I need to get a pair of Quad II working for a friend. He has 3 working old KT66 but doesn't want to pay silly money for a fourth.
What other output tubes do you suggest? I have 5881 and EL39 for example - may also have some EL34. EL39 have top caps, but that's OK - nice tube!
What other output tubes do you suggest? I have 5881 and EL39 for example - may also have some EL34. EL39 have top caps, but that's OK - nice tube!
Andy,
The KT66 is a "glorified" 6L6. An obvious replacement is the SED (=C=) 6L6GC. Tweak the amps' voicing by "rolling" small signal tubes.
The KT66 is a "glorified" 6L6. An obvious replacement is the SED (=C=) 6L6GC. Tweak the amps' voicing by "rolling" small signal tubes.
I'll have to look at the current crop of KT66.
Meanwhile, as to substitutes:
Quad circuit says
anode 340v 65mA 22.1W
screen 330v 7mA 2.3W
cathode 26v resitor 180 (shared)
Is 5881 a bit wimpy? Data states 23W anode max and 3W screen
That would put it on max? Max anode and screen voltage is covered at 400v
I actually have some 6550 just remembered. That shuld be OK. Except that would add 1.2 amps to heaters. Probably OK if preamp not used. I'm open to correction on this.
EL39 should be interesting - trying to find details.
Meanwhile, as to substitutes:
Quad circuit says
anode 340v 65mA 22.1W
screen 330v 7mA 2.3W
cathode 26v resitor 180 (shared)
Is 5881 a bit wimpy? Data states 23W anode max and 3W screen
That would put it on max? Max anode and screen voltage is covered at 400v
I actually have some 6550 just remembered. That shuld be OK. Except that would add 1.2 amps to heaters. Probably OK if preamp not used. I'm open to correction on this.
EL39 should be interesting - trying to find details.
EL39 is an odd-duck tube - I've seen EL38s labeled as 'EL38/EL39'.
you may want to check out the EL38, though it is a true pentode and requires a plate cap.
you may want to check out the EL38, though it is a true pentode and requires a plate cap.
Rewire the socket for 6BG6GA. The later Sylvania ones have large 30 watt anodes just like 6L6GC/7027A/7581A. Heater current is only 0.9 amp, which your power transformer will appreciate. These come up on that auction place regularly and sell for about 4 USD each.
Wade
Wade
My pal 7N7 reckons put in a 5881 and it should bias up fine. I'll have a look!
Still curious about the EL39, except looks like screen max is 325v - needs to get to 330v. Not a huge difference. Not sure if it's one of the 18 watters or 25. I have the ST shape ones with black glass
Still curious about the EL39, except looks like screen max is 325v - needs to get to 330v. Not a huge difference. Not sure if it's one of the 18 watters or 25. I have the ST shape ones with black glass
tubewade said:
Wade....
That is the best and most usefull idea I heard in a long time, regarding QUAD II tube substitution....
I like the part about the reduced heater current.....
I will look into this...
Chris
Rewire the socket for 6BG6GA>>
Data says 20W dissipation, so it's a case of distinguishing which are which, if later ones were 30W
Data says 20W dissipation, so it's a case of distinguishing which are which, if later ones were 30W
Yes, you will have to distinguish between the 20 watt and 30 watt anodes. It's easy to tell if you can see the valves, like comparing 6L6G or 807 to 6L6GC. The socket adapter is also a great idea if the additional height is not a problem. The same gent selling the adapter also sells the 30 watt valves for 8 dollars each. A tremendous value, I'd say.
Wade
Wade
Few points:
As Eli said, there is less than 10% difference between 6L6 and KT66, with heater current advantage. (This can be less than the spread in some KT66s!)
Here in RSA Russian valves are cheapest, but they can be free with numbering. E.g. the innards of Sovtek KT66s are exactly 6L6 in construction and ratings (including 0,9A heater), but they cost about twice as much! Avoid any "Coke-bottle" shapes; the glass comes too close to the transformers and will heat them up. I have used tens of 6L6EHs in refurbished Quad IIs with great success.
The 5881 should be OK, as the dissipation in the Quad is only 20,5W at normal mains. Unless money is a major factor, I am not sure that I would go to conversion, whether by rewiring or using an adaptor.
Then one must be careful of the power transformer rating. KT88 and EL34 is out for that reason, unless one lives in Canada or Greenland or similar. EL34 is also out because it is quite different from the 6L6 type beam tube - square wave response with those in Quads shows overshoot.
When refurbishing there are a few other considerations (old resistors have drifted etc.), but I will suffice with this for now.
As Eli said, there is less than 10% difference between 6L6 and KT66, with heater current advantage. (This can be less than the spread in some KT66s!)
Here in RSA Russian valves are cheapest, but they can be free with numbering. E.g. the innards of Sovtek KT66s are exactly 6L6 in construction and ratings (including 0,9A heater), but they cost about twice as much! Avoid any "Coke-bottle" shapes; the glass comes too close to the transformers and will heat them up. I have used tens of 6L6EHs in refurbished Quad IIs with great success.
The 5881 should be OK, as the dissipation in the Quad is only 20,5W at normal mains. Unless money is a major factor, I am not sure that I would go to conversion, whether by rewiring or using an adaptor.
Then one must be careful of the power transformer rating. KT88 and EL34 is out for that reason, unless one lives in Canada or Greenland or similar. EL34 is also out because it is quite different from the 6L6 type beam tube - square wave response with those in Quads shows overshoot.
When refurbishing there are a few other considerations (old resistors have drifted etc.), but I will suffice with this for now.
I'll also add that one of my favorite modern production 6L6GCs is the SED. I have an old quad around that I use for testing and sonically its up there with the NOS stuff. Its also tough as beans and has a 500V screen - it seems more like a mini 6550c.
http://www.uraltone.com/pics/datasheets/svetlana/sv6l6gc.pdf
http://www.uraltone.com/pics/datasheets/svetlana/sv6l6gc.pdf
andyjevans said:
What about selling the 3 working KT66 for silly money on eBay and using the money to cover the cost of a quad of current production KT66, 6L6GC, NOS 7581 or ? (Matched and burned in a good idea.)
Being a huge fan of the Quad II myself I can heartily recommend the KT66 Retro from the tube store. I doubt I could tell the difference between that and an original in a blind test.
You might want to check out the Quad II yahoo group - one of the guys responsible for over-seeing production of this reproduction hangs out there.
GECs are great of course, but unless you have matched pairs the chances of having the dreaded glowing red plate syndrome is increased (has happened to me). Trouble here is a 'bad' valve will take out the 'good' one. I'd rather go for the peace of mind (and price!) of a freshly made modern replica.
Cheers,
- John
You might want to check out the Quad II yahoo group - one of the guys responsible for over-seeing production of this reproduction hangs out there.
GECs are great of course, but unless you have matched pairs the chances of having the dreaded glowing red plate syndrome is increased (has happened to me). Trouble here is a 'bad' valve will take out the 'good' one. I'd rather go for the peace of mind (and price!) of a freshly made modern replica.
Cheers,
- John
Joan of Arc
I realise I'm asking to be burned at the stake for saying this, but the Quad II really isn't that good an amplifier (undersize output transformer). The only amplifier I'd be prepared to expend NOS KT66 in would be a Williamson or something modern with a similarly good output transformer.
kevinkr said:
I realise I'm asking to be burned at the stake for saying this, but the Quad II really isn't that good an amplifier (undersize output transformer). The only amplifier I'd be prepared to expend NOS KT66 in would be a Williamson or something modern with a similarly good output transformer.
John of Arc
No, burning at stakes is messy (and, just briefly serious, we seem to have had enough of that in S.A. How tragic ... but back to topic.)
About Quad II,
Yes, the output transformer is relatively small. But the specs do say that it is good for 20 Hz, mainly because of the nested feedback in the power stage. (It has been tested!) The Williamson transformer only has a low -3dB point at about 6 Hz, while the amplifier itself has a low -3dB point at about 1,5 Hz. (The huge transformer is a very modest 20W.) So from that angle one might say that is over-engineered for audio. (H.f. -3dB is at 300 kHz.) Then again, l.f. stability was questionable with all the time constants at about the same frequency - a worrying peak of 7 dB at 2 Hz and viscious phase shift there, as also at 300 kHz.
Now this could easily be (and was) altered by some constructors, and it is still one of the best amplifiers around. I would say that it was (is) better than the Quad II, but that does not make Quad II questionable for blameless audio use.
Now for me in the firing line: I would not use KT66 anywhere in place of (a good) 6L6GC. From what I looked up, 6L6 was popular enough to have been structurally upgraded several times and some of the modern ones are really well constructed - while KT66 has remained the same since the 50s. But yes! Its appearance is still classic!
So, no fires (or hopefully bullets), just an honest discussion which would hopefully be interesting to some. If it means anything, I myself am using the Quad output topology (own designed output transformer) but with the Williamson topology driving it, in some quite high-end 100W amplifiers. (And would that some later fancy designs had the basics right as Williamson did despite the slight stability question. But that is another story.)
No, burning at stakes is messy (and, just briefly serious, we seem to have had enough of that in S.A. How tragic ... but back to topic.)
About Quad II,
Yes, the output transformer is relatively small. But the specs do say that it is good for 20 Hz, mainly because of the nested feedback in the power stage. (It has been tested!) The Williamson transformer only has a low -3dB point at about 6 Hz, while the amplifier itself has a low -3dB point at about 1,5 Hz. (The huge transformer is a very modest 20W.) So from that angle one might say that is over-engineered for audio. (H.f. -3dB is at 300 kHz.) Then again, l.f. stability was questionable with all the time constants at about the same frequency - a worrying peak of 7 dB at 2 Hz and viscious phase shift there, as also at 300 kHz.
Now this could easily be (and was) altered by some constructors, and it is still one of the best amplifiers around. I would say that it was (is) better than the Quad II, but that does not make Quad II questionable for blameless audio use.
Now for me in the firing line: I would not use KT66 anywhere in place of (a good) 6L6GC. From what I looked up, 6L6 was popular enough to have been structurally upgraded several times and some of the modern ones are really well constructed - while KT66 has remained the same since the 50s. But yes! Its appearance is still classic!
So, no fires (or hopefully bullets), just an honest discussion which would hopefully be interesting to some. If it means anything, I myself am using the Quad output topology (own designed output transformer) but with the Williamson topology driving it, in some quite high-end 100W amplifiers. (And would that some later fancy designs had the basics right as Williamson did despite the slight stability question. But that is another story.)