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In Reply to: RE: Question about recapping a Dynaco ST70 posted by Alpha Al on July 10, 2014 at 13:13:47
...given the cap is in hand, I'd be inclined to put the 20uF at the rect and the 30 after the choke. However, the real world difference is pretty much negligible.
The max cap for a 5AR4 using cap input filter is generally speced at 60uF although there are a ton of factors that may alter this value. Also, contrary to what's stated elsewhere in this thread, the max cap is a limit not only during startup conditions but also during steady state operation unless special precautions are taken to limit peak repetitive current.
Follow Ups:
"... to limit peak repetitive current"
That's right!
And the more series resistance (DCR of the secondary winding or even added series resistors) the higher the cap value can be but if all one has is the minimum stated 'resistance per plate' then the max input cap is all you can use safely.
One way to solve the problem is to use a critical inductance choke input filter and then there is no current peaking and no minimum resistance per plate requirement.
It's all in the data sheets. One just has to know how to interpret them.
Start up surge is a different issue.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
After seeing a fireworks display on one of my amps putting a 60UF cap on the fier I now never use more than 40UF.
The cap that is a direct replacement for the Dynaco is a bit on the anemic side. I usually add more to the stages(about double) and I try to use film caps when I do.
...but I've seen 5AR4s handle 60uF w/o issue. It all "depends". One example is the Heath AA151. I've had an early Sovtek in mine for ~15yrs of occasional use and no fireworks yet.
The ST70 asks a hell of a lot out of a single 5AR4. Put in 4007s before the rectifier plates.
If that's the case then why didn't Hafler just use a SS rec?
Probably related to the selenium rectifiers of the day not being as robust as the 5AR4. Not the case anymore. 1959 was a long time ago., the 1n4007 came out in 1966.
When the ST70 was designed and introduced, 1959 or thereabouts, selenium rectifiers with the nec current and voltage ratings were large and had high voltage drop. Silicon rectifiers new and really expensive and usually required 2 in series to get the nec voltage rating. So, considering the budget nature of the amp, the 5AR4 was a logical choice even if run pretty much at absolute max rating. Too bad Hafler didn't opt for dual 5AR4s and more filtering beef like some other stereo amps of the time did. That would have been a giant killer.
So you are saying that two parallel 5AR4's would do wonders to that amp? I have pulled out the choke on many occasions and it woke the amp up much more than I thought.
Think solid state in an ST70
"Wonders" might be a little bit of hyperbole but dual rectifiers and beefed up PS would be a big improvement. Or, if you're not committed to 100% tubes, go silicon for the rectifier with due precautions for higher B+. Dave Gillespie covered the SS route in some detail @ Audio Karma a while back. Worth checking out!P.S. Don't mean to imply a stock ST70 can handle 2x5AR4...PT isn't up to addl filament load. Just that ideally, dual rectifiers would have been the way to go at the time. SS can be done with the stock PT.
Edits: 07/11/14
Hi,
Dave Gillespie is a very frequent poster on AudioKarma. Can
you please provide a link to the specific SS discussion you
mention????
Thanks and regards to all,
John
...a long thread but worth a read. Interesting conclusions about the stock circuit of the amp.
Edits: 07/12/14
Thanks, Steve. I appreciate you.
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