Self Biasing ST35?

[DortoH]

Is it possible to convert baby blue's circuit to a self biasing circuit?

[Shannon Parks]

Is it possible to convert baby blue's circuit to a self biasing circuit?

Technically speaking, it already is self biasing. But it has a limited adjustment range for tweaking the cathode resistance. I'm not sure why I made the circuit like I did other than I remember changing cathode resistor values all the time in my old point-to-point proto ST35 and thought adjustment would be handy.

A person could ditch all the circuitry there (the three resistors and the pot) and just replace it all with a 390 or 430 ohm, 1W resistor from cathode to ground.

Or the cheapest method would be to use one 200 ohm, 2W resistor from cathode to ground and one cap. Then tie a jumper wire from the other push pull pair's cathode to that cathode. Just use matched pairs of tubes. Two components replace ten.

Shannon

[erichayes]

Hi All,

Speaking of cathode resistors...

I got a new Gibson GA-15 in the shop under warranty a couple of weeks ago with one of the 6BQ5s intermittently going into superconduction. Gibson's running this thing about as class A as is possible, so it doesn't take much to push an output tube over the edge. I changed the coupling caps, which were of marginal voltage, but the amp still had the problem.

Cut to the chase: Gibson opted to use a common cathode resistor for the output tubes, to save a couple of pennies. They also opted to use those cheap-ass Chinese ceramic sockets we've all come to know and love. Turns out the "good" tube socket would intermittently lose contact with the cathode of the tube, thus causing the "bad" tube to receive twice as much cathode current as normal. I reworked both output tube sockets, retubed the amp and sent it on its way.

A couple of morals to the story: First, aside from the sonic superiority of using separate cathode resistors on the output tubes, the chance of blitzing an output tube is eliminated if the socket fails on the cathode.

Second, don't trust cheap-ass Chinese ceramic sockets. If they can fail on the cathode pin, they can just as easily fail on the plate pin, causing the screen grid to become the de facto plate and causing a rather remarkable light show. Take a fine, sharp tool, such as a dental explorer, and gently crimp each pin contact so it's making a good connection with its corresponding tube pin.

[Shannon Parks]

Turns out the "good" tube socket would intermittently lose contact with the cathode of the tube, thus causing the "bad" tube to receive twice as much cathode current as normal. I reworked both output tube sockets, retubed the amp and sent it on its way.

Good point, Eric. I've seen this, too. The separate cathode resistors do make trouble-shooting much easier and might save a tube.

Second, don't trust cheap-ass Chinese ceramic sockets. If they can fail on the cathode pin, they can just as easily fail on the plate pin, causing the screen grid to become the de facto plate and causing a rather remarkable light show. Take a fine, sharp tool, such as a dental explorer, and gently crimp each pin contact so it's making a good connection with its corresponding tube pin.

Saw this exact problem with Procrip's amp a while back. I couldn't hit the off switch fast enough and sent an EL84 to tube heaven.

BTW, Thermion has shown me a different style Chinese ceramic PCB style socket which he bought from Handmade (2-011,G):
http://www.hndme.com/storesocketsceramic.html

It seems to be a much better design than the standard two-piece ones. I might stock up on these.

Shannon