If the grid resistor connected there looks OK it is probably just fine. But the cap has to go. Sounds like you are getting the bugs out. Cheers.
One other point... Are you using a high voltage filter cap for the 33uF B+ ? The diagram shows a 400v cap with 442v applied. Don't know how that would have been missed for as long as this circuit has been passed around. Should be 500v cap to handle the cold start voltage that is higher than the working voltage. Coupling cap too.
Yes i did notice that, my cap is F&T 500v with a peak rating of 550v. I'm using a 5u4gb rectifier so I get a lower B+ of 415v and I've also placed a 16ohm thermister in the transformer primary. So that should be OK.
My coupling cap is 400v though.
My coupling cap is 400v though.
Why not indeed, but as stated I managed to source 39k 9w ones so went with them.
Last item for consideration is the coupling cap and whether 400v is adequate?
IMHO even a 250V would squeak by in this amp. 400V is fine.
One thing I wanted to add, I built this amp and in playing around with it, I found it sounded better with 300K used in the feedback loop and in triode mode sounded better at 750K or just removing it. Experiment around as this value has a large impact on the sound.
I saw a question about the grid leak resistor value? The datasheet shows a max of 1M, I'd go with 500K just to be safe, if you need to raise it over what a 100K pot would be.
https://frank.pocnet.net/sheets/010/e/ECC85.pdf
https://frank.pocnet.net/sheets/010/e/ECC85.pdf