So what's this varistor doing here?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Leonc, Jan 28, 2012.

  1. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    Indeed. I think Gary Croteau (Juke Amps) pointed that out to me a couple years ago. I believe he's separated them out in the AM / FM circuit he's developed for his amps.
     
  2. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    Okay, may as well get into this too. Here's the schematic for the Dearmond-Rowe Martin 112T amp. There's another varistor used in the tremolo circuit here. This circuit looks a bit closer to some used in some Fenders?

    [​IMG]

    Is the varistor here involved in changing the bias on the 2nd triode in that 12AX7?
     
  3. diagrammatiks

    diagrammatiks Member

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    well ya but I was merely putting out that the fender circuits don't actually do vibrato much at all.
     
  4. diagrammatiks

    diagrammatiks Member

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    ya that's modulating the bias of the gain stage.

    From what merlin says the full vox 30/6 vibrato channel also does true vibrato.
     
  5. trobbins

    trobbins Member

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    Ha ha, Gary C - what a character - a man of many personna's!

    Schobur even used the Bonham varistor vibrato circuit in their 'cheap range' spinet organ of the early 60's - in a circuit that appears to be more clinically 'vibrato only'.

    Dorf's vibrato technique in the Wurl 44 and Vox AC-30 and many later incarnations also appears to be much more a clinical vibrato.

    I got someway along the track of measuring the parasitics of the old 'Maggie' varistors, and part way along cloning the dc characteristic with zeners. I think Bonhams implementation in the Maggie is a bit more than just the resistive modulation.
     
  6. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    OT questions:

    - what is meant by the term "left handed taper" on the Depth pot in that Martin 112T schemeatic? Does that mean audio taper? or reverse audio taper or what?

    - Wonder why they specified the the two vol pots as .35M (weird value too, WTF?) and the Depth pot as 350K?

    - Am I reading that right on the Tone pot... .65M?? Other than the trem circuit, the amp looks quite a bit like a 5E3, eh? Wonder why they specified such odd valued pots?
     
  7. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    One more question about the Martin 112T schematic (in posts 22 and 24, above) and varistor usage, for anyone who'd care to take a guess. It appears to be modulating the bias of the 2nd triode (6,7,8) in the gain stage. It's identified as a Globar 331-BNR-7, but of course I can find nothing indicating the value of a varistor labeled that way.

    Is there a way to "back into" or deduce the value of varistor that might work in this amplitude modulation role?
     
  8. Warbler Muse

    Warbler Muse Silver Supporting Member

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    The Globars made by Carborundum in the 50's and 60"s numbers aren't listed on the data sheets which I got back in the 90's. I know it's a disc style but their is no cross reference info concerning the part numbers used by the manufacturer and the numbers used by the electronic companies that purchased them including Workman. If I had one I could put it in the calibration circuit and determine it's characteristics and then it's a matter of finding and calibrating available nos disc in hopes of finding a match. 20 years ago it wouldn't have been difficult but today it would be a long shot.
     
  9. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    Thanks Gary...actually, I think it's the same axial style varistor that we find in our favorites :).

    It's quite visible and recognizable in this photo of the Dearmond R15T.

    [​IMG]

    Of course that doesn't help much in terms of identifying its specification...
     
  10. Warbler Muse

    Warbler Muse Silver Supporting Member

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    As I said my data sheets are from the 90's and all the 300 series are discs but for all I know they didn't produce discs in the 50's or 60's.
     
  11. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    I have a few of the type that work in Magnatone circuits lying around here, so short of a more scientific understanding of what's required, I'm probably just going to experiment with them first and see if the circuit works with them. What I have here are a couple FS1205s (.05ma @ 80 volts) and the ones I got from you, which may be similarly rated, yes? I'll probably buy a few other types of different values and see if I can figure out what might work.

    Can you think of any measurements I can take in this circuit that may help me narrow down values that may work? E.g., the voltage at V1 pin 8 or something like that?
     
  12. Warbler Muse

    Warbler Muse Silver Supporting Member

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    If it was me I'd try the tremolo with the varistor shorted, because I think it real purpose is to do an end run around Fender's patent #2817708 which has an oscillator and driver very similar which modulates the cathode bias of a pre amp section.
     
  13. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    haha!

    I guess Fender didn't have a patent on the rest of the 5E3 then, huh?

    Thanks Gary, I'll certainly try that. Which models did 2817708 apply to? I was thinking I'd have to have plan B in place if I couldn't get this working. I recalled that this looked Fender-ish but couldn't recall the model...

    (BTW, I just spent an hour going through schematicsHeaven.com looking for a cathode-biased Fender where the pre-amp tube's bias got modulated. I did run into the Silvertone 1482...but that uses a 7-pin 6AU6 pentode...)
     
  14. Warbler Muse

    Warbler Muse Silver Supporting Member

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    Similar modulators are found in all Fender amps but only connected to a pre amp tube in the Tremolux 5E9 & Vibro Champ. Most of the Valco amps and some Silvertones and Wards amps use a similar modulator. though the pre amp stage it attaches to may vary. The tremolo in my amps are based on the same type of circuit.
     
  15. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    Thanks Gary. Yeah, I knew I'd seen Fender use it on a pre-amp tube before...but couldn't remember which. Hmmm...they didn't have the 5E9 at SchematicHeaven.com; no wonder I didn't find it, LOL. I just found it elsewhere. Gotta love the internet...

    Anyway, looks like they include a switch (on the speed pot)...appears that when disengaged, you're basically taking the oscillator tube out of the circuit? That would be nice as I understand it robs a little gain in the Martin 12T (which you can get back by pulling the osc. tube).

    It appears that they're modulating the bias on the PI, huh? As opposed to modulating the initial gain stage as in the Martin 112T. But anyways, I'll keep this in my "back pocket" in case I can't get the stock circuit working.

    Thanks a bunch for the tips! I very much appreciate it, as always.
     
  16. trobbins

    trobbins Member

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    Taking the LFO tube out would lower the modulated stage DC cathode bias more than just stopping the LFO from oscillating (using the jack). I think the 5E9 speed pot switch just stops the LFO from oscillating.

    The 5E9 PI is an olde voltage-divider type.
     
  17. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    Thanks Tim, I always appreciate hearing your take on things. Earlier in the thread, in reference to DoLuca's use of the varistor in the trem circuit you said:

    I can see the varistor is in a different place in the trem circuit in the Martin 112T, but do you think it might be fulfilling a similar role of minimizing of LFO going through the main amp?
     
  18. trobbins

    trobbins Member

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    No. As I see it, the bias wiggle technique transfers the LFO signal (whether at grid or cathode) in to the anode signal and then downstream (sometimes attenuated along the way, as per Gibson GA-40 - but often ending up with some LFO beating of the speaker).

    Gary may have some comment on the design of the original schematic extract you posted.
     
  19. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    UPDATE

    Got the Martin 112T built. I used one of the same valued varistors that is used in Don Bonham vibrato...tremolo is working and is quite nice. I did manage to wire it bass-ackwards in that I've got trem from the non-trem channel and vice versa, LOL, but that'll be easy to suss out. Clips within a few day. (Won't have cabinet done for another week or so).
     
  20. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    hahahah!!! Gary - you're a genius, man, LOL. Trem sounds 5 times better with the varistor shorted.
     

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