[telstar]

Originally Posted by Leon Crampin

Mike, I wouldn't recommend making this change. The '81 lacks the internal screen between triode sections which is fitted to the '88, has a lower gm and differing internal capacitances. This will play havoc with a neutralised stage and the performance (which is good on FM, less so on AM) will suffer.

ECC85s are not impossible to source, but a possible alternative is the ECC88 which is pin-compatible.

At risk of stating the obvious, make sure the string drive to the FM permeability tuning is functional...

A Hunts eviction cruisade is usually necessary on these sets, switch slider contacts can give trouble and, unfortunately, leakage from contacts carrying HT to other switch contacts via the very close spacing can cause crackles and odd results. For some reason, the ECH81 seems to be prone to poor operation due to changed resistor values around that stage.

Best of luck - make a few measurements on FM and report back. Can you use a signal generator to check the gain at various points?

Leon.

Thanks Leon, I will check that out.
Cheers
Mike