Why Opto Trem
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Questions and Answers
Why Opto Trem
Why Opto TremFrom SRSN81--(at)--rodigy.com Tue Jan 2 14:02:05 CST 1996
From: SRSN81--(at)--rodigy.com (Joseph Pampel)
Newsgroups: rec.audio.tubes
Subject: Re: 5881query/new Trem thread
Date: 2 Jan 1996 04:59:59 GMT
Distribution: world
stephen.delf--(at)--elcom.gen.nz (Stephen Delft) wrote:
re: Why change from power-stage bias modulating trem to opto-isolators..
>
>If you wish, I will start off the thread here. Fitting an opto and
>a neon (and saving 1 or 1 1/2 12AX...tubes and a lot of R's and C's)
>reduces assembly and component cost. Now....what _else_ were you
>considering?
I could take a few WAGs at that one..
#1 failing optocouplers will never take out your power tubes.. ;-)
#2 optocouplers stop LF/DC pulses from getting through to the
outputs/speaker. To see what I mean, check out a Princeton Rvb (or
similar) sometime when the trem is on. The speaker moves very slowly in
and out on the trem osc wave and if you crank the thing up, I think
you'll find it a lot easier to blow the speakers..
Joe
From stephen.delf--(at)--elcom.gen.nz Tue Jan 2 14:02:36 CST 1996
From: stephen.delf--(at)--elcom.gen.nz (Stephen Delft)
Newsgroups: rec.audio.tubes
Subject: Re: 5881query/new Trem th
Date: Tue, 2 Jan 1996 09:54:00 GMT
Distribution: world
(Joseph Pampel replying to Stephen Delft's thread: Fender trem circuit
changes.)
JP>#1 failing optocouplers will never take out your power tubes.. ;-)
JP>#2 optocouplers stop LF/DC pulses from getting through to the
JP>outputs/speaker. To see what I mean, check out a Princeton Rvb (or
Quite correct on both counts Joe. But this thread flows from
comments by Dr Distortion, and I _thought _ he was referring to the
_other_ old Fender trem circuit, using a gain-modulated pair of triodes
at an early stage in the amp. ( eg Fender Super 6G4 or 6G4A)
Why Fender gave up on _that_ one is still a puzzle...it sounds nice, and
doesn't take the output stage with it if it goes. I suggested that
perhaps someone decided it cost too much.
BTW, I have here a NZ made Jansen guitar amp, which bias modulates the
output pair of EL84's in the way you were considering, ...but this one
applies the trem waveform via a 0.5uF filmcap. So if the trem oscilator
dies, it can't do anything bad to the output stage. I will check the
speaker for LF wobbles when I get it on the bench for repair.
Regards, Stephen Delft.
From SRSN81--(at)--rodigy.com Wed Jan 3 10:40:10 CST 1996
From: SRSN81--(at)--rodigy.com (Joseph Pampel)
Newsgroups: rec.audio.tubes
Subject: Re: 5881query/new Trem th
Date: 3 Jan 1996 05:10:26 GMT
Distribution: world
stephen.delf--(at)--elcom.gen.nz (Stephen Delft) wrote:
> But this thread flows from
>comments by Dr Distortion, and I _thought _ he was referring to the
>_other_ old Fender trem circuit, using a gain-modulated pair of triodes
>at an early stage in the amp. ( eg Fender Super 6G4 or 6G4A)
I thought you were referring to the type in the Vibroverb, Tweed Vibrolux
etc. And of course since I wasn't looking at the schematic (trying to be
a hero and all.. :-) ) I neglected to notice that .1uF cap that isolates
the trem osc from the bias supply in the Vibroverb/Princeton... Oops. So
much for my credibility. ;-)
I still can't help feeling like it's pretty abusive to swing those power
tubes around like that though.
That 6Gnn type trem circuit that you meant is one of the nastiest to
build in my experience. If you check out the layout diags of those amps
(from the Vibrasonic on down) the trem takes up better than 1/2 the
eyelet board. With the advent of surf music (and reverb) I think Fender
dropped the 6Gnn trem for a bunch of reasons, not the least of which was
cost and complexity( as you mentioned), but also so that they could stuff
a reverb circuit into their professional combos. Otherwise, it's hard to
explain from a mfring standpoint why a mfr would change
chassis/faceplates/knobs/etc and retrain for a whole new series of amps
after only 2 years of production. The brown chassis did not have much in
common with the BF ones.. even the pots have shorter shafts, so changing
over to the BF amps was a fairly big expense. They must have really
wanted/needed to do it.
Just so it's clear I'm not talking completely out of my butt , I
have built/restored a bunch of amps of that era using that circuit and
they sound great but take at least twice as long to build as anything
else out there (short of a univac..)
Sounds a lot like a univibe at some settings too. (the trem does..)
Regards,
Joe P.
From SRSN81--(at)--rodigy.com Thu Jan 4 16:31:24 CST 1996
From: SRSN81--(at)--rodigy.com (Joseph Pampel)
Newsgroups: rec.audio.tubes
Subject: Re: 5881query/new Trem th
Date: 4 Jan 1996 12:59:42 GMT
Distribution: world
stephen.delf--(at)--elcom.gen.nz (Stephen Delft) wrote:
>Apart from the cost and time to assemble this kind of trem cct. have
you
>had any problems with getting them to work properly ...different
>samples of tube etc.
The only thing I've had a problem with is if I use an osc tube with too
high a gain: I get loud clicks in the audio. This happens occaisionally
with some NOS tubes and most 12AX7WXT's. Going to 12AX7WB's solves the
problem. Other than that, the circuit has been no trouble at all. It's a
really neat design, with a lot of hidden details. Not a book design, it
is clear (upon experimenting with it) that it was designed by that time
honored method of tweaking away..
Everything there is there for reason. Gotta admire that. :-)
>Any significant differences between the two versions of this trem
>(simple versus more complex oscillator/buffer sections) ?
Dunno actually. I've only built the more complicated one (the -A circuit.
) But it appears from messing with the circuit a bit that the buffer is
there to help maintain the trem wave integrity. They go to great lengths
(what, 4 .1uF caps and a .03uF to ground?) to get a nice sine-wave out
of the osc, but as you change osc speeds you run into various problems,
mainly the weakening of the trem signal though. Keeping the osc output
buffered and using that huge (10-Meg RA!) intensity pot keeps the trem
signal strength pretty constant over the speed range. Another advantage
of the buffered circuit is that it also incorporates the cathodyne phase
inverter to obtain the 2 out of phase trem signals. This is almost
certainly a better balanced circuit than the paraphase PI used in the
earlier examples, especially in the SLF region of interest in this case
(3-12Hz or so). Surprisingly, the trem signals do not cancel in the
mixer tube though. You get a really funny looking artifact at the mixer
output.. part of the charm of this circuit.. :-) (and why they use a
really small coupling cap to the output stage.. IMHO)
Nice to hear from you!
Joe P.
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