Power Trans Problem?

[drewl]

About a year ago I repaired an old Flot-a-tone amp, cool old combo with two cathode biased 6L6's, tremolo and 6V6 driven reverb.

It came back with power trans shorted primaries.
I remember the trans getting hot.

Found the exact match, 6K7VG, installed it and everything is fine.
After a few hours testing the new PT is warm, but not too bad.
The specs are 750v CT at 150ma, I put a meter in series with the CT to ground and measured 190ma, is that the way it's supposed to be measured for that rating?

[nickb]

+1 for mentioning Flot-aTone. A new one on me.

The current measurement must be made with a true RMS meter to have any meaningful value. If the RMS current in the center leg is 190mA then the current in each winding is 190 x 0.71 = 135mA. I guess this is at idle. It'll be more when running at full volume so it's a bit tight for my comfort.

[drewl]

It is a true RMS Fluke, why multiply by .71 for each leg?

You going from peak to RMS, multiplying by .707?

Thanks.

[nickb]

It is a true RMS Fluke, why multiply by .71 for each leg?

You going from peak to RMS, multiplying by .707?

Thanks.

Nothing to do with peak vs RMS. RMS values add as the root of the sum of the squares i.e. Center^2 = Outer1^2 + Outer2^2 in this case.

[mozz]

Just measure the voltage across the cathode output tube resistor, other tubes are only drawing a few mil's.

[nickb]

Just measure the voltage across the cathode output tube resistor, other tubes are only drawing a few mil's.

That gives you the DC current. However it's the RMS current in the transformer that matters. Typically it will be about twice the DC current, or in each leg of a full wave rectifier (as we have here) about 1.4 x the DC current.

[drewl]

I'll have to read up on transformer theory, never had to use it before.

Tried different output tubes and rectifier, filter caps were redone.

It pulls about 30v across a 50ohm cathode resistor which is connected to the filaments of the first two preamp tubes like Silvertone used.

[mozz]

I'm lost. If you have 2 tubes that draw 100ma total, how do you order the proper transformer? Say they sell one 350v/150ma, so that is undersized? Or are the manufactures rating the transformer for ac volts yet dc current?

[nickb]

I'll have to read up on transformer theory, never had to use it before.

Tried different output tubes and rectifier, filter caps were redone.

It pulls about 30v across a 50ohm cathode resistor which is connected to the filaments of the first two preamp tubes like Silvertone used.

That's 600mA. Doesn't sound right.

I think the easiest thing to do is put your meter in one outer leg of the HV secondary an measure the Irms at idle and again at full power. It's then up to you to choose a PT spec that lies somewhere between those two extremes of "too small" and "very conservative". When you're playing the average current drawn from the PT will be a closer to the idle figure as average output power is quite low, even when set to full volume. Some meters have a an averaging function. If you do then use it: Play through it at max vol for a minute or so and take the reading. That would be the ideal rating.

PS: I estimate the ideal figure will come out to about 2/3 * idle current + 1/3 * max current.

[drewl]

I doubt any of you will ever see one of these in the real world, but here's the schem that was stapled inside the cabinet.

[eschertron]

It pulls about 30v across a 50ohm cathode resistor which is connected to the filaments of the first two preamp tubes like Silvertone used.

What's the drop across the resistor alone? Less than the total 30v, I'll wager.

[52 Bill]

Thanks for posting the schematic.

That's quite an amp, dc preamp filaments, 6V6 drive to the reverb tank.

[nickb]

I sorta de-skewed it a bit...



PS: A tube rectifier has higher internal resistance and uses a smaller filter cap than I was thinking of. This will improve the ratio of DC current to center leg Irms a LOT. Probably closer to 1.6x than 2x (or about 1.1x for the outer legs).

I'd guess the 6L6 are 75mA each since they supply the 12AX7 heaters.

[drewl]

Yeah, I don't know why it was upside down....

[nickb]

It is a true RMS Fluke,

Which model number is it?