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We are completing our rebuild of a Dynaco ST-70 (Tubesforhifi board, our power supply mod) and one channel wouldn't produce signal. Since we're training a new person, I went over the build, found a couple of small errors and things to neaten, and the channel still wouldn't operate. So I started measuring resistances, etc.
On measuring the grid leak resistor, it behaved like a capacitance (resistance going up over time as it is measured). There ISN'T an input capacitor. Turns out between the time we selected tubes for the amp and the time we were ready to fire it up the input tube developed a hard grid to plate short, so we were charging the 47uF bypass cap. Changed the input tube and viola (yeah. I know, but I used to play viola) the amp works.
Now just a couple of little tweaks and we'll see what it sounds like. This is the amp we're trying the diode-isolated semi-dual power supplies on (how's that for a massively split infinitive?).
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Lee of Omaha,
I used the boards you used for years and they are really good. If you really want to hear that amp sound good or as good as it can take out the stiffness and put back some flab in the bottom end. I have a friend that has always been of the rule to put as little capacitance in just to keep it quiet. David Hafler was a great designer in his day and if he didn't do the designing then he had someone that did. He used very little capacitance in that amp and I always liked the way it sounded better that way than with loads of electrolytic capacitance. Also, as Mike Samra suggested, go with film caps if you can find places for them. The lower the value the better but again large enough just to get it quiet.
I built clones to the ST-70 on a larger chassis to be able to use film caps and it does sound better with them as compared to electrolytics. If you want a stiff sounding tube amp why not just go with solid state. Tubes are supposed to be romantic sounding.
Sorry, gotta disagree. I don't think an amp should sound "romantic" or have any other adjectives describing its sound. To me an ideal amp has no sonic character of its own--it just amplifies.
I'm also agnostic when it comes to tubes vs solid state. To paraphrase Duke Ellington, if it sounds good, it is good.
I personally can't stand flabby bass. Our ST-70 has the best bass I've heard to date from a tubed amp, and I very much want to keep it that way.
Lee
The 47uf cap you may want to remove if you have excessive gain..I wire all my boards for 12BH7s for the PS/driver and the voltage amp for a 12AU7.
Those are excellent boards but they can be tricky but playing with different configurations help.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
The 47uF cap is the input triode's filter cap. In attempting to measure the input load resistor, we were also measuring the filter cap through the grid-to-plate short. Too bad, it was a nice Sylvania chrome dome 6SN7.
Between TubesForHifi's generous on-board filtering and our power supply, out ST-70's power supplies are stiff as a board. If you unplug it while on, it plays for about 20 seconds (until the cathodes cool).
When you said bypass,all I could think of was the 47uf bypass cap that Roy used to put on the ST70 and MK3 boards to set gain and now he has done away with it.I try to keep a few on hand being they are so perfect.I usually buy 10 at a time and it takes a couple years to go thru them.
He does give you a generous amount of filtering.That is the most brilliantly designed ST70 board ever..You have lots of filtering,separate bias pots along with AC balance pots,and you can set it up for different tubes.
Did you ever try a 35 to 40uf film cap off the GZ34 in those? Even in the ones I use Schottky diodes in I use a film cap in the charging stage..It really tightens the bass and cleans up the mids and lower highs.
"
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
"This is the amp we're trying the diode-isolated semi-dual power supplies on (how's that for a massively split infinitive?)."
I'm happy to boldly split infinitives, but your example isn't a split infinitive but a dangling preposition:
This is the amp on which we're trying the diode-isolated semi-dual power supplies.
Your yore you're right!
mt
No, metal film. But the resistor wasn't the issue, it was a shorted tube.
I prefer shorts in the summertime.
I tried that diode isolated thing once on the front end of a diy pp guitar amp. Measuring b+ to the preamp tubes, it did prevent b+ from sagging as drastically under a heavy transient. YMMV... JH
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