Is there a possibility to increase the gain in this circuit? (Marshall DSL40/100)

[Pedro Vecino]

Hello everyone. This is the circuit that the modern Marshall DSL40/100 uses before the last two switchable master volumes. The output goes directly to the phase inverter.

Having a clean channel somewhat limited in terms of maximum volume, would it be possible to increase the gain in this circuit while maintaining its linearity in order to have a higher overall output level?

The common HT1 power node for both transistors is taken from the cathode follower node (this should be around 270V, I haven't measured it) and a 15K resistor in series along with a 22uF/450V capacitor to ground .

Can it be feasible? The interest is to maintain the current relationship between the channels.

Thank you so much.

[Helmholtz]

Rather not. At least not appreciably.

I don't understand the problem. Can't you get full output?

[glebert]

Rather not. At least not appreciably.

I don't understand the problem. Can't you get full output?

I believe these amps are known for being much quieter on the clean channel than on the lead. That may have as much do with with how we perceive clean vs dirty as anything. I would consider putting a clean boost between the guitar and amp. If you can get more clean volume out then there is still some headroom. If it starts to break up you know the circuit is maxed out for clean headroom. Maybe try the boost in the effects loop too.

[Helmholtz]

But increasing gain or using a clean booster would increase gain of both channels, so not change the difference between clean and lead?

Isn't it possible to get a good channel balance using the 2 masters?

Still don't understand.

Please post full schematic.

[glebert]

But increasing gain or using a clean booster would increase gain of both channels.

I meant just as a test to see if the clean channel has additional headroom, but one could presumably back off the gain of the lead channel to compensate for the boost if that got someone closer to the desired result.

[Helmholtz]

Isn't the clean channel loud enough with its master fully up?

[Pedro Vecino]

I have not measured it, but guitar in hand the clean channel does not seem to develop all the desirable potential or is at the limit. The amp is new (DSL40 combo with 2xEL34 and a single speaker).
I was wondering if there would be any margin in that area of the circuit as an easy route but knowing that there wasn't, I'd better forget about it.
I don't have any more sheets than that. A little boost pedal in the loop for sure that also works for when needed.
Thank you very much to all!

[loudthud]

You might be able to get 6dB more gain by bumping R53 up to 470K.

Edit: Or just remove R53 and see what you get.

[Chuck H]

Agree with Helmholtz position. Measure the output of the clean channel with a scope, master at full to see if it's delivering full output. And I would agree that if it's not then a modification is in order. Otherwise it's just a matter of setting up the amp correctly. There are a few amps that have a master for just the higher gain channel while the clean channel only has a single volume control. This always seemed like a smart design to me. But the ultimate in versatility would mean having a "master" for each. That doesn't mean that just because there's a master volume for the clean channel that it must be at some level of trim only because the dirt channel is. Just set the controls where they work best. If the clean channel is anemic even with the master at full then there IS a problem. Otherwise just set the master on full for the clean channel.?.

[Mick Bailey]

I would scope the drive channel to see where clipping occurs when the controls are set for maximum output. I'd be looking at the input at R23, the wiper of the relevant MV, the output of the PI and the speaker output to get a full picture of how the amp behaves. If the signal is clipped on the PI outputs, then increasing the gain of a pre-PI common stage will make the drive channel clip earlier and alter the relationship between channels. You'll get more output on the clean channel at the expense of the drive channel if this is the case and the gain would need increasing further back and only on the clean channel. A schematic would be useful, but the ones floating around all appear to be for the earlier DSL40.

+1 on increasing R53 to increase gain of this stage, but I'd want to see what's going on beforehand.

[Helmholtz]

Oh yes, increasing the feedback resistor R53, good point.
I overlooked that possibility. Might introduce some transistor distortion, though.

[g1]

From what I've read, Pedro has extensive experience with modifications and is very specifically asking about this one particular solid-state stage.

[Pedro Vecino]

Thank you very much to all again. The feedback resistor that Loudthud mentioned is exactly the same one that I first paid attention to as a possibility. But I don't have the knowledge to calculate the possible margin in that circuit.
I have connected in series a 250K logarithmic potentiometer and there is the solution. It fits like a glove. It has no strange deformations and is a natural extension of the volume. Finally I will use a 220K fixed resistor in series to compose 440K.

This question that I have raised has its reasons:

- I think that a user of a Blues Junior (it is one example among many possible ones) would never appreciate it. This amp has more volume and there is nothing to calculate. The same in the case of playing at home and never getting to use that volume threshold that I'm talking about. But the same does not happen if one uses other more powerful equipment, as is the case of the owner (Bogner Ecstasy, Friedman BE100 and some others). He wanted a smaller amp to accommodate certain scenarios. The starting reference is completely different.

- Another reason is the existence of the two master volumes when the second one is used for solos. It forces the first to always be below the maximum in order to achieve the necessary difference in volumes.

- Added to this, the amp is a single speaker combo (compared to the others that use 4x12 and 2x12 cabinets), with a narrower focus, which intuitively draws more attention to these volume-related things.

This is how I understand it.

One last question is what to do with the two attached capacitors. I figure the 22pF needs to be halved, but what to do with the 47n connected in series with the resistor?

Thank you very much to all!

[Helmholtz]

C27 passes all guitar frequencies - I'd leave it alone.
C43 attenuates frequencies above 33kHz. When doubling R53, C43 should be halved.

[Pedro Vecino]

Thank you very much Helmholtz. Tomorrow I'll look in my boxes of infinity capacitors and finish it