front end, pushing the output tube(s), pushing the speakers.  It could be a Champ, an AC30, or a Major, you got to make them work for a living!  No amount of cascaded gain stages and scooping or modeling can replicate that sound.  Sorry SG, but I will never buy into your love of solid state and modeling, techno music and rap.

Jim

[Jack1962]

Here is the only 40 watt DSL diagram I have it's the DSL401 , but it use's EL84's not EL34's , jhowever Mike if it is biased correctly it is putting out 40 watts if it isn't maybe your OT is faulty .

[HotBluePlates]

...  Sorry SG, but I will never buy into your love of solid state and modeling, techno music and rap. ...

But could you buy into solid-technical rap-state? All the cool kids are doing it...

[SILVERGUN]

Et tu, Brute?
I was hoping that as moderator, you would take note of the misdirected hostility that I get subjected to every time this guy wakes up from his afternoon nap,,, and log this on his permanent record....
I swear someone must go out and dig this fossil up just to come here and piss on my rug!....and no I don't mean hairpiece  

JustMike, please excuse the obvious distraction tactics.
The only metal that is acceptable around here is the kind in his hip replacement.
I'm not a metal guy, but I've been commissioned to build a metal amp,,,so I did my homework.
That DSL sent me back to the drawing board, and that was a good thing because I needed to know what was out there.

Solid state, modeling, techno, rap and Deep Purple all have one thing in common....they all make me want to poke my eardrums out.

[JustMike]

 Wow what have I started here? But anyway, I think the DSL-which is quite old by the way-kind of started Marshall into the SS/IC front end into a throttled OT section. They aren't designed to sound any better at high volume than they are at low levels. In my experience with them AS A GUITAR PLAYER, not a tech- they don't get nearly as loud as an old Superlead or JMP either. This new DSL40c doesn't sound any louder than a 22w Deluxe Reverb.

[SILVERGUN]

This new DSL40c doesn't sound any louder than a 22w Deluxe Reverb.

The one thing we didn't talk much about is the max. amplitude of the signal hitting the grids of the power tubes.....

Those 2 EL's will only amplify the signal that they are presented with.....smaller signal voltage, lower power output.

That might be a design technique to be able to keep all of that gain under control and provide stability......? ...considering that most guys will never do more than annoy their wives and/or mothers from the basement with these amps 

[Ritchie200]

Fossil?!?! Why you whipper snapper you!  Get off my lawn!
(ps...so you have FINALLY admitted that Morse sucks!  Congrats!  We are making progress! )
 

Naw, I dont think it has anything to do with stability.  You have done enough cascaded gains to know that you can bust the output without a problem.  Like JustMike said, the DSL comes in all wattage flavors.  I still think it has EVERYTHING to do with cost.  Like you said, most of these amps never make it out of the bedroom.  Put a wicked front end on it and Marshall can sell a boatload.  Nothing wrong with that, unless you are trying to keep up with the guy who has 15 pedals pushing a 50 or 100 watt half stack in the garage.

JustMike, if you put that Superlead or JMP through a single 12 you would certainly notice a difference!

Fossil Jim just up from his nap!

[SILVERGUN]

Naw, I dont think it has anything to do with stability.  You have done enough cascaded gains to know that you can bust the output without a problem. 

I don't know.....the most recent beast has 4 gain stages before a cathode follower fed tone stack into a tube fx loop
I can't get within 3 feet of the speaker or it starts squealing like a pig, and at idle it sounds like I'm running a nuclear power plant  .....now some of that has to do with the fact that it's all sitting out on a big piece of plywood in front of me,,,BUT the DSL was 4000 times more well mannered, as I sat right in front of it and it's connected 4x12 cab and couldn't believe how quiet it was.......even though I had the gain set through the roof.

They have to be dumping signal somewhere....maybe a lower value Master pot?....late stage voltage divider?
Either way, I do agree with you (reluctantly  ) that it comes down to cost,,,,BUT, from a tech viewpoint, I'd like to figure out how you can throttle back all of that gain and not lose the aggression.

On the DSL401 schematic that Jack mentioned earlier, I did see a conjunctive filter across the output after the PI, and I know that will attenuate some..
I'll have to try that and report back.

[SILVERGUN]

And looking closer,,, these 2- 220K series (meaning in series with the signal) resistors must be helping to "choke" things down a bit....right?

I do realize that I am looking at a different model, but I imagine there is a common theme here...  

[HotBluePlates]

... I did see a conjunctive filter across the output after the PI ...

That's not a "conjunctive" or "corrective" filter; that tem is reserved for the thing which goes across a transformer winding, and offsets the reflected rising speaker impedance.

What you circled in yellow is simply a network that trims signal, especially highs. There probably isn't significant loss even when the 4.7nF is a short-cicuit because the 2.2MΩ is so large compared to other circuit impedances. Beadboard testing would be the fastest way to check, but I'm guessing it loses about 10% of signal strength, tilted to favor lows.

And looking closer,,, these 2- 220K series (meaning in series with the signal) resistors must be helping to "choke" things down a bit....right?

220kΩ leading to a 200Ω pot... it's like a 420kΩ master volume pot that you can only turn up as high as 200kΩ, which is ~8.5 on a log pot.

[SILVERGUN]

That's not a "conjunctive" or "corrective" filter; that tem is reserved for the thing which goes across a transformer winding, and offsets the reflected rising speaker impedance.

What you circled in yellow is simply a network that trims signal, especially highs. There probably isn't significant loss even when the 4.7nF is a short-cicuit because the 2.2MΩ is so large compared to other circuit impedances. Beadboard testing would be the fastest way to check, but I'm guessing it loses about 10% of signal strength, tilted to favor lows.

Thanks HBP,,,,call it a terminology fart 
I tried it before I left the shop today and the volume drop was hardly noticeable...but I did notice a slight trimming of highs .
I couldn't go full-bore because there were still innocent inhabitants.

220kΩ leading to a 200Ω pot... it's like a 420kΩ master volume pot that you can only turn up as high as 200kΩ, which is ~8.5 on a log pot.

So I'm onto something there?

Can we call it a collective trimming "effect" from different sources that don't exist on a stock JCM800?
Partially because trimming highs will give the perceived effect of less volume.....?

Or please just tell me that I might be "more right" than Ritchie and that will be cause for a drink tonight 
I don't think you should be allowed to comment on anything modern when you're still playing the same amp you did in 1972 

[SILVERGUN]

220kΩ leading to a 200kΩ pot... it's like a 420kΩ master volume pot that you can only turn up as high as 200kΩ, which is ~8.5 on a log pot.

So I'm onto something there?

So, I can look at VR12a and VR12b as a standard PPIMV?....right?.... definitely not a JMP/JCM800 feature

When I first spotted that dual gang 200k pot in the schematic, I initially pictured it as a PPIMV,,,but then I noticed the insertion of the bias at that point, and I got thrown off because it didn't exactly match the picture of a PPIMV in my head.....

I still don't necessarily get it because,,,how does the setting of that pot affect bias voltage?

OR,,,is it a dual purpose pot that not only varies the amplitude of the signal, but also creates an (inversely proportionate?  ) variable bias supply that rides up and down with the signal voltage, based on where you set the pot????

OR,,, am I giving myself unnecessary brain trauma?  

[Ritchie200]

Or please just tell me that I might be "more right" than Ritchie and that will be cause for a drink tonight 
I don't think you should be allowed to comment on anything modern when you're still playing the same amp you did in 1972 

I have been trying to install a conjunctive filter on your "musical" output for about a year!

Some things you just can not improve on.  Like a fine wine...  Ya know son...back in MY day everyone threw a treble booster on the front of their fav 50, 100, or 200 watt NON master volume stack and wailed away at deafening volume.  Marshall built the MV, but still not quite there.  The 800 cut like a knife with the top boost 80's grind.  900 got too ragged when they tried to keep up the Mesa-Boogie mush.  Now, the DSL is scoop scoop scoop and sounds pretty wicked - getting closer to that treble booster in front of the NON master volume stack.  Imitation is the sincerest form of flattery - thank you!  Your history lesson for today Grasshopper.

Jim

[HotBluePlates]

I still don't necessarily get it because,,,how does the setting of that pot affect bias voltage?

It doesn't.

What is Ohm's Law? Voltage = Current x Resistance.

The is a changing resistance between the output tube grids and the bias supply, but what is the current there? As long as the grids are not positive, there is no current.

So if there's no grid current, there's no voltage change when the master volume pot resistance changes. Therefore, it doesn't impact bias.

[SILVERGUN]

.... if there's no grid current, there's no voltage change when the master volume pot resistance changes. Therefore, it doesn't impact bias.

Ah-Ha, I see........
Ok, while we're on the subject....
would that filter network have a varying effect with the setting of the PPIMV?..I assume it would

I ask because one of the least appealing features of the PPIMV has been the washed out tone at lower settings....could this be a "fix"?

I have been trying to install a conjunctive filter on your "musical" output for about a year!

When you come up with something, make 2, 'cause I'm sure my wife would like one of those  

[Ritchie200]

Silly question, on the back of the dual pots there is a Screen_D designation on one and a small tic on the other.  Obviously the cap values are under the yellow oval.  I am having a hard time deciphering the schematic in this area.

Jim

[Ritchie200]

So if there's no grid current, there's no voltage change when the master volume pot resistance changes. Therefore, it doesn't impact bias.

Ok, then why insert it here?

Jim

[SILVERGUN]

Silly question, on the back of the dual pots there is a Screen_D designation on one and a small tic on the other.  Obviously the cap values are under the yellow oval.  I am having a hard time deciphering the schematic in this area.

It looks like they are blending in the reverb return signal there....

[SILVERGUN]

So if there's no grid current, there's no voltage change when the master volume pot resistance changes. Therefore, it doesn't impact bias.

Ok, then why insert it here?

It's as good a spot as any....it's gotta get to the grids somehow  

[Ritchie200]

Silly question, on the back of the dual pots there is a Screen_D designation on one and a small tic on the other.  Obviously the cap values are under the yellow oval.  I am having a hard time deciphering the schematic in this area.

It looks like they are blending in the reverb return signal there....

How is it connected to the pot?

Jim

[SILVERGUN]

How is it connected to the pot?

 
a forth tab ??

[PRR]

> these 2- 220K series (meaning in series with the signal) resistors must be helping to "choke" things down a bit....right?

Yes. Using EL_84_, that type driver is easily capable of more-than-twice the signal which will throw the sensitive EL84 into grid-block. A 220K+200K divider makes a lot of sense- you can still get into grid-block but not so that it stays blocked for the whole bar.

> I do realize that I am looking at a different model

I doubt there is much point in endless discussion of a quad-EL84 amp when the real question was a two-EL34 amp. They are VERY different tubes. The designer will use/abuse them differently. The number-echo is an unfortunate coincidence.

[PRR]

Not sure why there is confusion between Output and Dissipation. Output warms your ears, dissipation just warms the room.

Look at a simple system. A light switch can control 120V 10A of lights, 1,200 Watts. But the switch does get warm when passing 10 Amps. Good wall switches, not nearly enough to notice. But just say the switch dropped 0.1 Volts when passing 10 Amps. It is heating itself with 1 Watt of waste power. The average cheep wall-switch probably shouldn't be asked to throw-away 3 Watts of power, the plastic softens and bad things happen. So it could have "1,197 Watts max output, 3 Watts max internal dissipation" on its data. But that does you no good because they don't tell you the voltage drop; and if they did, the average handyman wouldn't know how to do the math.

But switching is "simple". Your exquisite musical fingerings need more power manipulation than a simple on/off. For simplicity the amp starts with a "fixed" voltage then carves-away the part you don't need at each sonic instant. Some power goes to the speaker, some power doesn't get sucked from the supply, and some power is wasted in the tube.

Simple Class A (single tube) handing a Sine (we don't play sines but it is the best simple reference) can be at-best 50% efficient with a perfect tube. Tubes are far from perfect several ways so 40% is doing good. A 25 Watt Pdiss tube can output 10 Watts. (If there is an 11 Watt SE number on your EL34 data, note that it is grossly distorted.)

With two tubes we can run entirely class A, all the way to Class B, or in-between. A *perfect* Class B amp on Sine is 78% efficient. 100 Watts in, 78 Watts to load, 22 Watts of waste heat. This means that two 25-Watt devices could output 177 Watts!

No available device is perfect. Lean transistor amps can approach 70%, but most are lower. Strict design tube amps with appropriate "small" tubes can do 55%-60%.

NOTE that if a design comes to 50% efficient, the output power and the waste power are the "same". 100 Watts from supply, 50 Watts to speaker, 50 Watts of dissipation. So it is not "way wrong" to suspect that two 25W tubes might output 50 Watts.

However four 8417 at high voltage can output 200 Watts to load with just 140 Watts of available dissipation, 59% efficiency.

The EL34 is not even a particularly "good" power amp. It was designed to be low-cost, not good. It comes from a long line of low-cost tubes which sold better than their "more efficient" cousins. The knee is so soft that where you might like 400V 400mA swing you really get 300V 300mA swing. It also has a hungry screen grid (not accounted in pure Class B theory). A good "sane" databook suggestion gives 107 Watts supply power for 54 Watts output, 50.5% efficiency.

Yes, the Philips 1969 sheet shows 100 Watts output for 156 Watts from supply, 64% efficient. And note that at this point the dissipation is 20.5 Watts per plate, 7.6 Watts per screen. However to get to this point you need 800V(CT) with 3% regulation, *NEVER* going over 800V(!!). While this might make sense in a transmitter (where regulation may be useful for other large circuits) it can't-be-done in a stage amp (makes much more sense to use a Quad to get your 100W at less-insane voltage).

And ANY amp can be designed "smaller". I had a hot little one-6L6 amp which had been mis-loaded to 2 Watts output (catalog said 9 Watts). I've used an '80 Watt' transistor as a mike-amp delivering 0.2 Watts. 50C5 can be built up for 2 Watts, but millions of $9.99 radios delivered sub-Watt output.

"Usually" tube-cost encourages building "near" the best you can get, so as to put a bigger number on the amp sales-sheet and justify a higher price.

But guitar amps can be funny and Marshall and their EL34s can be funnier. Since they have boo-coo 50+ Watt EL34 amps, they can divide their market by also offering a 30-Watt amp with maybe the same EL34 flavor at different price/weight. (Or maybe not the same flavor.... they may have been side-tracked adding preamp distortion, or they may just have lost their thought like the way the Edsel came together.)

[jazbo8]

Yup, the thread has gotten pretty OT, does anyone know the screen voltage used on the DSL 40c? 40W from a pair of EL34 is pretty low considering the B+ ~ 450V, Marshall must have dialed down the screen voltage quite a bit... Browing the Marshall forum seems to suggest that swapping out the speaker is a very popular "mod", that and the C19 mod (removing or changing the value of the bright cap).