On Oct 11, 6:49 pm, "Phil Allison" wrote:
"Sander deWaal"
"One 6BQ5 grid had been shorted to ground since the thing was built,
so the sound must have always been off."
This leaves open a possibility that's not yet mentioned: a magnetized
core due to a possibly large DC current.
** The amp uses cathode bias.
A 100 ohm power resistor serves all 4 tubes.
Magnetised OTs are vary rare things.
........ Phil
I'm not a restoration purist, but I would immediately modify that
single 100 ohms to 4 separate 400 ohm (but see below)resistors (say, 2
watts each) with 4 separate bypass caps of at least 100 MFD each.
Single, shared Rk's are not good as the tubes begin to deteriorate
unevenly.
Then I'd check the exact ratios of the OPT's (P-P to speaker winding),
viz:
Use a 12 VAC, or so, "wall-wart" P/S from a variac, to the OPT
secondary and measure the OPT voltage ratios - keep the "ww" voltage
below about 6 VAC. Then reflect that to the P-P secondary impedance,
e.g. a ratio of 30:1 with 8 ohm secondary gives 7200 ohms P-P load.
While at it, check the screen tap ratio.
Then, while listening to it playing Mozart piano concertos into my
classic Ditton 44 speakers, I'd figure out a good working point for
the 6BQ5/EL84's given the B+ available (360 VDC, I think, draw load
lines on the tube plate curves - details beyond scope of this short
note) and (after switching it off ! g) consider if I needed to
change the Rk's a bit. Or quicker, just take a design from another
proven schematic - I'd use the Mullard 5-10 O/P design, see
http://www.bonavolta.ch/hobby/en/audio/el84_7.htm )
Good luck!
Cheers,
Roger
Disclaimer: potentially lethal voltages under the chassis - don't do
any of the above unless you're experienced in fixing tube gear.