60 Hz Quest

[trem]

I worked on my friends AC100.
I rebuilt most all of it.
The amp seems to function fine. Plug in a guitar, and it sounds OK.
BUT...there is a hum.

If I put my meter across the speaker jack, it says 60Hz.
If I pull V1 and/or V2 the amp still hums.
If I pull V3 (the phase inverter) the hum stops.

I have 3 new JJ AX7 in the amp. All 3 hum in the same way.
This amp has a separate heater supply for the preamp and power amp.
I lifted the 6 VAC to the preamp filaments, and ran them from a 6 VDC supply.....the hum is still there.

The power tubes are new JJ...all about 3-4 mA apart, and are biased about 55%.

How do I search for the source of this 60 Hz.?
Thank You
pics below.....

http://i.imgur.com/iMKThJD.jpg
http://i.imgur.com/yXn579x.jpg
http://i.imgur.com/9pACXri.jpg
http://i.imgur.com/prQWe7A.jpg

[pdf64]

Might the hum be 120Hz rather than 60Hz? ie originating from the B+ rather than heaters.

[g1]

Or if not the B+, perhaps from excess ripple on the bias supply, which would then be 60Hz.

[trem]

Thanks for the suggestions.
They may be over my head...or maybe I am too hung up on:
1. Pulling the PI kills the hum...so I think it must be after V2 and in or before V3.?
2. The reading at the speaker jack says 60 Hz.

As for the bias supply, I am not even sure how to check that. With a scope at idle on the Negative supply.? Or maybe just change/increase the bias supply cap.? It is 10 mics per the schematic.
I guess, first, I can parallel another cap onto the bias supply....see if the increase in capacitance helps the humming. I will give that a shot first.
Thank You

[trem]

Yeah, well...OK.
I paralleled a 22-450 cap onto the 10 Mic bias cap. That cut the hum by 70% probably.
I do not mean to sound stupid, but.....is more better.? Do I add more mics and hope the hum goes away 100%.?
Is it that simple, or should I consider something else in addition to just adding capacitance.?
Thanks

[g1]

That's funny, after you reminded us pulling the PI killed the hum, I thought it wouldn't be the bias supply then .
The original cap there may be shot, so adding in parallel may not be increasing the value, just putting back what is missing. You can check by pulling the original and leaving the new one there, does the hum come back?
Not sure what the stock value or circuit is, do you have schematic to post?
Most amps have more than 10uF, 50uF seems more common.

[trem]

The schems.....
http://www.drtube.com/schematics/vox/ac100jen.gif

http://www.drtube.com/schematics/vox/ac100jp.gif

[trem]

I almost rebuilt the entire amp.....I SHOULD have. Just a few parts in the preamp are original.
The PT, Choke and OT are original.
The bias cap and all parts in the power amp and supply are new. My current bias supply cap is 10-150.
Thanks

[g1]

The bias cap and all parts in the power amp and supply are new. My current bias supply cap is 10-150.
Thanks

This was the bias cap that made the hum when used by itself?

[nsubulysses]

Yeah, well...OK.
I paralleled a 22-450 cap onto the 10 Mic bias cap. That cut the hum by 70% probably.
I do not mean to sound stupid, but.....is more better.? Do I add more mics and hope the hum goes away 100%.?
Is it that simple, or should I consider something else in addition to just adding capacitance.?
Thanks

I'm sure G-one or someone would know this answer, but if you increased capacitance wouldn't it also affect bias voltage? In this case of increasing capacitance would it push the bias more negative? I know a smaller value coupling cap will adjust your bias voltage. I had that problem once when I put a .0047 in an Ampeg VT40 instead of a .047 by accident. Took a minute to figure out why my bias voltage was totally out of range.

[52 Bill]

I'm sure G-one or someone would know this answer, but if you increased capacitance wouldn't it also affect bias voltage? In this case of increasing capacitance would it push the bias more negative? I know a smaller value coupling cap will adjust your bias voltage. I had that problem once when I put a .0047 in an Ampeg VT40 instead of a .047 by accident. Took a minute to figure out why my bias voltage was totally out of range.

You're comparing two different things here. Try thinking of it this way. A capacitor will allow ac voltages to pass while blocking dc.

When you use a cap to filter the dc bias supply, the cap allows the ac component (ripple) to pass to ground leaving the dc part. The higher the value the more ac that passes through, the better the filtering.

When you use a cap to create a small dc bias supply, the cap is used sort of like an ac voltage resistor. It will allow a certain amount of ac voltage to pass through it depending upon its rated value. The higher the value the more ac voltage that passes.

When you used the wrong value of cap in what I would guess was an Ampeg bias supply, the bias voltage dropped because not enough ac voltage was passing through the cap to the bias supply. In this case adding capacitance will not increase or decrease the dc bias voltage in the same way.

[Tom Phillips]

I almost rebuilt the entire amp...

Maybe the amp didn't work at all before you started your work but, if it did, was the excessive hum already present?

[trem]

Maybe the amp didn't work at all before you started your work but, if it did, was the excessive hum already present?

It "worked'...but had low power and excessive distortion.
The amp had been worked on/modded by a couple of "techs"...it was a mess inside.
But it did not hum like this... as fas as I could tell.
Right now I have 2x 22-450 caps in parallel with the 10-150 bias cap. The hum is a lot better, but it is still there.
I put my DMM on the speaker jack and it says Zero Hertz (0 Hz). Is that possible.?

Unless you guys think it is a bad idea, I am going to order a (round numbers) 100mic 200V cap for the bias, and see what that does.
Having one cap on the board with two other caps on jumpers is not ideal. I really do not want to proceed until I have a "real" cap in the proper place on the circuit board.

Thanks BTW

[loudthud]

The phase inverter is supposed to be 12AU7. A 12AX7 won't bias properly unless the circuit is modified.

The bias circuit has two zener diodes regulating the bias at -35V. Is that working? (This circuit is crying out for a mod.)

Are all four diodes in the full wave bridge working? Is the ripple on B+ 60Hz or 120Hz?

[nsubulysses]

You're comparing two different things here. Try thinking of it this way. A capacitor will allow ac voltages to pass while blocking dc.

When you use a cap to filter the dc bias supply, the cap allows the ac component (ripple) to pass to ground leaving the dc part. The higher the value the more ac that passes through, the better the filtering.

When you use a cap to create a small dc bias supply, the cap is used sort of like an ac voltage resistor. It will allow a certain amount of ac voltage to pass through it depending upon its rated value. The higher the value the more ac voltage that passes.

When you used the wrong value of cap in what I would guess was an Ampeg bias supply, the bias voltage dropped because not enough ac voltage was passing through the cap to the bias supply. In this case adding capacitance will not increase or decrease the dc bias voltage in the same way.

Ah I see what' you're saying. Thanks Bill