Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mresistor on January 07, 2024, 10:27:08 am

Title: Hammond AO-35 volume potentiometer related problem
Post by: mresistor on January 07, 2024, 10:27:08 am

Hammond AO-35 conversion volume pot woes. The volume pot makes this loud clicking sound when turned down all the way. I've replace the pot to no avail. There is no significant DC on the pots. The conversion has been totally recapped. It is running at 10.6W at idle.   I really have no idea what would cause this. I also have no oscilloscope to look at the signal.


Anyone ever run across this problem?

Title: Re: Hammond AO-35 volume potentiometer related problem
Post by: mresistor on January 07, 2024, 10:38:01 am

The tone stack is from the Phoenix layout. I need to change a couple of componenets.  Change the 68K grid stopper to a 33K.
On the Hammond AO-35 schematic I will drop R9 to 220K and change C8 to .001uf.


https://www.flickr.com/photos/49108115@N04/4501456977 (https://www.flickr.com/photos/49108115@N04/4501456977)


http://www.webdatasystems.com/images/AO35_3.jpg

Title: Re: Hammond AO-35 volume potentiometer related problem
Post by: tdvt on January 07, 2024, 11:32:03 am

CTS pot?

I have had several questionable 1M CTS pots with bad connectivity right at the bottom.

Just a thought.

Title: Re: Hammond AO-35 volume potentiometer related problem
Post by: mresistor on January 07, 2024, 11:35:22 am

It's an Alpha   and two of them have done the same thing.

Title: Re: Hammond AO-35 volume potentiometer related problem
Post by: tdvt on January 07, 2024, 11:50:39 am

Quote from: mresistor on January 07, 2024, 11:35:22 am

It's an Alpha   and two of them have done the same thing.

Oh well...worth a mention.


I've not had those issues with Alphas.


I had a noisy, perceived "dead" spot (extreme upper rotation) on a small pre-amp board I built recently & I eventually tracked it down to lead dress with the input wire just a little to close to the B+ feed for that stage.


A shielded run fixed it & I moved the grid stopper right to the tube socket pin as well, as it was board-mounted.

Title: Re: Hammond AO-35 volume potentiometer related problem
Post by: mresistor on January 07, 2024, 12:16:07 pm

Both of those are present in this amp. Stopper right on V1-2 and shielded wire from vol pot to V1-7.   I wonder if a snubber cap would stop the click?

Title: Re: Hammond AO-35 volume potentiometer related problem
Post by: mresistor on January 07, 2024, 01:17:24 pm

Well, interestingly, a small snubber capacitor from signal output to ground on the volume potentiometer does eliminate the click at the lower limit of the pot.
I'd still like to know what is causing it.


Oh I forgot to mention that it's only doing this with no instrument plugged in.

Title: Re: Hammond AO-35 volume potentiometer related problem
Post by: PRR on January 07, 2024, 10:30:08 pm

> it's only doing this with no instrument plugged in.

Switch jack or not?

Title: Re: Hammond AO-35 volume potentiometer related problem
Post by: mresistor on January 08, 2024, 08:15:00 am

Hi PRR  yes it has a S-Craft 12a and I double checked it and even shorted it with a jumper with no change.

Title: Re: Hammond AO-35 volume potentiometer related problem
Post by: mresistor on January 12, 2024, 07:30:45 pm

OK this is odd.   I connected a meter to V1 pin 7 to see if the vol pot was flakey and it varied from 0v to 67mv or so and did not make that clunk sound at the total ccw end of travel.
So,  get this  I unplugged the meter lead from the DVM and it's just a test prob  a wire   hanging there in the air and the vol pot does not make a clunk with it attached to V1 pin 7.


what the heck is going on here?