Output Transformer causing "red plating."

[kc2eeb]

Has anyone encountered a case where unequal turns in the primary winding of the output transformer made the bias applied to the output tubes unequal and, when driven, caused too much current to be drawn
in one tube? This of course assumes that all other components, screen grid resistors, grid resistors, tubes, etc. are operating correctly.

[Alexo]

Nope. Sounds kinda impossible, but easy to test - swap the primary leads and see if the redplating swaps sockets.

[Cliff Schecht]

Yeah the imbalance is small and almost always negligible. The amount of imbalance depends on how the transformer is wound (what method of interleaving and such).

[selloutrr]

when I've experienced red plating on one tube in a push pull pair. It's always been, a bad solder point, bad tube, or to wide of a spread between tubes to be "matched" and an incorrect bias.

What you are saying is possible in manufacturing but if it's that great of a voltage difference on the legs of the transformer I would assume it wouldn't make it into production, and would be considered a faulty tranformer.

[TheGimp]

Are you determining the turns ratio by measuring the resistance of the windings, or by applying a signal to the secondary and measuring the resulting signal on the Primary?

Most transformers will have a resistive imbalance due to the winding topology (outer layers have more resistance for the same number of turns as inner layers) unless they are very expensive Audiophile grade OPTs.

If it is the resistance that is imbalanced, this is normal. No biggie.

If the turns ratio is not equal, you may have a shorted winding in the primary.

[kc2eeb]

I want to thank you all for your responses. I'm well aware that with the turns ratio being correct, one side of the primary will measure resistively different to the other side relative to the center tap due to simple physical
dimensions i.e. the "inner coil" will have lower resistance than the "outer"
one. I have another O.T. coming. The amp in question is a Randall RM50,
MTS or modular system.
I have swapped/replaced the common items, screen grid resistors, grid
resistors, etc. and checked current draw without the P.I. tube.
The tubes check very close on a Longhin Lo460 tester which allows one to
run the plate voltage up to 460 and vary bias, and the plate current is within 1 ma.
I will test the transformer with an ac signal and I'll post my results.
Thanks, once again.

[Richie]

if the PI cap is leaky for that side of tube or tubes, it can throw off the bias and depends on how bad the cap is.
It could cook the tubes or will effect the bias for that side. Have you measured voltages with tube in and out? Were they the same or still different. Also a bad OT can cook tubes,depends on how or what way it is bad.
Also measureing the resistence of the plates leads of the OT.-> A good quality transformer like a hi-fi will be almost dead on. Others may be off some. if its way off,most likely the transformer is bad,or will cause low output,and distortion sound.
It may still sound ok at low volumes,and light picking. But cranked up and hit chords hard and it will not have much output,and very distortion. Pretty much sound like crap.

[kc2eeb]

I am waiting for a O.T. to arrive, at which point I will apply all the suggestions. And I'll post what I find. Thanks to all.

[Roe]

PIs can be unbalanced, leading to redplating on one side only. this is quite common on marshall 100watters with high voltages

[greekie]

PIs can be unbalanced, leading to redplating on one side only. this is quite common on marshall 100watters with high voltages

+1

[Roe]

PIs can be unbalanced, leading to redplating on one side only. this is quite common on marshall 100watters with high voltages

+1

balance can be improved by increasing negative feedback, upping the tal resistor, and decreasing the 82k and increasing the 100k plate reistors