100Hz Hum issue - hmmmm.... please help!

[jimmyflanders]

Thank you to everyone for your help, I actually redid the wiring for the preamp tube sockets (better twisting and used the ampwizard style of wiring to the lugs), I also redid the grounding wiring, and added a PPIMV which I think is great, and a better impedance selector switch. Hum is no longer an issue I think - even with a gain pedal in front its now much much quieter.

However.... I now have a new problem!!! I think only one side of one of the tubes is working as I've not got much gain. It's pretty clean suddenly, even with the high and gain on full. Now, to be honest I quite like it, as it takes my pedals well but its definitely not right so want to fix it.

I've gone in and done the usual and can't feel for any loose joints, I've taken readings to each of my tube pins and think there's an issue around tube 2? If I plug into the low input I get the same clean tone but quieter, and there's still plenty of actual volume level (its a beast in terms of power amp volume).

Ive swapped both tubes 1 and 2 for 3 different tubes and still then same issue, heaters all seem fine.

Here's the readings:

Pin 1 2 3 4. 5 6 7. 8 9
V1. 250 0. 3v. 3.13v 3.13. 204v 0v. 1.87. 3.13
v2. 161. 0. 0v 3.13 3.13. 289v. 161. 162. 3.13
V3. 235. 23.7v. 38. 3.13v. 3.13. 223v. 24.8 38v. 3.13v

Should pin 3 of v2 show a DC voltage?

thanks again amp masters!

[jimmyflanders]

Its been 'clean' since I redid the wiring, and was like it before I put the PPIMV in. I've checked, double and triple checked the diagrams... but can't find a wiring issue.

[jimmyflanders]

I think the 24v on pins 2 and 7 shouldn't be there on v3!! and pin 1 v1 seems high?

[sluckey]

Voltages look fine except for V2 pin 3. That should not be zero. Pin 3 may be shorted to ground.

[jimmyflanders]

Hi thanks for that, it isn't shorted to ground, but you mentioning that pin and me looking had prompted me to look at the 820k resistor that is tied directly to that - I uprated a couple of resistors and just did a colour coding check and its a 10k instead of 820k, not sure how that happened!!!!! So might have something to do with it. I'll have to check I haven't put an 820k somewhere else where a 10k should be, but I can't see it, looks like all the 10ks are correct.
I'll swap it out in the morning but hopefully that's something to do with it

thanks again - if that's its then I'll be really chuffed!
Still a novice really at this stuff but really pleased this time with how its come out, the layout is so much tidier now I've redone some of it.

[jimmyflanders]

Although, that 820k runs to ground from the pin, so actually it might not be that (although still needs a swap!)

[pompeiisneaks]

820k to ground is a MAJOR difference from 10k to ground.

~Phil

[jimmyflanders]

I was hoping it was Phil, but didn't want to assume. You guys would know much better than me. Could definitely explain why it works great but is just very clean if I'm dumping a ton of that part of the preamp stage to ground (I'm sure that's not technically correct!).
But be great if that's all it is!

Jimmy

[pompeiisneaks]

It's not about dumping tone to ground. It's the cathode of the tube, which helps set the bias point. if the tube's bias is off it will perform radially different from what was planned/expected.

I do want to confirm something, per the amp you detailed, the cathode resistor is either 820R (not k!) or a 100k depending on if you're talking about the first or second half of the cathode follower for the tone stack.
Make sure you're doing that right. If the cathode was at 10k instead of 820 then it woud likely severely throttle that tubes operation and lose gain for sure.

the 10k might have been what's supposed to be on V1a's cathode?

At any rate, just make sure to triple check the resistors values on the schematic/layout and yours and ensure they're right.

~Phil

[jimmyflanders]

Hey Phil thanks for that I've just double checked and yes I've got a 10k where the 820 ohm one should be, so looks like its the culprit.

I'll give it a whirl tomorrow hopefully and double check everything else at the same time.

Thanks again

[jimmyflanders]

Hi guys!
So - I've replaced that 820r resistor and I now have 1.93v on pin 3, which I think is much better than 0.1-0.2v that I had before, however, the preamp is now much louder, but no more gain than before in terms of saturation! When the amp was working previously it had that nice medium/highish gain crunch (not crazy amounts, its an 800) but this is no-where close, it is still predominately clean.

Gone over every solder contact again, all the voltages on all tube pins are now in the right range, and the power section is biased fine (between 73-75% dissipation).
Not sure what to try next!!!!!

I've ordered some new screened cable to replace what I've got going to the 68k resistor on tube1, and also the run from the tube to the volume pot, could that make a difference?
Totally at a loss