Heathkit A7(c?) Rebuild - diyAudio
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Old 26th June 2008, 04:40 AM   #1
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Default Heathkit A7(c?) Rebuild

I have a Heathkit A7 amp, that seems mostly similar to version C....
The output transformer is a poor match and also not the original.
Attached schematic was leeched some time ago from Audiophool.
His original version is clearer. Forum limits my attach 1000pix wide.
Thanks to Phool's advice, at least its working now (mostly).

Anyways, it still don't sound too healthy and occasionally shocks
me! It desperately needs all new caps, and I am about to tear
seriously into that rats nest up underneath.

But looking at the A7C schematic, something looks odd about the
choice of triodes in the design? Why is a high Mu 2mA triode used
as the concertina splitter/driver? Why is a medium Mu 10mA triode
used to boost a tone circuit? Should the roles of these two triodes
have been swapped?

Ohyeah... I got an Edcor OPT (unlisted 10 Watt CXPP) coming to
replace the mismatched open frame (guitar?) tranny that some
clown stuck on here, presumably cause the original burned up....
Its gotta be physically small, as there ain't much area to mount it,
but I did want bell covers, UL taps, and 4-8-16 secondaries. Edcor
said it would have 5K primaries...

It needs a Fuse, but I gotta update to polarized power cord first,
or I might accidently fuse the return instead of the hot wire!

Would 12L6's have made better choice for output tubes?
If I abuse 6L6, can I string both heaters in series (for 12.6V)?
Or do unbalanced series heaters always hum in audio circuits?

I got some Novar compactrons might make improved outputs,
but I'd have to change from octal sockets... I'm not opposed
tearing the whole thing down far enough to do so if needed.
Nothing but perhaps the chassis and front panel are sacred...
The original power transformer seems adequate.
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File Type: gif a7c.gif (68.7 KB, 174 views)
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Old 26th June 2008, 10:12 PM   #2
kevinkr is offline kevinkr  United States
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The power transformers in these are pretty marginal so I would not contemplate any experiments with 6L6..

The 6SL7 was used as input and concertina because it had sufficient gain, and as the output stage is UL connected Cmiller is low. The 6SL7 can comfortably swing far more voltage than a 12A6 needs for full output.

The tone control circuit needs to be driven by a low source impedance, hence the choice of 6J5 and some gain was needed.

Note that different versions (A7 - A7D) differed in significant ways electrically. The ones I owned had a slightly different tube line up from the one you provided a schematic for, and used passive mixing instead of a source selector.

These amplifiers were near the bottom of the heath barrel and used cheaply available parts - mostly WWII surplus at the time these amplifiers were released.

Both of mine had good opts, but one had a bad power transformer, I had a physically identically sized replacement built, worked perfectly, and sold a few to other A-7 owners. Stupidly I seem to have lost all of the design information for that transformer.. (Sorry)

There were variations with and without a phono/mike stage. (Might have been an option you could add to the kit.) I had one of each..

Here is a link to the A7 on my site, not a lot of useful information, but it will give you a sense of what a good one looks like.

http://www.kta-hifi.net/projects/amp_page/a7amp/a7.htm

Your amplifier is comparatively rare, I would recommend cleaning it up, restoring it, and giving it a listen - if not your cup of tea pass it along to a collector. I actually got quite decent money for my working pair - substantially more than I put into them IIRC when I sold mine on eBay about 9yrs ago.
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Old 27th June 2008, 04:15 PM   #3
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I have no plans to throw any of the original parts out,
I'm a packrat when it comes to irreplacible parts....

But I'll probably try to squeeze all I can out of it's new
10W OPT. I understand your concern for the original
power transformer is marginal at best? How bout I run
filaments from a 12.6V external supply... Would that
ease up the burden enough to get a little more from
the original power transformer's high voltage windings?
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Old 27th June 2008, 11:49 PM   #4
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If you use a 5AR4 rectifier working into a modest 1st filter cap., say 4.7 μF., and follow with inductance (real or a gyrator ), the conduction angle the power trafo services will increase. That allows you to safely squeeze a few extra mA. out of the B+ supply.

The 5 VAC draw of both the 5Y3 and 5AR4 is 2 A. However, the forward drop in the 5AR4 is smaller than that in a 5Y3. You easily gain enough to compensate for the voltage losses a small 1st filter cap. and a gyrator introduce.
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Old 28th June 2008, 06:26 PM   #5
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I took some warm idle measurements before tearing anything down.

117VAC primary
357VAC+357VAC secondary#1
5.0VAC secondary #2
13.0VAC secondary #3

390V B+ OPT-CT
384V Plate(s)
329V Screen(s)
23V / 470R = 49mA Cathode(s) Tail Resistor

185V B+ Pre-Amp and Driver tubes....

Subtracting 23V bias from plate and screen of the 12A6's
I measured 361V on the plates and 306V on the screens!
Way hotter than RCA design center maximums! No more
than 250V max . 8.8W dissipated (per screen+plate) just
sitting there and doing nothing. Heath must have known
something about those beam pentodes that RCA didn't???

I don't think these values I measured are wildly wrong
for the components involved. The caps may sound old
when played, but seem to be working well enough to
hold up the B+ and cathode bias.

I am a little worried that I ordered an OPT with 5K imp.
And I'm not sure if thats per plate or combined total!
I may have to use the 4ohm speaker tap (with an 8 ohm
speaker) to fudge a better load line. These tubes want
about 7K per plate.

Perhaps the plate impedance will be slightly less in UL???
I may still have to burn off some portion of the UL voltage
to the screens. Perhaps I should forget "upgrading" from
12A6's to 12L6's at these sort of absurd voltages!
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Old 28th June 2008, 08:46 PM   #6
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Design center max. values are very conservative. 10% more is usually not a problem. JMO, design maximum data sheets are extremely useful, as they tell you exactly how far to "push the envelope".

Current production EH 6V6s are tough and should survive the voltage regime you describe. PP triode wired 6V6s will yield between 6 and 8 W., depending on how close to Class "B" operation you go.
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