Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dbishopbliss on January 14, 2023, 10:03:54 am

Title: Can someone explain these JTM45 output transformer specs
Post by: dbishopbliss on January 14, 2023, 10:03:54 am

I'm thinking about rebuilding my Supro-ish amp into a JTM45-ish style amp. The original amp is fine, but not the magic I was hoping for. I have recently been using a MESA TA-30 in the HI 1 channel. It has a lot of headroom and breaks up without getting into the high gain territory. It CAN get into high gain but that isn't how I use it. And while it is a nice amp, its a combo and I have a lot of speaker cabs that I think it sounds better with. In addition, the amp has a ton of features that I don't use so better to let it go and build something I think I will use more.


Since Classic-tone went out of business I used a Fender Tweed Super OT. The specs were similar to the Supro OT with a 6K primary. The Mojotone JTM 45 OT says that it is 3.4K for EL34 and 6.8K for KT66. I'm hoping to run 5881s. Do you think I could use the Tweed Super OT and get good results? Would I be better off getting the JTM 45 OT?


https://www.mojotone.com/British-Style-45-Output-Transformer


BTW, I am using a JTM 45 power transformer for the Supro. Perhaps that is why I wasn't able to get the sounds I was hoping for.

Title: Re: Can someone explain these JTM45 output transformer specs
Post by: brewdude on January 14, 2023, 10:32:57 am

I built a single channel 6G6B sort of amp using 5881’s and that 6K OT from Classic Tone.  It works great, but I don’t have any JTM45 knowledge or experience.

Title: Re: Can someone explain these JTM45 output transformer specs
Post by: pdf64 on January 14, 2023, 11:01:28 am

JTM45 have beefy OTs, high quality / spec, wide frequency response.
Is the tweed super OT as big and heavy as that of, eg a super reverb  :dontknow:
Or is it the size and weight of a bandmaster / vibrolux OT, ie low spec?
JTM45 use about the most negative feedback of any valve guitar amp, and a high spec OT is probably necessary to support that.

Title: Re: Can someone explain these JTM45 output transformer specs
Post by: dbishopbliss on January 14, 2023, 11:20:01 am

Quote from: pdf64 on January 14, 2023, 11:01:28 am

JTM45 have beefy OTs, high quality / spec, wide frequency response.
Is the tweed super OT as big and heavy as that of, eg a super reverb  :dontknow:
Or is it the size and weight of a bandmaster / vibrolux OT, ie low spec?
JTM45 use about the most negative feedback of any valve guitar amp, and a high spec OT is probably necessary to support that.

Its hard to tell, but the Tweed Super is open frame and the JTM45 has end bells. I suspect I should invest in a new OT to get the sound I am looking for.

Title: Re: Can someone explain these JTM45 output transformer specs
Post by: brewdude on January 14, 2023, 12:29:38 pm

The tweed OT is small probably more suited for 30 watts (I think Classic Tone called it 40 watts).  I think I’m only running about 365v on the 5881’s.