Behringer tube effects

[gui_tarzan]

I bought a couple of rack mount B'ger effects from a guy just because I got them CHEAP and I wanted to tear them apart for parts. What I noticed was the very low plate voltage that they run their 12AX7 tubes at (less than 40v) so my question is how do they even work? And don't say "badly", I know that already.

[eschertron]

In my limited understanding, emission is possible because of the heaters, and electrons are 'free' of the cathode once warmed up. The really low plate voltage will have an effect on the gain of the tube (lower than expected from the published mu) and standard charts are useless at the lower voltage. But electrons are still able to stream across the cathode-anode junction. That's all the farther I'm willing to stick my neck out on this one

[Malcolm Irving]

Merlin Blencowe has written an interesting article about triodes at very low voltages:

http://www.valvewizard.co.uk/Triodes...s_Blencowe.pdf

[J M Fahey]

At lowest voltages, 12AU7 are the best tubes, as in the ones which suffer less under those conditions.
12AX7 can work (sort of) as low as 12V DC but almost do nothing, gain is about 1 and to pass any reasonable current ( near 1 mA or thereabouts) instead of a couple microamperes or less, they need positive bias
They can distort, of course, that's what various Blue/Tube drivers do.
Actual gain comes from despised Op Amps.

With 40V supply those Behringer tubes may have gain about 5 to 10 and still behave like (weak) tubes.
Personally under those circunstances I ditch tubes and go straight to FETs .... but of course "it has a 12AX7 tube inside" works wonders for sales.
In fact probably *all* sounds (including the "tube" ones) are made by DSP .

[raiken]

.... but of course "it has a 12AX7 tube inside" works wonders for sales.

I call this the "market follower" tube configuration...

[Justin Thomas]

I just had my friend's 65 Ventura open. I texted him asking about the low plate voltages; all the way down to 35V on all tubes. He asked if I had the Master all the way down, and when I said yes, he said crank it up and see if it changes. Sure enough, normal 385V. His description of "the MV works like an attenuator" was inaccurate, but I got the point. The amp obviously sounds best with the Master on 0, but it still sounds pretty good on 1, too. One channel is EF86, the other uses an AT7, I think, w. an AX7 PI & 2x6V6. Heaters stay @ 6V, and it's cathode-biased.

It was different from anything I'd ever seen... Is this a variant of the famous "Tower Flailing" I was hearing so much about a few years back? And I'm not as likely to toss voltage-starved tubes out the window, IF the circuit is right.

Justin