[Diabolical Artificer]

I tested everything yesterday and all worked well for a while. Previous to that I'd noticed the cathode follower resistors made up of matched 10k and 2k2 5w wire wound, were getting very hot, I could smell scorching. I measured the current and found they were running at 20mA, which is a bit too much. I'd also noticed that the cathode followers wern't biased quite right. They should have been at -10v but were at -3v.

I replaced the 2k2 R's with 5k6 which reduced dissipation to 15mA, the resistors still get warm, but not overly so. After some thought I realised I could set the bias with the adjustable CCS of the phase splitter. This meant moving the bias point of the ECC82 triodes a bit, but found with the ECC82 PS set to -6.6v ( they were running at -11v, center biased, but not running in the linear region), the cathode followers were now set to -10v.

So as mentioned at the start all was going swimmingly, I was testing and setting the bias of the OP stage. This is set by a dual ganged linear 10k pot which balances the two sides of the OP stage. I noticed that it wasn't working as well as it did on the "prototype", and after a while prior to switching off I unplugged the lead from the sig gen. A loud screech issued from somewhere, I hit the off Sw.

I noticed I had nasty oscillation on the OP and after some investigation I found the screech was coming from the OPT. I tested the OPT for shorts and whilst doing so I found the tag of the pin on the valve base that the screen tap of the OPT connects to, was broken. This accounts for the difficulty of balancing the OP stage. These are decent NOS Cinch octal bases so not rubbish, but I did think when i got them that the tags looked a bit thin, and I had to get off some scale off each tag with a file prior to soldering. This must have put undo pressure on the tag. I started to unsolder every tag, then realised I had a spare base, so nicked a pin/tag off it to replace the broken one.

The only problem with doing a good job of soldering leads to valve base tags, IE making a good mechanical joint by wrapping a few turns and cinching with pliers, is that it's a ****** to unsolder without wrecking the hookup wire. Anyhoo, did that, screech still there, OPT not shorted. So I took out the input, PS and cathode follower, screech gone.

So something is broken/amiss with one of these three stages. Today if i get time I'll see if I can get to the bottom of the problem. At least the OPT seems undamaged, thank the gods. It seems odd that disconnecting the sig gen should cause such a catostrophic failure. I've unplugged/turned off the sig gen gen loads of times before, without issue. However I was using a different sig gen yesterday.

Two steps forward, one back. this is a bit like playing snakes and ladders, A.