Output transformer miracle?

[Ericxke]

Good morning to you all!

I started working on a Crate 60W amp that I picked up some months ago.
After some minor things it is running, but with too much distortion at medium power.

When checking the bias situation, one of the two output tube draws approx. 30mA, the other one 60mA, regardless of the position of the output tubes, the same with another pair of tubes. They are 6L6GC's.
The amp has an automatic bias circuit, the bias voltage is the same (around -52V) on the grid of both tubes. Plate voltage was 493V.

The coating of the output transformer showed some bubbles and I thought that it might have been overheated for some reason.
Measuring the DC resistance on the primary side of the ouput transformer shows 73 Ohms for the "brown side" and 95 Ohms on the "blue" side.
So probable the transformer was damaged and would have quite some windings on the primary side shorted out.

However:

When checking the transformer by putting AC (10V RMS @ 50Hz, 5V RMS @ 1kHz and 5kHz) into the secondary side of it, it shows a perfect winding ratio and equal voltages on both the blue and the brown side.

Can anybody give me a hint on how this is possible?

Thanks in advance,
Eric

[alexradium]

DC resistance is often different between the 2 windings,but you should check inductance and for potential short windings.
Try reversing the taps to see if it's the OT Or something in the bias circuit or a socket.

[pdf64]

...The coating of the output transformer showed some bubbles and I thought that it might have been overheated for some reason.
Measuring the DC resistance on the primary side of the ouput transformer shows 73 Ohms for the "brown side" and 95 Ohms on the "blue" side...

That seems normal; there's no reason to suspect the OT is faulty at this stage.

Consider that if both plates are at 490Vdc, the difference in winding resistances is having a negligible effect, at idle anyway.

The length of wire required for each turn (and hence its resistance) gradually increases as the windings build up, layer upon layer. So for a simply wound transformer, the inner half section of any winding will have a lower resistance than the outer half. What matters is the number of turns, and your voltage test has shown that to be good (well done )

I suggest that you remove the tubes, put your meter's black probe into a power tube socket terminal 8 and check the Vdc at 3, 4 and 5; repeat for the other power tube socket.

[mozz]

You have a bad tube.

[g1]

You have a bad tube.

Sounds like he has 2 bad tubes, one in each set (replacement set had same issue).
As he said "the position of the tubes doesn't matter", then it can't be the OT.
If the fault follows the tube, then the tube is the problem. One in each set.

Or I'm mis-understanding what is meant by "the position of the tubes doesn't matter".

[Helmholtz]

The amp has an automatic bias circuit, the bias voltage is the same (around -52V) on the grid of both tubes.

What do you mean with "automatic bias circuit"? What you describe sounds like fixed bias. With automatic or cathode bias the grids are typically at 0 DCV.

When checking the transformer by putting AC (10V RMS @ 50Hz, 5V RMS @ 1kHz and 5kHz) into the secondary side of it, it shows a perfect winding ratio and equal voltages on both the blue and the brown side.

This indicates that the OT is most probably O.K.

Measuring the DC resistance on the primary side of the ouput transformer shows 73 Ohms for the "brown side" and 95 Ohms on the "blue" side.

OTs are often interleaved, meaning the secondary is sandwiched between the 2 primary halves. With the same number of turns, the outer half of the primary requires more wire length, resulting in higher DCR.


Please try to post a schematic.

[Jazz P Bass]

So what is the Crate Model#?

[Ericxke]

Great! Thanks for your answers, until now.
I never thought about the outer windings requiring more length.

"the position of the tubes doesn't matter" meant that if I swap the tubes, the problem (the measured idle plate current) stays with the socket.

@Helmholtz / Jazz P Bass: I will attach a link to a schematic that shows the bias circuit (fixed voltage controlled by a solid state circuit) as well as the model of the amp.

The schematic: https://www.dropbox.com/sh/4ok6um6wg...1WrQq2Ica?dl=0

Looking forward to more education!

[g1]

Ok, I had it backwards. The fault is staying with the socket, not with the tube.
What is your DC voltage at pin4 for both power tubes?
Do R90 and R91 both measure 12 ohms?

[Ericxke]

R90 = 12,8
R91 = 12,9

Voltages, no OT inserted:
pin 3 4 5
L 505 498 -29,6
R 508 498 -29,6

[g1]

The OT needs to be connected for proper readings. Not sure how you can have voltages at pin 3 without OT connected.
How about DC mV across R90 and R91.
Also clean the connections at F2 and F3, and swap them around.

[Ericxke]

My mistake: By "OT" I meant output tubes.....
So the transformer is connected and the two 6L6's are out, like pdf64 suggested.

[SoulFetish]

What do you mean with "automatic bias circuit"? What you describe sounds like fixed bias. With automatic or cathode bias the grids are typically at 0 DCV.

I think I know what he's referring to. In the Crate Blue Voodoo 60, they use a fixed bias configuration that allows the user to switch between 6L6s and EL34s without the need for a manual bias adjustment. So, to clear up the confusion, by "automatic bias" he means fixed bias with "no adjustment needed" by the user or technician (so to speak).
Here is the schematics for the BV60:
Crate_Blue_Voodoo_60.pdf

edit: sorry, I missed the reply where he linked to the schematic.