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Old 7th January 2017, 03:16 PM   #1
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Default Output Tube Into Op Amp

I have a conceptual question:

Would it be possible to have a high voltage op amp in place of the output transformer in a tube amp, with a resistor (one that mimics the resistance of an OT) in parallel to act as a load? Would the op amp couple the high impedance from the tubes to the low impedance of the speaker, or would it just explode?

I know this would take away any of the sonic benefits of the OT, but I'm just mostly curious.

Thanks.
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Old 7th January 2017, 04:22 PM   #2
PRR is offline PRR  United States
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The tube will swing quite differently if the DC feed is a resistor instead of a choke (transformer).

And 6L6 to drive opamp is a snowplow to move a pebble.

But in general: sure, you can do anything. This is what you end up with when a tube preamp drives a power amp. Or a Herzog (5W tube amp on dummy-load) into a big PA system.
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Old 7th January 2017, 04:27 PM   #3
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Well, no, they don't sell op amps that high in voltage rating that I know of. There are some 100 v ones like TDA7293. Output tubes have used 300-400 volts since the early fifties.
Alternatively, since the best tubes come from the invader of Ukraine and further south, why output tubes can't be replaced by high voltage FETs. The output transformer is a very useful device to protect your speakers from shorted devices. Output transformers are comparatively expensive, but if you have some good ones, great. Somebody apparantly did build a fet driver hv output transfomer instrument amp, but nobody has built one for hifi that I know of.
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Old 7th January 2017, 08:43 PM   #4
PRR is offline PRR  United States
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> they don't sell op amps that high in voltage rating

You don't have to feed ALL the signal voltage.

Voltage dividers are the audio person's best friend.
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Old 8th January 2017, 03:07 AM   #5
Enzo is offline Enzo  United States
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Or you can make up a discrete op amp circuit from high voltage parts. That is more or less what a SS power amp is anyway.
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Old 8th January 2017, 04:33 AM   #6
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Quote:
Originally Posted by PRR View Post
a Herzog (5W tube amp on dummy-load) into a big PA system.
If I was to go this route, would just a dummy load with a voltage divider in parallel to bring the output down to line level work to feed into the PA?

Thanks for all the help.
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Old 8th January 2017, 05:40 AM   #7
PRR is offline PRR  United States
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Here is the basic Garnet Herzog:

Click the image to open in full size.

This plan is missing the B+ filter caps(??). But it is clearly a Champ-like amp, with a 6r-10r 10W dummy-load, and a 1Meg pot between that and another guitar amp input.

Story is that Randy kept bringing-back his Champ-copy with a blown output transformer. Gar asked how he was doing that. Plug the little amp to a big amp and beat it. Without a load the distortion spikes destroyed the OT insulation. Gar saw it would work the same less destructively with a dummy load.
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Old 9th January 2017, 07:44 AM   #8
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Originally Posted by SirDuctTape View Post
If I was to go this route, would just a dummy load with a voltage divider in parallel to bring the output down to line level work to feed into the PA?
I've had some success with that approach. Guitar amp, dummy load, voltage divider, graphic EQ pedal, then feed that into the P.A. or DAW or what-have-you.

I found the graphic EQ a vital part of the equation. Guitar speakers roll off the harsh high-frequency harmonics for us, so when you take the speaker out of the equation, you need to do the same sort of low-pass filtering electronically.

Using a graphic EQ (rather than just a low-pass filter) also lets you experiment with putting in a hump around 3 kHz, as many guitar speakers have, cutting bass frequencies to "tighten" the guitar sound and/or keep it from stepping on the drums and bass, etc.

-Gnobuddy
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Old 9th January 2017, 04:33 PM   #9
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This brings up a question about output transformers. Would I be able to just put the dummy load in the place of output transformer, with maybe an inductor along with it?

I'd rather avoid the transformer for cost reasons, but if needed I can spend the money.

Thanks for all the input, it helps a lot!
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Old 9th January 2017, 05:41 PM   #10
JMFahey is offline JMFahey  Argentina
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Quote:
a resistor (one that mimics the resistance of an OT)
Such a resistor does not exist by definition, so you started with the wrong foot.
Quote:
I'd rather avoid the transformer for cost reasons,
Sorry to hear that but sadly, you canīt.
And once you start adding inductors to the mix, you might as well go for a real OT and call it a day.

Or go full SS and emulate what a transformer does or how it sounds, which is tghe point after all.

DSP algorithm proficiency does certainly help but worst case you can do it all analog.
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