humming loudly

[myker]

hello, i turned a jtm45 reissue into a twreck/jtm45 with the jtm45 power section and the twreck pre. being new to the plate driven tonestack thing, i used my regular buss on the pre/star on the power grounding scheme and i have an extremely loud hum that goes away when i remove v1 stage 2's cathode resistor, i cant seem to ground it anywhere without hum. and lots of it. can anyone show me their grounding technique?
miike

[Mark]

Dear Myker

I can only suggest that you check out some of the photo's that have been posted up as well as having a look at the drawings in the files section.

That hum does sound quite unusual. Perhaps you can take some photo's of your amp and people here can look it over. To be honest, I'm not sure which part of the circuit is giving you trouble though I assume it is the first preamp stage.

[myker]

hey mark, thanks for your reply, it is a strange hum. i am now almost convinced that it isn't a grounding issue, or a tube issue. i have many tubes and have tried them all (almost) and the thing is wierd is that the whole chassis is microphonic. all the grounds, every wire in the thing is like touching a record needle. i think that i have traced it to stage 2 because the noise stops when i pull V2. (i stated on V1 triode #2) maybe the gain is just too high with the 22uf cap on there. i usually use 1uf on the cathodes on other amps, but this amp sounds so good with the 22uf there.
mike

[Mark]

Dear Mike

Rather than simply pulling the valve. Have you tried removing the hot end of the volume pot (attaches to the wiper of the treble pot) and tried adjusting the volume control?

If the problem is in the second stage and gain related, you should hear silence and then the noise will gradually kick in.

[myker]

hey mark, i got the amplifier to behave and be quiet, now riddle me this...
why would i not be able to use a cathode capacitor across the 2.2k resistor?? if i take the cathode cap off, the amp is normal, if i add the cap the amplifier goes to about half volume and has no gain.
what gives?
mike

[Elcabong]

Mike,

just a hint. Check the last paragraph here http://www.aikenamps.com/BlockingDistortion.html about blocking distortion in preamp stages.

[Dai H.]

besides oscillation possibilities the cathode is the return point for the AC plate current from the plate of that tube and the cathode bypass will couple the AC coming from the plate through the local filter cap back to the cathode. Possibly some of the grounding is off. Putting the ground in the wrong place?

[69SG]

That hum does sound quite unusual. Perhaps you can take some photo's of your amp and people here can look it over. To be honest, I'm not sure which part of the circuit is giving you trouble though I assume it is the first preamp stage.

put a 10k grid resistor right on the first stage socket between the grid and the input.
Add a 1 meg attenuator inline with a 100k grounded resistor between the first stage coupling cap and the second stages voltage divider.