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Old 23rd May 2013, 09:10 AM   #11
scrimpu is offline scrimpu  United States
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Location: Grand Rapids, Mi
I have about 310 on pin 6 and I do have the 30v, it was just hard to get a reading for some reason. I can hear the tremolo thumping very lightly in the background but not in the signal, not noticeably anyway. I have shorted the 2 lines to bypass R711 and the thump is slightly more noticeable but just barely.
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Old 23rd May 2013, 01:02 PM   #12
JMFahey is offline JMFahey  Argentina
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When I read voltages with tubes I get about 265 B+, with only the recto I am getting 420v B+, is that normal? It seems like quite a big difference.
No, your unbiased tubes are passing excessive current and pulling +B down , but now you solved that.

I don't see a C107, do you mean C702?
Yes, sorry, I mistyped.

installed the 140R and all voltages are right on.

As of the tremolo, I think Enzo is spot on.
Being an organ, probably most of the circuit lives in a chassis in the bottom of the unit, but wires run to some control panel by the keyboard.
That's the part we're missing.
Don't you have the rest of the schematic?

but the OP's problem is that the amp can't be used 'stand alone' as the bias voltage is dependent on whatever is connected to pin 2, and how much current it takes.

Easiest option would be to redesign the bias circuit sensibly, adding a small mains transformer, rectifier etc. and a preset to adjust the bias.
Agree, and my first reaction was to redesign a conventional bias source, using that 25+25VAC winding already present.
But then I got intrigued by the low value high dissipation R701 , which probably *is* needed for some other purpose (tremolo or feedback related?) so I preferred to "play along", simply connecting to that supply what it expected to see; another 140r 5W to ground.
It worked while keeping everything else stock.

It *is* a weird circuit after all
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Old 23rd May 2013, 06:17 PM   #13
scrimpu is offline scrimpu  United States
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No I do not have the rest of the schematic. I suspect the wires ran to a switch or a pot, but I have yet to even find a picture the organ where the controls are visible. Its does seem that the tremolo is working, I can hear it in the background but it is very faint. These old organs usually had a very intense tremolo effect.Could it just be a bad cap?
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Old 24th May 2013, 06:37 AM   #14
scrimpu is offline scrimpu  United States
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I managed to ahold of a more complete schematic and I can see the 2 mystery wires from pins 1 and 4 go to a vibrato on/off switch and a full vibrato switch. The lead from pin 1 also runs to a transistor vibrato unit in the tone generator, so I give up on the original circuit. What is a good tremolo circuit I can use with only 1/2 of a 12A_7?
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