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Old 8th June 2005, 09:15 PM   #1
mholz is offline mholz  United States
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Default stereo el34 + kt66?

Having built a few guitar amps, I've gotten hooked on tube electronics and I'd like to try my hand at hifi amp--and hopefully learn more along the way. I need to keep costs low, so I was thinking a simple stereo circuit would be the way to go (like the stereo EL34 circuit on the Angela site or John Atwood's Randall amp).

However, I'd really like to combine the sounds of an EL34 with a KT66. Would there be any problem in building a stereo circuit with EL34s in one channel and Kt66s in the other?

I haven't come across any examples of such a circuit, so perhaps it's just a dumb idea...
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Old 8th June 2005, 10:21 PM   #2
EC8010 is offline EC8010  United Kingdom
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Why would you want one channel to sound different to the other?

It's not just the output valves that make the difference in sound, it's how they're used and how they're driven.
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Old 8th June 2005, 10:34 PM   #3
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i agree, L and R channels should be as similar as possible. however, if you build for KT66, EL34 will probably drop in fine (same pin-out), so you could always swap depending on your mood.
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Old 8th June 2005, 10:36 PM   #4
EC8010 is offline EC8010  United Kingdom
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Be careful, although KT66, KT88 and EL34 are all pin compatible, they have different bias requirements.
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Old 9th June 2005, 12:26 AM   #5
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true, but an adjustable fixed-bias circuit would allow them both to run at their respective bias points. other requriements (drive V, output tx, etc.) aren't wildly different. or, pete millett has an amp which uses different tubes with NO bias adjustment but i haven't tried it.
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Old 9th June 2005, 12:49 AM   #6
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I just recently as in the last week or so read somewhere on the www of someone who did this except the tube of each output pair were different, I don't remember the types but it was somthing like EL34 & KT88 per channel, fixed bias or should I say adjustable fixed bias.

IIRC in the R.D.H. there was a PP amp doing the same thing but with a 2A3 & 45.


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Old 9th June 2005, 04:14 AM   #7
mholz is offline mholz  United States
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I figured that having different tubes in each channel might interfere with stereo imaging etc., but I'm still intrigued by the idea of mixing output tubes.

My memory is faulty here, but some time back I came across an article by Claus Byrith, where he mentions--as an aside--that his monoblock setup at home consists of one with EL34s and the other with KT66s. From my experience with guitar amps, I thought that would indeed be a nice combination. I can't find the reference now... perhaps I'm remembering incorrectly.

Of course, in a stereo circuit, you'd have to tweak each channel appropriately for the given tubes etc... but those are details. I'm really just wondering if this is even theoretically worthwhile.

Thanks for the references, in any case. I'll definitely check them out.
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Old 10th June 2005, 01:11 AM   #8
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Default Kt66 EL34

What about using the EL34 as a driver tube for the KT66, and switch places depending on your mood
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Old 10th June 2005, 03:12 AM   #9
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> EL34s in one channel and Kt66s in the other?

If you can find 1950s tubes in like-new condition, there may be a difference. But as patents expired and tube production ramped down, there was a lot of consolidation of details between tube types. On recent production, the two types may be trivially different, minor variation of grid spacing or plate-shape.

In push-pull under feedback below clipping, I suspect that any difference between tubes working below limits and at good bias is as much in the mind as in the signal. I know that just looking at an old-style KT66 sets up certain associations in my mind.

Without feedback or pushed past the limit, there are differences between the big bottles, which is why guitarists have preferences. Not just for type, but brand and vintage.

If there is a difference between left and right, I'd say "fake stereo". Feed a mono signal, and you will hear a "stereo stage" (probably narrow, but not one-point) which isn't in the recording.

As Andrew says, you can mix tubes in a push-pull stage. Works better than you might think. Of course you have to check pinout and bias. Bias is tricky: instead of biasing each tube for its optimum, you should probably bias for equal currents to minimize DC in the transformer iron. E-I cores will stand some unbalance; torroids hate DC and will probably demand equality of tube currents or suffer deep-bass trouble.

Or put in four sockets and a heater-switch to light-up one or the other pair (you may also need to switch bias etc). If you feed the "off" heater through a resistor so it gets about half heater voltage, you can hot-switch without interruption of sound.
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Old 13th June 2005, 04:44 PM   #10
mholz is offline mholz  United States
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Thanks for the response. I'll chew on that for a while.

Good stuff!
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