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Old 26th January 2011, 11:00 PM   #1
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Default Help identifying transformer wires.

Hi - I have an old thomas organ amp that I am learning to convert to a bass amp, and I'm now getting the hard(er) part. The Power transformer. I'd like to re-use the one thats on it if I can. I found someone selling the exact amp awhile ago, and he reported it to be 730 volts plate to plate (it was 700 something, I can't find it again). The amp has a 5ysgt, 2 6v6's, and 3 12ax's. Anyway, just curious if someone could explain one of the connections?
Most is pretty standard:
1 black to ac and 1 black to fuse
2 greens to heaters and lamps
2 brownish orange (they are old cloth covered wires) - Filaments - connecting to pins 8 and 2 on the 5y3gt.
2 white wires with red stripes (B+) running to 4 and 6 on the 5y3gt.

the question part is there is a third white wire with red stripe and a white wire with a blue stripe connected to the same lug on a terminal then connected to an OA2 regulator (which I guess I should eliminate?).
Is that white/red wire the same as the other 2 B+ wires? Could it possible that the white/blue wire is around 50 volts?

Thanks for any help.
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Old 26th January 2011, 11:55 PM   #2
PassFan is offline PassFan  United States
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Quote:
Originally Posted by Skullface View Post
Hi - I have an old thomas organ amp that I am learning to convert to a bass amp, and I'm now getting the hard(er) part. The Power transformer. I'd like to re-use the one thats on it if I can. I found someone selling the exact amp awhile ago, and he reported it to be 730 volts plate to plate (it was 700 something, I can't find it again). The amp has a 5ysgt, 2 6v6's, and 3 12ax's. Anyway, just curious if someone could explain one of the connections?
Most is pretty standard:
1 black to ac and 1 black to fuse
2 greens to heaters and lamps
2 brownish orange (they are old cloth covered wires) - Filaments - connecting to pins 8 and 2 on the 5y3gt.
2 white wires with red stripes (B+) running to 4 and 6 on the 5y3gt.

the question part is there is a third white wire with red stripe and a white wire with a blue stripe connected to the same lug on a terminal then connected to an OA2 regulator (which I guess I should eliminate?).
Is that white/red wire the same as the other 2 B+ wires? Could it possible that the white/blue wire is around 50 volts?

Thanks for any help.
White red should be the center tap of the main B+ winding. The other wire ...could it possibly be green and not blue? Is it connected at a different point on the OA2 ? Some organ transformers I have seen have blue wires for bias taps but they always seem to be a seperate winding (baldwin). With the amp shut down and the tubes pulled you should be able to match those two wires with a winding. As always, check for voltages and bleed down the caps if needed before ohming anything.
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Old 27th January 2011, 12:01 AM   #3
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Iím not familiar with the amplifier in a Thomas organ and I havenít seen a circuit diagram, so this information is largely the result of guesswork. However, does this sound plausible:

Black and black: primary, AC input
Green and green: 6.3v AC to heaters and lamps
Brownish orange and brownish orange: 5v AC to filament of 5YSGT
White plus red stripe and white plus red stripe: Secondary, HT windings, to anodes of 5YSGT

Guess 1: Third white wire with red stripe is the centre-tap of the HT winding, and thus (usually) connected to chassis.


Guess 2: White wire with blue stripe could be a centre-tap of the 6.3v heater winding. If so, this would also (usually) be connected to chassis.


If there is an OA2 voltage stabiliser in the power supply somewhere, the cathode of this would also (usually) be connected to chassis. This hypothesis could agree with your description of the wiring.
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Old 27th January 2011, 11:21 PM   #4
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Location: NJ
Thanks for the replies. On further inspection it looks like Huntingdon is right. They are connected to the chassis. I put together a quick picture. The wire with the blue stripe is slightly hidden under the one with the red stripe. I have been working on a new amp based off the 5f6A schematic, but with this transformer I have nothing to send to the turret board? Guess I will probably end up buying one, but since this is my actual first attempt at making a tube amp, I was hoping to reuse as much of the old stuff as possible. Thanks again.
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Old 28th January 2011, 12:52 AM   #5
PassFan is offline PassFan  United States
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Quote:
Originally Posted by Skullface View Post
Thanks for the replies. On further inspection it looks like Huntingdon is right. They are connected to the chassis. I put together a quick picture. The wire with the blue stripe is slightly hidden under the one with the red stripe. I have been working on a new amp based off the 5f6A schematic, but with this transformer I have nothing to send to the turret board? Guess I will probably end up buying one, but since this is my actual first attempt at making a tube amp, I was hoping to reuse as much of the old stuff as possible. Thanks again.
Click the image to open in full size.
Are you implying too short a lead length or what do you mean by that statement ? You know you can extend those leads with a few pieces of lead and some heat shrink. That is unless your wallet is a bottomless pit.
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Old 28th January 2011, 08:36 PM   #6
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No not that length of the wires. I mean I need a power connection to the turret board. Any way to reduce that connection to 50 volts? I'm still in the process of reading all the info I can find on Power transformers ( I like living), so bear with me. That and I still need to order a meter thats rated for enough volts. I'm more of a solid state guy, and when a friend found an old Thomas organ on the side of the road I decided to learn tubes. What the hell was I thinking?
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Old 28th January 2011, 10:58 PM   #7
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Quote:
Originally Posted by Skullface View Post
I'm more of a solid state guy, and when a friend found an old Thomas organ on the side of the road I decided to learn tubes. What the hell was I thinking?
If you don't stretch your brain, it shrinks. Would you rather do this, or learn to speak French?
BTW, on the headers I'm putting 100 ohm in all my HT circuits to the cathode of the rectifier, because new rectifier tubes arc a lot more than the old ones did. Make sure the resistor is rated (long enough) for 700 V ie 2 or 3 watts, these days.
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Last edited by indianajo; 28th January 2011 at 11:01 PM.
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