transformer PI

[uneumann]

So my recent project with an interstage transformer got me thinking about why we don't see transformers as PI stages anymore.
Some people here sent me pointers to Gibson and a Musicmaster bass amp that all have transformer PI stages.

fender musicmaster_bass_schem.pdf

Does anyone know why the transformer PI seems to have died in modern amps?

I see at least some potential advantages if offers over a common LTPI..

- simplicity - only one component
- no caps so no blocking distortion problems
- option to set the winding ratio to create a range of driving impedance and head-room options for driving power tube grids

Does anyone recall any serious downsides to this idea or why it died out?

[nickb]

I would hazard a guess: cost and poor frequency response. Seems you would need either a very large primary inductance or drive with significant power to get the low frequency response to a good place.

[Enzo]

What does an interstage transformer cost? What does six or seven resistors and maybe four caps cost? Compare.

If you decide to make changes, which is easier and far cheaper to implement: changing a few resistor values or designing and having made a different transformer?

Resistors, caps tubes all stock items on the shelves of any supplier. Interstage transformer made to order with lead times appropriate.


In the old days when these were common, no one was overdriving amps, so I think blocking distortion was a lot less of an issue than to today's amp tweakers.

[teemuk]

They arent exactly "generic" components so decent ones are quite expensive. Yes, that much expensive.

Also, they add another phase shift to the output stage so applying negative feedback becomes even more difficult. They also add distortion. Probably not issues in a guitar amp but guitar amps are a niche market considering amplifiers. Mostly designers just found ways to make do without them.

[Mick Bailey]

I suppose they're more prone to picking up EMR and also induced hum from the PT. Never heard of a high gain amp that uses them.

At one time it was usual to find both interstage and PI transformers in amps - but that's going back to the 30s.

[uneumann]

Thanks - all makes sense. But still I wonder...

The cost issue is not intrinsic - it's more of a chicken-egg problem. Consider the reverb driver - it's at or under $15 and it passes far more power to the tank than an output stage requires. That's got to be about what a tube stage costs - tube, socket, RCs, connections. If they were used widely, and made in bulk at the right power range (lower than an reverb driver) they could be cheap. But that's doesn't solve all the other problems you've described. IMO - the NFB issue is probably the hardest to crack.

Anyway thanks for your thoughts

[Enzo]

$15 is huge. Make 10,000 amps, and a $15 part in each is $150,000.

And I don't know that you will ever get economy of scale on the transformer. A regular tube PI uses resistors and caps they use all through the amps and are bought for dirt by the millions. There would be one PI tranny in an amp, but a pile of sockets, tubes, resistors.

[NateS]

One of the quirks of economies of scale, is that it does depend on scale (Fancy that!) The curve does some funky things at Qty=1, so go knock yourself out.

[R.G.]

The cost issue is not intrinsic - it's more of a chicken-egg problem.

Well, there's maybe a lesson in history. At one time, interstage transformers were in fact the - or A - preferred solution, and were made in quantity. So they were part of the mainstream design solutions for amps in general. But they died out. In this case, the chicken existed, and its eggs failed to thrive.

Some of the issues really are intrinsic. A transformer of a given size has some amount of iron, some of copper, and some of various insulation materials. On top of that, there is processing of the materials into the finished form and assembly labor on each sub part as well as the assembly labor on the finished part. Economies of scale can affect things only to the extent that they reduce the marginal costs. The market for ferrous materials, copper, and insulators won't be affected by more transformers being built, so the cost of raw materials and finished sub-materials would not change noticeably even if a progressive and far thinking government edicted that all tube amps in the future must be made with transformer PIs and all previous ones be retrofitted. The only real variable here is labor, and the world has already pushed labor into the lowest labor cost pools available.

It's been conventional wisdom in the transformer industry that the cost of a transformer is the cost of iron and copper, which you can often estimate by weight, plus the labor. Because PI transformers have very special frequency response requirements, they tend to need hand winding, not machines on bucket-bobbins, and the cost of labor skyrockets because hand winding for good frequency response demands *skilled* hand winders. Not only that, frequency response issues often demand more iron, or special nickel-iron, or high-price thin laminations as well.

So Occam's razor gets into it. It's not just that they can't be cheap because they aren't cheap. We (as a species) have solved that problem before. It's that there are cheaper ways of doing it with the same or better performance, and that is what killed off this particular dodo.

Consider the reverb driver - it's at or under $15 and it passes far more power to the tank than an output stage requires. That's got to be about what a tube stage costs - tube, socket, RCs, connections. If they were used widely, and made in bulk at the right power range (lower than an reverb driver) they could be cheap. But that's doesn't solve all the other problems you've described. IMO - the NFB issue is probably the hardest to crack.

And you're right - the frequency response and phase shift issues mean you can't use a $3.00 transformer like the ones still available from Mouser from Xicon for interstage use, or the $3.00 power transformers from similar companies. There are reasons that the audio industry abandoned transformers as much as they could, and are still working on getting rid of the rest of them.

Occam and Einstein. Occam had his razor, and Einstein was reputed to have said something like "Everything should be as simple as possible - but no simpler."