Wawasee JB150 issues "low bias"

[Shockwave]

You're making progress however, don't stop here or the tube won't like it. The -35 volts is where you want the 8417 bias in transmit. The -24 in receive is actually biasing the tube harder in RX. Bypass the existing bias contacts on the relay and connect those wires so the tube always sees the -35 volts of bias. Use the vacant relay contacts to turn the 350 volt screen voltage on in TX only, as Nomad suggests. This time proven method works well on these amps.

The main difference between the 6L6 and the 8417, are the bias levels in grid driven operation. Where -31 volts puts a 6L6 in class AB1, it puts a 8417 in class C. It only takes -12 volts to hold an 8417 in class AB1. The 6L6 needs more negative bias to run in the same class of operation as the 8417 and is why it runs better in the newer modulator with -56 volts in TX. "Modulators" tend to run these tubes closer to class C, in order to exaggerate forward swing on AM. It's also why they should sound funky on SSB, when running as a class C modulator.

 

[Danzik]

You're making progress however, don't stop here or the tube won't like it. The -35 volts is where you want the 8417 bias in transmit. The -24 in receive is actually biasing the tube harder in RX. Bypass the existing bias contacts on the relay and connect those wires so the tube always sees the -35 volts of bias. Use the vacant relay contacts to turn the 350 volt screen voltage on in TX only, as Nomad suggests. This time proven method works well on these amps.

The main difference between the 6L6 and the 8417, are the bias levels in grid driven operation. Where -31 volts puts a 6L6 in class AB1, it puts a 8417 in class C. It only takes -12 volts to hold an 8417 in class AB1. The 6L6 needs more negative bias to run in the same class of operation as the 8417 and is why it runs better in the newer modulator with -56 volts in TX. "Modulators" tend to run these tubes closer to class C, in order to exaggerate forward swing on AM. It's also why they should sound funky on SSB, when running as a class C modulator.

Yes, thank you Shockwave yeah I think I will go with the fixed -36 bias and put that screen voltage on the center tap of the relay, thank you !

 

[Shockwave]

We used the center pole of the relay to interrupt the screen-voltage supply to pin 4 on the big tube in receive mode. The setups that switch the grid bias higher for receive, lower for transmit just don't seem to be as stable.

At least shutting down the screen grid does a decisive job of turning off the tube.

73

Nomad, after reading your posts for many years it dawned on me that you are are entitled to a big "Thank You" from this forum. I've seen few people put as much effort in as yourself, to answer other people's questions. You do it consistently and provide the correct answers. I'm pretty sure we can all thank you for that.

 

[Danzik]

Nomad, after reading your posts for many years it dawned on me that you are are entitled to a big "Thank You" from this forum. I've seen few people put as much effort in as yourself, to answer other people's questions. You do it consistently and provide the correct answers. I'm pretty sure we can all thank you for that.

You got that right Shockwave! Thank you Nomadradio!!!! For all you do here !

 

[Danzik]

Nomad, after reading your posts for many years it dawned on me that you are are entitled to a big "Thank You" from this forum. I've seen few people put as much effort in as yourself, to answer other people's questions. You do it consistently and provide the correct answers. I'm pretty sure we can all thank you for that.

Not to mention his great aftermarket boards and products he puts out there for guys like me !!!! I am a big Tram fan and those BA boards he has are the tits ! I run those in every Tram D201 that I have ! The keying relay board is another great addition amongst others

 

[Danzik]

Hey, I can't thank both you fellas near enough! I wired her up with a fixed bias and ram the screen grid through the center of the relay and now I can even run a 6L6 which is exactly what I wanted to be able to do switch back and forth between tubes for driver use, working perfectly