Re: I'll probably get flammed !
- From: Mike Schway <mschway@xxxxxxx>
- Date: Sun, 12 Feb 2006 08:50:04 -0800
In article <1139751149.239632.34700@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
"Newbie Doobie" <savy@xxxxxxxxxxx> wrote:
Ya know I've been screwing around with the insides of my little old
Fender Princeton Reverb 2 combo trying to coaxe a bit of the classic
fender sound to come out for about three weeks now and have explored
just about every mod suggested in this forum (and countless others) to
finally tear it all out.... go back to the beginning... stick in a pair
of 6L6s in place of the grundgy 6V6s to get brighter and cleaner
headroom that doesn't break up so harshly early on the volume.
This amp (now equipped with the 6L6 pwr tubes) could be capable of
handling 35 watts but could only manage to be adjusted to 10 watt range
and maintain sufficient clean headroom. Also the power supply's
highest B+ node topps out at 360 volts (schematic says 400v).
Is this due to the higher demands placed on the tranny from the extra
current
required by the 6L6s ?
Also pecular :
when biased for fixed AB with -33 volts to get around 30mv reading at
cathode
to ground 1 ohm R the plate voltage reads around 335v (somewhat less
that the
schematic value of 380v.The resulting output is only 10 watts per tube.
With bias
adjusted for 15 watt output (60% for 6L6) the plate voltage sits at
275v and
all the while the grid voltage value follows suit matching the plate
volt per
volt.The amp was designed for 6V6 finals so I assume that is the reason
for
never attaining the varying voltage values as charted for 6L6s ie: in
the 15
watt range charted values typically show a difference of over 100 volts
between the plate and grid values for AB fixed bias. The only values
from the 6L6
spec charts that list equal plate and grid voltages are for class A
self bias at 250v and 250.
Should I alter the screen grid supply by stepping down the B+ ? so that
the
plate and grid values reflect more like typically stated for 6L6 rather
than always
have roughly equal plate and grid values ?
I want clean headroom with good tone and find I have to bias quite cold
to get it.
In fact this amp seems to sound really good when only putting out 2 or
4 watts !
That reflects a 400v plate (and grid) with only 1.5mv bias reading at
the cathode
1 ohm R. I would leave it there for sure If I thought the output it
could cut through the
loudness of drums and bass.
So again...the question of adjusting the B+ lower at the grid.
Will this help me set things for better clean headroom at higher
wattages ?
Incedently...I've already bumped up the B+ to the Phase inverter so it
should not be topping
out before the finals.
1.5mV???? Have you seen the output on a scope? Tons of crossover
distortion! Low output too.
Power output depends on more than just what tube you use. You've
already found that out. Now, if you had the correct output transformer,
and could somehow have 440V on the plates while idling at at somewhere
aroun 15W per tube, you'd be getting the 35-40W you want, but this won't
happen with your amp for several reasons.
First off, the princeton reverb has a pretty wimpy power transformer
which can just barely provide the required filament currrent even with
the stock setup. Move to 6L6s and you're going to start heating up the
transformer leading to further core losses (i.e. lower B+). Even if you
weren't overtaxing it, you can only get c.410V with the stock tubes at
20-25mA per tube. Up the static dissipation to 35mV per tube for 6L6
operation and you'll see further sag. You may get better results with a
solid state rectifier, but still not enough HT voltage for 6L6s.
Secondly, the output transformer is "optimized" (if I can even use that
word for a PR output tranny!) for 6V6 operation. This means a primary
impedance of around 7-8K ohms. 6L6s want to see about half that. You
can try running into a 4 ohm load to get a better impedance match, but
it's still going to be a 15W tranny. However, if after performing some
miracle (i.e. power tranny swap), you DO get 30W from your amp, you'll
toast the output tranny in no time.
Now, if it were my amp, and I want to get the optimum clean sound from
it, I'd stay with 6V6s, but perhaps try the JJ/Tesla 6V6S. Brighter and
cleaner than most. You've already done the p/i voltage mod, and there's
also the "Paul C" (grid biased p/i) mod to get a smoother sound when the
amp is driven hard. Finally, a speaker swap to a more efficient one
will certainly help. The Weber 10F150T is a great match for this amp,
but it'll set you back around $90 or so.
--Mike
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Mike Schway | [Picture your favorite quote here]
mschway@xxxxxxx |
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