Trainwreck tube options

[apache]

Hi! I'd just like to say how much I love this forum before I start.

Heres my question: I've started building a trainwreck express clone and noticed that the amp can run either 6V6s or EL34s. I have a great set of Mazda NOS 6v6s, but my EL34s aren't very good. Am I right in thinking that the only change needed to run 6v6s in the trainwreck is to change the bias with the trim pot?

I had a look at the recommend plate voltages on the two power tubes of the trainwreck and it seems like they are meant to be at over 400V, isn't that a lot higher than what 6V6s can handle?

[martin manning]

6v6's, especially the older production ones, can take 400V... they have in Deluxe Reverbs for a long time, but those amps are known to be hard on power tubes. Swapping 6V6 for EL34's at 6k6 load impedance, 400V+, and fed by a robust SS-rectified power supply is a bit over the edge for plate dissipation, though, even if you re-bias. Even though a Deluxe Reverb also runs at 6k6, I think you should save your Mazdas for something else.

[FYL]

Swapping 6V6 for EL34's at 6k6 load impedance, 400V+, and fed by a robust SS-rectified power supply is a bit over the edge for plate dissipation, though, even if you re-bias.

Static plate dissipation is set thru the bias control, dynamic diss. can be calculated and is AOK for a proper Express clone. Any good quality 6V6GT will be happy there.

OTOH, a Kelley FACS works hot and high - more than 490V plate with today's mains. A good test for high quality 6V6GTs!

[martin manning]

My comments are based on a 6k6 load line at 400V+ running at >200% of a 6V6's max dissipation when passing signal... This would seem a little reckless without a bit of power supply sag to cool it off.

[FYL]

Dynamic <> static, particularly for a class B/not too deep AB push pull amp. Full dynamic dissipation happens only on peaks, and only on one half of the period - remember, we're dealing with a PP amp.

Adding time in the equation and integrating these params for a complete period give a dissipation envelope app. halfway between idle and max.

Say the stage draws 20 mA at idle and 100 mA on peaks per half-period, current drawn is only 50 mA or so on peaks for a full period. In a nutshell the plates can cool for 50 % of the time.

Deep class AB (high idle) and of course class A are different stories.

[martin manning]

Yes, I understand all that. My point is that the Express is supposed to be running 415V with EL34's, and the supply has the capacity to drive them without too much voltage sag. The max plate dissipation along a 6k6 load line from 415V is 26W, or 217% of the 12W-max for a 6V6. With the 6V6's installed, the quiescent voltage will come up a bit, it won't sag as much, and there is some duty cycle modulation to be expected. That sounds like it's going to be pretty close to the edge for over-dissipation.

[apache]

Thanks for the replies, I'll have a go with my El34s when i finish the amp.

[Lonely Raven]

Thanks for the replies, I'll have a go with my El34s when i finish the amp.

JJ 6V6 can handle the beating. I used them in a couple Super Reverbs and Express for a couple years of casual play.

[Colossal]

JJ 6V6 can handle the beating. I used them in a couple Super Reverbs and Express for a couple years of casual play.

I'll second this. I've run JJ 6V6s at 440, even 460V on the plates (biased to around 70%) and I haven't had a set fail yet. They've continued to operate long after the tone starts to degrade with age and abuse.

[M Fowler]

So do I.

The JJ 6V6s are not giving me any problems.

Mark

[martin manning]

I agree, JJ 6V6-S are tough. Not like a typical 6V6, though... they list 14W max plate dissipation, 500V Va, 450 Vg2.

[2tone]

i have run RCA and Sylvannia 6v6's in my express for over 20 years...they hold up great at 395-400 volts...bias for around 30 ma..