Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dude on September 09, 2011, 05:05:00 pm
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Bruce over at Mission Amps uses 15v, 5watt zeners to drop B+ voltage when needed.
He just puts them band to ground off the center tap, leaving the leads long and no tube shrink.
I have had good luck for years with this method, they do get hot but if you go no higher than 15v each and put them in series with long leads not a problem for me, 5 watts works well.
But many knowledge people have told me not to mess with the ctr tap?
Just curious how you guys feel?
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http://www.el34world.com/Forum/index.php?topic=11506.0 (http://www.el34world.com/Forum/index.php?topic=11506.0)
That thread talks about a lot of the different ways to skin the cat. I'm sure others more knowledgeable will chime in soon!
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I've used 'Bruce's' method with 5W axial zeners in a few builds and it works fine as long as you watch the dissipation, which in most cases, if you are dropping 9-15V over the CT, isn't much. If you're concerned about it you can 'heat sink' the zener leads by soldering plates of metal to them (IIRC Bruce has sometimes used a copper penny with a hole drilled in the middle which he slips over the ends of the lead and solders in place), but if you mount the zeners on a decent tag strip, it achieves the same thing.
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http://www.el34world.com/Forum/index.php?topic=11506.0 (http://www.el34world.com/Forum/index.php?topic=11506.0)
That thread talks about a lot of the different ways to skin the cat. I'm sure others more knowledgeable will chime in soon!
Hey thanks for that thread, I guess I'm still in the dark ages. Your right, seems like several methods to skin a cat but the zeners off the ctr tap seem to be fine.
Cheap at .36 cents each.
But probably best to get the right PT. I do a lot of conversions and Zeners seem to do the trick.
al
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I often use 10 volts , 5 W zener and it works very well. I solder them on a terminal in safe way. Smetimes I need 6 in series to drop enough voltage.
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I often use 10 volts , 5 W zener and it works very well. I solder them on a terminal in safe way. Smetimes I need 6 in series to drop enough voltage.
Exactly, keep the leads long on a terminal and put them in series in a safe place away for anything touching them and no heat shrink.
al
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I've come to like a bucking tranny and an inductive power resistor. A typical bucking tranny will drop 6.3 or 12.6 VAC of incoming AC wall voltage. That drop will be multiplied by the PT windings. I recently used an inductive 330R power resistor to drop an additional 24VDC in a PP EL84 amp. Overall I dropped over 100 plate volts from 410VDC to 290VDC with this method. I guess this method will contribute to sag; zener method instead to voltage regulation (I think). Just another alternative.
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Yes a resistor could do the job but sometimes we don't like the sag this give to the amp.