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>> Just so happens I've got an old(but healthy & stock) BF Dual Showman
head,
> 2 matched quartets of Russian firecrackers with the same match #'s(8
> matched tubes), an ample supply of flameproof screen resistors, some
fuses
> rated 1a lower than called for, & a Halon fire extinguisher! I'm planning
> to do 'The Experiment' this weekend.
>
Did The Test!
Disclaimer: The results won't stop any flame wars(didn't expect that), are
somewhat inconclusive, and would be statistically insignificant anyway(1
amp, limited # of tubes).
Conditions: I replaced the 27K resistor in the bias circuit with a 20K to
provide better adjustment range. 4 new matched Phillips ECG 470 ohm FP
screen resistors were installed - also replaced the 470 ohm resistor in
bias circuit with a 470FP. All 8 new matched Sovtek 6V6GT tubes were tested
in low voltage tube tester - one scraped for poor characteristics.
The Test: After twice blowing up a tube & screen resistor trying to switch
on B+ after a 5 min warmup--(at)--117VAC with maximum negative bias, I decided
to start at -45V bias with the Variac set--(at)--110VAC & got the thing on.
Twiddled' the bias & Variac gradually to 115VAC, 20ma/tube(combined plate &
screen) - all tubes were within 1.2ma & plate voltage was around 455V.
Played my Strat through it(stock Dual Showman bottom) for 15 minutes with
volumes between 3 & 6. All 4 tubes had oval-shaped areas of moderate plate
glow.
After a short cooling off period, I put a signal generator, scope, & dummy
load on it - raised AC to 117VAC & rebiased to 17ma/tube(combined plate &
screen--(at)--idle) - 465V+ plate/460V screen. Output power with volume on 8 was
right around 50W. Crossover notch was minimal. Played it again for 15
minutes with volumes between 4 & 6 and ended with a 5 minute blast at
8(those poor neighbors)! Some plate glow was still present on the inner
pair of tubes - outers were very faint. Switched to an 8 ohm speaker
load(D120F in my DR) & played again for about 20 minutes with volumes
between 4 & 6 - no plate glow!(but no blast on 8 either).
I planned to raise the AC to 120, rebias strictly by the crossover notch
method, & try again, but got interrupted by a 'Honey Do' errand. Two hours
later at power up(old settings) one of the tubes had a grid/screen
failure(short) and took out the 470 ohm bias circuit resistor(zero bias
when this happens) - lost 3 tubes & 2 screen resistors before fuse blew -
amazing that 1 tube actually survived! Replaced all screen resistors - bias
circuit resistors, cap, & diode. Put the 7581As back in it, biased it to
40ma, & all is as Leo intended!
Sound: (Subjective) Stock, the BF Showman has a very clean preamp compared
to BF amps with Reverb. With single coils, distortion doesn't start until 8
& with 7581As you don't get much at 10! So with 6V6GTs we're talkin all
power tube distortion!
At 115V/20ma the clean sound was bright but still acceptable I would
describe it as sounding like a stock BF Pro Reverb. Distortion started at 5
& was crisper, crunchier, & more saturated than a Pro Reverb. Note
definition was still good when playing chords - nice Keith Richards
'Stonesy' sound.
At 117V/17ma the clean sound was too glassy & strident for me. Distortion
was interesting though, saturated & bright - not quite a plexi Marshall -
but the Strat neck pickup tones were definitely getting interesting. At 8
there was a lot of compression as well. I normally like to play on the edge
at lower settings and I found that the amp broke into distortion very
quickly & was hard to control. Through the JBL in the DR, the attenuated
bottom end made the amp seem even more strident.
Conclusions: At bias settings with acceptable tone(to me anyway), I don't
see how one could avoid glowing the plates. Had I gotten to 120VAC with
even less idle current and a 4 ohm load, I think the glow would have been
solved, but the amp would have been brighter still, and I question whether
the screens could take that much voltage. Maybe the tone is a whole
different story with some hot humbuckers, but in no way shape or form does
it sound like a Deluxe Reverb - of course the Showman lacks the extra
preamp stage & has much higher voltages throughout. With the same
transformers as a Twin Reverb, but 2 less tubes to power the Showman is
probably a worst case test amp. A couple of days ago I expected the 6V6s to
produce a darker fatter distortion - the bright crunchy sound was a
surprise - perhaps it was because of this amp's clean preamp.
As for reliability, I'll let you be the judge! I did break open &
dis-assemble one of the 6V6s that fried a screen. About a 1/4" of screen
winding near the center of the assembly had vaporized! All that was left
were little globs of melted metal on the plate & on the grid winding. These
particular tubes were all from the early 90s & were packed in the dark
blue/yellow boxes with all Russian lettering & '6V6GT' written on the ends
by hand.
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