AC15 Volume Issues

[ebulakowski]

Hey gang.

I have a 90's AC15 on the bench. I am getting hum that comes through when the volume is up.
I am also experiencing issues with the volume control. It acts normal in the first 30% of its sweep. Then I experience a strong volume drop for most of the remaining sweep.

No crackle and the pot measures normal. I'm working with new tubes. All other controls function predictably and moving the vol to zero gets rid of the hum.

What should I be looking for?

[Latole]

Issue with input jacks ; check ground,
Filter caps

Any mods in the amp ?

[PRR]

> normal in the first 30% of its sweep. Then I experience a strong volume drop

Oscillation. Which leads back to Latole's question.... what has changed?

[ebulakowski]

Issue with input jacks ; check ground,
Filter caps

Any mods in the amp ?

Thanks guys. The amp is stock. As far as I can tell I am the first one inside. Jacks are secure. All of the caps tested withing spec on a DMM, but I certainly haven't ruled them out as a potential problem. I just wanted to see if there were any other things I should check before changing them out.

[Latole]

Filter caps ; check AC voltage, yes AC not DC

Have a look here ;https://el34world.com/Forum/index.php?topic=28461.new;topicseen#new

[ebulakowski]

Thanks guys. I need to order some caps and then i'll dig in.

[ebulakowski]

I'm still struggling through this one. I read the entire linked thread. I Tested for AC voltage. My readings after a moment of contact on the plates was <.005vac and usually then crept down to .001-.002vac.

I have replaced all of the filter caps. The volume know continues with the same problem. It act/sounds similar to when I have bias transistors in pedals. a whooshing/static when turning, and only a small window of function. And I get a smooth resistance change with a meter.

[ebulakowski]

any other insights out there? I'm still getting volume drop after about 30-40% rotation on the volume pot.

[valvetones]

Check the potentiometer terminals for DC voltage. If you have any DC on any terminals, follow the lead back to the coupling cap that is connected to it. It may be leaky.
Just something I would do.
Haven’t looked at a schematic, don’t know ac15 layout.
Just a hunch

[ebulakowski]

Check the potentiometer terminals for DC voltage. If you have any DC on any terminals, follow the lead back to the coupling cap that is connected to it. It may be leaky.
Just something I would do.
Haven’t looked at a schematic, don’t know ac15 layout.
Just a hunch

Thanks for this suggestion. Yes, I found VDC on the two non grounded lugs of of the volume pot. I will look into swapping the caps in that area. I imagine C17 and C27 are the ones to look at from the schematic above?

[dwinstonwood]

ebulakowski, please note that the schematic you posted in your first post has at least one area that's missing from the drawing.
There is a connection between "BA" - the second, or "B" annode, pin 6 - of the second triode of the first input tube to R8, the 100K plate load resister.
I found and have attached a different copy of your schematic that shows this connection.

[ebulakowski]

ebulakowski, please note that the schematic you posted in your first post has at least one area that's missing from the drawing.
There is a connection between "BA" - the second, or "B" annode, pin 6 - of the second triode of the first input tube to R8, the 100K plate load resister.
I found and have attached a different copy of your schematic that shows this connection.

Thank you for noticing that and sharing a better schematic!

[ebulakowski]

Check the potentiometer terminals for DC voltage. If you have any DC on any terminals, follow the lead back to the coupling cap that is connected to it. It may be leaky.
Just something I would do.
Haven’t looked at a schematic, don’t know ac15 layout.
Just a hunch

Thanks to everyone for the help! valvetones hunch was right. I replaced c17 and c27 and i'm back in business!

[valvetones]

Excellent. Stay groovy