Assessing a PP transformer to use as SE

[systemtruck]

Pardon this question, which surely some of you well seasoned folks have had to begrudgingly entertain for the newbies once in a while.

The reason I’m asking is because I have my hands on some pretty beefy output transformers that are difficult to find specs / info about. They were made by HP in the dawn of their development.

I’m also asking because david at Cinemag mentioned to me recently that one of their transformers may not even bat an eye at some DC current because they’re so robust. This leads a new guy like me to think, hey, if these other HP transformers I have are quite literally 2X the physical size and weight of those Cinemag’s, or more, maybe they can handle some DC current.

Is any of this sounding like it has some mathematical backing?

I know that inductance is key for low frequency signal. I assume that gapping transformers makes up for lack of strong inductance. So im guessing here that if a transformer isn’t gapped but has massive inductance, maybe they’ll pass DC without too much loss of bass. But how much?

A completely different take on this is a quote I found somewhere…

“”” P-P transformers can work in a SE fassion if you measure your idle current of the tube and feed the current through the center tap as well as an equal current that goes to the other end of the 'unused' winding and you put a load resistor that draws the same amount of idle current as the tube. “””

 

[Murdock]

There are far more experienced people here who will hopefully help you but I have to correct you on one thing.

I know that inductance is key for low frequency signal. I assume that gapping transformers makes up for lack of strong inductance. So im guessing here that if a transformer isn’t gapped but has massive inductance, maybe they’ll pass DC without too much loss of bass. But how much?

Gapping transformers is done so the DC current does not saturate the transformer. Gapping a transformer drastically decreases its inductance. Therefore SE transformers are typically larger than their PP counterparts to make up for the loss in inductance. As larger core and more windings lead to more inductance.

Some PP transformers still have a small gap build in to handle small current imbalance of the push pull tubes. So if the current of the SE circuit is small enough, some PP transformers could be used in SE configuration.

Do you know out of which HP units these transformers are? Maybe you can find schematics and get data from there.
Post some pics and maybe DC readings of the windings.

 

[emrr]

“”” P-P transformers can work in a SE fassion if you measure your idle current of the tube and feed the current through the center tap as well as an equal current that goes to the other end of the 'unused' winding and you put a load resistor that draws the same amount of idle current as the tube. “””

That can be seen in RDH3 and other ancient texts, never have I encountered it. It’s still a PP output stage with no signal feeding one side, hard to imagine what conditions would make it a favorable arrangement.

 

[systemtruck]

The HP transformer part number is 9120-0037 and it was in a handful of old oscillators.
Not much in the way of specs can be found, except size and then some cross part numbers..
https://www.parttarget.com/5950-00-...37.html/-C397F95D-AA8E-41A7-AF73-328350F3E2D1
https://mil-nsn-parts.com/parts-catalog/nsn/5950000784083-5950-00-078-4083-transformer-audio-f.html

Attached is a pic someone else took of one with the bells off, and there’s a ruler for size reference.
Below that is a pic of one of mine still installed in a unit.

My application would be for a parallel 12SN7, so single ended.

These come from a standard 6v6 PP config, to 600R line out. I’m assuming primary is in the 6-7k area. I could shunt the secondary some to lower the primary to what a 12SN7 parallel wants, though I’m not sure what that is yet. And since it’s a triode, not pentode, I guess there is some flexibility in the primary being high anyways.

But two 12SN7’s will kick out up to 20mA DC. I could do an inductor + cap parafeed setup but the inductor cost makes it a moot point. Just looking to make use of good parts I already have!

I was confusing saturation with inductance. Mixing them together. Now I understand that gapping helps reduce saturation, and that inductance suffers from gapping so has to be made up for with added size. So I guess I’m separately wondering if size itself also helps avoid DC saturation. This is the impression I get when someone experienced says thatsome given non gapped transformer won’t be affected by some DC current… that the size of it can absorb DC and gapping isn’t needed. So I’m wondering is 20mA is way too high of a number to throw at any non gapped transformer.

This applies to some other transformers I have too, so I’m looking to understand a little more about it. Thanks!

 

[Murdock]

20mA is a lot for SE… can I ask for what circuit it is?
You can try but I would say keep it for a project where it fits. 6v6 PP would fit to RCA BA-6A or other Vari-Mu compressors.
What you can do is check the lams from outside to see how their are stacked. If it looks like in that picture it has a gap. If that gap is big enough to handle 20mA you have to try.
If it has two lines it’s not gapped.

 

[systemtruck]

I think it looks non gapped. I see two lines. Unless I’m looking for the wrong lines. Pic below.

I may try some of my good PP transformers in parafeed mode with what parts I have on hand. I can toss a decent cap between plate and transformer and see how it runs. To the best of my understanding thus far of parafeed, the big inductor between B+ and plate is to filter supply ripple noise. If that’s all it does, maybe I can still get good audio through the setup using just a cap, and if it works well I can then check around for some decent inductors to filter supply noise.
I did however find an interesting article about adding supply ripple to the cathode so that it cancels out, and you don’t need a a giant expensive inductor for anything.

https://www.tubecad.com/2014/09/blog0308.htm

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