Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mresistor on October 30, 2024, 02:25:17 pm

Title: Fitting Hammond 269EX to a Princeton Reverb chassis PT cutout (JCM800 Micro)
Post by: mresistor on October 30, 2024, 02:25:17 pm

Hey all. I have decided to build RR's JCM800 Micro amp in a PR chassis and use the Hammond 269EX power transformer.  Found out that its a little bit of work to devise how to mount the little stand-up PT into a larger opening and bolt pattern on the chassis. I will be tracing the adapter cover plate pattern and keeping it if someone is interested in doing this.


So I had some time this morning and some aluminum sheet stock leftovers to do the job. Then I decided to cut a mounting piece large enough to cover the two unused octal sockets in the process. It will make it easy to cut a 3/4 inch hole for a 9-pin socket if so needed in the future as well.


Here's the chassis holes. and the 269EX sitting in position needed.

Title: Re: Fitting Hammond 269EX to a Princeton Reverb chassis PT cutout (JCM800 Micro)
Post by: mresistor on October 30, 2024, 02:28:24 pm

And here is the new adapter plate that will mount the 269EX and cover the unused octal holes in the chassis. It's not perfectly aligned in the picture. Problem solved and I might paint the plate too.

Title: Re: Fitting Hammond 269EX to a Princeton Reverb chassis PT cutout (JCM800 Micro)
Post by: Willabe on October 30, 2024, 07:04:13 pm

The aluminum is going to react with the steel.

Might be a problem.

Title: Re: Fitting Hammond 269EX to a Princeton Reverb chassis PT cutout (JCM800 Micro)
Post by: rake on October 31, 2024, 02:35:14 am

If you are fearing a galvanic reaction then use SS hardware and on every bolt between the plates use a SS washer to act as an insulator between the dissimilar metals.
It will raise the plate slightly but there won't be a galvanic reaction. I've been using this trick on motorcycle engine grounding for decades and no more corrosion.

Title: Re: Fitting Hammond 269EX to a Princeton Reverb chassis PT cutout (JCM800 Micro)
Post by: AlNewman on October 31, 2024, 02:50:20 am

I've seen a lot of aluminum chassis...  I've never seen an aluminum jack.  I think the dissimilar materials make a big difference in an engine...  or a boat...  And even then over many years and different types of climate.

Title: Re: Fitting Hammond 269EX to a Princeton Reverb chassis PT cutout (JCM800 Micro)
Post by: rake on October 31, 2024, 04:34:26 am

A galvanic reaction can happen between dissimilar metals whenever there is an electrical current present.
nickel and stainless steel are neutral and can block a reaction. Any type of "electrolyte" can accelerate a reaction.
I played in a band with a guy that sweat electrolyte!  :sad2: I'd cringe every time he touched one of my guitars.

Title: Re: Fitting Hammond 269EX to a Princeton Reverb chassis PT cutout (JCM800 Micro)
Post by: mresistor on October 31, 2024, 07:50:46 am

Thanks guys.  Both the chassis and the pt adapter plate will be painted, first with an etching primer then with gloss black enamel. It shouldn't be a problem.