Just when you thought you had it sussed.....
The amp is mostly together so I thought I'd power it up. I've started with the input and phase splitter. I've changed the input stage slightly. Am still using an ECC83 but instead of HT @ 250v and anode R of 82k I now have it at HT400v, anode R is 220k, but cathode R is still 1k2 with a 100u cap across. this bit works well.
However the bl
dy phase splitter which was working fine in the "prototype" is playing silly b
ggers. the CCS and circuit are here -
http://www.vintage-radio.net/forum/s...=134126&page=2 post 27.
The CCS supplies around 10mA to the tied cathodes which should sit at + 10v, it was at - 3v.
So did the usual, and triple checked everything and scoped the cathodes for HF oscillation. All connected as it should be, no oscillation. I noticed that increasing the value of the emitter R raises the cathode voltage. The emitter R, R5 on WD's schematic consists of a 330r fixed R and a 300r preset. I swapped this for a 1k pot, with it set to full resistance the cathodes went up to +2.8v.
But this means that cathode current is at about 0.0014mA. Looking at the load line for a 12AU7/ECC82
with a HT of 430v (350 + -80v) and anode R's of 36k the bias should be at - 15v. I can't figure this out, especially as the anodes are at around 74v. An anode voltage of 74v indicates a grid bias of over 0v according to the loadline.
Puzzled.
Andy.