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Theoretical question about interstage transformers
Theoretical question about interstage transformers
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Old 12th June 2018, 05:56 PM   #1
waxx is offline waxx  Belgium
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Default Theoretical question about interstage transformers

While studying the subject of SE amps, i see that most DHT amps use an interstage transfo instead of a coupling cap, and this is never the case with a se amp using indirect heated tubes. Is there a technical reason for that that escapes me, or is it just because of the higher costs of that transfo? DHT tubes are mostly more expensive than others, so that may be the case...
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Old 12th June 2018, 06:02 PM   #2
Lingwendil is offline Lingwendil  United States
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Theoretical question about interstage transformers
It's mostly anachronism under the guise of "superior technique" in many cases. It's part of the territory of Ye Olde Schoole amplifier design. It takes a rather costly interstage to beat a solution with most tube driver topologies, and an interstage can be more simple to implement at times.
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Old 12th June 2018, 06:03 PM   #3
DF96 is offline DF96  England
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Probably partly nostalgia, but also DHT usually have quite low mu so need the extra boost from an interstage transformer. As soon as valves started having higher gain the transformer could be dropped. A coupling cap is cheaper, smaller, wider bandwidth, less distorting, less prone to hum pickup etc.
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Old 12th June 2018, 06:12 PM   #4
waxx is offline waxx  Belgium
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Quote:
Originally Posted by DF96 View Post
A coupling cap is cheaper, smaller, wider bandwidth, less distorting, less prone to hum pickup etc.
Well, i don't agree, i heared recently on a demonstration a custom DHT amp with a 26 trough a (Tango) interstage transfo to a 45 with again a Tango output transfo, and that beated all amps i heared before on sound (and i heared a lot). Sadly nobody makes those tubes anymore and they are rare and expensive. That guy also had a double 6N1P driver/EL84 SE also with tango transfo at the end and that, altough a wonderfull amp on it's own, did not came near in my opinion. Both were listened on the same speakers and in the same room with the same source.

But each to his own off course, this is a subjective opinion.

Last edited by waxx; 12th June 2018 at 06:14 PM.
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Old 12th June 2018, 06:14 PM   #5
analog_sa is online now analog_sa  Europe
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Theoretical question about interstage transformers
Quote:
Originally Posted by waxx View Post
this is never the case with a se amp using indirect heated tubes.
There is no technical reason. Amps using indirectly heated outputs generally tend to be more budget oriented, so this may be one reason.
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Old 12th June 2018, 06:29 PM   #6
cbdb is offline cbdb  Canada
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Quote:
Originally Posted by waxx View Post
Well, i don't agree, i heared recently on a demonstration a custom DHT amp with a 26 trough a (Tango) interstage transfo to a 45 with again a Tango output transfo, and that beated all amps i heared before on sound (and i heared a lot). Sadly nobody makes those tubes anymore and they are rare and expensive. That guy also had a double 6N1P driver/EL84 SE also with tango transfo at the end and that, altough a wonderfull amp on it's own, did not came near in my opinion. Both were listened on the same speakers and in the same room with the same source.

But each to his own off course, this is a subjective opinion.
Your disagreeing with facts. Just because you like the transformer sound dosnt mean it's more acurrate.
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Old 12th June 2018, 06:32 PM   #7
waxx is offline waxx  Belgium
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Originally Posted by analog_sa View Post
There is no technical reason. Amps using indirectly heated outputs generally tend to be more budget oriented, so this may be one reason.
Ok, so a 6922-EL34 SE aiming for 5w with a good interstage transfo may work well. I want to design and build one like that when i'm sure enough of my knowlodge of tube amps (not yet) so that may be a route to go...
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Old 12th June 2018, 06:35 PM   #8
cbdb is offline cbdb  Canada
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Don't get me wrong, you not alone. Rupert Neve says the sound of his ( still very sought after) 50 year old mixing desks was due to all the transformers. But most recording gear isn't wanted for accuracy but for effect. I don't believe a power amp should be in this camp.
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Old 12th June 2018, 06:37 PM   #9
waxx is offline waxx  Belgium
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Originally Posted by cbdb View Post
Your disagreeing with facts. Just because you like the transformer sound dosnt mean it's more acurrate.
Who's saying i'm aiming for accuracy. I'm aiming for a sound i love.

And i believe that solid state amps in class A (like the First Watt designs) are way more accurate than any tube amp with transfo's that color, so ...
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Old 12th June 2018, 07:16 PM   #10
GoatGuy is offline GoatGuy  United States
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Before this gets into a religious war, consider…

From the dawn of the vacuum valve era until about the late 1930s, the only 'foils' a manufacturer had at her disposal to create rolled-tube capacitors was either copper or tin. There really was no cheap aluminum foils, especially ultra-thin types. As a consequence, given also there were absolutely no plastics (for ultra-thin dielectrics), creating a capacitor with even single-digits worth of microfarads was an expensive proposition.

That's "first".

Second, as noted above, the mu of valves wasn't very high in the "big bottle" era. They performed quite nicely all in all, but high gain wasn't expected of them. The tolerances weren't present in the manufacturing lines to get millimeter-or-under cathode-to-grid–1 spacing. However, building a bigger bottle was easy, since for most intents and purposes the envelope was created from repurposed vacuum standard incandescent light bulb glass-blowing equipment. Bigger was easy. And it turned out that there were thousands of cottage industries eager to make whatever transformer(s) hit the fancy of the valve amplifier designing bunch.

Moreover, you could get at least a factor of 2 gain with an interstage. Even factors of 3, 4 or 5 weren't unheard of. The speakers of the day were so pathetic, frequency-response wise (and the available source-material also so pathologically non-high-fidelity), that if there were interwinding capacitance issues that'd limit amplifier bandwidth, well … it largely didn't matter. Bigger bottles + interstage step-up transformers got the job done. Also, without very much complexity, you also "got for free" the phase-inverter function with an interstage transformer, for increasing output power via push-pull designs.

No losers there! Win, win, win.

But also as noted above, with the "taming of the bottle", the relatively rapid development of specific-purpose tube-making equipment, came smaller more durable, less finicky "bottles". Came indirect heated cathodes to totally tame A/C hum (and extend life of tubes.) Came aluminum foil wound capacitors (even before electrolytics); then wet-type electrolytic dielectric aluminum foil capacitors were discovered, and rapidly transformed (couldn't pass up the pun) a revolution in power-supply mass-and-ripple reduction without needing fork-lifts to carry around electronic equipment.

The designers, subsequent to the expensive-capacitor and lots-of-interstage-transformer era, embraced the much lighter weight, smaller, higher gain tubes, lower power requirements and especially the magnetic-flux balancing push-pull idea for output power. Because the interstage transformer so handily could deliver, if wound thoughtfully, nearly full-audio-spectrum bandwidth and a bit of voltage gain and for-free complimentary signal inversion, of all transformers they remained designed-in for a long time. Well into the 1950s.

But by the late 1950s and especially early 1960s, “bottles were cheap” and transformers had become expensive, relatively. Capacitors were terribly inexpensive all in all. And resistors were “nearly free”. Hence designs edged toward minimizing transformers and special bias power supplies, and simultaneously toward “cathode bias + electrolytic” automagic methods that'd serve the consumer well, even tho' the plethora of bottle makers were notably loathe to actually make same-numbered bottles “to actual spec”.

Self-adjusting “good enough” circuits rose to rule the scene.
Because 99% of the consumers couldn't identify a bias screw from a toggle switch.
Nor could they be counted on not to screw “it” up.

With today's electron-beam evaporated aluminum-on-ultra-thin-film capacitors, we have “arguably” near-perfect capacitor solutions, for prices that are — in the historical context — breathtakingly low. And while us old fuddy-duddies like our valves, we also have a whole constellation of semiconductor devices which can markedly assist the non-valve side of nominally tube-amplifier design. We're basking in saunas steamed with bourbon.

Just saying.
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