Tube-ular, man

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Tube-ular, man

Postby rustyslide » Mon Apr 10, 2006 5:47 pm

Pulled some tubes out of an old military-surplus radio amp we're pitching at work, and was going to try them out in my Trace Elliot. The Trace Elliot only uses one tube, a 12AX7 in one of the preamp channels (uses FETs everywhere else, it's analog), so I only grabbed the ones I knew were pin compatable.

I snagged 1 12AX7 (Siemans), 2 12AY7s (RCA-Electra), and 1 12AT7 (dunno the brand, the writing's worn off - was in the 12AT7 slot in the chassis though). I've heard that 12AY7s are lower gain than AX7s, and I seem to recall the AT7 as a rectum-fryer tube. But other than that am pretty clueless.

Anything I should know before swapping tubes (unplug it!) ?

I don't think I have to do any biasing with one tube, do I? the manual certainly doesn't mention it.
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RE: Tube-ular, man

Postby ricochet » Mon Apr 10, 2006 6:32 pm

The tubes you mentioned are all compatible with the 12AX7, but of various lower gain levels. A rare potential problem is that much lower gain tubes like 12AU7s can flow too much current in some circuits that use 12AX7s, potentially overheating a cathode resistor, but you're unlikely to have a problem with this in practice.

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RE: Tube-ular, man

Postby ricochet » Mon Apr 10, 2006 6:32 pm

And no, there's no biasing involved.

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RE: Tube-ular, man

Postby rustyslide » Tue Apr 11, 2006 4:51 am

[updated:LAST EDITED ON Apr-11-06 AT 00:51 AM (EST)]Thanks!

I tried one of the 12AY7s tonight - worked like a charm. Gain channel is a lot more subtle now (it used to get 'crunchy' pretty quick).

Found this page and thought it was interesting:
http://home.comcast.net/~machrone/bjr/bjtubes.htm
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RE: Tube-ular, man

Postby ricochet » Tue Apr 11, 2006 4:25 pm

The 12AY7 is a special low-hum audio tube. It was the original front end tube on the tweed Fender Bassman. I think it's got an amplification factor around 60, compared to the 12AX7's 100.

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RE: Tube-ular, man

Postby rustyslide » Thu Apr 13, 2006 1:29 pm

Page I linked to says "45" vs the 12AX7's 100, but whichever.

I was talking with my dad (electrical engine-gear), and he was concerned that there would be too much grid current coming out of the 12AY7 for the amp if it was designed for a 12AX7, and that would potentially cause problems.
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RE: Tube-ular, man

Postby ricochet » Thu Apr 13, 2006 2:18 pm

'Tain't grid current that goes up with lower gain, it's plate current. It's unlikely to cause a problem, though, as 12AX7 guitar amps pretty much always have at least a 100K ohm plate resistor and 1.5k cathode resistor, which is going to keep the plate current low enough to not burn anything up with a 12AY7. Harpers stick 12AU7s in for 12AX7s willy-nilly all the time and get away with it, and that tube's mu is around 18.

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RE: Tube-ular, man

Postby rustyslide » Thu Apr 13, 2006 5:17 pm

>'Tain't grid current that goes up with lower gain, it's
>plate current.

It's unfortunately all Greek to me. He probably said 'plate current' too; my knowledge of electronics is limited to pretty simple circuits (i.e. without transistors).


>It's unlikely to cause a problem, though, as
>12AX7 guitar amps pretty much always have at least a 100K
>ohm plate resistor and 1.5k cathode resistor, which is going
>to keep the plate current low enough to not burn anything up
>with a 12AY7.

I'll poke around the board some more and look for big resistors. I was aware that people swap in lower gain tubes (or wouldn't have had the idea in the first place), but I've not read about people trying it in hybrid amps, and am not sure how that changes anything.
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RE: Tube-ular, man

Postby ricochet » Thu Apr 13, 2006 8:43 pm

It might. On that hybrid amp, they might have used a lower plate resistance to get lower gain out of the 12AX7.

Lots of high gain tube amps are built with preamp stages set up for pretty high gain, but dump most of the output voltage in resistance networks between stages. A typical 12AX7 stage can only take up a bit more than 1V peak to peak on the input before it begins to clip, but will put out something like 43-66V. That's why a simple amp like the Valve Junior with two 12AX7 stages (in a single tube) can overdrive so much. A P90 or really hot humbucker can just hit 1V at the beginning of a hard picked or strummed note, and momentarily just about saturate the first stage. A Fender single coil won't come close. Maybe an eighth of that voltage. But every subsequent stage can get overdriven hard if some gain's not dumped.

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