How to replace the interstage transformer with tube PI?



In the interest of adding to the amp content here, I'm hoping this will kill
two birds with one stone. Of course, the other bird is that I'm trying to
understand what I really need to do here. In advance, I apologize for the
ramble here, but I'm thinking that saying what's on my mind might illuminate
the questions, which might be buried in the subtext. It's become apparent
that I might be in over my head here with respect to the design
modifications I want to make.

I've got a nearly complete set of iron and an unpolulated board for a Gibson
GA15-RVT Explorer. I say nearly complete set because the pictures I've got
of someone else's amp clearly shows there is a choke, which is not on the
schematic http://www.schematicheaven.com/gibson.htm , which shows R32 as a
270 ohm 2W resistor where the choke should go. I've got a suitable choke
(Stancor 4H 250ma, big hunk), so this is not an issue, but I thought I'd
throw it in just for the heck of it. BTW, the reverb input tranny is also
missing from that schematic -- there is a 10mfd +-/ cap at C11 instead. I
have pictures of an Explorer showing the reverb tranny, too.

I want the interstage PP tranny for my GA20-RVT Minuteman. It is the
identical part -- same part number, same appearance, same size, making it a
rare find. The Minuteman i/s tranny went open on one side and I replaced it
with a Hammond 1:3. The Gibson is 1:2.4, so I think the Hammond with 240v
in would put out 80v and the Gibson tranny would put out 100v. I think that
might explain why the Minuteman sounds wimpy with the Hammond. Stealing
this small transformer is the root of the problem.

So, what to do with the left overs? I would like to build a
FrankenMinuteman, just the Reverb channel, which includes the tremolo. But
the optoisolator for the Minuteman tremolo is made of unobtainium, so I
prefer to eliminate that. I thought maybe I could use the tremolo circuit
from the Explorer, which doesn't use an optoisolator. But, then reality
sets in. The Explorer uses the CT on the i/s tranny secondary as the tie
point for the tremolo. Once I eliminate that tranny, I'm lost on where to
input the tremolo. It looks to me like the Minuteman injects the signal
from the LDR back into the second triode, after the tone stack.. But I'm
getting ahead of myself here.

First, I've got to figure out how to insert a tube PI into the design
effectively. LTP or Cathodyne? The Reverb channel of the Minuteman with
R&T will require one double triode for the preamp, one double triode for the
reverb input, and one double triode for the tremolo. Once I insert a 4th
tube, I figure I might as well do the LTP, but it's a lot more parts and I'm
thinking I've already made it more complicated than a project really ought
to be. After the PI is set, then we can figure out what to do about where
to inject the tremolo.

I'd appreciate some suggestions on how to tackle the design problem without
simply throwing money at it. This is, as is typical for me, an economy
build. It will be a head. I'm prepared to spring for the correct reverb
tank (pun intended) from Accutronics. Apparently, those guys actually have
records of what when into what, dating back to when the company was Gibbs,
and I've received a very nice email from someone there. I am not prepared
to buy the tank until I have the design worked out.

Thanks.

Phil


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Relevant Pages

  • Re: How to replace the interstage transformer with tube PI?
    ... I have pictures of an Explorer showing the reverb tranny, ... I want the interstage PP tranny for my GA20-RVT Minuteman. ... FrankenMinuteman, just the Reverb channel, which includes the tremolo. ... circuit from the Explorer, ...
    (alt.guitar.amps)
  • Re: How to replace the interstage transformer with tube PI?
    ... I want the interstage PP tranny for my GA20-RVT Minuteman. ... FrankenMinuteman, just the Reverb channel, which includes the tremolo. ... R&T will require one double triode for the preamp, ...
    (alt.guitar.amps)
  • Re: How to replace the interstage transformer with tube PI?
    ... I want the interstage PP tranny for my GA20-RVT Minuteman. ... resistor they would just have drawn it without the resistor, ... leak resistors in the LTP or similar spot on a Cathodyne. ...
    (alt.guitar.amps)
  • Re: Preamp tube as power tube
    ... You could easily make one using a 12AT7 and a reverb driver tranny. ... Using the reverb tranny as OPT? ... As I understand it, the main advantage of 6N1P is lower plate impedance, allowing for a lower impedance OPT. ...
    (alt.guitar.amps)
  • Re: Gibson GA20RVT questions
    ... the exception of the reverb and tremolo. ... sound much better than this amp does. ... I am thinking, for both the reverb and tremolo, there is something about the tubes. ...
    (alt.guitar.amps)