Noise from 1 channel. I have tried the following so far and no luck. Suggestions?

Hi All, I have had my ST-120 and SP-14 for about 1 year or so now. Everything has been fine until today. I had the Weber SS Rectifier go bad and 1 right side (front) output tube go bad. I replaced them both (put original tube rectifier back in) and powered it back up. I set the bias too. But I noticed a hum noise from the amp even with no input. It doesn't sound like the hum gets louder when I turn up the input volume. Also I put in a switch to turn on and off the house power ground inside. It doesn't make a difference either way. I have tried these steps and not sure where to go.

I also have the yellow paper diodes installed.

1) took off input interconnects
2) Swapped output speaker wires. Right speaker noise moved to left side. Meaning it stayed on right amp output.
3) Swapped around driver tubes.
4) Replaced all 3 driver tubes. No change so I put my other back in.
5) Swapped around Output Tubes, even though Bob said they don't cause hum.
6) Took all tubes out and checked pins 5 and pins 6 of output tubes to ground. With bias pots in the middle. All 8 pins about -47vdc to -50vdc
7) turned off and checked 10 ohm resisters on each output tube socket. All good.
looked inside for "browned" components
9) I never used the volume knob, so I removed it from the circuit. No change.
10 Also I put in a switch to turn on and off the house power ground inside the cabinet. It doesn't make a difference with the Right Side Noise either way.

I got these voltage check hints from searching this forum. I wasn't sure something to try before swapping the output wires from the VTA circuit board. If that would show anything.

Thanks, depressed something isn't right, with my right side... Scott

Scott, read the STICKY above on the home page : Quick troubleshooting guide. Also, use SEARCH. Update back and we'll get this. Dale

Read the hints again and found suggestions on the internet too. I added the following but not resolved yet. Still only the right side and hum does not change with volume. Next I may swap the PCB L/R outputs and try a star pattern ground setup. Since it is only on one side I know that this amp is capable of being dead silent just like my SP-14 preamp is.

11) moved power switch from front to rear. Shortened wire to be just where the switch is.
12) removed incoming power earth ground switch and went straight to the chassis.
13) triple checked tightness on all the chassis ground nuts.
14) Replaced the three 12 AU7 tubes with new ones
15) Replaced the 4 6550WE tubes with new 6550 tubes
16) Replaced the rectifier tube with a new one.

What do you think if I get jumper wires and jump all the ground points together. I wasn't sure if that may cause ground loops or not. I also have not touched the quad cap because hum is only on 1 side and doesn't change with volume.

Did you check the input jacks for solid connection? A loose connection on shield?

Rec wrote:Did you check the input jacks for solid connection? A loose connection on shield?

Yes I did. I made sure that the ground parts of the RCA jacks are not touching anything. This right side hum happens with no input connected too.  I read somewhere that the LM334 (?) on the driver board can cause issues. Not much detail on it, except others saying it goes bad sometimes. I don't have a scope anymore unfortunately to look for noise.

I had to replace an LM334 on my ST120 a while ago.

During a listening session, I heard, IIRC, a brief crackling or static /rustling sound. From then on, volume was greatly reduced on one channel. I moved the speaker cables side to side to confirm the channel. I changed the driver tubes side to side, new center tube. Nothing fixed it. Checked all solder joints, no improvement . Had a back and forth with Bob and he suggested the LM334. That fixed it for me. I didn't respond earlier to this as my amp never had a hum.

An LM334 on the VTA octal board was the problem in my markIII. See my thread from 8/28/21. No output other that a soft noise/rustling sound.

E.

Well things have gotten worse now after trying to fix the 1 channel noise issue.

I forgot to put the A/B switch over and took out one of my tubes. At this point I took the ST-120 back to the bench. I have been reading about noise from EMI wires off the tube power wires. The wiring I did on the one side with the noise, didn't look as good as the non-noise side. That and the output wire from the board was close to the heater bias wires. I put new bias wires (newly twisted tighter than before) and moved them to be farther away from other things.

I connected up hoping I had found and fixed the 1 channel noise issue. Well that same noise was there on the same channel. BUT the other channel which was perfect before, now sounded like the "rustling leaves" that the others have mentioned. I never checked the output after replacing the tube that went bad (same channel as new rustling noise) so I don't know if something happened from forgetting to flip the A/B amp select switch. I did not make any changes to the amp side where I now have rustling leaves while inside because it was dead quiet.
I have ordered up some LM334Z transistors to replace it on the board.

This is one of those, maybe I should of left well enough alone.... But, I hate having that noise on the one channel. Scott

Don’t know if you tried connecting the amp directly w/o the A/B switch to isolate the problem? Also, these amps don’t like not having input and or outputs disconnected?

New2Tubez wrote:Don’t know if you tried connecting the amp directly w/o the A/B switch to isolate the problem? Also, these amps don’t like not having input and or outputs disconnected?

I actually added the A/B after the issue, so I could easily switch to back my Onkyo SS amp while working on this amp.  I won't forget again... haha

While waiting until my LM334Z's come in, I figured I would double check all soldier joints and wires. There was one wire on the side where I lost a channel, looking a little bit "dull". It was one of the ones at the driver tube. I hit it with the soldier again there and a couple other places to be sure. Well that got my channel back! So now I am not sure what to do next. Do I just rewire everything up again from scratch to get rid of the hum/noise on my right channel. Or go just buy a Rogue Audio amp...

Thanks Bob. Wife and I are listening now and it still sounds amazing! I have our daughter's (our first grandchild) Baby Shower this weekend, family converging on house starting tonight. I will get on this Monday/Tuesday. Thanks!

UPDATE:  WooHoo!!! All silent now. Thanks Bob and Roy for helping me out via emails! Great support from both of them as I pulled my hair out. Final fix was to order a new VTA Driver Board. Roy suggested I get the NEW octal version using 6SN7 tubes. I did move the option Russian Caps I had on the 12UA7 board over to the new board. I did put in Cardas RCA jacks and used the shielded wire left over from my Pre Amp SP-14 build for the input wires to the driver board.