UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post

Diabolical Artificer · 22nd Apr 2017, 2:05 pm

You've got me confused there David, an ECC82 is a triode and has no screen grid. It was the tied cathodes of an ECC82 LTP phase splitter fed by a CCS that was sitting at the wrong bias voltage.

I found the cause of the problem, but still have no idea why the "fault" occurs. I've attached the schematic so we know what's what. Sorry, forgot to label the other valves, but the ECC82 should be obvious, the cathode follower are 12BH7's.

So why with no 12BH7 inserted in the circuit does the ECC82 PS cathodes stay at -3v, but with the 12BH7 in does the cathode tail voltage rise to +10v?. Could it be that because the cathode followers are direct coupled that their grids are pulling more current from the PS anodes, therefore raising PS anode current and thus cathode current?

Andy.

22nd Apr 2017, 2:05 pm	#169
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,662	Re: 807 (maybe) amplifier build. Now EL34 You've got me confused there David, an ECC82 is a triode and has no screen grid. It was the tied cathodes of an ECC82 LTP phase splitter fed by a CCS that was sitting at the wrong bias voltage. I found the cause of the problem, but still have no idea why the "fault" occurs. I've attached the schematic so we know what's what. Sorry, forgot to label the other valves, but the ECC82 should be obvious, the cathode follower are 12BH7's. So why with no 12BH7 inserted in the circuit does the ECC82 PS cathodes stay at -3v, but with the 12BH7 in does the cathode tail voltage rise to +10v?. Could it be that because the cathode followers are direct coupled that their grids are pulling more current from the PS anodes, therefore raising PS anode current and thus cathode current? Andy. Attached Thumbnails __________________ Curiosity hasn't killed this cat...so far. Last edited by Diabolical Artificer; 22nd Apr 2017 at 2:10 pm.