Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: rdrgtr on October 08, 2010, 11:14:29 pm
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I just came across some schematics for some Ken Fisher inspired builds with 3 or 5 zener diodes (10v 5watt) coming off the ground of the power transformer. So, how does this work? 10 volts drop per diode? All of the builds were using bridge rectifiers. Can they also be used with a tube rectifier (in series with the center of the power transformer to ground)? I also noticed a bypass switch on some of them.
Thanks
http://www.blueguitar.org/new/schem/blue_gtr/blues_expr/blxprsch.pdf
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Yup that's a switchable 30V drop with 15W dissipation capability. Not that it needs that much - that amp probably has 100mA peak on the HT winding at most (so with a 30V drop its only 0.3W). Use 5W axial zeners and mount them on a tag strip away from the chassis
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If you want that the voltage drop will work on these schematic diagram, please mount them in the opposite direction.
Now these zeners will only drop 0.7 volts each. :grin:
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In the schematic posted the PT is labeled KT-17620. Anyone know what company produced that PT? I can't find any info on it at all.
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If you want that the voltage drop will work on these schematic diagram, please mount them in the opposite direction.
Now these zeners will only drop 0.7 volts each. :grin:
No, in this application you want the cathode toward the case.
It effectively puts the power transformer center tap at approximately -30 volts relative to the ground of the rest of the amp. (All voltage is relative, right?) So it subtracts 30 volts from B+ relative to ground in the rest of the amp.
Ground layout is REALLY important with this. Do not ground your entire power amp at the top of the zener stack. I don't recall exactly what happened, but it had very unexpected results when I did that a few years ago. You may be able to ground the first ("reservoir") cap on the top of the zeners - that's what I did, but it was a single-ended amp with the plate supply on the second node of the power rail.
Also, I recommend strongly against throwing that switch while the amp is on. A SP/DT standby switch with On/Off/On setup would be good so that the amp is in standby whenever you switch the B+ voltage.
HTH
Chip