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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: Dynaco ST-70: upgrading using mod kits or replacing existing parts? posted by newbie on July 24, 2002 at 10:26:20:
My ST70 renovation started with new tubes out jof necessity, as it arrived with an incorrect rectifier, a bad 7199, and a mis-matched set of outputs.Before installing the new tubes (Svetlana EL-34s, JAN Philips 7199s, and a good used Mullard rectifier), I cleaned the sockets with Caig D-5.
I used the amp for couple of years with just the above service until recently, when I read a post here that explained that the Selenium rectifier used for the negative bias supply tends to spew vaporized Selenium when it fails, so that got replaced with a generic SS diode. The resulting voltage put the first bias filter cap right at its rated working voltage, so after a couple of days use, the caps and resistors in the supply were replaced. I used Mr. Curcio's idea for changing the range of bias control, and calculated new resistor values to replace the 10k carbons, although the old resistors measured very well.
I next replaced the input connectors with som nice gold ones from Radio Shack, which I selected for their mounting hardware as much as anything else. I was able to remove the old connectors from the phenolic boards, and modify the boards to accept the new connectors. In the process, I also relplaced the two 470k resistors with some metal films, although once again, the original resistors measured well within tolerance. In addition, I installed some very pure copper wire in the signal path from the connectors to the PCB.
I next replaced the 4 .1uF driver coupling caps on the PCB with some AXONs. The old caps, although they measured and worked well, were a tiny bit leaky.
Today, I will replace the can cap with a new manufacture one from Antique Electronics Supply. They sell these for under $35.00 (there is still a 10% off sale in effect for the next few days!), and even though the old cap seems fine, I thought it time to let it retire after more than 40 years of faithful service.
Next will be the octal sockets on the top of the amp with ceramic ones. As you can see, I am working my way toward the PCB, which I will probably replace entirely with a new reproduction from Curcio, stuffed with parts of my choice.
So far, not counting the tubes, I have well under $100.00 in parts; the Curcio replacement PCB will add another $40.00 plus parts. As is, the amp sounds wonderful, and I suspect an improvement from replacing the can cap.
This 'little bit at a time' technique has been beneficial to me, as it has allowed me to evaluate each change along the way, and it has kept the cash outlay spread out over a period of time.
IMO, this is a wonderful sounding amp, and well worth the effort.
Cheers!
Jim
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Follow Ups
- Here is what I am doing... - Jim Carlon 11:41:33 07/24/02 (1)
- Re: Here is what I am doing... - airtime25@comcast.net 19:05:06 07/24/02 (0)
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