Quad II, KT66 substitutes. - UK Vintage Radio Repair and Restoration Discussion Forum

flyingtech55 · 8th Nov 2006, 11:20 am

Hi all

Can anyone tell a suitable alternative for the KT66 output valves in the Quad II valve amp. KT66s seem to be pretty expensive or of dubious quality.

TimR

mickjjo · 8th Nov 2006, 11:29 am

I think the 6L6 is probably the cheapest satisfactory substitute :-

http://members.aol.com/pmobbs/Site2/...5___new_valves

Regards, Mick.

BassoonBloke · 8th Nov 2006, 12:21 pm

Hi TimR,

Just as a mild warning, the Quad II's tend to be a bit of a KT66 equivalent eater !!

I have found that the Russian Sovtek 5881/6L6WGC's work well and have a fairly good life.

I can also reccomend the newly released copies of the original GEC KT66's but as you say these are quite expensive.
If you have'nt checked, have a look at the biasing resistors and caps in your amps and make sure they are in good condition, also the grid feed caps. This will ensure that you don't ruin your nice new valves when you fit them.

All the best,
Alan.

ppppenguin · 8th Nov 2006, 12:35 pm

I put EL34s in one of the pair of Quad II amps I had years ago. Had to strap down the suppressor grid and I may have tweaked the cathode resistors to give correct bias current though I don't remember having to do this. It was a long time ago. The amp worked perfectly. It even worked OK with a KT66 and an EL34. I admit that my ears are not super golden but I couldn't find any problems.

EL37 should also work well.

These amps are very non-critical of valve characteristics - a testament to Peter Walker's excellent design.

Don't ignore Alan's comment about checking the coupling caps and bias.

sparkymike · 8th May 2007, 7:15 am

Hi. I have used 6L6GC valves in place of the KT66's with no ill effect. The best 6L6GC's are the RCA version but are like hens teeth and probably as rare as the KT66's The RCA's are often to be seen on US ebay hoiwever.
Mike.

Studio263 · 11th May 2007, 1:23 pm

Be a bit careful whatever you use, the mains transformers in these are very marginal so make sure the valves you pick have the same (or lower) heater current as the originals. For valves produced recently, actually measure this with a meter as they don't all match the original specifications. A dodge you can use is to disconnect the GZ34 heater and wire in a couple of 1N4007 diodes (with resistors) underneath, this frees up a bit of current for the other valve heaters. Not really recommended though...

I'm sure you don't need me to tell you that the cathode resistor is under-rated too, make sure it's not changed value. Not a great design really.

8th Nov 2006, 11:20 am	#1
flyingtech55 Octode Join Date: Nov 2005 Location: Ayrshire, UK. Posts: 1,096	Quad II, KT66 substitutes. Hi all Can anyone tell a suitable alternative for the KT66 output valves in the Quad II valve amp. KT66s seem to be pretty expensive or of dubious quality. TimR Last edited by Darren-UK; 24th Oct 2007 at 3:16 pm. Reason: Error in Quad model number corrected.

8th Nov 2006, 11:29 am	#2
mickjjo Rest in Peace Join Date: Jul 2004 Location: Dartford, Kent, UK. Posts: 1,661	Re: KT66 substitute I think the 6L6 is probably the cheapest satisfactory substitute :- http://members.aol.com/pmobbs/Site2/...5___new_valves Regards, Mick.

8th Nov 2006, 12:21 pm	#3
BassoonBloke Pentode Join Date: Feb 2006 Location: East London, UK. Posts: 231	Re: KT66 substitute Hi TimR, Just as a mild warning, the Quad II's tend to be a bit of a KT66 equivalent eater !! I have found that the Russian Sovtek 5881/6L6WGC's work well and have a fairly good life. I can also reccomend the newly released copies of the original GEC KT66's but as you say these are quite expensive. If you have'nt checked, have a look at the biasing resistors and caps in your amps and make sure they are in good condition, also the grid feed caps. This will ensure that you don't ruin your nice new valves when you fit them. All the best, Alan.

8th Nov 2006, 12:35 pm	#4
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: KT66 substitute I put EL34s in one of the pair of Quad II amps I had years ago. Had to strap down the suppressor grid and I may have tweaked the cathode resistors to give correct bias current though I don't remember having to do this. It was a long time ago. The amp worked perfectly. It even worked OK with a KT66 and an EL34. I admit that my ears are not super golden but I couldn't find any problems. EL37 should also work well. These amps are very non-critical of valve characteristics - a testament to Peter Walker's excellent design. Don't ignore Alan's comment about checking the coupling caps and bias.

8th May 2007, 7:15 am	#5
sparkymike Octode Join Date: Dec 2006 Location: Colchester, Essex, UK. Posts: 1,076	Re: KT66 substitute Hi. I have used 6L6GC valves in place of the KT66's with no ill effect. The best 6L6GC's are the RCA version but are like hens teeth and probably as rare as the KT66's The RCA's are often to be seen on US ebay hoiwever. Mike.

11th May 2007, 1:23 pm	#6
Studio263 Octode Join Date: Apr 2007 Location: Hampshire, UK. Posts: 1,578	Re: KT66 substitute Be a bit careful whatever you use, the mains transformers in these are very marginal so make sure the valves you pick have the same (or lower) heater current as the originals. For valves produced recently, actually measure this with a meter as they don't all match the original specifications. A dodge you can use is to disconnect the GZ34 heater and wire in a couple of 1N4007 diodes (with resistors) underneath, this frees up a bit of current for the other valve heaters. Not really recommended though... I'm sure you don't need me to tell you that the cathode resistor is under-rated too, make sure it's not changed value. Not a great design really.