Traynor YBA-3C bias adjust

[Bub]

Hi Gentlemen,

This is amp #3 for me so Im still on a major learning curve.
YBA-3C bias adjust on the schematic has a note to set the voltage drop across the screen resistor.
I have 555V on the plates with a new quad of el34s
When I set the voltage drop to 8V, this is what I believe the schematic says I have 11mA.
When I set the bias to 30mA I have a 26V drop.
Why would the bias be set to the screen resistor voltage drop?
Thanks for any help.

Rob

[Bub]

Sorry Gentlemen, Ive tried to resize the schematic but with no luck.
Its on the Free Information Society site as the schematic on this site doesnt show the notation.
Thanks again

Rob

[sluckey]

When I set the voltage drop to 8V, this is what I believe the schematic says I have 11mA.
When I set the bias to 30mA I have a 26V drop.

8V across a 1K resistor is 8ma. That's 2ma screen current per tube. Sounds kinda cold to me.
26V across a 1K resistor is 26ma. That's 6.5ma screen current per tube.

Why would the bias be set to the screen resistor voltage drop?

Screen current is a good indicator of proper tube operation too. The screen resistor was just a convenient way to get an indicator of tube current. I prefer measuring cathode current via 1Ω resistors in the cathode circuit for safety reasons and because I can calculate static plate dissipation too. We've become accustomed to wanting to know static dissipation mainly because we like to fiddle things a lot. Back in the day, most manufacturers were more concerned with a bias adjust procedure that was straightforward, easy, and repeatable and gave good, safe tube operation, AND was cost effective without adding additional components/cost.

Or they may have just had a whacky engineer!   

[Bub]

Thanks again sluckey for an explanation. I didnt know if it had anything to do with the way Traynor tied the suppressor grid to raw bias.
Ill stay with 30mA.

Rob
 

[PRR]

> Ive tried to resize the schematic but with no luck.
Its on the Free Information Society site....

Don't re-size a readily available schematic but DO give us a link so's we don't have to hunt to help you. 

http://www.freeinfosociety.com/electronics/schematics/audio/traynor_customspecial_yba3c.pdf

> set the voltage drop across the screen resistor

That's a bit short-sighted. Traynor didn't give a handy way to measure cathode current. There is a place to (dangerously) read screen current. Screen current is a *rough* fraction of cathode current. Therefore if you know screen current, you know cathode current *roughly*.

But the screen/cathode current-ratio is different for every batch of tubes, and many tube-types made today will be significantly different from the ones Yorkville got in 1972.

Use a bias-checker or add 1-ohm cathode resistors, read the actual cathode current.

It is not impossible that Traynor shipped these amps at around 10mA cathode current per tube. My little BassMate seemed to be biased 9mA per tube. This gives cooler operation and a different sound than the hot/rich amps of a decade before. But I think they went too far. And very-cold bias did not become popular.

30mA cathode current per tube is reasonable for this HIGH plate voltage. Do not go over 45mA; 40mA is probably the richest you should set.

[Bub]

Thanks for the information PRR.
I did use a bias-checker to set the tubes. I just couldnt figure out why Traynor would set the bias for four tubes across one screen resistor at that voltage drop.
On the list of things to learn tomorrow is how to post a link.

Thanks

Rob

[PRR]

> I just couldnt figure out why Traynor would set the bias for four tubes across one screen resistor at that voltage drop.

I think it was just a hasty hack.

We never used to think about "bias". But as tube production and quality declined, Traynor may have had problems with some batches running too hot (or cold). Reading across the screen resistor gives a *rough* idea if you are in a ballpark. I agree that reading the sum of four tubes via an indirect method is pretty hit/miss.

> how to post a link.

Open the web page you want to point to. Put your mouse over the Address Bar at the top of the broswer (where it says "http:....gibberish"). Right-click or Long-Click, a drop-menu shows "Copy", click that. Now switch back to your forum compose window, Right/Long-Click, "Paste".

There's other/better ways. Especially for very long links, but this is the basic.