The original HF-87 had a thermal delay on the B+ that took about a minute or so to kick in. This provided protection against excessive HV on start-up. In my opinion, there should not be 400V on any 12AX7, if the circuit is working.
I'll bet that your thermal switch is missing, defective, or bypassed . . .
I'll bet that your thermal switch is missing, defective, or bypassed . . .
And you would win that bet, it's been gone for over thirty years, seemingly with no ill effects. I've been thinking about putting in a 555 timer and a relay to delay the B+, but can't see any reason to since there haven't been any problems.
Jim
quote:
I believe those little ones are just a couple of silicon diodes. The copper ones are supposed to have some series resistance added so that they don't result in a lot of extra B+ and probably an inrush limiter. I've never taken one apart, but that is the impression that I got.
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Couldn't be said any better than this. You can make the same thing with an empty tube base and a few parts for a few bucks. Do they sound any better?
Not in my opinion but I do not wish to get something started here on SS verses tube rectifiers. I can tell you that a home made version sounds the same as a copper cap.
I believe those little ones are just a couple of silicon diodes. The copper ones are supposed to have some series resistance added so that they don't result in a lot of extra B+ and probably an inrush limiter. I've never taken one apart, but that is the impression that I got.
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Couldn't be said any better than this. You can make the same thing with an empty tube base and a few parts for a few bucks. Do they sound any better?
Not in my opinion but I do not wish to get something started here on SS verses tube rectifiers. I can tell you that a home made version sounds the same as a copper cap.
The Weber Copper tube replacement(s) are several to choose from to best match the need, are an ok replacement. It is most true that they do not have the same slow warm up time as a tube, and that they do not suck life out of the 5 volt winding. Different versions have different voltage drops at the rated currents to try to simulate different tubes. I've designed one into a KT88 amp, I did not have a 5 volt winding to power a tube and I did not want to add another transformer. You can end up with a higher B+ to stretch the voltage ratings on the caps and tubes, so just know that before you play with them..
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