Tie both cathodes together and bias with one LED?

[nashvillebill]

I'm using a 6SL7 for my first two gain stages. If I go to a LED for the cathode biasing, can I just tie both cathodes together (like a 6SC7, only exernally connected) and use a single LED to set the bias voltage? As opposed to using a separate LED for each cathode.

Thanks!

[nashvillebill]

Well I tried it, and it seems to work. It now seems that I've got so much gain in these two stages that I really have to attenuate between the two stages to keep from overdriving.

[Tage]

Yeah it should keep the cathodes of both triodes at the same voltage, although how are they connected in the circuit? are they cascaded?

Glad it works though.

[catstrat]

Interesting. I saw a thread about using dioodes in place of cathode resistors and caps, or something like that. I can't find the thread now and i'm not sure what forum it was, and i'm not sure even if i remember it correctly. Would you be kind enough to give us some details? Thanks

[Steve Conner]

LEDs are so cheap, why in the world would you want to share one between two circuits?

[Tage]

Hey steve do you think if the stages are cascaded that out of phase signals would appear at the cathodes? maybe causing some swirl/occilation?

[nashvillebill]

LEDs are so cheap, why in the world would you want to share one between two circuits?

Because I'm cheap AND lazy!!

Actually it was just more convenient to try it with one LED. I've attached the schematic...

This build is an offshoot of an old Webcor tape recorder, which I gutted and saved the transformers and tubes. Originally it had a 6SC7 for V1, with grid leak bias. That tube was incredibly noisy, and I didn't have another one, so I rewired the socket for the 6SL7 and tried keeping the cathodes tied together again with grid leak bias. Wasn't real thrilled so I figured I'd try the LED bias. But this is a learning exercise for me, hence the various things I've tried.

I've scoped the amp out now and it's the output stage that is driven into clipping, not the preamp stages. After all, the output is a 6SN7 in push-pull!

[Chuck H]

Hey steve do you think if the stages are cascaded that out of phase signals would appear at the cathodes? maybe causing some swirl/occilation?

I'm no Steve, but... Since there's only a fraction of a volt in AC that isn't bypassed to ground I wouldn't expect much interaction other than a little NFB gain reduction. But that fraction could be more important if the two stages were of the same phase (PFB). Possibly causing instability.

[loudthud]

... I wouldn't expect much interaction other than a little NFB gain reductiion.

Actually, it's positive feedback

[Tage]

seems like an alright idea. although I gotta say I like tone shaping with R/C combos on cathodes. but good to know it works.

[Chuck H]

Actually, it's positive feedback

If the two stages are cascade one in series with the other it should be NFB.?.

[loudthud]

If the two stages are cascade one in series with the other it should be NFB.?.

That's true if the output is taken from the plate of the second stage, but not when it's taken from the cathode.

[Tage]

so if they are both inverting stages, and cascaded one to the other, there is a possibility of positive feedback causing occilation or instability?

[nashvillebill]

If there is indeed positive feedback, as loudthud's schematic indicates there should be...wouldn't the 6SC7 be problematic? It's cathodes are tied together internally and I saw nothing in the 6SC7 datasheets suggesting any instability issues to address.

[Chuck H]

If the two stages are in series the signal is flipped 180* from one triode to the next. So the cathodes should be out of phase with each other. And that should be negative feedback. Of course you could always omit the issue altogether by fully bypassing the diode. If that's even needed. I've read that it isn't but I also know that diodes drop about a half a volt. I think that could show up as head voltage on the cathodes. So just parallel a 47uf cap with the diode and any feedback, positive or negative, is eliminated.