Old tubeamp question - Page 2

Smart thinking!
Voltage conversion: I have an AA-100 and it has no provisions for 220V AC. Most likely the AA-151 is the same.
Adapters come in two varieties: transformer-based, which should weight a few kgm for 100 watts, or diode-based, which "chops" half of the sinewave. Unfortunately, the latter is not ideal, since the chopped AC has high harmonics (i.e. noise).
Vintage tube issues: You must follow a sane procedure before attempting to operate vintage equipment. The fact that it's only 28 watts does not mean it can't start a fire or electrocute a person. It's just as dangerous as a 100 WPC amp!
1) check all tubes with a tube tester.
2) replace all sellenium rectifiers (and compensate for the voltage difference!)
3) replace all coupling caps in the audio path. Twenty years of high voltage and high temperature stresses take their toll!
Bad caps are the main reason for distortion (if you're lucky) or an overheated tube (if you're not).
Ceramic caps in the tuner section would last much better over time.
4) Check electrolytics for shorts (including in the bias supply) and replace as necessary.
5) with all tubes removed, power on the unit at half voltage and watch for smoke.
If nothing happened, then let it "cook" and slowly raise the line voltage in steps (using a variac).
If the unit has a tube rectifier then repeat step 5 with the rectifier in place. Measure voltages in the supply and make sure they are withing a reasonable range, depending on the amp.
6) Turn off the unit and wait for caps to discharge (measure!!!!)
Now plug in the small signal tubes. Power up the amp at 70% of the line level (to compensate for low current and low voltage drops in typical RC supply filters) and measure tube voltages. You need the schematics, tube pinout and typical operating conditions (or a service manual checklist).
Verify that the bias voltage to the finals is within range.
7) Turn off the unit and wait for caps to discharge (measure!!!!)
Now, the acid test... Plug in the power tubes and instead of speakers you may use power resistors.
Bring it up slowly to full voltage, watching that the plates don't go red.
Measure everything once more and feed some music. Check with a scope what kind of signal you get on the power resistors.
8) Plug in the speakers and enjoy your "new" amp!

In the process of doing all that, you may find bad resistors, dirty switches and potentiometers, bad traces or cold solder joints (especially in kits!)

If it seems like too much to do, then don't mess up with vintage tube gear. That is what it takes to get the real good sound from that equipment. Cutting corners is not only dumb - it's utterly dangerous.

Do it right the first time and you'll have a long and rewarding service from your vintage gear. Have fun!!!

Frankly, there isn't that much progress in newer designs. The circuits are quite similar and some "newer" circuits are just that, newer. Not necessarilly more musical...
Components like capacitors have improved, due to advanced materials and more precise manufacturing. It's a good idea to replace these.
Vintage transformers are not bad at all. There are new core materials but I still like overall the good old M6 iron for a full-range tranny.
The specific Heathkits are a bit "complicated". If you want purity of sound then you would probably want to bypass all the different options for individual gain controls per input, output phase inversion, tone controls, etc.
Even without massive changes as such, updating components and going over the checklist would give you a very competitive sound compared to any new amp I've heard recently. You may not have the same power levels as the bigger amps, but the vintage units do have the "tube sound", and I mean a good tube sound - transparent, fast, extended and simply musical.
Unfortunately the cost to update these amps are quite high if you pay a lab to do that for you. However, if you learn to take care of the maintenance yourself then these amps are worth the investment and then more!
Mine has 7591 finals, similar to 6L6. I personally like the sound of smaller tubes like 6V6 or EL84. IMO the EL84 is a "perfect" tube if all you need is 10-12 watts per channel. It has all the sonic virtues of the 25-35 watt tubes but it also seems to have excellent bass control, which is surprising given the low power.
Enjoy it!