UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post

cathoderay57 · 11th Apr 2021, 4:27 pm

I'm dubious about replacing the ECC85 with an ECC88. The latter valve has a max spec anode voltage of only 90v whereas the former goes up to 250v. Yes, the difference in gain isn't surprising since the latter has a gm of 12.5mA/v whereas the former is only 5.9mA/v. I would be concerned on 2 counts: the higher anode voltage would require alteration of the HT line dropper resistor(s) and, whilst the increase in gain could be seen as beneficial, it could also lead to instability. Jerry

11th Apr 2021, 4:27 pm	#6
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,384	Re: Bush VHF61 FM band dead. I'm dubious about replacing the ECC85 with an ECC88. The latter valve has a max spec anode voltage of only 90v whereas the former goes up to 250v. Yes, the difference in gain isn't surprising since the latter has a gm of 12.5mA/v whereas the former is only 5.9mA/v. I would be concerned on 2 counts: the higher anode voltage would require alteration of the HT line dropper resistor(s) and, whilst the increase in gain could be seen as beneficial, it could also lead to instability. Jerry