greetings...help with T1-RVT tremolo circuit?

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I hope some of the knowledgeable folks here can help. I'm working on Thunder1 -RVT that's been somewhat hacked sometime in the past. There's no collector value, its in another cab, so my interest is in getting good functionality and tone.

I've got the schematic PDF that's around the web, unfortunately, many of the values are illegable, and some parts don't match my amp at all. (Reminds me of my vintage Gibson project).

My problem is in the tremolo circuit, part of which was cut out. I've got everything together so it looks right, except the pot labeled strength in the schematic.

Obviously, the wiper goes to the cathodes on the output tubes. One leg (which, left or right, if the pot is sitting on edge with the shaft pointing away from me?) goes to the plate of the tremolo triode through a cap and a resistor (values), where does the other go?

The schema shows it going to a little power supply off a third set of transformer windings, but there's nothing at all like this on my amp, and I can't read the voltage values on the line to the pot, so I cant hack something up.

Anybody seen one like this that's been built properly, or at least have a clear schematic?

Thanks for any help...
Andrew
 

capnjuan

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OlAndrew said:
I've got the schematic PDF that's around the web, unfortunately, many of the values are illegable , and some parts don't match my amp at all.
Hi Andrew; it sounds like you have the same schematic everyone else does; blurry.

OlAndrew said:
My problem is in the tremolo circuit ...the wiper goes to the cathodes on the output tubes. One leg (which, left or right, if the pot is sitting on edge with the shaft pointing away from me?) goes to the plate of the tremolo triode through a cap and a resistor (values), where does the other go?
Yes, it ultimately gets to the cathodes but first it's connected to the bias resistors; possibly maybe 220K ... I have the same schematic you have ... the 'left' leg goes down to the footswitch - if you have one - and the 'right' leg goes to the plate of the of the oscillator tube; 1/2 of one of your 12AX7s.

OlAndrew said:
The schema shows it going to a little power supply off a third set of transformer windings, but there's nothing at all like this on my amp, and I can't read the voltage values on the line to the pot, so I cant hack something up.
Well my new friend, this is the biggest problem you have. The 'little power supply' you are referring to is the bias supply. If it isn't there and the amp has worked since the 'little power supply' went missing, then your amp has been converted from a 'fixed bias' to a cathode-bias amp. The missing supply provides negative voltage to ensure that, via the 220K(?) resistors and the probably 1K 6GW8 grid resistors, the grid is always negative with respect to the cathode - that is, the cathode always has a more positive voltage appearing on it. If you have a resistor between the two cathodes and ground, you have a cathode bias amp and, if it would work that way at all, you wouldn't have any tremolo.

Like nearly every amp from back in the day, the tremolo feature was an oscillator pulsing and what gets 'pulsed' is the voltage on the grid resistors; the result is the wavy gravy thingie. As you can see from your schematic, the voltage from the missing bias supply goes first to the wiper of the tremolo 'Strength' control and then to the bias/grid resistors. If that connection isn't there, 'cause there's no bias supply, then there's no tremolo - and I might add, no way to make sure the grids of the output tubes are negative. The only other way to accomplish it to put a resistor between the cathode and ground thus the guess that someone cobbled your amp into a cathode-biased amp.

OlAndrew said:
Anybody seen one like this that's been built properly, or at least have a clear schematic?
Yes, I've seen one and we have several members who own this amp; no on the clear schematic.

I wish I had better news for you; if you can take and post some pics, that would help confirm what's there and what's not. Sorry I can't be more positive about your amp. "Not every mod is a good one." Famous saying of Leo Fender. Good luck. CJ
 

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Yupper, a nice gut shot would help. Is it the same tube lineup as the schematic?
 
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It's for sure cathode-biased. I've worked on those before and the parallel high-watt resistor and cap pair are there running to ground. Didn't figure it was a hack, 'cause the workmanship and the patina/dust on the connections is the same as the rest. The resistor is one of those long skinny things that looks like a wirewound dipped in brick clay, just like the rest.

There's an obvious ground loop, though, and much other wierdness I won't try to describe.

Then there's some much later work, new can caps and resistors, some small electrolytics that seem right circuitwise. and the thing did work.

Godawful bright, though, even with the treble turned all the way down, and I was hoping for some mellow jazz sound....think the intro to "Cry Me a River", or maybe Larry Carlton.

I did put in new tubes, contemporary Tungsol 12ax7s and NOS Amperex 6gw8s.

I'll try to do a schema of what's there, some of you might be curious at some of this, and maybe pics this weekend.

Thanks for the quick and detailed replies, much appreciated.
 
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Madness!

Been really busy at home & at work, finally got a pic together to post:

madness1.jpg


There was a nice black Death Cap, running from hot to chassis, that you won't see anymore. Those twisted leads from the xfrmr that are flapping around loose? They used to be soldered to two of the lugs that attach the can cap to the ground. For a miracle, I've still got 138 VAC across them, so I guess that's where my bias power comes from, once I work out the circuit values. There's a 12v filament pair out of the xfrmr that runs up to a 12AX7, not seen on the schematic (the weird leads, the 12ax7 belongs there). Maybe I've got a non-stock power xfrmer. There's that little blue electrolytic in parallel with the main power lead to teh can cap....hey ya can't have too much capacitance, can ya? There's a ground loop or two around the power tube cathode grounding stuff ( the cathode bias conversion is real obvious ) that don't show very well. Then there's that brown lead that runs from the wiper on the filament ground balance pot to the power tube cathodes !? I guess that's so I don't have to look at the pilot light to know the thing is turned on.....seems to work just fine.

Based on what I've seen so far, I'm gonna have to trace out each and every little bit and compare it to the schematic (as best I can) and first principles, if I'm to have any hope of getting this thing running right. Oh great, grand, joyous gruntcakes! What fun.

Oh well, onward through the fog.....
 

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Re: Madness!

OlAndrew said:
Those twisted leads from the xfrmr that are flapping around loose? They used to be soldered to two of the lugs that attach the can cap to the ground. Hi Nigel, those are the center taps for the secondary and heater windings; they need to be soldered to ground. I guess that's where my bias power comes from ... No; the output tubes are biased by the resistor and cap shown below. There's a 12v filament pair out of the xfrmr that runs up to a 12AX7, not seen on the schematic (the weird leads, the 12ax7 belongs there). No there isn't; all the heater power is coming off the hum-balance pot. There's a ground loop or two around the power tube cathode grounding stuff ... Then there's that brown lead that runs from the wiper on the filament ground balance pot to the power tube cathodes !? I see it and it doesn't make any sense.
Shown below - your amp on the left and a working T1 RVT on the right with transformer secondaries identified:

Slide2-1.jpg



Your amp with bias resistor/capacitor identified and preamp and reverb amp heater supplies identified:

Slide3-1.jpg



Correctly built 'fixed-biased' T1 RVT with bias diode, cap, and supply to trem control ID'd (+ lead of bias cap to ground):

Slide1-1.jpg


Really can't tell what's going on in your amp; the cathode-to-hum balance pot connection and the lifted center taps looks like someone trying to find and kill hum. Not really sure what to tell you; if your goal is to convert this back to a fixed-bias amp just to rescue the trem function, I'm not sure I'd go through the trouble. I'd suggest grounding the center taps; if it creates more noise then lift them and insulate the ends. I'd also suggest lifting that pin 7 cathode to hum balance wiper connection; that isn't the center tap for purposes of connecting the heater center tap to the cathode bias resistor/capacitor; a common way to abate noise - the heater center tap is the white 'flapper' and if it's going to be connected the bias R/C, then it's connected ahead of them, not on the ground side. Not sure this helps, not sure what you have, not sure what so-and-so was trying to do. John
 
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Wow, you really put some work into that, thanks. I was waaay off base on the transformer wires, now I understand what OUGHT to be going on 'round there. Couple of questions....

Your shot of a working amp shows the lead from the bias supply going to the wiper of the pot, the schematic shows it going to a leg, and the wiper going to the powertube grids. Any idea why or what's the practical difference?

Bias voltage...from what I've read (haven't worked on a fixed bias amp before) bias voltages are usually in pistol range of -50 VDC. Some authors speak of a voltage as little as -2. If the xfrmr leads that should be providing the bias supply are running a 12 volt filament, and i rebuilt it to use as a bias supply, given losses and resistors etc, I'd expect to see maybe -6 to -8 on the bias line. Any idea what the bias voltage (-vdc) ought to be?
 

capnjuan

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OlAndrew said:
Your shot of a working amp shows the lead from the bias supply going to the wiper of the pot, the schematic shows it going to a leg, and the wiper going to the powertube grids. Any idea why or what's the practical difference?
You are correct, my error; supply goes to leg of pot, wiper connection to output tubes.

OlAndrew said:
Bias voltage...from what I've read (haven't worked on a fixed bias amp before) bias voltages are usually in pistol range of -50 VDC. Some authors speak of a voltage as little as -2. If the xfrmr leads that should be providing the bias supply are running a 12 volt filament, and i rebuilt it to use as a bias supply, given losses and resistors etc, I'd expect to see maybe -6 to -8 on the bias line. Any idea what the bias voltage (-vdc) ought to be?
This can get awkward; you have the amp in front of you ... but the two brown bias supply legs in your amp appear to be tied to the red high voltage; the red wire 'emerging' from under the blue capacitor and to the right in the picture, and connected to the large brown resistor. If that's the case, they aren't supplying heater current anywhere.

Depending on where they are taken, 'bias' voltages can range from -50vdc at the source of supply to -50mvdc at the grid. The public domain schematic actually prints better than it appears on your pc monitor. You might want to consider calling a local printer and ask them whether they'll print it for you oversize. You can email to them; ask them to try it at 12"X18" and 18"X24". The schematic has the raw bias supply voltage on it.

But you will have no tremolo or corectly-biased amp if you don't disconnect those two brown secondaries and wire them according to the schematic. Your alternative is to clean up what you have - while still disconnecting the bias supply, grounding the center taps, and disconnecting that cathode to hum balance mess.



6GW8 tube data sheet:
http://www.pmillett.com/tubedata/HB-3/R ... _ECL86.PDF
 
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Well, it ain't blowed up or caught fire yet...pics and schem

Got it up and running. I think I've got a real oddball here...those brown transformer lines are 12.7 VAC, and feeding a 12AX7 filament. The output of the tremolo feeds the grid of the V2 tone stack recovery stage. Everything looked stock, same workmanship and materials throughout.

I changed the tone stack somewhat, 'cause this thing had 3 basic tones, too bright, painfully bright, and peel-the-chrome-off-the-Harley bright. Changed some small electrolytics that were starting to let the magic out, and put in push-pull DPDT switch pots in place of the non-existent footswitch. Got rid of some ground loops and that "filament ground" running to the power tube cathodes, there's an order of magnitude less hum, now.

Its in a Twin Reverb style cabinet with a pair of Weber Signature 12s. Actually sounds pretty good, gives a very nice jazz tone from a 335 clone (Epiphone Riviera Deluxe) with GFS Fat Pat humbuckers.

trempic.jpg


th1_rvtschema.jpg


That big pot at the bottom of circuit pic is a sort of master strength for the tremolo, it has to be turned fairly
far down. I still haven't got the strength control that appears on the front working well, it doesn't have much effect
in the current incarnation. More guess and try, I suppose. Think I'll try a 1 meg with the wiper to V2 grid, trem output to one leg and ground to the other. It's really a nice smooth sounding trem, though.
 

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Hi Nigel; glad it's coming around ... still scratching my head over the brown transformer leads; but, if it ain't broke ....

John
 
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Hey capn---gotta love that gibsonette in the pic...one of my fave sounds in a little amp with the unusual circuit that runs two 6V6's in single ended parallel instead of a push pull arrangement.
 

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count_scrofula said:
Hey capn---gotta love that gibsonette in the pic...one of my fave sounds in a little amp with the unusual circuit that runs two 6V6's in single ended parallel instead of a push pull arrangement.
Hi Count; sorry but that's an understandable near-miss on the amp ID; it's a Gibson GA18 Explorer; twin 12AX7s plus push/pull 6V6s as opposed to the Gibsonette's single 12AX7 and parallel outputs ... one tube short but who's counting. I did thread the fix-'em-up of a GA8; Click Here for more than anyone in their right mind would want to know about a GA8. I recently added a tweed Gibson GA30RV 'Invader' (where did they get those names ... :oops: ). Two channel, reverb, 8" and 12" Jensens; Gibson's rough equivalent of a Fender 5E5-A Pro or 5E4-A Super which I hope to find time to thread. Welcome back! CJ
 
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