Tube Power Dissipation - 6V6
Peter Alerich
I have a question. I'm trying to wrap my head around why these two
sets of 6V6 GT tubes behave so differently in the same amp circuit.
The tubes in question are a set of old Sylvania pulls I have from an
old mono hi-fi amp and the other set are more recent Groove Tubes that
came installed in the test amp - a Marshall Studio 15. It contains a
mixture of both fixed and cathode bias. I don't see how that bias
scheme figures in to this but it might:
http://www.alerich.com/studio15.gif
With the Sylvania tubes in place I can set the adjustable bias to the
minumum (least negative voltage setting) and the tubes only dissipate
a little under 6W each. With the GT tubes I have to increase the bias
voltage (more negative) and they still dissipate about 9W each, even
with more bias voltage than the Sylvanias. I won't just pop the GT's
in with the amp setup for the Sylvanias but if I did I would expect
the GT's to be working pretty hard in that case. Both sets of tubes
sound great - different in their own ways but still great tone. You
can tell that the GT's are in since the amp is a bit louder. Duh.
I know one set is from the old school "glory days" of tube technology
while the other set are the proverbial "polished turds" so let's not
get into that discussion!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
My question is why are two sets of 'relatively' similar types of tubes
behaving so differently in the same amp with the same settings?
Physically inside the tube (impedances, etc.) what is going on to
account for this? Are the Sylvania tubes just shot?
Peter
Lord Valve
You have two sets of tubes which are different from each other.
If you're only eyeballing the idle current, that's the main difference
you're seeing. It's not at all uncommon for a group of tubes from
the same manufacturer, built on the same line by the same people
on the same day, to exhibit idle currents (given the same voltage
and loading for each) that vary over quite a large range. This
will be seen with NOS as well as currently produced tubes.
And, as you've found, it's only necessary to adjust for the
sweet spot for either set and forget about it - assuming, of
course, that after setting the bias by ear, you take a set of
measurements to assure yourself that you haven't set the
static wattage to some ridiculously high figure - like, say,
20 watts (for a 6V6). You'd also want to eyeball 'em while
they're in use (playing the guitar) to make sure they're
not red-plating.
I don't think you have any kind of a problem - just a couple
of sets of tubes that differ from each other. And, as you
say, they both sound great. Imagine that. ;-)
Lord Valve
Expert (please obsess)