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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

UHF tubes for audio?

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bst

bst

Member
Joined 2005
I've been accumulating tubes at an alarming rate -- according to wife, anyway. (Last count was somewhere north of 6000...) I've found it easy and inexpensive to work with 'oddball' heater voltages and compactrons. However, I haven't tried using any of the dozens of UHF triodes and pentodes in my collection.

I've read that these are unusable due to their tendency to oscillate at the drop of a hat. But I've also read some glowing reviews of commercial amps which use them in their designs. Can anyone shed some light on this seeming contradiction?
 
Miles Prower

Miles Prower

Member
Joined 2005
dolphin-hsl.com
bst said:
However, I haven't tried using any of the dozens of UHF triodes and pentodes in my collection. I've read that these are unusable due to their tendency to oscillate at the drop of a hat. But I've also read some glowing reviews of commercial amps which use them in their designs. Can anyone shed some light on this seeming contradiction?
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There is nothing to this at all. Remember, VTs aren't like solid state, particularly BJTs, which are at their best at DC, and it's downhill from there. Any VT, even if it's strictly an "audio" type will operate into the bottom of the VHF band, at least.

As for RF oscillation when using VHF/UHF types, it's no worse than any other type. Just pay attention to layout, and include grid stoppers as needed. The main danger for RF oscillation is high gain. Thus, pentodes and cascodes require a bit more attention to detail. Install your bypass capacitors across the socket with the outside foil grounded so that it does double duty as an electrostatic shield. If you have sockets with a central metal pin, be sure to ground it as well. It's included as a shield. Keep your wiring as short and direct as possible. Use the same type of construction techniques that would use if wiring an actual RF circuit. I did a design that used cascoded 6BQ7As (a type good to 300MHz) and had no RF problems that a grid stopper couldn't fix.

If you can make a stable transistor circuit, a stable VT circuit is easy.

Some of these UHF types, particularly the 6GK5, have pretty good audio linearity, and make some excellent voltage amplifiers, either SE or LTP. The ones I used, 6BQ7A, doesn't list any possibility for audio in the spec sheet, but they do work just great as cascoded LTP splitter/amplifiers. Take a look at the plate characteristics, draw some loadlines, and see what you get. Don't discount those TeeVee T00bz automatically.

Let the audiophoolz pay audiophool ridiculous prices for magical tubes. Remember:


As one tube becomes popular, i.e. expensive, we'll use something else. Because we can.

-- Fred Nachbaur
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