Dynaco PA509 Power Transformer: Anyone know specs?

[Wharfcreek]

Does anyone know what the specs are on the subject Power Transformer? More specifically, I'm wondering what the HV Current load capability is? As this came out of the old Dynaco FM tuners, I'm wondering if the PT is up to a 50ma current draw on the HV side? Looking to construct a little SE EL84 guitar amp, one 12AX7 and one EL84. Shouldn't be more than about a 45ma HV draw in total, but if the PT isn't up to that I'd like to know. Thanks, WC

 

[BinaryMike]

This thread has information on transformer current rating measurement: http://audiokarma.org/forums/index....wer-transformer-operating-temperature.833267/

 

[Wharfcreek]

Hey Tom, thanks for that reference. Sadly, I’m ill equipped to perform testing at that level. So, like those posted on such web sites as Hammond and others, I’m just looking for a number. Hopefully someone has an old Dynaco FM manual around that has such a spec in it. If not I guess I’ll either use that diagnosis method, or just put it in a circuit and see if it survives.

 

[Tom Bavis]

If the HV winding is less than 400 Ohms DC (end-to-end) it's good for at least 50 mA - that would be a watt dissipated in the winding. 6V winding should have plenty of capacity.

Recall that Dyna sold a PUSH-PULL 6AQ5 amp module that fit inside this tuner, and was powered from the same transformer.

 

[BinaryMike]

...If not I guess I’ll either use that diagnosis method, or just put it in a circuit and see if it survives.

If you're using the actual application circuit as a load, then all you need is an ordinary ohmmeter to check temperature rise in the high-voltage winding. In this way, you can quickly judge whether the transformer will survive long-term.

 

[Wharfcreek]

Hey Tom, again, I really appreciate your time and info. I can hit the secondary with an ohm meter tonight when I get home from work. That said, it sounds like it ought to support one 6BQ5 and one 12AX7 on the HF side, 6BQ5 in Class A. It will have those plus a 6CA4 on the filament side, but sounds like it make it.

 

[GordonW]

OK- I just quickly went through the original Dynaco FM-1 schematic (that unit used this power transformer).

The voltage on the first section of the can cap (right after the rectifier) is specified as 285V DC. After going through the first dropping resistor (500 ohms) to the next cap, the voltage drops to a specified 255V DC.

If you do the math- you can get the current draw from the power supply. 30 volts divided by 500 ohms, equals 60 ma DC.

That means that it should obviously be able to supply 50ma DC, no problem... that's less current than it's original load.

Regards,
Gordon.

 

[Wharfcreek]

Hey Gordon~ !! Nice to hear from you. Oddly, I just thought about doing the same thing, only I did it on the FM-2. I got as far as seeing the same 285 that you did, but then decided to also check the specs on the 6V4 Rectifier. Appears that's good for about 90 ma. The little Champ circuit I posted above is sadly missing the voltage specs.....so I'm going to have to do some swagging at it. That said, I did find a similar era amp, the Princeton, which does show some voltages. According to that schematic, the voltage going to the OT is 305. Fortunately, it also shows the cathode resistor (470 ohm) as having 19 volts on it. If I'm doing my math correctly, that means that there's about a 40 ma current draw on the output tube. Pretty 'hot' for a 6V6, eh? Anyway, I'm thinking that even if I'm down to around 250 volts going 'in' to the Output Transformer, I can try to emulate the same kind of 'ratio' used in the Champ's circuit with the Screen Grid having the the same B+ going to it as is going to the OT. Assuming the DC resistance in the OT will eat up some of the voltage going to it, I also assume the Screen will actually have a higher voltage than the plate? Anyway, hopefully I can just adjust the cathode resistor and still get the same basic bias current draw. Or, 'close' anyway. Actually, I don't think it would hurt the amp if it were even a bit less. Probably good for the tube anyway! These old circuits are known for being 'crunchy'.... which I think is somewhat dependent on this output stage being run hot like it is. At least I believe I'm in good shape using the PA-509 for the PT in this project!

Many thanks to Tom, Mike, and you for the help!! Best wishes over the holidays!!

Tom D.