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Looks like I found a completely original ST-70 - the tubes are labeled Dynaco Mullards and all caps appear original. It's a little dirty and the chassis has a small amount of rust.

My question to the group, should I clean it up and leave it alone - save for a simple check up to make sure all is working properly - or start the modification process - new board, etc.

I am leaning toward cleaning it up and leaving it alone and finding another chassis to mod. They can only be original once.

I would replace the filter caps. Otherwise, leave it original - and enjoy it.

If you want something more modern, possibly better, buy a brand-new amp.

dcriner wrote:I would replace the filter caps.

Replace those black "Pyramid" coupling caps, too. Oh, and the two 50uF electrolytic caps underneath that are part of the bias supply. The selenium rectifier in the bias supply also needs to be replaced with a 1N4007 or UF4007 type silicon diode.

Simple answer if the unit in question is generally more than 10 years old then replace ALL the caps. If it has been in use usually the unit could be considered to be in better condition cap wise. If it has set for years I don't even consider plugging it into the AC without first reforming the caps. Caps are cheaper than replacing output tubes and or rectifier tubes.

It is subjective as to whether you want the stock sound or a more modern one. However, the components in your st-70 are very old, and the stock driver board was cheaply made. The traces could tear, or it could disintegrate if you try to replace the caps on the board.

There are originally designed driver boards available (on ebay for $15). Unless you want fireworks in the future, the caps need to be replaced and the selenium rectifier replaced with a UF4007.

Definitly replace that can cap, there is a better one that is rated 80-40-30-20 @525v, just wire it 30-80-40-20 and get new 6.8k and 22k resistors because you will destroy the old ones trying to get them off.

Replace the bias rectifier and both caps!

I would really consider replacing the driver board 1: the traces lift easily when you try to replace parts, 2: it's most likely already cooked and warped, 3: good 7199 tubes are getting hard to find. BUT if yours looks good and not too discolored around the tube sockets you could try replacing the six coupling caps.

Another consideration is the hookup wire- 22awg solid, it's 50 years old now and brittle. I like using 22awg 7-strand silver plated teflon wire, you can find it cheap on ebay.

I recapped a PAS-3. When I finished, I checked all the original film caps with a cap tester - no leakage. I then disected all the film caps - they were all mylar. The original caps could have lasted indefinitly.

If you decide to replace the caps on the driver board, I would snip off the originals, leaving a pigtail connected to the board - then splice replacement caps to the pigtail. If you try to desolder the original caps, you run the risk of overheating the board and lifting a trace.