18W Modern Classic TMB build random pops and cracks

[3jaw]

I just finished the build and fired it up through a dim bulb tester and to a dummy load without tubes. No shorts, so I turn it off, discharge the caps and add pre-amp tubes one at a time and check for shorts again. All is well so far. Added both power tubes and 150 watt bulb barely lights, which I expected. Turned it off, plugged in a speaker and the trouble starts.
The first problem I ran into was severe static at one volume setting and random chirps (birdies) on the TMB channel. I thought I might have a bad volume pot but after chopsticking, I found that if I moved the pin 7 lead on the PI tube that the static and noise disappeared. I shut off the amp and inspected my solder joints and they were sound. The tube socket pin holes were tight as well. On a whim, I changed tubes and the static and chirps disappeared. Okay, I thought, problem solved. Now there are random pops and crackles coming through at just about any volume level above zero. I chopsticked some more and the only reaction I get are thumps when I tap the .022 and .01 coupling capacitors from both channels into the PI. I suspect that I have bad pre-amp tubes (new JJ's, I know, I know ) but I want to ask the experts here.

Attached is the voltage sheet and a gut shot. The line voltage here averages about 125V.

[Bieworm]

Hi

Looks like a decent job all in all, but I would start chopsticking for bad solder joints after you've ruled out bad tubes.

[3jaw]

Thanks for your reply, Bieworm. I found the problem. It turns out that the amp was picking up rf signals from my cell phone and wi-fi in the shop. My first clue was when there was a series of "machine gun" noises coming through the amp. I knew that wasn't random, so I unplugged the router and moved my phone away from the amp and all the pops and cracks disappeared.

I did add a 20v and a 15v zener between the 1st and 2nd filter caps to drop the voltages. The voltage readings are for the most part better but there are still a couple that are a bit off, particularly V1 plate voltages are about 13-15 volts high and the PI plate voltages are 10 volts low. Power tube screen and plate voltages are 4 volts low and 5 volts high respectively.

I am going to have to change the bias resistor. I'm only getting 29.8 mA plate current per tube. That's 80% dissipation. Too cold per Rob Robinette's bias calculator.

[Bieworm]

Hi

the voltages don't look out of the ordinary. Raising the plate dissipation on the power tubes will lower overall voltages to some degree, but I wouldn't bias the amp much higher than 85% (the EL84 tubes will last longer) and don't get fooled by people that claim 100% bias is ok in cathode biased amps. Those break down frequently and give EL84 amps a bad rep. I.E. Voxes. The sweet spot for EL84's sits somewhere between 75 and 85% plate dissipation.
I suggest you try a 180 ohm bias resistor and see where that brings your amp voltage wise.
I wouldn't go into the fine tuning B+ dropping resistors rabbit hole. You can keep the 200R resistor too if it sounds good to you.
I know out of experience it's tempting to experiment with component values, but in the end you'll have a messier build and introduce more problems/noises in the process. So I'd say try a 180R bias resistor and evaluate. When all is fine settle down and start playing the snot out of it

[3jaw]

Thank you for your help, Bieworm. I will order a 180 ohm power resistor and try it.