dbw said:
=CB= said:
No... build yer own bias probe for a few bucks, and learn to bias it yourself! cost... under $15, if you already have your own multimeter.
Do you have a good link that explains how to do this?
Its pretty simple really. Get an octal base from www.tubesandmore.com (Antique Electronics). Get a socket from them too. The Russian ceramic sockets seems to fit the bases fairly well. Take some 22gauge bare wire about three inches long and solder it to all the pins on the socket, except pin 8 - IMPORTANT - loop the wire thru the hole in the socket pin (tab really...). Make two holes in the octal base, close to pin 8. Pass a test lead wire thru the hole closest to the bottom of the base, and solder it into pin 8 on the base. Pass another test lead wire thru the hole closest to the socket, and solder that wire to the pin 8 tab on the socket. Now take those 22 gauge wires and pass them through the holes in the pins of the base, keeping them in order! (HINT: staggering the lengths slightly makes this go REALLY easy). Pull all the wires snug as you do this... 22gauge bare wires, and test lead wires should have the minimal "play" inside the unit. Solder all of the remaining wires to the pins - keeping them tight as you do. (HINT: work fast on each wire, so you dont undo the soldering on the socket side of the wire). Let it all cool. Now put a banana plug that fits your multimeter on each test lead wire. Many folks like to epoxy the unit at this point (pot it). You can use just about any consumer grade 2 part epoxy, but I suggest the model aeroplane 30 minute epoxy, as it has great temperature conformity and flows well. What you want to do, if you pot it, is to mask off all the pins on the socket, and press a hole in the masking tape in the middle of the socket - fill it thru that hole using a little paper funnel you can make yourself. Once its set - undo the masking and make sure there's no epoxy in the hole in the middle of the socket (if there is, just use a small screwdriver to scrape it out).
Its all done.
To use it, plug it into the amps socket and put the tube into the probe. Put one lead into the ground of your multimeter (common) - hopefully thats the lead that went to the base, not the socket. Put the other lead into the amerage (milliamps) input on your multimeter. Set the meter to read someplace in the 200ma range, max scale.
Turn on the amp and observe the meter, which is now reading all the current going through the tube. After some warmup, adjust the bias to read about 60 percent of max dissipation (you can figure this out by getting the plate voltage from the schematic). Barring that, set 6L6's to about 35-40 milliamps at idle - close enough for most practical uses.
Be sure to let it all cool before you take it apart. Note - extended playing with potting compound (epoxy) in the base is not suggested. Just get the job done, let it warm up about 5 minutes or so, set it and shut down.
And there ya have it.
If you IM me... I might have access to some under the radar items for ya <gg>, good will, and braggin' rights inclusive of course.
and suddenly something arced visibly in one of them... YIKES. So I immediately hit the power switch. Nothing seemed to be wrong with the amp, but I didn't know what was going on so I pulled the plug. The plates were red-hot but this is normal, right?
