Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Diverted on July 09, 2019, 12:46:37 pm

Title: Accessory jack Hoffman AB763
Post by: Diverted on July 09, 2019, 12:46:37 pm

Hi all,

Awhile ago I built a Hoffman AB763 reverb amp. It has a 2/4/8 ohm impedance selector, the friend I built it for usually runs it at 8.
I did not add the accessory jack. My question is this: He would like to run two speakers simultaneously out of this amp, likely two at 8 ohms each. Would I just wire in the secondary jack as shown in the Fender/Hoffman schematics (looks like they are in parallel) and then switch impedance selector to 4 ohms when using two 8 ohm speakers?

Thanks!

Title: Re: Accessory jack Hoffman AB763
Post by: EL34 on July 09, 2019, 01:27:37 pm

Yes, but one jack always short to ground in case no speaker is plugged in
If you have an impedance switch, you are good to go

Title: Re: Accessory jack Hoffman AB763
Post by: Diverted on July 09, 2019, 07:21:46 pm

Thanks EL34. Just wanted to be sure, didn't want to fry his expensive Heyboer output transformer.

Ted

Title: Re: Accessory jack Hoffman AB763
Post by: d95err on July 10, 2019, 06:55:49 am

A lot of old Fender amps had extension speaker jacks without any impedance switching. So apparently, a bit of impedance mismatch was not considered a problem.

I wouldn’t worry too much about it.