Dynaco ST-70
The Dynaco ST70 amps were available in either factory built or in kit form. Most were kits and 90% of the ones I have seen were assembled by amateurs that did a terrible job of wiring and soldering. The easiest way to repair and restore these amps is start from the bare chassis and rewire the whole thing using new stranded wire.
After stripping the amp down to bare chassis and cleaning it, I install all new tube sockets for the 8 pin sockets using gold plated ceramic sockets.
Next the main power supply cap was under rated on these amps and you can go as high as 50uf on the first section after the rectifier tube. Since it's hard to find a multi-section can cap that can go above 500VDC and has enough capacitance,I etch out a printed circuit board with 4 sections of capacitors much higher than the original can cap and with a higher voltage rating.
The picture above shows a cap board mounted in the amplifier. The board has: 50uF/900v , 165uF/500v,100uF/450v,47uF/450v. The neg bias selenium diode and caps also get replaced with a 1n4007 diode and filter caps which also get mounted on the circuit board.
There are more sources available these days for a mult-section can cap if you want to pay around the $50-$75, or you can use a JJ 500v with a few sections and then install a few caps below the chassis. At the time this was rebuilt a decided to build a cap board to replace the original can cap.
Next is to wire all the transformer leads. I cut them all back to new clean ends for soldering to the new sockets. Then all new 22 guage wire is used for the rest of the underneath wiring.
The predriver section of this amp is constructed using a printed circuit board. Dynaco was a bit a head of some of the other kit builders from that era as their kits had less construction required using pre-assembled circuit boards, which the builder had to do less wiring to make the amps run the first time verses some of the other kit builders which involved a complete point to point wiring. I do not prefer circuit boards with tube amps. They are flimsy, traces are small and excessive heat while soldering can have them lift off the board and then you are having to add small wires underneath to repair the broken traces.
The best fix I have found for these driver boards is to order a new bare board for about $25. The board material is almost twice as thick and has larger thicker traces.
Then install new ceramic tube sockets and choose new quality components to stuff the board.
The amp above shows one of the new driver boards available. I stuffed this one with PRP audio resistors and 715P coupling caps. You can choose higher quality coupling caps
like MIT or Auicaps. This board uses the 7199 tube which is the original design. Search the internet and there are at least 10 different boards available to replace the original
design using the 7199 tubes. The 7199 tubes are getting harder to find and expensive and the new manufactured ones don't seem to be the same as the old style rca black plate.
I still prefer the original 7199 design with the new boards.
To finish up the amp, I installed new gold plated rca jacks on the front, got rid of the stereo/mono switch and installed new gold plated speaker binding post on the back along with a new power switch and power cord.
For power tubes, the original Dynaco EL34 mullards are long gone these days, so the new russian manufacturers do make some nice tubes. In this one are the 6CA7 large bottle Electro Harmonics. Another choice is the new production mullards which also seem to be quite good.