Ampeg B15 Hum issue

[Arbutt]

Hello again
I Have an issue with hum when i turn up the volumes on both channels on this amp, but when i turn the volumes down its quite

It is much to much for this amp if I can get the picture uploaded you will see what I'm talking about

This is B15N 110v, All original and in poor state, so i replaced all the caps and as 2 of the 3 6SL7 were rattling , i replaced those and fitted new power tubes, Customer is thinking about a rewind of the Power tx

Replaced 40-40-40uf for 50-50uf 500v and an additional 47uf - Grounded to the same point( Cap Clamp mount )
Replaced 30uf 500v ground to original ground location
Replaced 22uf 25v
Replaced bias caps
All new tubes 3 x 6SL7 ( JJ), 2 x 6L6( TAD) , Gz34(JJ)
Tried to fit extra caps - no change
Hum balance working
Cleaned pots and switches

Ac 117v
Heaters 6.1v
V1 Pin2 200v V1 Pin5 192v cathode 2.5v
V2 Pin2 200v V2 Pin5 183v
V3 Pin2 285v V3 Pin5 278v
V1-V2 supply 410v
Pi supply 415v
power tubes anodes 473v
Adjust the hum balance and with the volumes down you can get it silent
Bias is ok at 35ma per tube



Amp is working except for this excessive HUM when you turn up volumes

I have a picture of the scope But i can't seem to get it up loaded today nor can i get the schematic diagram up
I will tried to upload via my phone

[Arbutt]

Won't let me upload or insert a picture

[Dave H]

Try a shorted jack plug in each input in turn.Is the hum reduced?

[Arbutt]

Thanks dave h. It's not the input Jack's. I've tried all that. It's really annoying as I can't seem to upload the scope picture. A

[The Dude]

If the amp is quiet with volumes turned down, I wouldn't be looking at the output section or power supply. It must be something in front of the volume controls.

[Arbutt]

Yes with the pi tube out its OK quite. I can can see the problem with the output from v2 - v1. I guess it's back to it tomorrow

[Arbutt]

Finally got to get this to upload This the the wave form I'm getting with the vol up full on pin 2 of v1 or v2.
Many thanks
Arbutt

[Arbutt]

I have checked today. Now my thinking is it has to be related to psu as its the same on both channels
I can see this waveform on pin 5 v1 or pin 5 v2.
When I scope the other side of anode resistors it's good.
Too be sure I disconnected the heaters and hum then I wired up my test 6.3v heater tx with ct. No difference.
Decided to bypass the tone/vol no change

If anyone has any ideas
I would be grateful

[Helmholtz]

Finally got to get this to upload This the the wave form I'm getting with the vol up full on pin 2 of v1 or v2.
Many thanks
Arbutt

Sorry, can't positively read scope settings. Do you use a 1:1 or a 10:1 probe? What is the hum frequency 50Hz or 100Hz?

What happens to hum if you short input tube grids (pins 4) to ground?

Quite = quiet?

[Arbutt]

Hi helm it's 1:1 probe. I can remember shorting the 2 resistors( 270k) into the pi tube to ground. Went all quite. But I will check tomorrow
Thanks

[Helmholtz]

Hi helm it's 1:1 probe. I can remember shorting the 2 resistors( 270k) into the pi tube to ground. Went all quite. But I will check tomorrow
Thanks

Again, do you mean quiet?

I meant the input triodes, not the PI.

[The Dude]

......What happens to hum if you short input tube grids (pins 4) to ground?......

^^^^^^Yes, that. You said in post #1 that turning the volumes down eliminates the hum. The volume controls are just after the first gain stage of each channel, so the problem must be in the first gain stages.

[Arbutt]

Hi dude. I will have to check but I think I can see it on the first stage . I don't know if this has any relevance. But if I scope the output of say v1 and adjust the hum balance it changes the sine wave in the picture, but if my memory serves me correctly it don't change the level of the hum. I have also tried another humdinger 470r pot to ground. Which made no difference
Just want to say that the hum is excessive.
I will try and post up pictures tomorrow

Thanks again. For all the help. You are slowly teaching this old dog new tricks

[The Dude]

Also, check that the hum balance pot is good. I've seen them burnt up, particularly in Ampeg amps. If the pot is bad, you can just use a couple of 100 ohm resistors like the typical Fender circuit.

[Arbutt]

Again, do you mean quiet?

I meant the input triodes, not the PI.

morning
tested this morning if i ground pin 4 no change, but when i ground pin 1 hum goes, this is with the volumes up
I have previously swapped out the decouplin caps and as no difference was made original caps put back
the resistors were replaced on v1
it is being created by the first stage as i can see it on pin 5