12BH7 max plate voltage

[jimboyogi]

I've seen in a few locations on the webs that 12BH7 has a max plate voltage of 300V, while the 12BH7-A has a max plate voltage of 450V. But the GE data sheet for 12BH7-A has a max plate voltage for Class A1 use as 300V, and 450V for use as a vertical deflection amplifier.
http://www.hifitubes.nl/weblog/wp-co...ic-12bh7-a.pdf

I am currently running one as a Class AB1 P-P output valve, cathode biased, with ~330V plate to cathode, and it seems happy enough so far

My question is three-fold -

Does the 12BH7-A actually have a higher max plate voltage then the 12BH7 for Class A1 amplifier usage?

Is this Class A1 max still the most aplicable one when the 12BH7 is used as a Class AB1 P-P output valve?

How high a plate voltage can they really take without getting uncomfortable ie. has anyone run them higher than 300V and had no problems

Thanks for any and all help

[Ronsonic]

I've run them over 300V without problem.

My general rule is that there are specs for max voltage, current and dissipation. Only violate one of those at a time. Some tubes are terribly under-spec'd and will do far more than the datasheet suggests but those are a special case, for everything else I'll only break one of the three and have never been punished for it.

[Jazz P Bass]

Plus the datasheets are for "perfect" operation.
The indication is: "Here is how to operate this tube to ensure excellant performance, durability & longevity of tye life of the tube."
ie: not for guitar amplifiers.

[jimboyogi]

Thanks for the replies.

I like that "only break one spec at a time" rule ronsonic, ATM I am breaking 2 - over-voltage and over-dissipation. I'd better check myself!

One of these days I'll try one at ~400V plate-cathode and see what happens

[MWJB]

If the conservative data sheet suggests 300vdc as the limit for class A, why not run AB? If you come down on the current, you should get more leeway with the voltage...up to a point.

Kendrick claim to get 7W from a single 12BH7 wired PP, as I'm getting half that from a 12BH7 at 292vdc from plate to cathode (470ohm cathode resistor, ~4W per side idle dissipation...though have run up to 320vdc for short periods) I have to assume that they are running a lot more B+?

[kg]

overdissipation will kill a tube far quicker than will overvoltage.

in fact, the only thing overvoltage will do is arc the tube, most of the time a non-fatal warning that things have gone too far.

[jimboyogi]

Thanks again for the help MWJB and kg.

MWJB, I am currently running a 12BH7 as a PP AB1 output stage, 330V plate to cathode, 560 ohm cathode resistor, into 8K ot Z, and dissipating just under 4W per triode, which is too high. I am only getting 2.5 WRMS pre-clipping. But raising the bias voltage always seems to give too early crossover distortion, so I really find the higher dissipation sounds a lot better.

Blackstar also claim to get 5W out of a 12BH7, but I've got no idea how they get this much output. Maybe they are rounding up to the nearest 5

I'm thinking that with higher B+ i might try fixed bias, to keep the dissipation under control while minimising crossover distortion.

[MWJB]

I'm using a 16K primary on one amp, 32K primary on the other, Kendrick use ~17K/33K depending on whether one or two tubes are installed. It's typical to use a primary with 2-4 times the tube's resistance. 8K should "work" but maybe try the alternatives?

At 4W dissipation I'm not seeing any signs of redplating.

[jimboyogi]

OK, so over the last two days I've tried a 12BH7 out running 400V+ B+, and have a couple of observations. All at 1KHz into an 8K OT load, 100R plate stoppers.

First fully cathode biased,

Rk = 1K, Vk = 17.6V, Vp = 411V. Pp = 3.46W, Wa = 3.5W, so biased at 99% Wa. Pout (max. pre-clipping) = 1.4 WRMS, lots of compression on the sine wave well before clipping (not surprising with such a high value of Rk)

Then combination zener/resistive bias,

Zener = 11V, Rk = 470R, Vk = 17.8V, VRk = 6.8V, Vp = 407V. Pp = 2.8W/3.5 = 80% Wa. Pout = 3.0 WRMS.
Much less compression visible pre-clipping, both on the scope and when listening while playing guitar through.

Zener = 15V, Rk = 220R, Vk = 18.2V, VRk = 3.05V, Vp = 409V. Pp = 2.7W/3.5 = 77% Wa. Pout = 3.6 WRMS.
Hardly any compression discernable. Ran very overdriven for ~ 5 mins into a dummy load, with all lights off I could see a very faint glow on one side of one plate, glow dissapeared as soon as I backed the input signal back to more regular levels of overdrive.

No sign of arcing, or any other signs of distress. All of the above sounded very good to me, better than running at Vp = 330, due to what sounds like more treble response. It just depends on how much compression and power output I was after to determine a favourite.

[kg]

when you say "combination" bias, does that mean the zener and resistor are in series or in parallel?

[jimboyogi]

Zener and resistor are in series.