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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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In Reply to: Stock driver probably wont cut it..... posted by Jim Doyle on November 4, 2006 at 16:05:55:
I agree, the stock circuit doesn't have the testicular fortitude
to drive 6B4G's to full output.Straight fixed bias is a problem for a couple of reasons, first, the ST70 only has a 55V bias winding, second, 6B4's, by the book, want 50K or less of DC grid resistance.
Cathode bias has its problems, too, as you're straining the 5AR4 (and
the power transformer, if it's original) coming up with enough juice to feed 4 6B4's running cathode bias, and you're drawing enough
current that the choke is running saturated and not effectively any
better than having a 55 ohm resistor. It'd probably be more effective
for hum reduction to replace the choke with a 50 ohm resistor and
instead use a small low current (20 ma or so) choke in place of the
6800 ohm power supply resistor.There's another option: make pseudo fixed bias by bypassing the
cathode resistor with a zener diode.Frankly, I think (bear in mind I have a conflict of interest) the
Shannon Parks diytube.com board with the long-tail pair phase splitter
would be better for this, as the effective plate supply voltage on the
driver tubes is about 100 volts higher because there's a lot less
voltage loss across the cathode resistor.One other thing: because 6B4G doesn't use pins 4 or 8, it's possible
to wire the amplifier up to use either EL34 or 6B4 interchangably.
across the cathode
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Follow Ups
- Re: Stock driver probably wont cut it..... - Uncle Ned 18:24:58 11/04/06 (0)
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