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Output transformer DC resistance

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Hi list, This is a dumb question but I need some technical info on 2 tube output transformers..

I need to know the 'normal' DC resistance I should read with an ohmmeter at the primary of an output transformer (each side of B+ tap) driven by 4 X 6L6GC and also for an other transformer that is driven by 2 X EL34.

For now the 6L6GC transformer read around 14ohms DC at primary on each sides of that center tap (red B+ wire).

For the EL34 transformer I read 24ohmsDC and 28ohmsDC at primary on each sides of that center tap (red B+ wire).

I think both transformer have shorted primary inside. But I can be wrong. Any help. Thanks:dead:
 
Further tests showed that the EL34 transformer is shorted.

For the 6L6GC transformer: One technician i just spoke to told me I should not measure less than around 50ohms DC res. on each side of the B+ tap on both type of transformers. And both sides should read equal values which is not my case here (24ohmsDC and 28ohmsDC). The four 6L6's that drive one of the suspect transformer have a good signal on there input grids and there screen grids have correct high voltages. But when I measure the 6L6's plates on a scope there is only the B+ ripple voltage with few signal add to it.

Looks like the transfo is short.. :dead:
 
I wouldn't be so quick. DC imbalance of that magnitude is perfectly normal (I'll let you ponder why that should be). Try my suggestion before tossing. If you can't pass a known-good ac signal, then you know you have a bad tranny.

FWIW, I checked the DCR of a 100W transformer in my iron pantry and it had pretty similar numbers to yours. And an on-hand datasheet for the Sowter U059 (made for ppp EL34) shows 21 ohms on either side.
 
I once measured the DC resistance between the primary leads on a Dynaco Z565. It's tapped for UL operation, and is intended to be used with a push/pull pair of 6BQ5 output tubes.

Z-565
===========
blue stripe 208
green stripe 53
green 44
blue 172

All measurements are in ohms, with respect to the center tap (B+). Plates go on blue leads. Screens go on green leads. This transformer is in perfect working condition.
 
I've had some SE transformers as low as 12ohm DCR. It really just depends on the wire length and gauge the manufacture uses. As long as no compramises were made in order to achieve Low DCR, Low DCR is a good thing as there is less I^2 x R losses.

As everyone else suggested, test it with a signal. The Variac idea is a good one as long as your carefull.
 
Imbalance in transformer windings

There is a good reason why nominally identical windings in a transformer show a difference in resistance. Most of you will know the reason but here goes anyway!

Take the primary winding of an output transformer as an example. The winding starts with one of the anode connections, the required number of turns are wound onto the bobbin then the wire is brought out for the centre tap which goes to the HT supply in the amp.The wire is then returned into the bobbin where the same number of turns are wound again, Finally it is brought out of the bobbin to for the other anode connection. Start - Tap - Finish. As each layer of wire is wound onto the bobbin the diameter increses, meaning the wire wound over it has to travel further to make a complete turn. Since the second half of the winding, from the centre tap to the finish is wound over the top of the first half there is more linear length of wire in it, which explains why it's resistance is higher.

The actual resistance in a given transformer winding depends on the gauge of wire used. The gauge is calculated according to the expected current in the winding and the allowable temperature rise of the winding. The thinner the wire the higher it's resistance for the same linear length. Low VA transformers use thinner wire so one would expect big differences in resistance between windings in a 30W output transformer and windings in a 150VA transformer both designed to operate at the same voltage.
 
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