Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: six el six on July 14, 2012, 06:58:34 pm

Title: 60hz hum in Ampeg SVP preamp
Post by: six el six on July 14, 2012, 06:58:34 pm

I've got 60hz hum in this Ampeg preamp I'm trying to eliminate

The voltages in the circuit all look good and they match up with the schematic nicely.

I'm using a cheater plug on the preamp and running an amp that does have it's ground connected.

The 60 hz hum increases significantly when turning the master up.

Wherever it's coming from I'd like to be able to locate it.

Does anyone have any advice on the most effective method of hunting it down?

Thanks,
six

Title: Re: 60hz hum in Ampeg SVP preamp
Post by: six el six on July 14, 2012, 10:55:42 pm

Forgot to mention.

The hum can be increased or decreased via the master volume.

I first tried "divide and conquer" by pulling tubes. After pulling v1 the hum remained. After pulling v2 the hum quit and I can then turn the master up and down with no hum anymore.

I think I'm going to try "injecting" a signal onto pin 2 (grid) of v3 and see if the hum is gone.

If it is I'll then be figuring out how the 60hz hum is getting into the circuit at v2.

Hmmm....

Title: Re: 60hz hum in Ampeg SVP preamp
Post by: stratele52 on July 15, 2012, 10:24:01 am

So the hum is comming between V1 and V2 , or from V2. Did you try a good tube in V2  ?

The schematic is not easy to read.  I don't know if the two 220 ohms resitor for heater's artificial ground will work better ( quiet ) with most see 150 ohms ?

 

Title: Re: 60hz hum in Ampeg SVP preamp
Post by: six el six on July 15, 2012, 05:10:56 pm

Quote from: stratele52 on July 15, 2012, 10:24:01 am

So the hum is comming between V1 and V2 , or from V2. Did you try a good tube in V2  ?



 

Yes. If I pull V1 I can still turn the master up and down and get hum.

When I pull v2. Voila! No hum.

Title: Re: 60hz hum in Ampeg SVP preamp
Post by: stingray_65 on July 15, 2012, 06:54:03 pm

Make sure it is 60Hz

plug a cord into an amp and put your thumb on the tip, this is 60Hz.

60Hz, to me at least, is hard to distinguish from 120Hz.

60Hz comes from the heaters or a sensitive input close to a mains or heater line.

120Hz is from the B+ rail or a sensitive input close to the B+ rail.

Title: Re: 60hz hum in Ampeg SVP preamp
Post by: six el six on July 15, 2012, 07:57:37 pm

Quote from: stingray_65 on July 15, 2012, 06:54:03 pm

Make sure it is 60Hz

plug a cord into an amp and put your thumb on the tip, this is 60Hz.

60Hz, to me at least, is hard to distinguish from 120Hz.

60Hz comes from the heaters or a sensitive input close to a mains or heater line.

120Hz is from the B+ rail or a sensitive input close to the B+ rail.

Yep. It was 60hz.

Anyway, I got 'er squared away.

I found a center tap fron one of the heaters under the PT that was exposed and possibly coming into contact with the laminates so I put heatshrink on it.

But probably just as important I changed V2 and V5 to 12aU7s.

Hum gone.

A google search on these things indicates that this a very common problem with this preamp.

Thanks for the help folks.

six.