Bias question

[msclford]

I am getting ready to modify my Peavey Valveking for adjustable bias, but I have a couple of questions first.

1 - These amps have their patented Texture circuit, which is supposed to go from Class A to Class AB (or Class A to Class AB sounding). Will this affect how I should set my bias (will it run the tubes hotter in Class A mode)? What do I need to take into account?

2 - I have JJ 6L6GC's in there. The amp calls for 6L6GC's. I have seen that the rating for 6L6 tubes is 23 or 25 watts, while the rating for 6L6GC's (according to Eurotubes and the Tube Store) is 30 watts. Should I go by the 30 watt dissipation rating?

[msclford]

Update: I bought a bias probe from Eurotubes, and when I plug the black lead into the COM input and red lead into mA input of my multimeter, I get no reading when the multimeter is set to 200 mA. However, when I set the multimeter to 600V, I get a reading of around 440. Any ideas why I'm not getting a reading for mA? It's supposed to measure in mA. It also appears that the red lead goes to pin 4 or 5 on one side of the probe, while the black lead goes to pin 4 or 5 on the other side of the probe.

[Enzo]

The texture control affects the SOUND of the amp, not its class of operation. It has zero effect on the bias.

Most everything you find in the market is a 6L6GC. An actual 6L6 is a metal tube, not glass. Poorly suited to audio amps, these are more collector items than viable tubes. 6L6G was the glass version, and you won't see those much either.

I think you are not using the bias probe correctly, call Bob and explain your dificulty.

[msclford]

Yeah, I was going to call Bob today. Thanks, Enzo.

[MWJB]

Sure you're not supposed to measure in "mV" rather than mA? Many probes house a 1ohm cathode resistor, mV converts near enough to mA in this case.

Blown mA fuse in your meter (my favourite trick).

[JC@]

Blown mA fuse in your meter (my favourite trick).

That was my favourite trick also, untill I bought a Fluke and blew the fu$e. From then on I NEVER use my fluke for measuring current. Never

[msclford]

So I did find that the fuse was the problem - I must have adjusted the setting on the meter while it was plugged in. You would think that I would be biased and happy now, right?

I got the bias trimpot installed, adjusted it, and was getting ready to put everything away when I broke the key on one of my 6l6's when putting it back in. I ordered a new set of tubes, installed them today, and found that my amp is now very quiet (like I can dime it without even getting loud) - the tubes aren't glowing abnormally. It also makes a constant tapping or popping sound from the speaker, about every second or 1/2 second. It was working fine before I put in the new tubes (or before I broke the old ones). Any ideas?

I don't really have a spare set of tubes to stick in, since I broke the old ones BTW, are these a good idea? http://www.tubeampparts.com/Merchant...roduct_Count=8

[MWJB]

What current do you have now?

Have you eliminated the preamp tubes and trim pot installation as the cause of the popping?

If the new 6L6s are noisy the vendor will replace them.

[msclford]

No, I figured out what the problem was. I added a presence control, and a wire from that was shorting against a lug of that pot. Easy enough! I biased the tubes at 50 mA each. Can you believe that the factory had them biased at 18 mA!!! Crazy.

[MWJB]

Yes, I can. They'll last for years at 18mA but probably never sound their best, however it is unlkely that anyone at the factory specifically checked the bias current anyway.

Your amp should sound fine at 30-40mA. No reason not to run the JJs at 50mA but I'd keep an eye on the heat generated if you have 6L6 sockets that are fitted directly to a PCB (no flying wires/stand-offs between socket & PCB).

[Garydean]

MSCLFORD,
Those keyway adaptors work great. I've used them a few times. Tubes and More sells a pack of 4 for $10.

Gary