I have been enjoying the vintage audio hobby for close to 20 years. I have always taken my tube gear to techs for repair but my favorites have either moved or unfortunately passed. I figured that an ST70 would be a good start to learning how to do some of this work myself.
I recently purchased a vintage system from an estate sale. Dynakit PAS2, Dynakit ST70 and a pair of AR2a speakers.
The Pas2 and st70 appear to have been serviced sometime after 2001 judging by the date on the CE Quad cap. Lots of new yellow caps on the boards along with many new resistors. No selenium. All original boards.
ST70 has GZ34 rectifier, two Electro Harmonics EL34s, two EL34s (no white lettering but I found XF1 on one dual Halos - Mullards?) and two unknown 7199 tubes. The Mullards are in the right channel.
I powered up the amp and preamp with a variac over the period of a few hours. Adjusted the bias. The Left channel seemed like it was running out of adjustment on the bias pot Everything otherwise seemed fine and sounded good. Then after a few days of listening I saw and heard that the Right channel was motorboating. While I was looking up the causes for this, the Quad cap let go with lots of smoke and noise. Thankfully I was sitting in front of it and I cut the power.
I did some checking and found that my main voltage is 123VAC. This is pretty consistent. So when I had my variac set to 110 it was actually putting out 120VAC - Lesson learned and I will be adding a meter to my variac. Setting the variac at about 108 gets me 115VAC
I ordered a new 525 volt 80/40/30/20 quad cap. While I was waiting for it to arrive I removed the resistors from the quad cap and checked them. The 22K was at 20K and the 6.8K resistor read more than 13K. I checked all of the 1K resistors on the tube sockets and the one on V2 was 901 ohms. The others were very close to 1K. The Bias resistors were spot on. I picked up all new 1% resistors for the amp.
I wired up the quad cap with the diagram provided including new 22K and 6.8K resistors. I brought the power up slowly with the variac making sure to stop at 115VAC. I biased the tubes to 1.56. This time there was more adjustment available with the left channel. The amp sounds really good.
I touched the Quad cap to see if it was getting warm and was surprised to feel it vibrating. If the volume is increased, the vibration seems to modulate with the music. No vibration can be felt anywhere on the chassis or transformers. Maybe some on the output tubes. If I use a pen to touch the quad cap or the tubes I can feel the vibration but not as much. Is this normal? Is it just that the quad cap is tall and the chassis or transformer vibrations are being mechanically amplified?
I have been checking all of the forum posts and do not find anything about a quad cap vibrating. I did see in several pictures that the 6.8K ohm resistor is replaced by a straight wire. Not sure if this is only for VTA-70 amps. Should I delete the 6.8k ohm resistor?
Thanks for reading this long post.