Marshall DSL 401 - NFB???

[Lynxtrap]

Removing fizz and livening up treble is an act of balance in a multi stage amp. It's likely no coincidence they went to such length to attenuate highs, with the snubbers and caps between plates and cathodes. Still, you might do fine removing those if you only use the clean channel.

If I was planning on pushing the output stage a lot, I might put in higher value screen resistors, like 1-2K, 2W. Some argue that this also makes for a more touch sensitive amp, but I have never A/B'd that. But it would make life a little easier for the tubes.

Put the lower gain tube in V1 or V2. It won't bring down preamp overdrive if you put it in V3.

[Littlewyan]

This amp as others have stated has a lot of drive on tap. It has 4 gain stages before the cathode follower and by the looks of it they're getting hammered pretty hard, which is probably why it's so 'flat' sounding or lacks touch sensitivity. I would personally start by changing the 2.2K Cathode Resistor (R123) on V2 to something like 4.7K. See if you like the change, if so, maybe take it even higher like 10K. Then look at tone shaping like removing snubber caps or changing values etc.

[Littlewyan]

Actually just looking at the Marshall DSL15 schematic I can see that Marshall lowered the gain a fair bit when they updated the circuit (still tonnes on tap though). Maybe try going straight to 10K and see how it sounds.

Or if you're feeling really adventurous you could partly turn this into an SLO100

Also increasing the screen resistors would be nice as others have said.

[diddymix]

Thanks guys this is all helpful stuff! Could I ask anyone... are the screen resistors the 4 x 100 ohm tied to each EL84 (pin 9)??? If so is it safe to just increase them.. to how much?

[pompeiisneaks]

yes those are the screen resistors. Yes it's fine to increase them. I think I've seen all the way up to 2k, but more commonly it's 1k. Just make sure they're 5W resistors. It helps drop the voltage and keep the screens from taking too much of a beating. (older marshalls used 470 ohm I think as well? At any rate it's a common addition/mod to keep the power tubes happier longer.)

~Phil

[Lynxtrap]

Yes, like I wrote earlier, between 1K and 2K on the screens. Some people recommend 2.2K for EL84's.
470 ohms is the typical Marshall value for EL34's, but I wouldn't go under 1K.
Here they put a 470 as dropping resistor to the screens, but that doesn't function as a screen resistor.

5W might be overkill, but I guess it's better to be on the safe side, also considering how hot these amps run.

[diddymix]

Thanks everybody! I have decided to give the pentode/triode switch a bash. A gentleman kindly put up a diagram on page 1 of this thread on how to do it, but I am confused after studying his suggestion (in red marking)... Forgive me if I am wrong in my assumption or missing something but if someone could point out whats going on I would most appreciate it!
He has done 'modified' the schematic in red showing to remove the B+ supply from the the EL84 screen grid resistors.. and then have the two pairs switchable to either the B+ for pentode, or the plates for triode... The only thing I do not understand, is if the screens are connected to the plates via a switch... then I can't see how the screens recieve the B+ power.. as that connection has previously been isolated for the switch as he's drawn with the red 'x's' ..... Do the tubes no longer need high voltage DC power if triode connected???

Thanks people

[sluckey]

When switched to triode mode the screens get B+ from the same place the plates get B+, ie, through the OT.

[diddymix]

Hey thanks! What so the OT carries the B+ on both sides not just the middle CT??

Also with regard to the switch is it a DPDT i need?? I'm having trouble finding one thats over 240V... is this a problem as there will be over 300V going through the switch??

Appreciate any help!

[tubeswell]

What so the OT carries the B+ on both sides not just the middle CT??

An OT primary is one winding. On a push-pull OT, the winding has a wire connected at the 1/2-way point in the number of winding turns - i.e. this is the centre tap. If you apply a DC voltage potential to one part of the winding, (more-or-less) the same potential will be present on all other parts off the winding, because it is all connected. Hence each end of the winding will be at the same potential as the centre tap.

[Lynxtrap]

Hey thanks! What so the OT carries the B+ on both sides not just the middle CT??

If it didn't, where would the voltage on the output tube's plates come from?