UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Quad 22 KT66 to 6L6 with Cathode resistor increase?

Valvepower · 16th Sep 2022, 9:45 am

Hello All,

This old chestnut…

I’m looking to do some valve swapping to release some GEC KT66’s from a pair of down at heel cosmetically challenged 22 power amplifiers, so I’m thinking of fitting the Quad 22 power amplifiers with 6L6GC’s, which I have in the toy box. The older red information booklet states the KT66 and the 6L6G are interchangeable, which make sense given the nature of the KT66 and 6L6, however there is no mention of changing anything other than the valves.

However, I have some old information from a reputable engineer saying when using 6L6GC increase the value of the cathode bias resistor from 180Ω to 220Ω. Alright it's not a massive increase, but enough to question the reason.

I just wanted to ascertain the consensus as regards the substitution of KT66’s with 6L6GC along with increasing the value of cathode bias resistor in Quad 22 power amplifiers.

I may be wrong, but I feel Quad (now IAG) now fit 6L6GC or similar marked as KT66 when they repair and restore Quad 22 power amplifiers, however..., if they do fit 6L6GC’s, do they change the value of the cathode bias resistor?

I’ll not be changing the values of anything else..., however I’m guilty as charged, M’lud for doing unspeakable things (in the eyes of the purest) to Quad 22 power amplifiers back in the 70’s, but back then old Quad 22 systems were well out of favour and could be bought at jumble sales (yeah, got my first Quad system at a jumble sale!) or the radio TV and record player adverts in the local papers etc., for 20 quid or so.

Regards
Terry

16th Sep 2022, 9:45 am	#1
Valvepower Octode Join Date: Aug 2008 Location: Rayleigh near Southend-On-Sea, Essex, UK. Posts: 1,880	Quad 22 KT66 to 6L6 with Cathode resistor increase? Hello All, This old chestnut… I’m looking to do some valve swapping to release some GEC KT66’s from a pair of down at heel cosmetically challenged 22 power amplifiers, so I’m thinking of fitting the Quad 22 power amplifiers with 6L6GC’s, which I have in the toy box. The older red information booklet states the KT66 and the 6L6G are interchangeable, which make sense given the nature of the KT66 and 6L6, however there is no mention of changing anything other than the valves. However, I have some old information from a reputable engineer saying when using 6L6GC increase the value of the cathode bias resistor from 180Ω to 220Ω. Alright it's not a massive increase, but enough to question the reason. I just wanted to ascertain the consensus as regards the substitution of KT66’s with 6L6GC along with increasing the value of cathode bias resistor in Quad 22 power amplifiers. I may be wrong, but I feel Quad (now IAG) now fit 6L6GC or similar marked as KT66 when they repair and restore Quad 22 power amplifiers, however..., if they do fit 6L6GC’s, do they change the value of the cathode bias resistor? I’ll not be changing the values of anything else..., however I’m guilty as charged, M’lud for doing unspeakable things (in the eyes of the purest) to Quad 22 power amplifiers back in the 70’s, but back then old Quad 22 systems were well out of favour and could be bought at jumble sales (yeah, got my first Quad system at a jumble sale!) or the radio TV and record player adverts in the local papers etc., for 20 quid or so. Regards Terry