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Old 26th June 2008, 04:56 PM   #1
bbuz is offline bbuz  United States
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Default Heathkit AA-32 modifications / upgrades

I have a Heathkit AA-32 and it is working fine. However, I was wondering if people had ideas for upgrades to the circuit.

Here is the schematic link:

http://www.nijnkonijn.nl/machmat/sch...p/heatha32.gif

Specifically I was wondering if people felt the original design had room for some improvements.

Perhaps making the circuit UL with new transformers?

Power supply improvements?

Component changes to alter/improve signal( resistor values to change voltages?, capacitor changes?)

Complete redesign?

don't want to spend too much time or money on it but since I have 2 new transformers on it that are UL capable( see edcor below) my immediate idea is to make it UL .... and slowly mod over to Baby Huey?


http://www.edcorusa.com/products/tra...pp15-8-8k.html

All ideas welcome : )
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Old 26th June 2008, 09:44 PM   #2
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As with my Heathkit A7-C, the schematic is darn near unreadable
by attachment limits in this forum . Where would I go to find the
original size document? (for your drawing, not mine)

I have a slightly smaller Edcor coming, as I have very little space
on the A7 chassis for the OPT. 10W CXPP 5K UL-4,8,16. I'm not
sure if thats 5k per plate, or the 2.5k per plate??? I neglected to
ask! Whatever primary Z, I'll find a way to make it work. Thats
partly why I ordered multiple secondary impedances.

At least you have a 6.3V heater supply, I got 12.6 which severely
limits my choice of tubes unless I run a pair of 6.3V's in series....
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Old 26th June 2008, 10:10 PM   #3
SY is offline SY  United States
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That could be the base for a pretty decent phono preamp (that will take some major redesign, but the right number and size of tubes are already there). I'd shift some of the gain structure and input around, as well as design for lower distortion and noise; your cartridge choice should be driving all of your other design decisions. It's an intermediate-to-advanced project which will take some bench testing and many design hours, so gauge accordingly.

The power amp part is simple and pretty good as is. If it were me, I might replace the R||C biasing the first stage with an LED, but that's a niggly detail. Active regulation of the screen supply is a very big bang-for-the-buck. The compensation network will certainly have to be changed to accommodate the new output transformers.
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Old 27th June 2008, 02:17 AM   #4
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Quote:
At least you have a 6.3V heater supply, I got 12.6 which severely limits my choice of tubes unless I run a pair of 6.3V's in series....
As long as the pair of series wired heaters are: of the same type, from the same manufacturer, and from the same production run there is nothing to worry about.

New Sensor's Saratov, Russia, plant makes 2 different 6V6s (1 labeled TungSol and the other labeled ElectroHarmonix) that will be highly satisfactory in your Heath amp. You could also try to buy matched pair NOS 12V6s, which don't command nearly as high a premium as NOS 6V6s command.
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Old 28th June 2008, 04:25 AM   #5
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Default Re: Heathkit AA-32 modifications / upgrades

Quote:
Originally posted by bbuz
I have a Heathkit AA-32 and it is working fine. However, I was wondering if people had ideas for upgrades to the circuit.

Here is the schematic link:

http://www.nijnkonijn.nl/machmat/sch...p/heatha32.gif

Specifically I was wondering if people felt the original design had room for some improvements.

Perhaps making the circuit UL with new transformers?

Power supply improvements?

Component changes to alter/improve signal( resistor values to change voltages?, capacitor changes?)

Complete redesign?

don't want to spend too much time or money on it but since I have 2 new transformers on it that are UL capable( see edcor below) my immediate idea is to make it UL .... and slowly mod over to Baby Huey?


http://www.edcorusa.com/products/tra...pp15-8-8k.html

All ideas welcome : )

Unfortunately, the tubes Heath used are scarce. The 6EU7 problem is academic, as the triodes in that type are identical to those in a 12AX7. Build/buy an outboard 12 VDC supply for the phono section heaters and all is well there. AFAIK, there is no simple answer to the 6GW8/ECL86 problem. Cathode biased PP finals require matched pairs of tubes. Singles of the 6GW8 are tough enough to source and are costly.

The 6BM8/ECL82 might work for you, at reduced power O/P. The 8 KOhm, end to end, O/P trafos you have are on the low side for 6BM8s. Still, that might be your best long term option.

In any event, I'd "ditch" the signal shaping circuitry and the gain stage that supports it.

A look at the ECL86 data sheet suggests that the power trafo might have the capacity to support 2X PP channels of triode wired EL84s. The "extra" tube socket could hold a 12AY7 serving as the voltage amplifier in both channels. DC coupled MOSFETs (ZVN0545As ?) would serve as the "concertina" phase splitters.
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Old 15th September 2014, 10:09 PM   #6
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I found a more legible Heathkit AA-32 schematic. I think this integrated amp can be, as SY suggested, a good candidate to be converted to a full function preamp with phono and line stages. I like it that it is all hardwired and probably structurally better than any Dynaco PAS3. I would add DC filament to combat hum though.

Click the image to open in full size.

Last edited by directdriver; 15th September 2014 at 10:16 PM.
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