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Thread: power transformer problems

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    power transformer problems

    Building a 5E3 clone on an old Hammond 6v6 push-pull chassis. Chassis is unidentified, just using the steel box and the power transformer. No ID markings on the transformer except for a hand written date, 7-17-51. Power transformer gets very hot very quick when power tubes are in and standby switch is in the operating position. The tranny puts out 340vac, and I'm using a 5y3 rectifier tube. With 6v6 tubes in, I read 426B+, 405 plate vdc and 396 on the screens(pin4). I read 27 vdc at the cathodes with a 400 ohm cathode resistor. The amp uses a 25uf bypass cap I have a 100 ohm pot in cathode section so I can adjust between 400-500ohms. That gives me about 3v adjustment. The heater windings read 5.6 and 7.8, with the 7.8 sagging to about 7.3 with all tubes in. The transformer gets very hot and started to smoke a little after about 5 minutes of operation. The amp has a quite a bit of hum but sounds excellent and very loud with a guitar plugged in. I have two pairs of old made in USA blackplate 6v6's, one pair is RCA and one pair is labeled Hammond, made in USA. The tubes have tested ok and are in fair to good condition. Same problem with each pair of tubes, though the plate and screen voltages are a little lower with the rca's. The primary of the power tfx has 3 leads, which I shall call blk1, blk2 and brn. Blk1-blk2 reads 4.7ohm dcr, Blk2-brn reads 5.1ohm dcr. Blk1-brn reads 0.9ohm dcr. I have blk1 and blk2 hooked up to the AC mains, and brn not hooked to anything. At first I thought that the problem may be a filament short, but the tranny had no problems just running tube heaters, as soon as the standby switch is engaged in the operate position(standby switch opens and closes 2ndry ctr tap to gnd) the tranny heats up very quick. Not quite sure which primary leads are meant to be hooked up to the ac120 mains. yep, i'm a noob.

    Please Help!

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    Old Timer soundguruman's Avatar
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    Quote Originally Posted by Mr Johnny Birchwood View Post
    Building a 5E3 clone on an old Hammond 6v6 push-pull chassis. Chassis is unidentified, just using the steel box and the power transformer. No ID markings on the transformer except for a hand written date, 7-17-51. Power transformer gets very hot very quick when power tubes are in and standby switch is in the operating position. The tranny puts out 340vac, and I'm using a 5y3 rectifier tube. With 6v6 tubes in, I read 426B+, 405 plate vdc and 396 on the screens(pin4). I read 27 vdc at the cathodes with a 400 ohm cathode resistor. The amp uses a 25uf bypass cap I have a 100 ohm pot in cathode section so I can adjust between 400-500ohms. That gives me about 3v adjustment. The heater windings read 5.6 and 7.8, with the 7.8 sagging to about 7.3 with all tubes in. The transformer gets very hot and started to smoke a little after about 5 minutes of operation. The amp has a quite a bit of hum but sounds excellent and very loud with a guitar plugged in. I have two pairs of old made in USA blackplate 6v6's, one pair is RCA and one pair is labeled Hammond, made in USA. The tubes have tested ok and are in fair to good condition. Same problem with each pair of tubes, though the plate and screen voltages are a little lower with the rca's. The primary of the power tfx has 3 leads, which I shall call blk1, blk2 and brn. Blk1-blk2 reads 4.7ohm dcr, Blk2-brn reads 5.1ohm dcr. Blk1-brn reads 0.9ohm dcr. I have blk1 and blk2 hooked up to the AC mains, and brn not hooked to anything. At first I thought that the problem may be a filament short, but the tranny had no problems just running tube heaters, as soon as the standby switch is engaged in the operate position(standby switch opens and closes 2ndry ctr tap to gnd) the tranny heats up very quick. Not quite sure which primary leads are meant to be hooked up to the ac120 mains. yep, i'm a noob.

    Please Help!
    Yeah, you can have a working transformer that overheats.
    It can be an internal breakdown of insulation, or the windings were originally defective, etc...the design of the winding causes excessive eddy currents. The Super Champ transformer has a similar defect, it works but gets hot as Hades.
    You are apparently randomly connecting the primary wires without a spec sheet or instructions... maybe that's why.

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    Quote Originally Posted by soundguruman View Post
    You are apparently randomly connecting the primary wires without a spec sheet or instructions... maybe that's why.
    With no ID markings on the transformer except the hand written date 7-17-51, and limited information on the provenance of the chassis, it is impossible to find any spec sheets or instruction. If I HAD a spec sheet or instructions, if they were AVAILABLE, then I would USE THEM. LACKING THOSE, one must employ TRIAL AND ERROR.

    I think the 4.7ohm pair of leads are the 110v taps, Got it hooked to the 5.1ohm pair and it still gets hot, but not smoking, fry-an-egg-on-the-transformer hot.

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    Old Timer olddawg's Avatar
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    Seems like to me if you hook up what you think to be the primary and can measure a filament voltage on what you think are any of the secondaries, then you have the problem pretty much resolved. You can then identify the other secondary windings with an ohm meter, then label the supplies by measuring the AC with the mains on. You might want to use a lightbulb or a variac on the mains to limit the current. Once you have the leads identified unloaded you can work from there. Be careful with the HV. If the PT runs cool unloaded but gets hot during operation something in the amp may be drawing excessive current or it may be normal for a 60+ year old transformer. I have a 1960 Ampeg Rocket that runs hot as hell. It doesn't seem to mind.

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    Quote Originally Posted by olddawg View Post
    Seems like to me if you hook up what you think to be the primary and can measure a filament voltage on what you think are any of the secondaries, then you have the problem pretty much resolved. You can then identify the other secondary windings with an ohm meter, then label the supplies by measuring the AC with the mains on. You might want to use a lightbulb or a variac on the mains to limit the current. Once you have the leads identified unloaded you can work from there. Be careful with the HV. If the PT runs cool unloaded but gets hot during operation something in the amp may be drawing excessive current or it may be normal for a 60+ year old transformer. I have a 1960 Ampeg Rocket that runs hot as hell. It doesn't seem to mind.
    THanx, been tinkering with the filter section and the cathode section of the power tubes. I can operate the amp at full power for about 20 minutes, but the transformer starts to smoke after that. I don't know exactly how much current the transformer was intended to handle, but the chassis used a 5u4 rect, 6v6gts push-pull, as well as a number of pentode pre-amp tubes, so I assume that the 5y3, 6v6's and a 12ax7 and 12ay7 won't draw more current than the hammond amp was. But you are right, I just gotta go through it and calculate the current draw through each tube.

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    "If you can get the smoke back in the amp, it will work."

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