Output transformer DC resistance..

[jpdesroc]

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

[SY]

Wouldn't it be coincidental that both sides are shorted the same amount?

If I were worried about it, I'd apply an ac voltage across the primary, measure it at the secondary, and see if the ratio looks OK.

[jpdesroc]

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..

[SY]

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.

[Ty_Bower]

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.

[Bandersnatch]

hey-Hey!!!,
Check the ratio with a known signal. It should be between 20 and 30 to one step down. With a variac apply a bit of line voltage and measure both primary input and secondary output.
cheers,
Douglas

[Jeb-D.]

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.