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Old 16th Sep 2022, 8:45 am   #1
Valvepower
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Default Quad 22 KT66 to 6L6 with Cathode resistor increase?

Hello All,

This old chestnutÖ

Iím looking to do some valve swapping to release some GEC KT66ís from a pair of down at heel cosmetically challenged 22 power amplifiers, so Iím thinking of fitting the Quad 22 power amplifiers with 6L6GCís, 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 KT66ís 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 6L6GCís, do they change the value of the cathode bias resistor?

Iíll not be changing the values of anything else..., however Iím guilty as charged, Mílud for doing unspeakable things (in the eyes of the purest) to Quad 22 power amplifiers back in the 70ís, 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
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Old 16th Sep 2022, 8:58 am   #2
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Default Re: Quad 22 KT66 to 6L6 with Cathode resistor increase?

Quad sold a fair number of these amps in the US, back in the day. I seem to recall that their written advice was that 6L6xx's were a perfectly fine substitute and nothing else needed to be changed. I can't remember what xx variety they specified though, and Mrs GJ says we need to leave NOW for a long weekend away. It's worth noting that, in some respects at least, quite a few modern KT66's are really 6L6's (sheep in wolves' clothing). A 0.9A heater current is one giveaway.

Cheers,

GJ
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Old 16th Sep 2022, 9:28 am   #3
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Default Re: Quad 22 KT66 to 6L6 with Cathode resistor increase?

And dont forget, KT66, 6L6, CV254, CV255, and several others that I have forgotten at the moment, are essentially the same valve. OH yes!! different bases, different glass shape, and supposedly different magical sound. I would dare to suggest, IF you use REAL valves and not some chinese or russian junk, you cannot go wrong.

N.B. my bent against "newbie " valves is well earned!! I have had to wear warranty more times than I wish to remember OR mention, using modern "equivelents ".

Just my two cents worth.

Joe
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Old 16th Sep 2022, 10:27 am   #4
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Default Re: Quad 22 KT66 to 6L6 with Cathode resistor increase?

At the same voltages the KT66 passes more current than the 6L6/6L6G/6L6GC. At Va = Vg2 = 250 V and Vg1 = -10 V the KT66 passes about 120 mA against about 100 mA for the 6L6/6L6G/6L6GC.

The original 6L6/6L6G had a lower maximum anode dissipation rating of 19 Watt, while that of the 6L6GC and KT66 is 25 Watt (design centre rating) or 30 Watt (absolute maximum rating).

The CV254 is a cathode ray tube. I could not find any data for the CV255.
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Old 16th Sep 2022, 1:17 pm   #5
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Default Re: Quad 22 KT66 to 6L6 with Cathode resistor increase?

Quote:
Originally Posted by Robert Gribnau View Post
The CV254 is a cathode ray tube. I could not find any data for the CV255.
From memory, KT66=CV1075
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Old 16th Sep 2022, 1:54 pm   #6
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Default Re: Quad 22 KT66 to 6L6 with Cathode resistor increase?

I ran into this issue when using original spec 6L6G in a Williamson amp. The grid bias had to be increased so the combined anode and screen dissipation when strapped as a triode was no higher than 21.5W. Also gave reduced power output.

6L6GC has a higher anode and screen power rating of 30W and 5W respectively. It also has a higher anode and screen voltage rating which is comparable to KT66. Suitability depends on circuit configuration. Compare RCA data for 6L6/6L6G and 6L6GC.

https://frank.pocnet.net/sheets/049/6/6L6GC.pdf

https://frank.pocnet.net/sheets/049/6/6L6.pdf

I have had the misfortune to buy a batch valves marked 6L6GC only for them to fail prematurely. Most likely remarked 6L6GT.

Rich
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Old 16th Sep 2022, 4:23 pm   #7
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Default Re: Quad 22 KT66 to 6L6 with Cathode resistor increase?

Quote:
Originally Posted by Robert Gribnau View Post
At the same voltages the KT66 passes more current than the 6L6/6L6G/6L6GC. At Va = Vg2 = 250 V and Vg1 = -10 V the KT66 passes about 120 mA against about 100 mA for the 6L6/6L6G/6L6GC.

The original 6L6/6L6G had a lower maximum anode dissipation rating of 19 Watt, while that of the 6L6GC and KT66 is 25 Watt (design centre rating) or 30 Watt (absolute maximum rating).

The CV254 is a cathode ray tube. I could not find any data for the CV255.
Maybe a misquote for the 5B/254M, 5B/255M? (CV428, CV391)

ISTR these are favourites of Joebog
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Old 16th Sep 2022, 5:01 pm   #8
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Default Re: Quad 22 KT66 to 6L6 with Cathode resistor increase?

OK, that must be it.
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Old 16th Sep 2022, 7:49 pm   #9
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Default Re: Quad 22 KT66 to 6L6 with Cathode resistor increase?

5B/254M is often spoken of as an 807 in a smaller bottle, and the 807 spoken of as a 6L6 with top cap, ceramic insulators and a bit more space... same characteristics but allowed to extend higher in voltage. I've always understood that the 6L6 was derived from the 807 but not come across evidence.

The KT66 has differences, but were they deliberate to throw Philips' lawyers off the trail of beam tetrodes and kinkless tetrodes impinging on their power pentode patents. And vice-versa.

Waters were deliberately muddies.

David
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Old 16th Sep 2022, 8:45 pm   #10
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Default Re: Quad 22 KT66 to 6L6 with Cathode resistor increase?

Thanks Herald,
Slap, yes my fault and you are correct. The 5B/254 has the topcap, the CV391 does NOT have a topcap, but it does have higher grid plate capacitance. The loctal bases require the same chassis cutout as octals so there is no chassis butchery to happen.

http://www.r-type.org/exhib/abv0089.htm

http://www.r-type.org/exhib/aaa0035.htm

Either way, ( topcap or not ) they are excellent audio bottles, and both have full voltage ratings.

Joe
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Old 16th Sep 2022, 9:42 pm   #11
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Default Re: Quad 22 KT66 to 6L6 with Cathode resistor increase?

The early coke bottle KT66 (VT75) have a similar anode structure to the 6L6G but, as others suggest, the 6L6GC characteristics are likely to be different so you need to check the cathode current and adjust the auto-bias to set to the same as the KT66.
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Old 21st Sep 2022, 10:03 am   #12
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Default Re: Quad 22 KT66 to 6L6 with Cathode resistor increase?

Quote:
Originally Posted by PJL View Post
The early coke bottle KT66 (VT75) have a similar anode structure to the 6L6G but, as others suggest, the 6L6GC characteristics are likely to be different so you need to check the cathode current and adjust the auto-bias to set to the same as the KT66.
Many thanks for all the replies.

Taking measurements with both 6L6GC and KT66 and adjusting, if necessary, is a sensible route.

There are well documented performance figures for the Quad 22, so I can check a unit with 6L6GC against these as well.

The various variants of the 6L6(GC-WGB...etc.), 5881 and 7581A are a bit of a can of worms.

Regards
Terry.
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Old 21st Sep 2022, 6:14 pm   #13
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Default Re: Quad 22 KT66 to 6L6 with Cathode resistor increase?

Quote:
Originally Posted by Radio Wrangler View Post
Waters were deliberately muddies.
Philips even made beam tetrodes for some applications (TV hor output comes to mind) but explicitly called them pentodes with no mention of the construction whatsoever...
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Old 22nd Sep 2022, 9:40 am   #14
Robert Gribnau
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Default Re: Quad 22 KT66 to 6L6 with Cathode resistor increase?

Like you wrote, Philips kept on calling them pentodes but in later times sometimes added that they were 'beam pentodes'.

Two examples of this:

https://frank.pocnet.net/sheets/010/e/EL503.pdf

https://frank.pocnet.net/sheets/010/p/PL504.pdf
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Old 22nd Sep 2022, 2:29 pm   #15
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Default Re: Quad 22 KT66 to 6L6 with Cathode resistor increase?

Not wanting to go at too many tangents, there is the Mullard EL35, which making reference to The Valve Museum www.r-type.org the Mullard EL35 is down as an equivalent of the 6L6 and looking at the data I personally feel the EL35 was Mullardís Ďpentodeí take on the 6L6.

A couple of the EL35ís I have here in the toy box have a mesh anode (as per the pictures on the Valve Museum site, so you can clearly see the grid structure and its defiantly a pentode.

http://www.r-type.org/exhib/aaa1624.htm

I recently tried a pair of the EL35ís from the toy box in the fixed bias 25-Watt guitar amplifier project. They worked OK, but lacked the power I expected.

Sadly, the valves I have here, all of which came with some other stuff many moons ago, really arenít the best selection where two were duff another two were OK and Ďjust usableí with low emission (these are the pair I tried in the amplifier), however, the fifth is very good, however sadly, I have nothing useable to match with Ė If I saw another good EL35 at a sensible price I could revisit the EL35, but definitely not at the silly prices Iím seeing on the internet sales sites!

Terry
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Old 22nd Sep 2022, 3:32 pm   #16
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Default Re: Quad 22 KT66 to 6L6 with Cathode resistor increase?

As far as I know the EL35 is the Octal version of the EL5 which has a side-contact base.

Philips introduced the EL5 in May 1936, while RCA introduced the 6L6 in April 1936. So I don't think the EL5 and the EL35 were based on the 6L6. The characteristics of the EL5 and 6L6 are close indeed but I personally would not call them electrically equivalent.

Later many variants of the EL5 were introduced. Some had the anode lead coming out at a top cap, so could be run at much higher anode voltages. See:

https://frank.pocnet.net/short/054/2/224.pdf

When I just rediscovered this hobby I breadboarded a couple of amplifiers using the EL5. In push-pull ultra linear with cathode feedback they sounded very nice to my ears/brain, even though I was running them rather 'cold'.
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Old 23rd Sep 2022, 10:38 am   #17
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Default Re: Quad 22 KT66 to 6L6 with Cathode resistor increase?

Robert,

Many thanks for the background on the EL35, which is very interesting.

I’ve never seen the datasheet with the EL5 and the EL35 and thought it was introduced in 1945 after the 6L6 – this is as per the info on The Valve Museum www.r-type.org site as in post #15 above.

The EL35 data I went by can be found in the data sheet below and also in the Mullard early maintenance manual:
https://frank.pocnet.net/sheets/084/e/EL36_Mullard.pdf

Looking at the EL5 and the EL35 datasheet, Robert directed me to and also the later EL35 data I had sight of; maybe Mullard – as GEC did with the KT66 – reintroduced or remarketed the EL35 valve in 1945 with higher electrode voltages to appeal to the 6L6 users?

Sadly, as I previously said, using EL35's is all quite hypothetical as the examples I have here are a motley ‘ole crew and I’d need to find good one to match the good one I have here. I’m not going to shell out for a good one based on the ridiculous prices I’m currently seeing!

Terry.

Last edited by Valvepower; 23rd Sep 2022 at 11:04 am.
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