Early Phase Linear 700 Biasing issues.... Or stuck in biasland....

[Mark Allen]

I finished my WOPL conversion a few months ago and its been working just fine except I still had the NTE- 129A transistors in the bias sense. I could only get the bias down to about 400 mv and everyone on here said...."No! You need 2N5088's in those spots so it'll adjust correctly." So today I switch them to 2N5088 Fairchilds and now the bias is even slightly higher at about 420+ mv turned all the way down. The bias still behaves much as it did when the NTE's were in and it settles in nicely after 30 min and doesn't drift. Brand new MJ21196 outputs in here and the drivers are RCA-410.

So whats up guys? I just completed a Phase Linear 400 (Anita) for a friend and it adjusted perfectly, i used he same 2N5088's in the 400...

Mark

 

[Gepetto]

I finished my WOPL conversion a few months ago and its been working just fine except I still had the NTE- 129A transistors in the bias sense. I could only get the bias down to about 400 mv and everyone on here said...."No! You need 2N5088's in those spots so it'll adjust correctly." So today I switch them to 2N5088 Fairchilds and now the bias is even slightly higher at about 420+ mv turned all the way down. The bias still behaves much as it did when the NTE's were in and it settles in nicely after 30 min and doesn't drift. Brand new MJ21196 outputs in here and the drivers are RCA-410.

So whats up guys? I just completed a Phase Linear 400 (Anita) for a friend and it adjusted perfectly, i used he same 2N5088's in the 400...

Mark

What control board is in your early PL700 Mark?

 

[Gibsonian]

Quoting Lee

Mark,

In discussing bias on my WOPL 700(1000) and I think board level was a C, Lee said this:

With that much bias if it runs hot go to a 15 ohm 1 watt 1% MO for R38,R39

I never did a thing but may have to someday. I checked it recently and I think around 0.420 was as low as it would go.

Lee and Joe can comment with actual knowledge now!

 

[Mark Allen]

White Oak has a modification for this issue to bring the bias adjust into the center of range if you are using their driver board. Mine is a Rev E Board. It involves changing out one resistor and changing a diode in the boas string to a Schottky type diode. I did it to mine and it's fine now. A PL-400 I just restored already had those parts in the kit. Its neither difficult nor expensive to do.

Mark

 

[grapplesaw]

White Oak has a modification for this issue to bring the bias adjust into the center of range if you are using their driver board. Mine is a Rev E Board. It involves changing out one resistor and changing a diode in the boas string to a Schottky type diode. I did it to mine and it's fine now. A PL-400 I just restored already had those parts in the kit. Its neither difficult nor expensive to do.

Mark

This news to me. Always addusted bias with ease in the stock d and e boards. Do tell more