Transformer from Heathkit A-7 amp

[bryanz]

Does anyone out there have a transformer (model 54-25) from a Heathkit A-7 amp? Mine just died and I'm looking for either just the transformer or a junker amp that I can pull the transformer from.
Thanks,
Bryan

[kenpeter]

If you mean the audio output transformer? Yeah, I am about to pull
mine (open frame, not the pretty potted style). And upgrade to an
unlisted 10W CXPP thats identical in physical size and end bells to
my original Heath power transformer. Only the Edcor Blue doesn't
quite match Heath's (Hammond?) Black...... I can live with it.

My OPT I am about to pull from a Heatkit A7-C, It has an unknown
primary impedance which I may measure soon, and no UL taps...
It does have 4,8,16 ohm secondaries... Not the transformer shown
on the A7-C schematic. But I'm sure it is correct, I've seen another
A7 with the exactly the same.

If you want my power transformer, you are outa luck. I can't part with
it just yet. But I did take measurements, I can tell you all the voltages
when I get back to work on Monday...

Audiophool has several A7 schematics, which letter version you got?

[bryanz]

Unfortunately it's the the power transformer that I need. If you can send me the voltage, resistance, amperage, etc, I would greatly appreciate it. I just have an A-7, no letter model.
Does anyone have suggestions on a replacement? I think the outputs are supposed to be 390v, 12v, and 5v.

[kenpeter]

None of the A7's I've seen have an actual sub-letter
on the amp itself, but the sub-letter of the schematic
can be very different. Mine is neither C nor E, but is
similar to both and completely different. Go figure...

Your power transformer voltages are very similar to
what I measured, I think 385VAC, 12.9VAC, 5.1VAC.
No clue what amperages, but you can figure from
the tube spec sheets depending which tubes for
your specific amp.

I measured 45mA combined quiescent current in the
tail of the output tubes... If that helps. Mine may not
use the same bias current as yours.

Download those schematics, and figure out which A7
is the closest match to what you have.

Edcor can build whatever transformer you may need.
And I can say by this 10W CXPP they made for mine,
they do have exact matching size core and end bells.
Complete coincidence physical fit to my A7 was exact,
I didn't ask them to do anything special regarding fit.

Mind you this Edcor CXPP is 10W 5K-PP-UL/4-8-16 OPT,
not the power tranny. Complete coincidence the same
physical size and shape as Heath's power transformer
you might be trying to replace. If that helps you order
for the right physical size.

The Heath power transformer might be 2 lams thicker...

[Simcha]

Hi from New Zealand.
I've been lurking here for a year or two, very interested in some of the discussions, but I haven't needed to ask anything directly until now....

I'm building a simple p-p 12A6 amp to drive a very small monitor amp for amplified acoustic instruments. Doesn't need much power because it will be close by and at shoulder level. If it works ok, I expect I might need to make 2 or 3 more for friends. The obvious route would be to rebuild a couple of poor-condition Heath A7's and put them in cabs with a 6 or 8 inch speaker. But Heath amps are uncommon here, and A7's are scarce as hens' teeth in NZ.
I can wind 1-off trannys: and the PT is no problem - I already have one from another design which is very close. But for something that's just a 'fun' project, it's hard to justify the time to design a decent UL output tranny with a fairly high p-p load res.

So does anyone please have for sale/trade/whatever, one of the U/Lin OT's used in the later Heathkit A7's ( A7E?). - Either a working one, or I'd be just as happy with a faulty one which I could strip, blueprint and rewind. ( Or even a winding sheet/blueprint - though I think that's unlikely)

Best regards, Simcha

[kenpeter]

Sorry, mine (A7C) has no UL taps. And may not even be original.
I am about to stick a "standard" 5K p-p with 43% UL. If the Z's
turn out to be grossly wrong, I intend to connect the speaker to
one of the other secondary taps and maybe correct it.

NFB from resistance under the cathode will increase the output
impedance of a stage, NFB from plate to grid (or from plate to
the previous plate) might reduce that same output impedance.

You might be able to use small amount of such feedback to
engineer a better match for a standard transformer than your
plates by themselves would otherwise conspire to give you....

[Simcha]

Hi Kenpeter, Thanks for the reply, and the advice.
12A6's conveniently have low heater requirements - 300mA for a pair!
( So I already have a small and fairly lightweight PT which will drive two 12A6's plus preamp and PI) And they're not expensive yet.

But the data I've found suggests something like 10 or 12k plate-to-plate for a p-p pair. Could probably go a bit lower in a musical instrument amp - Fender 6V6 amps often use 6.5k p-p, when the basic data for 6V6's suggests about 8k. So you might try mismatching (say) 8 ohm spkr on 4 ohm tap.
Or as you suggested, a little plate-to grid NFB and a generously specified PI or driver would probably make the whole thing more tolerant of lower loads.

Anyway, please give us an update sometime of how the amp turned out.
Cheers, Simcha in NZ

[kenpeter]

Originally posted by Simcha
But the data I've found suggests something like 10 or 12k plate-to-plate for a p-p pair.

What data is this? For PP 12A6's in Pentode mode or UL?

UL feedback also lowers effective output impedance, exactly
how much it lowers I have no quick and easy way calculate.
Certainly somewhere between Pentode and Triode I expect.

RCA's triode curves for the 12A6 seem to suggest 2K3 per plate.
Judging by the slope of 0V grid from 150V to 250V at the plate.
Of course, true triode strapping means the plate can't go any
higher than the screen will tolerate.

But you could Plate to Grid (Schade - RCA 1938 - Beam Tubes)
For nearly the same set of curves without screen voltage issue.
Or plate to screen (by way of UL or pseudo-tapping....)

I don't think 5K (or less) Plate to Plate too difficult to achieve.

[gingertube]

The preferred replacement for a 12A6 is a 12V6.

I would then just stick to published 6V6 operating conditions.

Class AB1 Pentode or Ultralinear at 300V - Raa = 8K - use a Hammond 1608.
At 250V go a little higher, Raa = 10K use a Hammond 1609.

Also if you want to triode strap the output tubes the 1609 would be better.

Cheers,
Ian