advertisement


KT66 vs 6L6GC for Quad II Classics.

L

LPSpinner

pfm Member
Hi All,

Having recently moved and unpacked my QUAD II amplifiers (New Classics, not Originals), I found that one of the KT66’s did not fair well from the move. I am now trying to source a new set of four KT66’s. It wasn’t a simple fracture or breakage that can be attributed to the removalist. The tube shows no sigh of external damage so I can’t claim insurance on this one. I suspect that vibrations or shock may have internally dislodged a heater mounting bracket or dislodged the heater coating which caused a short in the heater, thus causing it to overheat and fail. Thankfully the amplifier checks out fine with no problems mechanically or electrically.

However with the current world situation, Pandemic and international boarder tensions etc. Here in Australia, reliable sources of KT66 tubes are not easy to find.

The tube that failed was a JJ’s KT66 and I cannot source another set of these within Australia. The original Tubes that Came with the amplifier are the QUAD branded as KT66.

medium


However looking closely, the original tubes do seem resemble a 6L6GC.
medium


Both tubes have very similar specs and appear to be interchangeable with the 6L6GC drawing slightly less heater current than the KT66 and the KT66 having a bigger "Coke bottle" glass envelope.

The 6L6CG is used in allot of guitar amplifiers so is a popular tube amongst instrument amplifiers. Purchasing from an instrument amplifier establishment means that it is a little more realistically priced as musical instruments don’t always operate on the peculiarly esoteric pricing structures of high-end audio. I intend to replace all tubes in both amplifiers as the JJ’s have done a few hours so cost can soon accumulate.

On paper this swap out should work fine as either tube is being operated well inside the maximum conditions listed on both data sheets.

What do you guys think?

Has anyone tried this swap out already?

Regards

LPSPinner
 
The 6L6 is an output pentode.

The KT66 is an output Beam "Kinkless" Tetrode, hence the KT title.

You cannot mix these two types as a push pull pair...

Sorry.

The Holy Grail is a quartet of NOS Marconi Osram KT 66s

Even older Marconi and Osram before they joined up and became "MO/Osram" are as good if not better.

Big debate ongoing as to the preference of clouded glass envelopes -vs- clear glass envelopes...

Choose yer corners Guys, let the battles commence.

Rage hard, we do need some distractions right now!

Nighty night all, yawn.
 
Hi Darmok.

The 6L6 family of tubes (like the KT66) is actually a Beam power Tetrode. Also I’m not mixing tube types, All output tubes will be replaced in one hit.

The Beam forming electrode in a beam tetrode behaves the same as the suppressor grid in a pentode. Both are designed to suppress secondary emissions from the screen grid element.

EMI originally developed the Beam forming electrode to circumvent the patent that Philips / Mullard held on the Pentode. Generally speaking. Beam Tetrodes and Pentodes can be used in the same circuit as long as the other parameters are similar.

Fun fact, some EL34’s(6CA7) manufacturers actually use a beam forming electrode rather than a traditional “wire screen construction” suppressor grid.

My Original post is asking for people that have tried using a 6L6GC (the GC is the “Glass Case” version) in place of the KT66 and any observations anyone is able to share. In the guitar community it’s a common swap out. It's just that I’m using Quad HiFi amp rather than a Marshall or a Fender guitar Amp.

Also; I don’t do NOS “holy grail” tubes they are ridiculously priced collectables with the cost of ownership being placed beyond normal people by well healed collectors that are hoarding any remaining stocks. The asking prices are well above any net performance gain. My amplifier is intended to be used for a couple of months a year during the winter season with my Quad Solid-state or Naim Olive kit serving listening duties during the Hotter summer months.

I’m looking for observations and experiences, not arguments and debates.

LPSpinner.
 
The information I was given is that 6L6’s are less tolerant of high voltages that tend to be used in amps designed for KT66’s. PYE list 6L6 as equivalent for their HF25 which runs at B+ 450V, which is well above the Quads. I have run 6L6G in my PYE’s and they were happy enough.
Subjective opinion: KT66’s sound better.
Offered as experience only - no conflict of interest: I’m currently running Watford Valves’ Relic GEC copies, which sound fine and have been very stable for a year now. You could buy 4 - 6 matched pairs for the price of a single NOS valve! They were delivered very well matched as well. For a daily use amp they are perfectly adequate.
 
@LPSpinner - I can't answer you question directly, but does the QUAD amp handbook/instructions not say anything about using alternative valve types, or have you tried asking QUAD if your proposed swap would be ok? Decent current production KT66s are available still from UK and US suppliers, so you could import some at a push?
 
Always much confusion and duff information on substituting valve types.

The valve museum site gives 6L6 and EL37 as "sensibly equivalent" to the KT66 despite Gm for the EL37 being more than double that of the 6L6 and nearly double the KT66.

The 1953 (ish) Mullard Maintenance Manual gives EL37 as a direct equivalent and does not mention the 6L6 despite it having closer characteristics in most aspects.

I suspect the Quad is fairly forgiving in most cases though, as Jez suggests.

I imagine Quad advice would be to only use KT66 (we can supply) .

I have a pair of NOS GEC KT66s supplied by Quad sometime in the early seventies to someone I have never heard of - I have no idea how I came by them......
 
As I said either 6L6 or KT66 will work just great. The best "6L6" in recent years is the Russian mil spec "button base" one which is often sold as a 5881 (a USA special quality version of 6L6) IME. 70's, 80's NOS ones no doubt much better than recent ones. Beware of suspiciously cheap Russian 6P3S in trad glass envelope without any suffix as although similar characteristics to 6L6 these are much lower rated.... Although actually the Quad II runs it's valves fairly gentle so you may get away with them even though they would have a life of maybe 1/2 hour in a 60W Fender guitar amp.

I suspect EL37's would need mortgage facilities!

With suitable adaptors made up 807's could be used and as not much/any demand from guitarists and audiophools they can be relatively cheap. They are a higher voltage capable ruggedised version of the 6L6 with completely different base and top cap anode. Prob still available NOS in "desirable" brands like Mullard, GEC etc etc at reasonable prices. Used as output stage in vast amounts of military and civilian radio transmitters from pre WW2 till 70's!
 
Arkless Electronics said:
With suitable adaptors made up 807's could be used and as not much/any demand from guitarists and audiophools they can be relatively cheap. They are a higher voltage capable ruggedised version of the 6L6 with completely different base and top cap anode. Prob still available NOS in "desirable" brands like Mullard, GEC etc etc at reasonable prices. Used as output stage in vast amounts of military and civilian radio transmitters from pre WW2 till 70's!
Click to expand...

Options options...

1Piece 4D32 instead KT88 6L6 KT120 tube converter adapter...

https://www.ebay.co.uk/itm/19344301...d=link&campid=5338728743&toolid=20001&mkevt=1

s-l1600.jpg


s-l1600.jpg


s-l1600.jpg


4D32

R.76200766b27cae3b26ae6412c1198bd1


Similar for the STC 5B/254M TO 6L6 Tube adapter...

https://www.ebay.co.uk/itm/19083916...d=link&campid=5338728743&toolid=20001&mkevt=1

s-l1600.jpg


STC 5B/254M
s-l1600-1-5.jpg




Both of the above available only from the "Peoples Republic of China."

Bless Them!!
 
This site contains affiliate links for which pink fish media may be compensated.
Hi All,
Thanks for the responses.

Arkless:
In Quad II's either works just fine and I've even seen KT66 and 6L6 in the same amp before with still adequate results! (not recommended).

I have also seen the same thing in the “Guitar Amp” community. It’s a common theme as these guys are legitimately trying to “tune” the amplifier for a specific sound as it’s a musical instrument rather than HiFi.

MikeMA:
I can't answer you question directly, but does the QUAD amp handbook/instructions not say anything about using alternative valve types, or have you tried asking QUAD if your proposed swap would be ok? Decent current production KT66s are available still from UK and US suppliers, so you could import some at a push?

Good point, The Handbook just says KT66's but I'm guessing there is an element of "liability management" and "ass covering" in that statement. I’m in Australia and QUAD will just hand me over to the Australian distributer who is not all that helpful outside of wanting to make another sale.

Revisiting my data sheets archive, both valves have a “design Max” plate voltage of 500V with the KT66 having an absolute Max of 550 Volts. The little Quad runs a 340 V HT rail which could possibly go as high 360 V if the 240 V Line voltage drifted up, well with in the design limits of both tubes. The interesting thing though is the 6L6 likes to see a little lower plate load than the KT66. I’m also thinking if the output transformer is set up for the KT66 (as the QUAD II is) then the current draw on the plate with the 6L6GC should be a little less.

This condition may increase distortion a little as the 6L6 is not seeing the correct load. I’m guessing this is why the Guitar amp guys are doing this swap-out as it produces a softer-warmer tone where the KT66 are reputed to have a bigger “grittier” sound.

From my perspective, the main reason I’m asking the question is due to availability of KT66 tubes in Australia. I wish to purchase from a local source for convenience and peace of mind as I have had bad experiences with overseas shipments. The GC variant of the 6L6 seemed a good option as local supplies are still available for reasonable money and as I said in the first post, the Original QUAD branded tubes looked suspiciously like re-branded Svetlana 6L6GC’s. Down Under we are coming out of the winter and moving back into a Summer. Queensland get pretty Hot and Humid in summer so the Amps will not be being used for another 6 months, currently there is no real hurry for resolution.

Thanks for the responses Guys.

LPSpinner
 
LPSpinner said:
Hi All,
Thanks for the responses.

Arkless:
In Quad II's either works just fine and I've even seen KT66 and 6L6 in the same amp before with still adequate results! (not recommended).

I have also seen the same thing in the “Guitar Amp” community. It’s a common theme as these guys are legitimately trying to “tune” the amplifier for a specific sound as it’s a musical instrument rather than HiFi.

MikeMA:
I can't answer you question directly, but does the QUAD amp handbook/instructions not say anything about using alternative valve types, or have you tried asking QUAD if your proposed swap would be ok? Decent current production KT66s are available still from UK and US suppliers, so you could import some at a push?

Good point, The Handbook just says KT66's but I'm guessing there is an element of "liability management" and "ass covering" in that statement. I’m in Australia and QUAD will just hand me over to the Australian distributer who is not all that helpful outside of wanting to make another sale.

Revisiting my data sheets archive, both valves have a “design Max” plate voltage of 500V with the KT66 having an absolute Max of 550 Volts. The little Quad runs a 340 V HT rail which could possibly go as high 360 V if the 240 V Line voltage drifted up, well with in the design limits of both tubes. The interesting thing though is the 6L6 likes to see a little lower plate load than the KT66. I’m also thinking if the output transformer is set up for the KT66 (as the QUAD II is) then the current draw on the plate with the 6L6GC should be a little less.

This condition may increase distortion a little as the 6L6 is not seeing the correct load. I’m guessing this is why the Guitar amp guys are doing this swap-out as it produces a softer-warmer tone where the KT66 are reputed to have a bigger “grittier” sound.

From my perspective, the main reason I’m asking the question is due to availability of KT66 tubes in Australia. I wish to purchase from a local source for convenience and peace of mind as I have had bad experiences with overseas shipments. The GC variant of the 6L6 seemed a good option as local supplies are still available for reasonable money and as I said in the first post, the Original QUAD branded tubes looked suspiciously like re-branded Svetlana 6L6GC’s. Down Under we are coming out of the winter and moving back into a Summer. Queensland get pretty Hot and Humid in summer so the Amps will not be being used for another 6 months, currently there is no real hurry for resolution.

Thanks for the responses Guys.

LPSpinner
Click to expand...

The Quad II was designed to be particularly insensitive to variations in valve characteristics;)

Ideally get a matched set of NOS GEC KT66's.....:eek:

The fact that IME maybe 70% of the Quad II's I've ever had in have had original GEC KT66's in them speaks for both the quality of the valve and the gentle usage of them the Quad II makes:)

From a longevity, reliability and price POV you won't do better than the Russian "button base" "5881" I mentioned up thread but I couldn't comment on them V KT66 on sound quality as I've not made the comparison. The Quad II does though seem to work well with whatever is bunged in 'em (EF86's as well) which speaks well of the original design intentions:)

Oh.. Call that a hot amp? This is a hot amp!
hero.jpg


Hot enough to boil a monkey's bum!
 
Arkless Electronics said:
The Quad II was designed to be particularly insensitive to variations in valve characteristics;)

...The fact that IME maybe 70% of the Quad II's I've ever had in have had original GEC KT66's in them speaks for both the quality of the valve and the gentle usage of them the Quad II makes:) ...

...The Quad II does though seem to work well with whatever is bunged in 'em (EF86's as well) which speaks well of the original design intentions:)
Click to expand...

... These statements make allot of sense, Peter Walker was a brilliant Engineer, an excellent Production manager as well a pragmatist. Most of His designs are very tolerant of component variations.

As a manufacturer he was always ready to "adjust" his supply chain and go with whatever suppler could provide a reliable source of parts for a given production run. You can't hold up production if Dubilier can't supply parts, you just go to Philips or Nichicon instead. I have seen quite a few 405's that have had differently sourced parts between the Left and Right channel boards. The amps still ran flawlessly and if you didn't take the cover of the amp you didn't know anyway.

The Russian "button base" "5881" sounds interesting.



I did find a local source for these but they are out of stock at the moment.

SOVTEK 5881 6L6 Guitar Amp Valves Tubes Melbourne Australia Fender (thevalvestore.com.au)


Arkless Electronics said:
Oh.. Call that a hot amp? This is a hot amp!
hero.jpg


Hot enough to boil a monkey's bum!
Click to expand...

Ouch ... Thats why we have global warming .... Glad I don't have to Re-Tube that little beastie.

Thanks...

LPSpinner.
 
Aren't there any "Saratov" KT66 available? (Check if kwtubes is still active on the bay). Used them in my old IIs. Good for everyday use, but on the weekends I inserted NOS GEC KT66s. Sublime. Or you can go out to Bletchley Park and nick some ;)
 
I appreciate you’re in Australia but is phoning up the Quad service department in the UK and asking them to send you Quad-branded replacement KT66s not an option? I’ve done that a couple of times for my II Classic Integrated.
 
It may be of interest to note that when the Quad Service Dept. exhausted their stocks of original GEC / MOV KT66 valves in the early 1990s , their approved high quality replacement for the Quad 2 was either the GE or Philips (U.S.A. ) 7581A .
 
Well we have a happy ending….

After searching the web for a set of KT66’s and failed to find a local supplier who was selling for sensible prices as well as considering the KT66’s compatible cousins. I came across a music shop selling tubes for guitar amplifiers, he had one single, old stock JJ manufactured KT66. The JJ’s I’m currently using would have little more than a hundred or so hours on them and it hurts me ethically, philosophically and fiscally to put them aside probably never to be used again.

Luckily for me, the QUAD II’s unique output circuit makes it very tolerant of tube mismatching (Thank you Peter Walker) and it runs the tubes way below the maximum ratings. So; after a bit of pondering, I pulled the trigger and ordered the single JJ’s KT66. It took a week for Aussie Post to deliver it across two state boarders but it has finally arrived in one piece. I have had it running for a couple of hours now and so far its behaved just as it should. I’m now about to finally hunker down for a good listening session.

Trying out the II's with a QUAD 34 preamp... it actually works quite well.
source


A close up of the Old and The New tubes together.
source


Thanks, guys, for all your ideas and input the solution was not in the original plan but it worked out well financially and with minimal waste.


Happy Listening.

LPSpinner.
 
brecklandboy said:
It may be of interest to note that when the Quad Service Dept. exhausted their stocks of original GEC / MOV KT66 valves in the early 1990s , their approved high quality replacement for the Quad 2 was either the GE or Philips (U.S.A. ) 7581A .
Click to expand...

This is what I remember, have five new GE 7581A's here as spares for my Quad II's
 
You must log in or register to reply here.


advertisement


Back
Top