Gibson GA20RVT Reissue Transformers

[reggiepe]

Long time lurker and extractor of loads of fantastic information on repairing and building. I have a question for the group that I am hoping that there is a bit more info that I can get from you to help me bring this amp to life.

I just recently acquired a Gibson GA20rvt reissue (circa 2006?) that had all of the original iron pulled and replaced. I have no idea of the history of why this happened, but the last owner installed replacement (exact replacements is debatable) The Turret board is intact with all wiring appearing to be original. The inside of the chassis was very dusty, so that leads me to believe that it sat for a very long time on someone's bench.

I have an original reissue and the original iron from that working GA20RVT has taped on pieces of paper that read the following with no other markings:

PT:

IMAG X7001
575VAC PWR
DV-130 0618

Choke:

IMAG x7003
100mA 5H

OT:

X6998
0618

Reverb Driver:

Imag X7004
3W Pwr
0618

The replacement iron that is in the non-working amp is as follows:

PT:

Hammond 290AX

Choke:

NSC125C3A
989 0403

OT:

Hammond 1760H:

Reverb Driver:

MOJO776
989-1620

The replacement Hammond PT is maybe half of the size of the original and makes me think that it is grossly undersized. The OT appears from the specs that it may work....but that is the only one that I think is correct. I will attach a schematic of the amp to help give a clue. The amp has (5) 12AX7's and (2) 6V6's and a full wave bridge rectifier. I had an email conversation with Shane Radke from British Audio and he told me that this amp does not have very much documentation out there on it and did not know the specs for replacement iron for these models.

Thanks so much.

Rich

[sluckey]

What's your question?

[reggiepe]

I guess I got so wrapped in the details, I forgot to ask the question. Will the replacement iron that is currently installed work for this amp?

[sluckey]

Everything looks OK to me except the PT. I think it's underrated for that amp. Your amp needs 2.4A current capacity for the filaments, but the 290AX only delivers 2A. And I would rather have 125mA to 150mA capacity on the HT winding but the 290AX only delivers 81mA.

[reggiepe]

Thank you!

That confirms my suspicions of the PT. Just the physical size had me cautious of the proper specs of the Hammond. So bumping that PT up to a 290BX or the like may be what I need to do.

[sluckey]

Good idea.

[SILVERGUN]

...The amp has (5) 12AX7's and (2) 6V6's and a full wave bridge rectifier.

The schematic shows a full wave (not bridge) rectifier.
Just want to make sure you get a correct PT since we know the amp has been fooled with.

It does show a full wave bridge for the filaments.

[reggiepe]

oh, I see that now. Just looked more closely at the schematic and see that now. It appears that the 6v6's are getting ac to their filaments and the 12ax7's are getting dc to theirs. Which raises a question....does that change the specs of the PT that Sluckey mentioned in an earlier post?

[sluckey]

no

[PRR]

Actually you want to up-size the DC amps to figure AC amps. As Sluckey said, what is in there now is already undersized. Go up and be generous. 3A AC is not excessive for the 0.9A AC and 1.5A DC loading.

[reggiepe]

Thanks to both of you!

With the PT in the unmolested amp reading 578vac, I guess I should shoot for a replacement with that voltage and 125 to 150mA's with the filaments at around 3A? sound about right?

[sluckey]

Hoffman sells this PT...

http://el34world.com/Transformers/files/BRITISH-18W-PT.pdf

[reggiepe]

Whoa, that one just about nails it. Have to do some measuring to see if I can make it fit somehow. The original PT is a vertical mount.

[sluckey]

Whoa, that one just about nails it. Have to do some measuring to see if I can make it fit somehow. The original PT is a vertical mount.

But that Hammond 290AX you say is in it now is a laydown PT. How is it mounted? Will you post some pics so we can see what you are dealing with?

[reggiepe]

Home made brackets

[sluckey]

Good job on the brackets. If you rather have a stand-up PT take a look at this...

     https://www.hammfg.com/files/parts/pdf/270EX.pdf

     https://www.tubesandmore.com/products/transformer-hammond-power-275-0-275-v-144-ma

[reggiepe]

Awesome! Thanks Sluckey!

The mounting centers are a little off but I think I can make that work. Will dig into the chassis today to make sure it can be mounted and not get in the way of anything beneath. It is a very tight fit in that corner.

[reggiepe]

I just opened up the unmolested GA20RVT and tested the the secondaries off of the PT....seems to be a 300-0-300. That opens up a whole new realm of possibilities for a vertical mount transformer in that spot. Seems to be a large abundance of that voltage out there. It would seem that the sticker with 578vac on the top doesn't hold much value regarding the specs. Thanks for all the help

[PRR]

> seems to be a 300-0-300. ... sticker with 578vac on the top doesn't hold much value regarding the specs

300V NO-load sure could be 289V under full load, the way transformers are normally rated.

[reggiepe]

Oh, so it begs the question....when you purchase a transformer, are the specs given as no load or load? I think you said that in your response, but want to be sure I understand.

Thanks