DIY Audio General - stuff you're making, tips, advice sought, etc


So having swapped in some cheapo RFT 81s, the result is the same. So…


I literally now have no idea why the level is down, so the next step probably needs to be the cavalry who own the sort of scopes that can measure milivolt signals at non zero DC conditions where a Hantek does not cut it.


And the cheapo RFTs may have shown possibly what the issue is. The reason behind the value of R17 was to drop the voltage to the point where the regulators dropped c. 10-15V at idle given an ECC81 running at 170V supply/4ma. However, with an RFT one section wanted to run at 165V for 4ma and one wanted over 178V and the valves @chelseadave sent me were still not dropping anything significant over the regs at 200V supply given the required current.

One properly balance matched ECC81 should therefore be on the way from Watford Valves and a selection of 5W Kiwames from 2K up to 4.3K to try in R17 given the particular requirements of the individual valve that arrives. Hey ho…