Interstage Transformer - Useful?

FlaCharlie

FlaCharlie

Super Member
So I've been scrounging around various bits and pieces for power transformers that might be useful in preamp builds. One of the possible donors is a Wollensak reel to reel chassis. I actually have a pair of these.

Unfortunately, the PT would only be useful with very low current tubes since it's rated at only 10mA. At first I thought this was a typo in the Sams but a look at the unusual circuit reveals that it only powered a few input tubes. It could also be used as a filament supply I guess.

The chassis has three transformers, which I assumed were a PT and a pair of SE outputs. The output tubes are 12AB5s. Turns out its a mono PP output stage and the third transformer is an interstage.

I have no experience with interstages or input transformers. It's a 1:1.2 with dual secondaries. Could this one be useful in a preamp or some other application?

Here's the specs listed in the Sams and the schematic of it's use in the original circuit:
Wollensak Interstage Specs.jpg
Wollensak Interstage Schem.jpg
 
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wired the right way it could sub as an interstage for an antique radio.

10ma might be enough for a simple line stage. Half a 12ax7 for gain and the other half as a cathode follower per channel perhaps. Or maybe something like the RCA RIAA circuit.
 
gadget73 said:
wired the right way it could sub as an interstage for an antique radio.

10ma might be enough for a simple line stage. Half a 12ax7 for gain and the other half as a cathode follower per channel perhaps. Or maybe something like the RCA RIAA circuit.
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I've got better choices for a PT so I probably won't be using the one from the Wollensak at this point. I probably wouldn't have mentioned it except that it's such an odd PS design - a transformer powering the input tubes and the section of the PS that powers the output tubes looks like a doubler coming right off the AC. I'm guessing this was dictated by the limited amount of space available.

It's the IT I'm mostly interested in. The specs don't mention a current rating but the output section draws 50mA according to the schematic so I assume it's good for at least that, if not a bit more. Obviously, it could be used in an amp just as it is here but I'm curious if it might have any use in a preamp, perhaps using only one of the secondaries?

Here's the wacky PS:
Wollensak PS.jpg
 
hm, a non-isolated B+ supply. Dicey stuff there. Yeah, looks to be a doubler straight off the power line. I'm guessing B- doesn't tie to chassis anywhere either. Makes sense that the input stuff would have a proper supply if that has an input jack where you might actually get in contact with the chassis. Also makes sense that it has an interstage. That would be where the isolated HV supply from the input tubes mates to the non-isolated supply.

Not sure where you'd need such a thing, other than as a phase inverter for something odd like this. There are generally better ways of getting inversion done though. I suppose if you wanted to add an aux input to a 1950s hot chassis radio, this would be just the ticket.
 
Interstage transformers have a few issues.

Frequency response, which is a direct function of cost. The parasitic inductance controls the LF response while the parasitic capacitance controls the HF response. This is why good interstage and output transformers are expensive. Multiple layers in the construction to keep inductance high and capacitance low.

Reflected impedance, if ratios other than 1::1 are used to perform voltage gain at the expense of current drive. So one must ensure the plate has the proper load.

Old radios used transformers for interstage coupling because capacitors were expensive, prone to failure, and, being electrolytic, caused distortion, and resistive coupling required a variety of offsetting power supplies which was also expensive, and prone to failure because of the dropping resistors. Once inexpensive coupling capacitors came along interstage transformer coupling was largely abandoned.

So the question is: what is the frequency response of that interstage transformer, particularly at the low and high ends. You could plot it and see.
 
Wollensak's odd design helped keep the weight down in this portable machine - the high power stage ran from an off-line doubler, so the "power" transformer was much smaller. (incidentally, it would be good for more like 20 mA with a full-wave bridge) The output and driver transformers provided isolation for chassis and speaker. Feedback used isolated winding too.

12AT7 driver was run at 7 mA, interstage transformer may not take much more than that. I never measured one, though the output transformer is surprisingly good, pretty flat from 20 to 20 KHz at low levels. (Obviously not enough iron for 10 watts at 20 Hz, though)
 
should be more than fine for interfacing an ipod to a hot chassis tabletop radio then. That wouldn't involve any DC current.
 
Retrovert said:
So the question is: what is the frequency response of that interstage transformer, particularly at the low and high ends. You could plot it and see.
Click to expand...
Unfortunately, I don't have the equipment to do that.

Tom Bavis said:
Wollensak's odd design helped keep the weight down in this portable machine - the high power stage ran from an off-line doubler, so the "power" transformer was much smaller.
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:rflmao:If that was the goal, they failed! All the "portable" R2R machines weigh a ton. A lot of weight packed into a compact design, like carrying around a box of bricks. I'm guessing it was more about space.

Tom Bavis said:
(incidentally, it would be good for more like 20 mA with a full-wave bridge) . . . 12AT7 driver was run at 7 mA, interstage transformer may not take much more than that. I never measured one, though the output transformer is surprisingly good, pretty flat from 20 to 20 KHz at low levels. (Obviously not enough iron for 10 watts at 20 Hz, though)
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Looks like the PT may be usable for a low current preamp then. I'll probably still go with another candidate first, though, since it will take some grinding to remove the Wollensak PT from the chassis.

The puzzling thing about the OT is its primary. It runs a pair of 12AB5s, which are a 9 pin that is electrically equivalent to a 6AQ5 or 6V6. One would expect the primary to be in the 8k to 10k range yet it's only 1.5k. What's up with that?

Wollensak OT Specs.jpg

Plus, in addition to the 6-8 ohm speaker secondary, it has a CT secondary rated at 125 ohm which seems to be part of the IT inverter design. If I decide to experiment with the IT in the future, I suppose I would need to substitute a resistor for the secondary or build something using both the IT and the OT.
 
possible thats a typo, 1500 p-p doesn't make a lot of sense unless maybe its cathode to cathode?
 
A mistake in Sams? Not the first I've seen... I measured one of those output transformers from a Wollensak 1500: 10,060 Ohms when terminated with 8. Effectively 12.5K if the feedback winding is used for cathode feedback, though that winding can be left unused. Though I see the one I measured was P/N D-131-5A - still likely the same.
 
Tom Bavis said:
A mistake in Sams? Not the first I've seen... I measured one of those output transformers from a Wollensak 1500: 10,060 Ohms when terminated with 8. Effectively 12.5K if the feedback winding is used for cathode feedback, though that winding can be left unused. Though I see the one I measured was P/N D-131-5A - still likely the same.
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Thanks Tom! I'll make a note of those measurements.

Did you ever use those OTs (or any of the Wollensak transformers) in an amp?
 
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