Re: Another Fender Twin question
- From: Chief_Billy@xxxxxxxxxxx
- Date: Tue, 30 Oct 2007 12:30:30 -0700
On Oct 30, 10:22 am, Yard Dog <rockin...@xxxxxxxxx> wrote:
I've read that I can run my Twin on (2) tubes, either the inner or
outer but what I can't get confirmation on is if I need to re-bias if
I do this. Also does it put any un-needed stress on the amp to run it
with (2) tubes?
TIA
You may need to re-bias because you will be reducing the current load
on your
PT and power section, and will likely see a rise in plate voltage
which may or may not
be significant enough to require re-biasing..but the bias needs to be
checked.
Removing a tube from each side will increase the impedance seen by
each side of the OT primary
(2 tubes in parallel < 1 tube by itself) . Many will insist that the
OT output impedance must be adjusted
to compensate, but read this (re: http://tone-lizard.com/Marshall_Myths.htm)
:
"Many people who own a 100-watt/non-master volume Marshall will remove
two of the output tubes to achieve a more manageable volume from the
amplifier. This isn't such a bad idea (other than the fact that you
have to keep tabs on a pair of output tubes that aren't 'aging' at the
same rate as the 'in use' pair), but a popular myth is that you must
reset the output impedance selector to one-half of what your cabinet
is rated for. This is 99% untrue. An 8-ohm load connected to an 8-ohm
output tap should achieve full rated power and full bandwidth. A
mismatch 'up' or 'down' will reduce the power output and the
bandwidth. With a 'good' transformer utilizing high-temperature wire
and 'properly' insulated between the coil stacks, no special care is
needed to ensure safe limits of current through the tubes or
transformer. You will achieve full wattage and the 'original'
bandwidth by adjusting the impedance, but the 'alternative' has
interesting possibilities. It's all up to your ears. If you aren't
100% comfortable about this procedure, play the amplifier and feel the
shell of the output transformer. Did you get a blister? You may want
to check Screen Grid voltages and bias currents. As for the output
tubes and reliability, you should have learned by now that output
transformers were wound for many different primary impedances over the
years. Using 6L6's as our example, the primary impedance recommended
has been 10k-ohms, 6600-ohms, and 3800-ohms. The tubes, transformers,
and speakers all lived long and fruitful lives. A better idea came
later when some Marshalls (and Fender's The Twin) featured what is
usually called by everyone except the manufacturer a Pentode/Triode
switch (which doesn't really switch the output tubes between pentodes
and triodes, but that's a long story). 'Triodes' will have a much
lower plate-to-plate impedance, and not even the average tube 'guru'
was brazen enough to state that the impedance selector must be
adjusted whenever the amplifier is run in the 'triode' mode. As a side
note, those Marshall amplifiers with 'High' and 'Low' power switching
(JCM900) have given service technicians very little trouble, while
JCM2000 amplifiers (without High/Low switching) are the source of many
output transformer headaches. Draw your own conclusions. "
If the Fender Twin OT is rugged enough, you may not need to worry
about OT impedance. And, considering that the Twin OT is not
adjustable,
this might be a good thing.
Never did it myself, though, so ask around.....
.
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