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Fender
6V6 In 6L6 Amp Test
In <5crjl7$8j9$--(at)--antuc.canterbury.ac.nz>
misc168--(at)--antua.canterbury.ac.nz (Hamish Hubbard) writes:
>
>I'm feeling really motivated at the moment, so I'm going to attempt to
>take this 'discussion' and turn it into a piece of useful knowledge.
>
>On one side we have Lord Valve, Tremolux, etc who theorize that
running
>6V6 tubes at 400 to 450V will toast them.
>Evidence:
>The design-maximum plate voltage for a 6V6GT is about 350V (from
memory).
>No empirical evidence that I am aware of (...if you're think they're
going
>to blow, why try it to be sure, I guess.)
>
>
>On the other side we have K.O. who believes this is no problem.
>Evidence:
>Empirical evidence dating back to the '70s that 'western' 6V6 and
eastern-block
>"6V6" operate OK for long periods of time at up to 450V B+ in amps
designed
>for 6L6 tubes.
>No dis-satisfied customers with this mod (?)
>
>It should be noted that Mr. O'Connor and Mr. Valve share the exact
opposite
>positions on the subject of overloading the heater windings of old
Fender
>transformers, which is rather curious IMHO.
>
>Would either side care to make any additions? I think it's too soon
for
>anyone to take either side as fact, but right now the evidence is in
>favour of Mr. O'Connor.
Lord Valve Speaketh:
I can speak (from experience) to the question of installing 6L6s in 6V6
type FENDER amplifiers...it works just fine. I've been doing it for
years, and I can't recall anyone coming back with a fried power tranny.
The Fender power transformer is very beefy. I have a customer who had
his Super Reverb (BF) outfitted with 6550s (NOT by me, that one's not
my style) back in 1973 (he says...I've only been servicing his gear
since around 1984), and although the trans runs hotter than normal,
he's been gigging with this amp for pushing 24 years on the same
transformer. As to the other question...Sovtek 6V6s at plate voltages
of 470 volts, and not one failure in 20 years (Mr O'Connor's words, I
believe)...I don't see that happening. There have been posts upon
posts to this NG by techs, amateurs, and engineers alike bemoaning the
low quality of the Sovtek 6V6, and its propensity to self-destruct at
the drop of a riff. And we're talking Champs, Deluxes, and Deluxe
Reverbs here, NOT Twins. SO...just for fun, I dug up the last 6 Sovtek
6V6s I had in my shop...the ones left over from the dozen I had back
when three customers in a row became VERY irate after their Deluxe
Reverbs lasted less than a set and a half, after I retubed them with
Sovtek 6V6s...and I shoved a pair into a BF Super Reverb. The bias was
first set to max negative; the amp was turned on and allowed to warm up
on standby for a generous 5 minutes. The incoming AC line was adjusted
to 120 VAC exactly. Upon taking the amp off standby, the left-hand
power tube put on a rather spectacular fireworks display, taking the
screen resistor with it. The amp was immediately powered down, the
dead tube removed, and the smoked resistor replaced. Another 6V6 was
installed, and the same warmup proceedure was followed. This time,
once the tubes warmed up, the amp was operated into a 2-ohm dummy load
and the output monitored on the scope. Due to the max negative setting
of the bias control, the wave was extremely notchy; I decreased the
bias voltage as far as it would go (without modifying the bias circuit)
but there wasn't enough range to remove the crossover notch from the
output. The measured voltages at this point in the experiment were
-45.5V bias, and +471.2 volts plate, with the screens slightly lower
than the plates by a volt or so. I turned out the lights in my shop,
and had a peek at the plates. It was necessary to do this in darkness,
as the glass on a Sovtek 6V6 is coated with a black substance on the
inside, making it very difficult to observe the plates. Large orange
spots were visible on the plates of both tubes. As I reached for the
standby switch to shut the amp down, the right-hand tube went south in
a major way, again taking the screen resistor with it. I removed both
tubes from the amp (which, by the way, were hot enough to make me get
rid of 'em really fast, and I was wearing WELDING GLOVES, as I always
do when pulling hot tubes) and installed two of the remaining three
untried tubes. Again, the bias pot did not have sufficient range to
remove the crossover notch from the output, and again the plates lit
up. I had a customer of mine plug his Strat into the head and play it
into the speakers in the cabinet. This time, the tubes fared a little
better; it took 15 minutes of playing (softly at first, and then
balls-to-the-wall at the end) to kill a tube. I will admit, when
overdriven, it sounded pretty good. The clean tone was WAY glassy,
though. Incidentally, RMS wattage measured across the test load worked
out to 26.3 watts. A Chinese 5AR4 was used for the rectifier. The amp
was completely stock in all respects. I have a feeling that if the
bias voltage could have been taken low enough to produce a
decent-looking output waveform, the tubes would have shorted out WAY
faster than they did, or the glass would have melted. (Don't
laugh...I've seen this, but usually only on 6550's in amps that were
over-fused.) My conclusion: not in a Super Reverb, they won't.
Somebody try it with a Twin Reverb and post the results. Trem?
Lord Valve
detritu--(at)--x.netcom.com
(Fat Willie)
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