What could this farty little distortion be caused by

[mikeydee77]

Would love to have some help on this one.

This clip at the link below is from a Sheldon TT3 amp; 12AX7 pre amp and PI and then a 1W output stage made up of three 12AU7. It's a lovely looking amp in red and sounds amazing although not at the moment.

http://youtu.be/WRyPTpmOx60

The story is that speaker connection was damaged and so the amp did have some time operated without load. It is now working apart from the annoying distortion, could that be form the output transformer? Wondered what others think.


Checks so far include:
replaced valves
checked cathode bypass caps for short
coupling caps for leakage - no dc present
Output transformer measured resistance: 126 0 166 ohm on the primary and few tenths of a ohm on the secondary.

[soundguruman]

you are replacing parts without measuring, which generally gets you nowhere...
well yeah, you run it without a load and the transformer can be damaged.
126 0 166 seems like the primary ot is way out of balance. But where is the schematic?

[Jazz P Bass]

The story is that speaker connection was damaged and so the amp did have some time operated without load.
Output transformer measured resistance: 126 0 166 ohm on the primary and few tenths of a ohm on the secondary.

How long of a time was the amp run without a speaker?
The 126 0 166 imbalance is not uncommon.
Is there any change when the "Freeze" control is engaged?
The best way to rule out the output transformer is to sub in another one.

[mikeydee77]

How long of a time was the amp run without a speaker?
The 126 0 166 imbalance is not uncommon.
Is there any change when the "Freeze" control is engaged?
The best way to rule out the output transformer is to sub in another one.

Hi Jazz P bass, thanks for looking....

I don't know how long it was run without speaker but I am assuming that it was several minutes as the owner had replaced the pre-amp tubes thinking that was the cause.

The Freeze is at mid way on the video clip. The control doesn't effect the extent of fuzz at all however when it is at max the amount of hum through the speaker is increased to noticeable levels.

I know substitution is best way to prove a point but I am dodging it. Although I have to say the chap that builds these amps is very helpful. I will call him tomorrow to see how much the OT are.

The failed output transformers I have seen have usually been open circuit or otherwise obviously the problem - is distortion common place problem too?

[mikeydee77]

you are replacing parts without measuring, which generally gets you nowhere...
well yeah, you run it without a load and the transformer can be damaged.
126 0 166 seems like the primary ot is way out of balance. But where is the schematic?

Hi Soundguruman,
Thanks for responding - I think you may have misunderstood.

I like to isolate the problem by substitution so when I said I replaced the valves I just meant that I had substituted them to rule them out of the equation. The other checks around caps are also measured, here are my notes unfortunately the formatting is a little jumbled.

Plate B+ V Plate Ia Cathode Resistor V Cathode I cathode
resistor
V1a 220000 R3 246 136 5.00E-04 2700 1.338 4.96E-04
V1b 220000 R6 250 106 6.55E-04 1500 0.986 6.57E-04
V2a (PI) 100000 R13 260 176 8.40E-04 1500 1.23 8.20E-04
V2b (PI) 56000 R16 284 216 1.21E-03 56000 67 1.20E-03


V1 cathode bypass capacitors removing cathode capacitor on V1 a and V1B keeps the cathode voltages the same so assuming that the caps are fine
Voltage on all pre amp grids is 0v

[Jazz P Bass]

The 'Freeze' control is supposed to take the output section from low wattage / single ended to high (relative term) wattage /push pull.
How this is accomplished I don't have a clue.
The fact that the amp hums more at full 'Freeze' may very well indicate a bad OPT. (as push/pull should cancel out hum)

[Mick Bailey]

You may have a shorted turn - use the neon trick to test the OT.

[Gtr_tech]

http://www.geofex.com/FX_images/xform_test.gif

[mikeydee77]

http://www.geofex.com/FX_images/xform_test.gif

Ok, thanks I will try out the Neon test and post results.

Cheers all...

[mikeydee77]

So I built the little transformer testing unit tonight and tried it out. The parts cost about 10 GBP but I had most to hand so only took about an hour.



I used the tester on the Sheldon amp under question and the light was quite dim on the secondary winding but gave a strong bright flash on the primary. Here is a video when testing the Sheldon transformer: Transformer Tester in action - YouTube. Actually the camera has compensated and the flashes look quite similar on there but in real life it was obvious. And then for For comparison I compared with a Peavey Classic 30 amp and both primary and secondary registered a strong flash (sorry no video but it was the same as the primary on the earlier video - i.e. a bright flash). Conclusion I feel much more confident swapping out the transformer now.

Loving that new bit homebrew test gear

Thanks M-E-F

[Gtr_tech]

Keep in mind that the secondary will not give as bright a flash on the neon as the primary. There's less wire there, so you won't get the same kickback.