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I removed the decoupling caps just to check them out. No change - the hum was still pretty loud.
For some desperate reason I switched the output tubes from side to side. And nothing else.
Now the hum only starts about 3 or so seconds after warm up, lasts for about 3-5 seconds and then fades away. Well, not completely away. But to the point of where you have to put your ear up to the speaker to hear it.
It's quite listenable for a garage amp. But it would be a problem with critical listening.
Any suggestions as to how this is happening and hopefully how to fix the remaining hum?????
Follow Ups:
Voltage across R19 and R21 --- 109v across and all four at 250vdc at plates
R5 bypassed --- music does pass undistorted. low hum both channels with Left a little worse.
Bypass R9 with 100uf cap --- Hum worse in LEFT then drops very low. Hum is still present in the RIGHT but very low. Music passes well
Bypass cap C13/15 to grd leg of R5 --- Hum starts real loud on LEFT then fades. Both have low hum with the LEFT a bit louder.
Disconnect NFB --- Hum starts real loud on the LEFT and fades with low hum in both channels. Music passes well.
I switched the output tubes from side to side and the loud LEFT side hum stopped but the low level hum remained.I went to check the bias and there was NO bias voltage on the front RIGHT output tube. Wiggled it and not only did the bias voltage come back, but the loud hum on the LEFT returned.
I'm also measuring 49vAC on the bell housing to ground on the power transformer.
I just reflowed and tightened all the output sockets and 10 ohm ground resistors.
I'm kinda stupmped? I checked every friggin connection in that amp.
Edits: 01/04/22
Sounds like you have some iffy socket/tube pin connections or dirty tube pins.
It's good practice to isolate the transformer frames and ground them to the (single) safety/chassis ground point.
" I switched the output tubes from side to side. And nothing else."
Again... have you replaced the CCS? In addition to isolating the PI from signal ground, its functionality is related to output stage balance.
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Buy Chinese. Bury freedom.
Yes, I did replace the CCS LM334 twice. What that means is I ordered two. O put the first one in and it still hummed AND the cathode was still measuring 13.5v (10.5v normal) so I replaced it again. The hum was still there with both LM334's.
I checked and rechecked to make sure the cathode ground wasn't interacting with anything. No luck - the hum continued. For some reason, mainly out of desperation, I switched the output tubes. You have no friggin idea how stupid I felt. Since swapping tubes was the FIRST thing I did.
I'm still battling a very low 120hz (I think) that is in both channels. When I have some time this week I'll see if it's the gain tube causing it. I still have a problem with the one channel (the start of this post) that I have to correct. But I don't really know if that problem could be causing a low hum in both channels????
Do you measure the correct voltage drop across R19 and R21?
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Buy Chinese. Bury freedom.
I have in the past checked that voltage and it was always somewhere in the 20% of the schematics value. And that was on a good day.
I will be able to get back to testing Tuesday. I also have a few things Trobbins wants me to try.
aren't your driver tubes in pcb mounted sockets?
I once gave myself some sweet 120HZ hum in a little used guitar amp changing an input tube on a pcb mounted socket ... don't know if it was me or the prior owner but someone lifted & broke a trace!
went nuts figuring it out ... well, actually I didn't, my cousin did, I still went nuts though!
it sure sounds like you've got a similar grounding issue with a socket
just a WAG because you said it improved plugging and unplugging tubes
do you have any freeze spray you could spray around the sockets & see if there's changes? might be worth a shot
with regards,
I've you've already cleaned and tightened all tube pins and socket contacts maybe do it again just to be sure.
If they are not seated or connecting and are just a smidgen off, tubes won't operate at their best, or almost instantaneously, from power on. Including cancelling transformer hum in the circuit.
That's why you hear the hum for a few seconds before it mostly goes away.
The only other thing that comes to mind is the inmate who was going crazy with a similar problem by having his cell phone nearby to take pictures of his bias settings. Sheesh that was a fun discussion. Look around for potential culprits.
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
Years ago I kept hearing a one sided hum that was only heard from my listening position. Weeks of getting up and putting my ear to the speaker - nothing. Even stranger was that it stopped during the day???? I tried everything to figure it out.
Long story short - it was one of those early LED lightbulbs in the lamp next to my head.
I remember the cellphone prrrpppp blu blu blu blerppp.An early V1505 breadboard that near enough filled my dining room and a hum I couldnt trace. Music sounds so much better when everything quiet, no industry, early hours of morning, maybe only the occasionally fridge freezer motor, but otherwise the best time to listen to music. I turned the lights off, listened to radio 3 FM not digital. In seventh heaven except for this hum.
Had a neptune potted choke, as the lights were off, I saw even through the pot (probably must have arced and burned a pinhole to see arc, not visible with lights on or daylight.) Might have saved my life turning the lights off. Power supply was a Boat Anchore 3kvct about a foot by a foot by a foot in domentions and almost my body weight with two big eyelets for crane lifting. The transformer wasnt the issue but the potential for it to provide fuel to the fire was life threatening.
My advise: just be satisfied with 45's 10y's 2a3's, and just because a choke or transformer is potted does not mean its not the cause of a house fire or at least hum!
Edits: 01/03/22
Sounds like it is working fine to me.
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