lack of volume and distortion

[JC@]

I'm trying to tune up an old Fender clone, 2 x 6L6, something very closely like to a Pro, the older model. I get lack of volume, and distortion even at low volume level. I've changed all the cathode bypass caps and the bias caps.
Everything looks right. B+, tubes, resistor values, no leakage on the coupling caps. The bias is set correctly. The vibrato isn't stuck on.

But it distorts even at 4. It sounds like a cranked all the way up amp but with relatively low volume, and even at ten far away the expected 50 watts.

Sine waves look fine at the grid pins of the power tubes. I have tried different power tubes with no change.

There is a certain imbalance between both 'halves' of the output transformer, but not too bad.

Could it be a defective output transformer?

[Sir Cuitous]

It could be the OT or you could try a different speaker unless you know the speaker is a good one. I got a similar response from a fender clone using an old "vintage" speaker. Once I put a new speaker on, the volume came up and the distortion went away.

[JC@]

It could be the OT or you could try a different speaker unless you know the speaker is a good one. I got a similar response from a fender clone using an old "vintage" speaker. Once I put a new speaker on, the volume came up and the distortion went away.

The speaker is good, I have tested the amp with my bench testing speaker.
I think I should have tested the amp with another transformer before asking. I will try that first.

[pdf64]

There's a very quick test for shorted turns, basically totally disconnect the transformer from the circuit, connect a neon light / series resistor across one winding and briefly connect a battery across another winding. If there are any shorted turns in any winding, the neon won't light. My fender clone has suffered an OT with a short in the primary - quite confusing trying to diagnose it. See
http://www.geofex.com/
Peter

[JC@]

There's a very quick test for shorted turns, basically totally disconnect the transformer from the circuit, connect a neon light / series resistor across one winding and briefly connect a battery across another winding. If there are any shorted turns in any winding, the neon won't light. My fender clone has suffered an OT with a short in the primary - quite confusing trying to diagnose it. See
http://www.geofex.com/
Peter

Thank you Peter. I have one very similar homemade device to test transformers. Anyhow as I have a spare transformer I will test the amp using the replacement one.

[MWJB]

"Everything looks right. B+, tubes, resistor values, no leakage on the coupling caps. The bias is set correctly. The vibrato isn't stuck on."

What are your voltages at the PI tube (plate, grid & cathode)?

Bias set correctly...what mA do you have?

[JC@]

What are your voltages at the PI tube (plate, grid & cathode)?

plates 235 and 216 dc
grid 64 and 70 volts dc
cathode (shared) 100 volts dc

Bias set correctly...what mA do you have?

32mA at 460 volts dc

[MWJB]

Well, nothing immediately jumps out there. In the case of a shorted OT primary I'd expect a significant mismatch in plate current, though I have seen OTs where everything measures OK, only goes out of kilter with a signal applied.

If plate & screen voltages are OK, then 1st stop is the OT, let us know how the spare checks out.

[JC@]

Well, nothing immediately jumps out there. In the case of a shorted OT primary I'd expect a significant mismatch in plate current, though I have seen OTs where everything measures OK, only goes out of kilter with a signal applied.

If plate & screen voltages are OK, then 1st stop is the OT, let us know how the spare checks out.

This must be the case, as I remember watching the bias current swinging abnormally when I strummed a few chords. As soon as I plug the spare I will post the results.
Thanks for your help.

[JC@]

...then 1st stop is the OT, let us know how the spare checks out.

Here we are.
The current on each power tube swinged outrageously when big signal was applied to them (chord strumming hard).

With the replacement output transformer the amp... just sings LOUD and clean.
Done it!

Thank you all.

[pdf64]

'The current on each power tube swinged outrageously when big signal was applied to them (chord strumming hard).'
This is normal, you are just measuring the current through the tube, it will go up in response to the input signal (that's what output tubes operating in class A/B do). You will get very similar measurements with your new OT. The difference is that the old OT had shorted turns, so there was nothing to couple that current modulation to the speaker. Hope that helps - Peter

[JC@]

This is normal, you are just measuring the current through the tube, it will go up in response to the input signal (that's what output tubes operating in class A/B do). You will get very similar measurements with your new OT. The difference is that the old OT had shorted turns, so there was nothing to couple that current modulation to the speaker. Hope that helps - Peter

Well if I recall right the measurements I was taking, and for some reason, when I say outrangeously means something I didn't see before. I think it will go up to 190mA per tube or so before I quickly unswitched the amp... That is way too much, isn't it?

[MWJB]

Probably best not to measure whilst playing...you'll just scare yourself! I've seen current readings like that whilst playing...so I just stopped watching the readings whilst playing and it doesn't bother me any more! :-)

When biasing an amp we're concerned with idle current.