Varistor in LFO driver

[Willabe]

What does the varistor do in this circuit?

Full schemo link;   

http://el34world.com/charts/Schematics/files/_Miscellaneous/martin-112t-amplifier-schematic.pdf

[kagliostro]

Interesting

too I'm curious

Ciao

Franco

EDIT: Here you can read something about (on page 2 there is the 112t)

http://www.thegearpage.net/board/index.php?threads/so-whats-this-varistor-doing-here.1023715/

[trobbins]

I'd say it adds some modulation 'character', compared to just straight resistance (another variant doesn't use the varistor).


The LFO cathode follower is powered from higher 300V, so if no varistor then another 10mA could be pulled through input stage cathode circuit.  Input stage would likely only have about 1mA in cathode for idle.  There is no info on that Globar varistor, so its a guess, but it would have a substantial affect on the cathode follower current - whether that noticeably changes the tremolo would be interesting.

[jjasilli]

+1.  Willabe's question was also asked on this thread:  http://www.thegearpage.net/board/index.php?threads/so-whats-this-varistor-doing-here.1023715/page-2


The idea was floated that the varistor contributes to a small Tremolo effect (LFO vol modulation) to work with the Vibrato (pitch or phase shift modulation).  This concept sounds intriguing.

[trobbins]

There is no vibrato going on, it is only tremelo.

[PRR]

> What does the varistor do in this circuit?

With other parts; it wobbles the preamp tube cathode.

My reading is that the varistor is, bizarrely, an "amplifier". When CF output swings low, varistor runs cool, varistor resistance rises; when CF output swings high, varistor runs hot, varistor resistance drops. A modest swing at CF output is a larger relative swing at preamp cathode. Mostly at "large depth", when Depth pot is set large the varistor resistance is swamped out. Varistor variation is slow, so it may do less for fast trem.

It is an interesting question. But: can you get that part today? Can you be happy with a wobbled-cathode trem?

Mock it up without the Globar. I think you will find that wobbled-preamp trems are a "gimmick". It puts two more knobs on the product. It promises you can play like (insert star-name here). It has to compromise overload and bass depth. It may not be able to give the deep lush wobble we expect from Fender output-stage shakes, or even the poor imitation from a roach.