Diagnosing sub low issues with modded Marshall clone

[jtgainor]

Will try to keep this brief. Had a buddy mod a Rockitt retro 1987 clone up to replace a Golub Crunch/Dookie amp I recently sold. When I received this amp, it seems to have excessive low end, mostly in the sub lows. Have swapped 3 different sets of preamp tubes in, and have also swapped in some brand new el34’s biased at around 65% (plate voltage of 440 volts read via Euro tubes bias probe). B+ voltage to the preamp is reading 279v. Trying to diagnose this issue before I drive 2+ hours to the nearest reputable tech in my area.

 

[tpruitt]

What the nfb at?

 

[jtgainor]

What the nfb at?

8ohm / 47k

 

[lespaul6]

Is that one done by Julio? I noticed it sounded really dark in the youtube clip

 

[jtgainor]

Is that one done by Julio? I noticed it sounded really dark in the youtube clip

Yes it is.

 

[lespaul6]

Yes it is.

I asked him about it and he mentioned he used a "dark" cap instead of the bright cap found in the crunch mod. You might want to ask him about it. It doesn't sound right to me.

 

[FourT6and2]

Where are you reading 279v B+ in the preamp? That's quite low. B+ feeding the preamp from the screen node should be the same as... the screen voltage and quite close to your Vp of 440v.

B+ before your first 10K dropper should be around 440V. B+ after the 5K6 dropper should be the same as your PI node. Is that where you're seeing 279v? If so, that's quite low. And I would simply remove/bypass that 5K6 to get your preamp voltages higher. This will tighten and brighten the amp. But I'm not sure what's causing any sub lows issue. Everything I can see looks fairly standard.

Your first pot has both a bright cap and a cap acting as a low-pass, dumping highs to ground. You can try changing the cut-off freq. for the low pass filter by changing the value of that cap or simply lifting the cap out of the circuit, which will bring more highs back. BUT... this won't affect your low end, if that's actually the problem you're experiencing.

The caps bypassing your V1a and V1b plate resistors are also doing the same thing—shelving high frequencies.

Everything else looks fairly tame for a Marshall mod.

 

[jtgainor]

Where are you reading 279v B+ in the preamp? That's quite low. B+ feeding the preamp from the screen node should be the same as... the screen voltage and quite close to your Vp of 440v.

B+ before your first 10K dropper should be around 440V. B+ after the 5K6 dropper should be the same as your PI node. Is that where you're seeing 279v? If so, that's quite low. And I would simply remove/bypass that 5K6 to get your preamp voltages higher. This will tighten and brighten the amp. But I'm not sure what's causing any sub lows issue. Everything I can see looks fairly standard.

Your first pot has both a bright cap and a cap acting as a low-pass, dumping highs to ground. You can try changing the cut-off freq. for the low pass filter by changing the value of that cap or simply lifting the cap out of the circuit, which will bring more highs back. BUT... this won't affect your low end, if that's actually the problem you're experiencing.

The caps bypassing your V1a and V1b plate resistors are also doing the same thing—shelving high frequencies.

Everything else looks fairly tame for a Marshall mod.

So I measured 279v at the red dot. 139 at the green dot, and 199 at the red dot.

 

[FourT6and2]

So I measured 279v at the red dot. 139 at the green dot, and 199 at the red dot.
View attachment 428448

The red dot is your V1 B+ node. That is the very end of the preamp B+ line. So 279v there is fine.