S.E.X. mono 6DN7(or 6SN7GTA)/6EM7, help requested

[keto]

Hey all, I've converted a pair of Paramount 12AT7 C4S-SR-C4S boards for a similar function in the S.E.X. mono circuit. Everything seemed okay on paper, but voltage readings were topsy-turvy upon briefly firing up. 400vdc after the CLC psu filter, 220vdc on the signal and SR plates and 350vdc on both cathodes. Yikes. Here's a pic of one board and the proposed connections (same that yielded those voltages). I see three possibilities: 1) I prepared the board incorrectly 2) I smoked some part or trace in re-purposing them and/or 3) the 12AT7 board is not the right vehicle for this circuit. Any suggestions? Thanks!

[Paul Birkeland]

No picture comes through.

Could you post a schematic of what you were attempting to build?

If you took the Paramount 12AT7 boards, changed no parts, then tried to run them in the SEX amp, then yes, you will run into some issues.

[keto]

Thanks for the response, Paul. I rebuilt the 12AT7 boards as a variation to the SR45 board. Just a few differences: signal triode biased to -6Vg and drawing 4.2mA (this part was working well in its C4S-only iteration), SR triode should draw about 15mA and the output (6EM7 triode #2) should draw about 34mA. My idea was to take the C4S-only "6DN7 or 6SN7GTA" driver and add shunt-regulation via the other triode (#2). For trouble-shooting the boards I built, I suppose I could disconnect the signal cathode and plate, and see if the SR and output and signal-LED's are working. Let me try that....

[keto]

Okay, I got it! I was missing a 2nd ground connection. This is only 6DN7+6EM7. I'd hate to overheat a nice old 6SN7GTA on start-up 

[keto]

I've adjusted a few values and ended up pretty close to the desired operating points.

The input, triode #1 of the 6DN7 is 188Vp-k, -6.3Vg and draws about 4.5mA

The shunt-regulating triode #2 of the 6DN7 is 270Vp, and 13Vk and draws about 20mA (a little hot).

The output triode #2 of the 6EM7 is 246Vp, -43Vg and draws about 30mA (a little cool).

Since Rbias is now 680 ohms, I figure going to 630 ohms will increase the output's draw from 30mA to 34mA and decrease the shunt-regulator's draw from 20mA to 16mA.

Overall current draw is about 63mA.