EL84 cracked?

[joshdfrazier]

I was recapping and doing some work on my lafayette/univox amp, and decided to move some wires around to allow it to accept EL84's as opposed to the 6973 it was designed to use. Old tubes of this type are too expensive, and the only current production is the EH 6973, which I don't like. Plugged in a sovtek el84m and an old RCA 6bq5, and it sounded great.

Turned it on again today, and the sovtek cracked open and failed. It was a brand new tube. I installed a matched hammond pair, and have had no issues so far.

There is something really unusual about this amp, though. The tremolo doesn't work, but the intensity knob actually controls the negative grid bias. at its lowest setting, -11vdc (no sound, but this setting worked fine for the 6973's) and -25vdc at its highest setting.

so, why would the tube fail, what is the deal with this knob, and is this a safe mod or should i revert to the 6973?

[Phil_S]

They call it bias wiggle tremolo. That's how it works!

As for the cracked tube, they don't make 'em like they used to?

[joshdfrazier]

haha yeah i guess so. anode B+ was over 450v so i guess that explains the bottle breaking.

[Gaz]

haha yeah i guess so. anode B+ was over 450v so i guess that explains the bottle breaking.

No, it doesn't. The high voltage on the internal element won't cause the glass to break. My guess is that the tube was hot and got jarred in just the right way. I've had this happen to a JJ as well. Sucks.

[Phil_S]

+1 on what Gaz says. Also B+ 450v is way too much for EL84. See if you can get old stock 7189-A which have a rated VAmax =440 or maybe the EL84M which is the English language designation for several Russian tube types, of which 6P14P-V or 6P14P-EV are the high voltage rated, see here: http://www.tubes.ru/techinfo/HiFiAudio/6p14pev.html

[Smokebreak]

They call it bias wiggle tremolo. That's how it works!

As for the cracked tube, they don't make 'em like they used to?

I love that tremolo. It's quite hypnotic. I find it curious that the tremolo knob "works" and is fluctuating the bias, yet the tremolo effect is inaudible...also, from my limited experience with a Traynor with this type of tremolo, I had to disconnect the tremolo circuit entirely to get an accurate bias reading, as even turning the trem knob to zero wouldn't keep my bias steady enough to set properly.

[joshdfrazier]

I'm sure the tremolo would function fine if I bothered to change all caps in the circuit, but I use pedals, so no need.

Phil, I will probably order a pair of the 7189a. Or, would a 6v6 be a sensible replacement? I'm not sure of the filament current draw but I was thinking of getting rid of the rectifier tube anyway.

[Structo]

I have had a couple tube failures where the tube turned white in color.
To me that means a loss of vacuum.

Unless I have beat the amp up with gigging or shipping, it is not explainable unless like I said, poor handling.

[Phil_S]

6V6 are trouble. You'll have to drill out the holes for Octal sockets. I'd try to avoid that.

[joshdfrazier]

I have the sockets, and the chassis has enough room below that I wouldn't need to disturb the existing 9 pins for the 1" octal.
I have plans to build a 1x12 cabinet for this amp to make it a bit more portable, anyway, so I don't mind doing a little drilling. The larger bottles look cool too.

If you mean trouble in other respects, would you mind elaborating a bit?

[cbass]

I'm no expert but I'm thinking you can find 6973's for less than 7189s