Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mark in pdx on March 13, 2026, 08:18:45 pm

Title: adding speaker jack to Ampeg GU-12
Post by: mark in pdx on March 13, 2026, 08:18:45 pm
My GU-12 combo has OT secondary wires that run straight to the Altec speaker. I'm trying to add a speaker jack between OT and speaker - not an extension jack, just making the speaker connect through a jack, Fender style. I soldered the OT wires to a Switchcraft #11 mono jack and installed it in a 3/8" hole in the amp chassis. Amp squeals when any combination of volume, treble and bass are turned up. Squeal can be silenced by turning back down.

Tried disconnecting the jack from the amp, and the squeal disappears and amp works normally. Tried swapping wires from OT to opposite jack lugs - same deal, squeals unless isolated from chassis.

This amp has no negative feedback. I can isolate the jack, but I'd like to understand what's going on... thanks if you can help me get it.
Title: Re: adding speaker jack to Ampeg GU-12
Post by: sluckey on March 13, 2026, 09:30:04 pm
Quote from: mark in pdx on March 13, 2026, 08:18:45 pm
I can isolate the jack, but I'd like to understand what's going on...
Me too!
Title: Re: adding speaker jack to Ampeg GU-12
Post by: BrianS on March 14, 2026, 09:11:19 am
I don't know exactly why, but it seems like a lot of Ampeg designs are sensitive to how they are grounded.  I've solved weird noise issues in Ampegs by following their grounding schemes exactly, as well as making sure every grounded part is secured well.
I'd say isolate the jack from the chassis and see what happens.
Title: Re: adding speaker jack to Ampeg GU-12
Post by: mark in pdx on March 15, 2026, 01:23:25 pm
[dim light bulb flickers]

Maybe I located the jack too close to the preamp. The way the squeal comes and goes when the collective volume and tone knobs reach an average level... maybe that's induction at work?

I ran this amp with a temporarily soldered jack dangling in the air for a short time so I could run it through a Fryette Power Station at home. never noticed the problem because I was making sure the jack was nowhere near the chassis. I'll confirm that isolating the jack makes this go away, and try moving the jack and see what happens.
Title: Re: adding speaker jack to Ampeg GU-12
Post by: JPK on March 17, 2026, 03:38:40 pm
You already confirmed that isolating the jack solved it in your earlier posts. Right?
Title: Re: adding speaker jack to Ampeg GU-12
Post by: mark in pdx on March 21, 2026, 02:27:58 pm
Quote from: JPK on March 17, 2026, 03:38:40 pm
You already confirmed that isolating the jack solved it in your earlier posts. Right?

I did, but I'm a measure twice guy, to a fault.

I got some isolating washers and installed the jack right in the same spot - dead quiet with volume and tone all the way up.

Moral of this story seems to be don't mess with Ampeg's grounding or lack thereof. Thanks for being there, people.