stancor A53c

[Andy Le Blanc]

Planning for the next project, I found an old interstage tranny salvaged a
while back. I am looking at fabricating a power side using this and a pair
of 6v6, like an old Gibson GA15rvt, or BR series.

Now I know this set up has a limitation as to how much DC current the primary
of the interstage will handle, around 10 ma..

I was wondering if any have had the chance to play with this set up?

I've seen it put together with a blocking condenser to keep DC off the primary,
and have read the tone actually better although I'll try the interstage and
measure the current on the primary first and see if its in line with expectation,
before complicating the circuit.

[Phil_S]

Andy,
I have a GA20-RVT, which uses the same interstage transformer as the GA15-RVT. A while back, the i/s tranny blew and I put in the Hammond 10K:90K (or 1:3), but it does not provide adequate volume, nor does it sound the same.

To diverge.... I was also able to acquire from eBay (of all places) a complete transformer set for the GA15-RVT (including the reverb). Naturally, like many guys in their 50's, I've got more projects on the bench than I'll finish any time soon. One of those projects is to put the right tranny into the GA20-RVT, which is a PITA because it requires lifting the board.

The Gibson i/s tranny is 1:2.4. I can find the ohms if you are interested, but I don't have an L meter. I feel like, if I knew what I was doing, I could compensate for the 1:3 by driving it harder. This probably involves changing the driver tube and/or rebiasing that tube. It is driven by half of a 12AU7 with about 290v on the plate. I mention this because I don't see any wisdom in raising plate voltage, even if it were possible.

My point, long winded as it is, is that looking at the GA15-RVT or GA20-RVT (essentially the same design) with the Stancor A53C (1:3, 10ma) probably places you in the same boat as me with the Hammond in the GA20-RVT. While that circuit might serve as a jumping off point, I suggest that it can't be used, as is. BTW, the tonestack in either of those amps leaves a lot to be desired and I'd pick another design for that.

If you have the know how (or just dumb luck), and you figure out the right way to put this iron into the circuit and function well, I'd sure like to know what you did. Just about anything would be better than lifting the board in that amp -- too many solders to R&R.

Phil

[Andy Le Blanc]

I appreciate the heads up. Its one thing to see a schematic and read a text,
its another to discover practical difficulties. I've had the chance to work on
a few gibsons too, the wire dress is tough. Mercury magnetics has a clone
but the price is $60, a bit rich. I'll fart around with the old stancor and see.
I like the idea of a tranny invertor, I know that it has draw backs.
I'll try a couple feed options to and around the instage too, could be fun.

[Phil_S]

Yeah, I know about the MM clone of that tranny. Apparently, there must be just enough of them around with burnt out iron that someone finally sent their old one to get a proper replacement made. I agree it is pricey. I think I got the whole set of iron for considerably less, but I don't remember exactly how much. I felt like I found a needle in a haystack.