From kee–(at)–ustin.ibm.com Tue Jan 9 13:17:01 CST 1996
Article: 5052 of rec.audio.tubes
Path: geraldo.cc.utexas.edu!ausnews.austin.ibm.com!keen
From: kee–(at)–ustin.ibm.com ()
Newsgroups: alt.guitar.amps,rec.audio.tubes
Subject: Re: Bias-Modulating Tremolo
Date: 9 Jan 1996 15:34:13 GMT
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In article <4ctsd3$ib--(at)--nrp1.news.primenet.com>, Mike Rejsa
> : I have an old Magnatone which has the most amazing tremolo you’ll ever
> : hear, it modulates the tone instead of the volume. It almost sounds like
> : the pitch changes as well (but that would be impossible with a tube
> : circuit wouldn’t it?) This is actually a characteristic of all Magnatone
> : amps I’ve ever heard that had tremolo. Do you happen to know how they did
> : it?
>
> The Magnatones do indeed modulate pitch. Its hard.
> There is one Fender circuit that does this, but I can’t remember offhand
> which one – Weber refers to it in his book.
>
They modulate pitch in the same sense that phase shifters modulate pitch,
by varying the phase delay of a signal through a filter section. This produces
the same small time delay that phase shifters do. The Magnatones did this
with a voltage variable resistor called a “varistor”. A varistor’s small
signal resistance varies with the voltage across it.
The Fender that did something similar was the Concert. That amp split the
signal into a high and low band, then did out-of-phase tremolo by the
bias modulation method one each band separately, and mixed them back together.
This approach was intended to give a better cancellation of the LFO signal
since it was out of phase in each band and any leakage cancelled when the
two bands were added together. The unintended (?) effect was that the
phase shift in the band split filters effectively varied when signals in the
crossover between the filters were added back together in the mixer. That
effect made for true phase/time shift in the crossover band.
I’ve listened to a Concert down in Workhorse Guitars, and the tremolo is
GREAT.
R.G.
