AA-100 Restoration With Pictures
OK. Here's John's AA-100, after restoration:
Chassis bottom shot:
From this, you can see the basic layout... amp board at the bottom left, power supply bottom right, and preamp board at the top. You can see the new bias pots (blue), that were added.
Here's another picture of the same, with some of the more major work annotated:
John's unit was supplied with a 5R4 rectifier (more voltage drop then 5AR4/GZ34), so the big 80 ohm 30 watt resistor wound up being not being permanently installed- it was not necessary. If you're wanting to use a 5AR4 (necessary if you want to use the unit with the lid on it- the 5R4 will NOT fit under the factory cover!), then some sort of dropping resistor is needed to get the B+ down to a reasonable level...
Here's a close-up of the amp board- I circled the bias-test points (cathode connections of the output tubes) with a black Sharpie, to make them more apparent:
You can see the tiny 10 ohm, 1/4 watt resistors (BTW, one of them is brown, the other three blue- that was how the batch of resistors I got, came- same brand, same model, just one of a different color- go figure...), next to the black-circled terminals. On this board, the traces were cut to allow the output cathode connections to have individual grounds (a couple of jumper wires were needed, to run those new grounds to the cathodes), and the traces were also cut to allow separate access to each grid-bias resistor on the four output tubes- individual wires were run from each, to the bias supply pots.
One other problem that has been apparent on both AA-100s I've rebuilt, is that the screen supply resistors for the 7199s (820K ohm) tend to get "cooked" by the heat from the 22K phase-splitter resistors near them, when the coupling caps to the output tubes short. When this happens, these resistors go way out-of-spec... too high of a resistance value (typically 2M ohms or more, instead of 820K!) When this happens, the amp output becomes very distorted and weak. These resistors, as well as the 22K phase-splitter resistors, were all replaced. All the coupling caps were also replaced!
Here's a close-up of the power supply:
You can see the four new bias pots (labeled to correspond to the tube numbering on the output circuit board), mounted on a new terminal strip, and said terminal strip mounted to a small bracket which is then mounted to one of the transformer mount-down screws. This bracket moved the mounting point of the terminal strip rearward, so that the strip no longer ran into the wiring for the heater hum pots.
Each pot is labeled with its corresponding tube number, as silk-screened on the top circuit board. I also etched in the corresponding tube number on the bottom of the board, by the 10 ohm bias resistor for each output tube, so it would be easy to determine which test point corresponded to each bias pot.
Also, note the extra decoupling caps (the small black caps) near the bias pots. This was to isolate the tops and bottoms of the pots from each other, so that there was no "crosstalk" between them. Without those, the 8.2K resistor between the pots and the bias voltage source, and the 3.3K resistor between the pots and ground, would allow AC voltage drop across them... which would allow signal from one grid to "leak" to another, reducing gain and channel separation! The caps "shorted" such "AC signal leaks" to ground... no more crosstalk.
In addition, note that in this shot, the 80 ohm, 30 watt resistor is still in circuit. It's connected between the rectifier and the first power supply cap (60uf stock, 70uf total in this amp now). If using a 5AR4 rectifier, this cap is needed since B+ voltage, once the bias current through the outputs is reduced to a sensible (less than 35ma per output tube) level, and with today's high wall line voltage (commonly 120V or more, as opposed to 115-117V in the past), will commonly be close to 500V! This resistor drops between 15-20v... a value between 80 and 120 ohms is usually the right ballpark for this. But, since John had a 5R4 rectifier tube, this resistor was not needed... and it was removed, and the rectifier returned to being wired directly to the first power supply cap.
Lastly, here's a picture of the preamp circuit board:
In the earlier picture above, I noted that there were broken connections beneath the preamp board. These included several breaks in where the "auxiliary circuit cards" containing the selector switches (the little boards sticking up off the main board, with the shafts connecting them) connected to the main board! I had to go in, and solder in little jumpers, to re-make these connections, before I could get any useful output from the unit at all, on either channel!
Also, the tube positions on the left of the picture (V5 and V6 by the factory labeling) are the most important places where replacing the 12AX7 factory tubes, with 12AT7 or 6201 tubes, makes the most improvement. There can be a small improvement in using them in the phono stage 12AX7 positions as well, but the line stage is the biggest thing that makes a difference, when swapping tube types here.
One problem I did run into on this unit, that I did not on my earlier subject, was that the 2.2M ohm screen-supply resistors for the EF86 tubes, had both gone effectively OPEN CIRCUIT. Strangely enough, the unit still passed sound- just with much lower gain than normal, and a "muffled" character to the sound (like there was a high-filter blocking everything above about 5KHz or so). However, once these resistors were replaced, it's now dynamic and tonally correct- plenty of highs and good gain.
Frankly, this particular AA-100 had more problems than any of these I had encountered before. Between an original kit-assembler who wasn't careful about how they soldered (too much heat and not enough solder!), and the fact that this unit has been dis-assembled and re-assembled a number of times... the boards had really taken a beating. I could understand someone wanting to throw out those boards, if they went through what I went through on this one! However, it does sound FANTASTIC now, once it's properly repaired and restored... and I'd expect that MOST of these units won't have NEARLY this many problem areas...
Regards,
Gordon.