Re: How to replace the interstage transformer with tube PI?


"RS" <RS@xxxxxxxxxx> wrote in message
news:ssvri410a9pikm5hfd9jtbn8bu36tli18o@xxxxxxxxxx
On Wed, 26 Nov 2008 09:08:29 -0500, "Phil S."
<psymonds_no_spam@xxxxxxxxxxx> wrote:


I just took a minute at the bench. The Gibson interstage tranny is center
tapped on the secondary (the power tube side). Input to the primary at
~5vac puts out ~2vac on the outer legs of the secondary. This is a step
down transformer. <light bulb goes on> So this is (more precisely) 2.4:1,
not 1:2.4. The primary DCR is 727 ohms and the secondary is ~3.5K ohms.

The Hammond 124C is a step up (?), CT secondary, rated 10K to 90K or 1:3
turns. sqrt(10K/90K)=1/3. This one is supposed to have primary DCR 403
ohms and secondary 1524 ohms.
http://morenclosures.com/124.htm

You had described the Hammond as sounding weaker than the Gibson,
which would have made some sense if they were both stepdown (1:3 and
1:2.4 , respectively). Given that the Hammond is a step-up, you
should be getting more voltage than you are. Current is not a concern
there until your driver's voltage swings wide enough to put the output
tube grids into conduction (they conduct kind of like a diode when the
grids go positive with respect to the cathode). The amp would already
be plenty loud at that point.

If you're using a 12au7 driver, that should work OK. Not sure what
else could be wrong.

Re the Hammond model 124C, that's just a coil with no core. You
probably have the 124A or 124B, right? The coil component of the A
and B may labeled as '124C', but the C itself has no iron.

Well, OK. My recollection is that it says 124C, but it has iron
laminations, so I guess it's what you say!

I am not in a position at the moment to fire up the Gibson to take a test
reading in circuit, and due to family obligations, travel, etc., probably
won't get to it until Sunday. It is a PITA to drop the chassis and get
this
done, but now I feel motivated to do it, as I want to know for sure.

I am confused by what I see here. Looking at the primary and secondary
DCR,
these trannies look similar. But 10K to 90K tells me the Hammond is a
step
up? That would put silly high voltage with low current on the grids of
the
power tubes. My recollection is that plate voltage was OK, but I really
need to check again.

You could try clip-lead'ing each transformer into the circuit to
compare voltage swings on the transformer primaries and secondaries
when you send a signal into the amp. That should tell you what's up.

I agree, but this is way easier said than done. The GA20-RVT is a
particular PITA to do anything to. Gibson engineers must have joked for
years about how they managed to layout that amp. Mine hummed like you
wouldn't believe when I got it. I was a novice and couldn't find an amp
tech. It's what got me started. It took months for me to learn what I
needed to do to solve the hum problem, but I did! Maybe when I return from
Thanksgiving travel, I'll put some pictures up on the web so maybe you can
get an idea of what I speak.


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