ASKAMPMAN
Trem Troubles
Jeff,
Thanks for your contribution to Premier
Guitar. I look forward to your column each
month. It is always the first thing I turn to
when I open my mail box. I really enjoy
working on tube amps and your column
has given me great insight on a number of
unique issues. Every month I add another
tool to my troubleshooting toolbox!
Here is a problem that has vexed me for
a while. I have a 1970 Fender Twin with a
finicky tremolo circuit. The tremolo works
at all depths, but I run into issues with the
speed control. The speed control seems
to have a bit of a hair trigger. If I start
tremolo in a Twin reissue in your March
2008 column. This seemed a bit different,
but maybe not. Anyhow, thanks again!
Hi Brandon,
Thanks so much for reading the column. I’m
glad it has helped you in the past, so let me
see if I can help you again.
I’m going to bet that your 1970 Twin is not
all original, and that sometime within the
last 38 years it has been serviced. Okay, I’m
not clairvoyant; it’s just that the odds are
in my favor. My guess is that for whatever
reason, at some point in time, the Speed
control has been replaced. At 3 megohms,
1 to 2 setting. It is generally a logarithmic
function, which is why an audio taper pot
is also referred to as a “log” or logarithmic
taper pot.
The next type is linear taper. This simply
means that the resistance change from say
2 and 3 on the knob is the same as it will be
between 3 and 4, 5 and 6, etc. This would
have been a better choice for the speed
control than a standard audio taper, but still
not the correct choice.
The pot that needs to be installed in the
amp for proper operation is a reverse audio
taper. This is usually a custom taper and is
used when the resistance change needs to
Inside a silverface Twin
with the speed at 1 there is no oscillation.
That condition stays the same as I increase
the speed until I get between 4 and 5, at
which point the oscillation jumps in at the
rate you would expect at that level. As
soon as I hit that threshold the oscillation
jumps in. I can tweak the knob between
4 and 5 to get speeds I would expect
below 5, but it is difficult to fine-tune and
the oscillations eventually stall out. I have
checked all the solder joints and replaced
the opto-isolator, but the problem has
persisted. I was considering replacing the
speed potentiometer next, but thought
it wise to ask Budda first. Why does my
tremolo disappear at speed settings
below 5?
Thanks!
Brandon
P.S. I reviewed your past columns for
some advice on this. The closest thing I
found was your comments about weak
the correct value is not very standard and
may have been replaced with a more readily available 1 megohm. While a 1 megohm
pot will change the range of speed that the
oscillator circuit can produce, the problem
that you’re experiencing comes from the
taper of the replacement pot. My guess is
that the replacement pot is a 1M-A, which
should be a standard 1 megohm audio
taper pot used for the volume control in
most amps. The correct pot would be a 3M-
RA; a 3 megohm reverse audio pot. Let me
try to explain the different tapers found in
some typical pots.
The resistance value of an audio taper pot
changes very slowly in the beginning as the
wiper is moved from the counter clockwise
(zero) position. As the knob is turned, the
rate of resistance increases as the wiper
moves closer to the clockwise (ten) position.
This means that while the resistance of the
pot may only change 100 ohms between 0
and 1, it may change 1000 ohms from the
happen quickly in the early stages of rotation. Where the resistance of a standard
audio taper pot changes slowly between
0 and 1, the resistance of a reverse audio
taper pot changes very quickly in that area;
the rate of change becoming less and less
as the control is rotated clockwise. The
tremolo oscillator circuit needs a good bit
of change in the early stages of the control
setting, while I’m betting the pot that has
been installed in the amp is giving it just
the opposite.
Change the pot to the proper value and
taper and be sure to re-install the 100k
resistor that is attached to the rear of the
pot. You’ll be amplitude modulating by the
numbers in no time.
Jeff Bober
Co-Founder and Senior Design Engineer
Budda Amplification
jeffb@budda.com or www.budda.com
©2007 Jeff Bober
