Setting Express Bias

[RockNRollDr]

Well everybody I just finished my first build and I am very pleased with the results, no hum, no smoke, no pops, everything worked with a flip of the switch. I consider myself very blessed for this to happen and now I have one more task, setting the bias. I need to know if anybody has any tips for setting the bias the way Ken had it. I'm sure this topic has been tossed around a few hundered times but hopefully someone can shed some light on this for me. I read something Ken wrote one time saying that only some experienced technicians can set the bias correctly and that a novice has a 1 in a million chance of getting it right. I have a meter and have access to an oscilloscope so if someone can help me out I would be very greatful. Also I am using Svetlana EL34 tubes, if that helps you any. Thank you for any information you can give me.

[geetarpicker]

You already got my message, but others might want to know what I do though it isn't rocket science nor the only way at all. I did have Ken Fischer himself tell me anything that's not replating is fine health wise for the tubes and then you can literally go by ear from there...there really is no perfect setting as it is a matter of taste within a safe range that is.

That said, I've done this enough with my amps with my specific tubes of choice that I have a system that works at least with my original and two clones. All three of my amps use 6.6k OT primary and run about 415-420 B+ when the wall is at about 125v around here.

I built myself a bias checker tool. Basically a "plug in" unit that the tube then fits into. I've seen similar devices on ebay. The one I made breaks the ground connection out to two test leads with banana connectors. Then I hook my meter in series and power up and read the current directly. I usually test both tubes to see if they are close in current draw, and most GT EL34Ms "matched pairs" are within 2-5ma which isn't great but workable. Then I measure the higher drawing of the two tubes in a set and use that one tube as my first reference and set that one for a 45ma draw. Then if I check the other tube I'll usually get about a couple 3 lower ma draw. If the set is matched dead on I'd just set it for 45ma each then. This method seems to work well. It seems that if an Express is around 400-420 B+ then a bias of 40-45ma works well.

I can't say that I really can hear a difference in real slight changes of bias when I'm testing it. 5ma yes, 1ma no. However recently I thought my amp was breaking up to soon on the gig and when I checked it the bias had dropped to about 42ma. I put it back to 45ma and all is well...

[Firestorm]

I put it back to 45ma and all is well...

Interesting. At 415V, that's almost 75% of max dissipation at idle. And you could hear the difference between 42ma and 45ma. Wish I had ears like that ...

[geetarpicker]

Yes an Express can be biased so hot I guess it could almost be considered class A operation. I would assume the moderately low B+ is the main reason this is possible. I've heard Allyn likes to run them even higher, up to 48ma?

[57goldtop]

All three of my amps use 6.6k OT primary and run about 415-420 B+ when the wall is at about 125v around here.

Before I blow mine up I'm just wondering what voltages can one expect to see on b+ when you're using 240v mains ?

[geetarpicker]

If the transformer was wound correctly you should see the same B+ voltages on the secondary side as we do here in the states. Low 400s

[Firestorm]

If the transformer was wound correctly you should see the same B+ voltages on the secondary side as we do here in the states. Low 400s

...meaning: you are using an international or "export" transformer or something like that, right?

[57goldtop]

The part # on mine is HTS-5199-EX , I suppose EX means export.. I don't really know. Sorry to hijack the thread, I guess we should get back on topic.

[paulster]

I put it back to 45ma and all is well...

Interesting. At 415V, that's almost 75% of max dissipation at idle. And you could hear the difference between 42ma and 45ma. Wish I had ears like that ...

Glen

Does your figure include the screen current (i.e. is it the exact cathode current), or have you subtracted the screen current first?

I have my cathode current at about 48.5mA but there's 5.5mA of screen current included in that, thereby bringing the figure for the plate current down to 43mA, which is around the 70% dissipation mark.

All three of my amps use 6.6k OT primary and run about 415-420 B+ when the wall is at about 125v around here.

Before I blow mine up I'm just wondering what voltages can one expect to see on b+ when you're using 240v mains ?

I get 405V on the plates with exactly 240V in, which correlates against Glen's 415-420V when putting 125V into a 120V primary.

[Firestorm]

I have my cathode current at about 48.5mA but there's 5.5mA of screen current included in that, thereby bringing the figure for the plate current down to 43mA, which is around the 70% dissipation mark.

Paulster --
You read my mind. I was about to ask if anyone had measured had measured their screen current. 5.5ma is higher than I would have expected, but suggests something interesting. At 405VDC on the plates, you should be seeing 399.5VDC on the screens at idle.
So has anybody measured the voltage drop across the 1K screen resistor with the amp wide open? Since the screen controls the operation of the tube (with the plate just going along for the ride, as it were) the reduction in screen voltage at performance levels should be a significant contributor to the "feel" of the amp.

[Lonely Raven]

You already got my message, but others might want to know what I do though it isn't rocket science nor the only way at all. I did have Ken Fischer himself tell me anything that's not replating is fine health wise for the tubes and then you can literally go by ear from there...there really is no perfect setting as it is a matter of taste within a safe range that is.

...

I'm going to go ahead and pat myself on the back for figuring that out myself.

[paulster]

At 405VDC on the plates, you should be seeing 399.5VDC on the screens at idle.

You would if the screens were also running of B+1, but they're running off the B+2 supply so they're lower. B+2 is about 388V in mine, so screens about 382V at idle.

[Firestorm]

You would if the screens were also running of B+1, but they're running off the B+2 supply so they're lower. B+2 is about 388V in mine, so screens about 382V at idle.

Ahh, yes. Duh! [Slaps head].