Anybody Else Want A Jim Kelley Amp?

[briane]

wow- jim kelly was here!

I feel unworthy.

Thank you for your great contribution to the world of amp building - I truly respect your heritage.

And hey if you ever in seattle Ill cook you up a first class steak!

cheers,
brian

[musant]

As much as one can feel sorry for a tube, yes EL84 or 6V6 run at elevated B+, OW! I'm more thinking "what were THEY thinking" referring to the designer/manufacturer. There's always some spec-sheet-reading wag who will sound off, "EL84 rated up to 800V plate, as long as you don't draw any current," or "6V6 were used in B&W TV Vertical circuits with voltages up to 850V." But we're concerned with guitar amps & looking for trouble-free operation as well as good tone & power. Now that you're here Jim, what's your solution A) for your current amp design and B) for the older amps. We now have JJ 6V6 which seem to put up with 500V plates, but techs & amp owners who try other brands or NOS 6V6 find themselves with a fireworks show instead of an amp. (And heaven help those who try it with Visseaux or Mazda 6V6 - these have no tolerance at all for B+ beyond 325V.) For the moment I'll select JJ's when re tubing a Kelley or other amp with elevated B+. At times in the past, I've been tempted to substitute a pair of 6L6 for the quad of 6V6, with bias set accordingly.

Excellent points. TV sets needed to be reliable, too, of course. I only ever used Sylvania, and subsequently Philips tubes in my amps back then. For some reason I never found tubes that were 'painted' black on the inside to be entirely trustworthy. I also found that almost anything would work fine in a Champ.

A good vacuum is crucial. Flaking cathodes are a big red flag. Grid emission - be very afraid. The screen grid is the wimpiest high voltage element in the tube, and it doesn't handle excessive current very well. Don't allow it. Don't run the plates at _high_ voltage without fixed bias, and don't use excessively large resistors in series with the control grids. And, to almost every rule there is at least one exception (though no one has yet found an exception to conservation of energy). Be on the lookout for spitting and sputtering at an idle, and red plates under a full load.

If you like the sound of the Kelley amp, then please put good 6V6's in it. If not, then please put good 6V6's in it anyway. Old amps should probably run on a Variac, or have the power transformer replaced with one that has a 120+ volt primary. High voltage is hard on filter caps and semiconductors. High current is hard on tubes. But in general it's the product of the two that we need to worry most about. AFAIK.

Thanks very much for all the kind words, guys. Really.

[Leo_Gnardo]

For some reason I never found tubes that were 'painted' black on the inside to be entirely trustworthy.

Roger THAT, and thanks very much for your reply & advice.

Recently I've seen some non-commercial amp builders using "pull-down" resistors on the screen grids. 220K 2W to ground from the SG, and 1k to the second hi voltage node, for instance. What do you think of this?

I'll have to keep eyes & wallet open for some NOS Sylvania / Phillips 6V6's. In the meantime JJ will have to do.

Thanks again Jim & a big hello to John Suhr.

[musant]

For some reason I never found tubes that were 'painted' black on the inside to be entirely trustworthy.

Recently I've seen some non-commercial amp builders using "pull-down" resistors on the screen grids. 220K 2W to ground from the SG, and 1k to the second hi voltage node, for instance. What do you think of this?

It's not a bad idea. Depends on what you're going for. I just haven't wanted to expend any of the finite supply of B+ current in quite that way. Ohms law works to our advantage either way. Just know that there's nothing magical about the number '470'.