It's mind blowing the amount of misinformation going on here regarding the restoration of the Heathkit aa-100. Most of these were build using good all caps which are survivors that don't need to be replaced. Everyone cutting good vintage caps out of their hifi please send them all to me!
It's a shame people like to carve this stuff up because they lack the real world experience to do a period correct original restoration. All of the mods and changes suggested are absurd. My Heathkit aa100 is 100% period correct parts and 95% original. It's plug and play with no issues and sounds fantastic. I have racks and racks of vintage hifi all with period correct and original parts. Every piece I own can be taken off the shelf plugged in and enjoyed. Its mind boggling how people dont realize the engineers who designed this stuff knew what they were doing and take a random forum post as gospel.
1) The individual bias mod is a necessity, if you plan to use any of the modern 7591 tubes. Even "matched quads" of current production 7591 tubes aren't nearly as matched as anything that you could get in 1965.The sound is measurably and audibly better, when the bias current is matched- and it's easier on the output transformers as well. Plus, the tubes last longer (which is a BIG plus when you're trying to milk all the lifespan out of expensive NOS 7591s), because the bias can be turned down to be within spec of the tube (most of these amps, unless the rectifier is changed out or a lot of resistance is added to the power supply, over-bias the output tubes- this was not accidental, but a result of the "horsepower wars" going on in tube amps in the mid 1960s- a somewhat lower tube life was considered worth it, to get a few more watts on the label).
2) It's well documented that the screen resistors on the 7199s on these frequently fail (much higher resistance than spec). Those, and sometimes the phase inverter resistors (for the same reason) frequently need to be replaced. When doing so, it's good practice to replace them with resistors with better thermal stability and freedom from drift, compared to the original types. Fixing the same problem again, later, isn't my idea of a good time...
3) The power supply caps- and any other electrolytic caps made in the 1960s- are junk now. Period. In fact, mine was popping (shorted) internally when I got the amp. It's a wonder someone didn't blow up the GZ34 or the transformer, it was so bad. Running one of these with original power supply caps is careless at best, and reckless at worst (there's this thing called FIRE)...
4) The circuit boards on these, were far from "state of the art construction". Bypassing failed connections at components, as needed is a necessary part of making many of these work at all, much less reliably.
5) It's well documented, as well, that power line voltages ARE higher than they were when these amps were built. Slightly reducing the power supply B+ voltage is a very prudent thing, to have the amp running within it's design specs.
6) Many of these amps were assembled with the green Sangamo caps. Those are known unreliable caps, compared to most of that era. Mine had those, and at least a third of them showed leakage- and a few of them were even cracked open and PHYSICALLY leaking. Hence, a wholesale replacement was warranted. I'm not sending out a time bomb. This, along with the higher-than-original power line voltage combined with the already high stock bias points, is the reason why you see many of these amps with burn marks on the covers (I had to carefully paint-match and refinsh mine, in fact- the paint was actually burned away over a couple output tubes), due to run-away output tubes at some time during the life of the amp...
7) When I rebuild one of these- unless requested otherwise, I'm using mylar film caps (not poly film). Those are a very similar sonic signature as the caps of the original era, if not functionally identical in sound- but they don't leak.
These amps, like almost any other consumer product, were all "price engineered" to an extent. We have an opportunity to correct the shortcuts that were made then, in the interest of bottom line and sales volume. They were built to be sold at a price, and to do so, some things that could have been done better were done "good enough", for the time- but that's all you get from them, in original form...
Regards,
Gordon.