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In Reply to: RE: For an extra $50 posted by Triode_Kingdom on September 24, 2021 at 10:57:36:
I would imagine you're right about voltage surge in a ST-70 if SS rectification is used. I thought I'd read that somewhere, so I checked the SDS Labs site, thinking it was there, but he was commenting about the Dynaco MK-III, even when using the 5AR4:
"The Dyna MKIII has a basic design flaw that none of the other Dynaco products have. The capacitors in the power supply are not rated at a high enough voltage for the power transformer. This results in the quad cap being run over it's rated voltage every time the amp is turned on. The first section of the original quad cap is rated at 525 volts and the B+ goes as high as 585 volts during start up. This is due to the fact that the rectifier heats up and begins conducting before the power tubes heat up and start drawing power from the high voltage supply."
Apparently the ST-70 didn't suffer the same over-voltage ills, at least when tube rectification was retained. Stokes also said something else which I found interesting about the ST-70, which is the point I was trying to make to the original poster regarding ordering a larger chassis from VTA.
"There are many modifications available for this [the ST-70] amp. There are two problems that most modifications fall into. First there are the total mods. Basically, everything in the amp is removed except the iron, and the amp is rebuilt. This is all well and good, but don't forget that this is a budget amp, and the chassis is not so great either. If you are going to dump that much money into a tube amp, why not start with a new chassis, and design it from the ground up instead of making compromises to fit the mod into this chassis."
Back to the OP's interest in the ST-120. I found this on the VTA site, "The power transformer instead of 360-0-360 secondaries like the ST-70 has 420-0-420 secondaries and puts about 485 volts on the plates (pin 3) of the output tubes. The ST-70 puts about 420 volts on the plates." Given that SS rectification is so widely used (apparently by necessity) in the ST-120, hopefully they're using the 550V (600V surge) quad cap I've seen available elsewhere. Some manner of time delay in that amp seems like a very prudent choice.
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Follow Ups
- "the slow warmup of the 5AR4" - 1973shovel 09/25/2105:52:13 09/25/21 (3)
- RE: "the slow warmup of the 5AR4" - Triode_Kingdom 15:37:58 09/25/21 (2)
- RE: "the slow warmup of the 5AR4" - 1973shovel 09:20:01 09/27/21 (1)
- RE: "the slow warmup of the 5AR4" - Triode_Kingdom 14:05:33 09/27/21 (0)
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