From SRSN81–(at)–rodigy.com Mon Nov 20 13:18:03 CST 1995
Article: 5593 of alt.guitar.amps
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From: SRSN81–(at)–rodigy.com (Joseph Pampel)
Newsgroups: alt.guitar.amps
Subject: Re: Vibrato retrofit
Date: 20 Nov 1995 07:03:02 GMT
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mgarvi–(at)–anix.com (Mark Garvin) wrote:
>
>In <48cunb$sh--(at)--ver.mhv.net> Dr.Distortio–(at)–bs.mhv.net (Dr Distortion)
writes:
>
>>Mark Garvin (mgarvi–(at)–anix.com) wrote:
>>: With a little ingenuity, the LDR version could be made to do this
>>: as well. If I were retrofitting an existing chassis, I’d personally
>>: use a solid-state oscillator and an LED rather than chop holes for
>>: tubes and use a click-prone neon.
The Vibrochamp just uses a single triode osc to modulate the bias on a
pre-amp stage. Safe, and neat sounding. 🙂 It’s the Vibroverb (and some
others..) that modulated the bias on the outputs, which I think sounds
great, but is a potentially dangerous retrofit I suppose.
>>I’m glad to hear I’m not the only person who feels this way. I’m all
for
>>all-tube signal paths, but I do have to laugh when I see the guitar amp
>>ads touting “all-tube tremolo.” Solid-state oscillators work just as
well..
>
>Yeah…why go thru all that just to light a little bulb?
>And neon bulbs come with a whole set of their own problems.
Just for the record I’ve had really good luck re-building the osc
circuits in BF amps with new high temp poly caps (the smallest I can get
in 600V, Mouser has some good ones) running the leads exactly as the
layout diag calls for and keeping all leads on caps etc. short. No more
clicks, and a better waveform avait ye who wouldst try this. Almost
guaranteed.
>Now that we’re on the subject of tremolos, why did Fender use a
>50k pot? Why not a 100k or even higher? Due to the reverse-log
>taper, it’s not as if 1/2 of the range would be wasted.
To optomize gain into the power amp stage? The amps get dirty in a hurry
without that 50K pot.. that’s for sure.
>>Fender used in the brown-Tolex amps (such as the Concert, Super, etc.).
>>This is the type that sends the guitar signal through a simple passive
>>crossover and trems the highs and lows separately, imposing the trem
>
>Yeah, they had some strange name for this (I forget). Pretty
>archaic-but-clever design. My brother has an old Showman with
>this type of tremolo. Nice long row of 12ax7’s in that thing.
They called it “Harmonic Trem” In addition to tremming the highs and
lows seperately, they combine them out of phase so you have a phasing
effect that responds to your pick attack. Sounds neat enough that a lot
of folks think it is FM mod, but it isn’t. VOX seems to have snagged the
idea & most of the circuit for the AC30, but then made it non adjustable
for some inexplicable reason.. Really complicated circuit just to get
some trem going, and also a somewhat tempermental one. Very sensitive to
B+ changes, individual tubes, phase of the moon. Fun to play with,
that’s for sure. I’ve been working with it for some time now, since
restoring a ’61 Concert for a friend and falling for the sound. I’ve
been finding out how much depth you can achieve before the mixer circuit
“craps out” and amazingly enough you can get a halfway convincing Leslie
sound out of one of these circuits with some tweaking. No kidding.
Only drawback to the circuit is a low mid cut that is an artifact from
the hi-low network.. although since most guitar amps build in a low-mid
cut, it’s not that big a deal. Anyway I’ve been busy down in the
dungeon with an old SF twin chassis that I’m building into my next
franken-amp.. A ’61 Concert pre-amp with the “harmonic” trem circuit and
a cathode biased 6L6 output stage for maybe 40W? That should do me for a
while.. 🙂
Joe
