Choke Voltage Question

[dorrisant]

I have a Univox U-1086 PA here... 4x6L6 at 600+ volts... Schematic says 15H 75mA for the choke. Anyone have a suggestion for a replacement? Looks like all of the Hammond chokes that will physically fit without modification and have roughly the right H & mA ratings are 400v max. I could move the rectifier board and try to fit in others that are slightly lager physically with modification, but they are still only rated at 500v. I cannot find voltage ratings from ClassicTone. For ease of install, 2 7/8" mounting centers would be best but I'm not going to be that picky. Not much room to mount a choke topside either... Any suggestions?

[martin manning]

The 500V rating is maximum DC voltage across the choke, I believe.

[dorrisant]

That makes sense, I was thinking that it was a rating for voltage to ground.

I did not even see until just now that the plate is on a doubler at 600+ but the screen, pi and preamp are all on a different branch that starts at half of the plate feed... So, even if it was for volts to ground I would still be good. Choke is only dropping 327-322=5v... or is it (312-305=7v)? I wonder what the second set of numbers relates to.

I'm going with the Hammond 158L... Looks perfect.


Thanks Martin!

[tubeswell]

,,,I wonder what the second set of numbers relates to?

The mA in parentheses may be the peak current seen at that point, and the other one may be the idle current.

[Malcolm Irving]

The 500V rating is maximum DC voltage across the choke, I believe.

I don't think that can be right. If the choke has (say) 100 ohms DC resistance, that would be
500x500/100 = 2500 watts of heat.

I think it's more likely the voltage rating from the coil to the frame.

[martin manning]

True enough Malcolm, and I think you are correct. 500V seems low, but there is also a note saying they are tested to 1500V. Dorrisant, you are correct, and in the schematic you can see the voltage at the input side of the choke is listed at 327, so you are ok. The second set of voltages and currents look like values at rated output power.

[roberto]

The wattage is implied in the current and resistance, while the voltage rating is related to B+ (so the maximum voltage at start up). I would say that you can use a 500 V rated choke safely by slowing down the charge-up of the first cap (like a bypassable resistor between the rectifier and the cap), so keeping lower the deltaV across the choke.

[JMFahey]

the plate is on a doubler at 600+

Not exactly, it is a conventional center tapped, bridge rectified supply, basically same as the popular "split supply" used on SS power amps, only one end is grounded instead of center point so instead of +/-300VDC you get +300VDC and +600VDC.
Only +300VDC has the luxury of a choke, and any conventional one will do there.

[dorrisant]

the plate is on a doubler at 600+

Not exactly, it is a conventional center tapped, bridge rectified supply, basically same as the popular "split supply" used on SS power amps, only one end is grounded instead of center point so instead of +/-300VDC you get +300VDC and +600VDC.
Only +300VDC has the luxury of a choke, and any conventional one will do there.

Not to argue, but what is the difference that makes this not a doubler circuit? I just want to understand.

[martin manning]

Really it could be thought of as two doublers...

[Ten Over]

To me, the fact that it does not double the voltage of the winding means that it is not a voltage doubler.

[roberto]

It was discussed something like 15 years ago in a forum (don't remenber which one).
I don't consider it as a doubler, but some considered it a doubler even at that time.

[dorrisant]

I thought it was a doubler when you connect the CT above ground, stacked on top of a cap.

Does not matter... call it a unicorn, it still works the way it does.

[roberto]

A nice advantage of the unicorn, on top of the scenographic effect of the shining rainbow thermoionically emitted by his tail,
is that you can easily do a 1/4 of the wattage switch à la Fender (pages 20 and 21):
http://medias.audiofanzine.com/files/tw ... 479723.pdf

[trobbins]

Dorrisant, that choke has had some serious thermal issue. Do you know the history of that, and if other parts had issues?

It's plausible that the screens failed, or were being seriously pounded. A guitar amp would typically protect the screens more than a PA. Output tube imbalance would also need frequent checking with a bias adaptor, or adding some current sensing. It's worthwhile trying to make the circuitry more bulletproof, as just a primary side fuse is likely not going to save the PT (or your new choke).