GEC KT66, what one to look for?

Sigs

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Seems to be various types of GEC KT66's, double halo getter, cv1075 and others, what's the best ?
I'm look for a quad set but difficult to get at a reasonable price, so what if I purchased a pair of matched tubes and later purchase another pair with the results of the second pair were almost the same as the first pair, would this be considered as matched set of quads ?
Would these run in an amp without any issues ? if biased correctly.
 
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Kelia

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Seems to be various types of GEC KT66's, double halo getter, cv1075 and others, what's the best ?
I'm look for a quad set but difficult to get at a reasonable price, so what if I purchased a pair of matched tubes and later purchase another pair with the results of the second pair were almost the same as the first pair, would this be considered as matched set of quads ?
Would these run in an amp without any issues ? if biased correctly.
Maybe you could do this an have a qualified tech to install a double bias circuit
in your amp so both pairs would be happy.
 

Sigs

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Maybe you could do this an have a qualified tech to install a double bias circuit
in your amp so both pairs would be happy.

I understand what you're saying, you bias the two middle and the two outer separately.
I don't have the amp, I'm in the process of gathering all the parts so just thought I'd find out what is the best GE KT66 to look for, seems the clear bottle ones are most sought after for guitar amps.

So if you have two tube with a measurement of-

IA: 73 GM: 6.4
IA: 75 GM: 6.4

and you buy another pair @-

IA: 72 GM: 6.1
IA: 76 GM: 6.6

Would these not be considered a matched quad ? the only ones I know that match the tubes to be almost the same is TAD, as they but tons of them, burn them in and match them, I doubt you will get that with any other tube.

I believe you may have a JTM45/100, that's what I'm looking to build, have trawled the old Metropoulos forum trying to get as much info as I can, also trying to find a source for some vintage parts.
 

Kelia

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I understand what you're saying, you bias the two middle and the two outer separately.
I don't have the amp, I'm in the process of gathering all the parts so just thought I'd find out what is the best GE KT66 to look for, seems the clear bottle ones are most sought after for guitar amps.

So if you have two tube with a measurement of-

IA: 73 GM: 6.4
IA: 75 GM: 6.4

and you buy another pair @-

IA: 72 GM: 6.1
IA: 76 GM: 6.6

Would these not be considered a matched quad ? the only ones I know that match the tubes to be almost the same is TAD, as they but tons of them, burn them in and match them, I doubt you will get that with any other tube.

I believe you may have a JTM45/100, that's what I'm looking to build, have trawled the old Metropoulos forum trying to get as much info as I can, also trying to find a source for some vintage parts.
I have a sixtet of GECs and they are all very far from each other so that's why I asked my
tech for a double bias and he told me it could be done , I'm not a tube expert so hopefully someone will chime in
on your tubes readings.
 
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StingRay85

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you could match them per pair, and make sure the push-pull is balanced. if I understand correctly it doesn't matter so much if you have 30 36 + 36 30 installed, while 30 30 + 36 36 would be a lot worse. So each side one tube of the matched pair, and don't make the difference between the sides too large. Expect to pay big bucks for a matched quad. There are no easy solutions
 

Sigs

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you could match them per pair, and make sure the push-pull is balanced. if I understand correctly it doesn't matter so much if you have 30 36 + 36 30 installed, while 30 30 + 36 36 would be a lot worse. So each side one tube of the matched pair, and don't make the difference between the sides too large. Expect to pay big bucks for a matched quad. There are no easy solutions


Yes, that's a possibility, but what's the difference from twin halo getters, CV1075 etc ?
 

Matthews Guitars

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Before you dump probably well over a thousand dollars on a NOS set of tubes, I suggest that you make a much smaller investment in a new apex matched quartet of Shuguang (Valve Art is one name they sell under) KT-66s and try them out first. I run a set in my '69 Plexi and they're great sounding tubes. I seriously doubt that I'd notice any difference if someone sneaked in a set of NOS GEC KT-66s while my back was turned.

This mania for golden age NOS tubes at incredible prices may not actually yield tonal benefits that are worth the high price of admission. I think that it might make more of a difference in your tone if you just put a fresh set of strings on your guitar!
 

Sigs

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Before you dump probably well over a thousand dollars on a NOS set of tubes, I suggest that you make a much smaller investment in a new apex matched quartet of Shuguang (Valve Art is one name they sell under) KT-66s and try them out first. I run a set in my '69 Plexi and they're great sounding tubes. I seriously doubt that I'd notice any difference if someone sneaked in a set of NOS GEC KT-66s while my back was turned.

This mania for golden age NOS tubes at incredible prices may not actually yield tonal benefits that are worth the high price of admission. I think that it might make more of a difference in your tone if you just put a fresh set of strings on your guitar!

You have a point, how would you know if the GEC's are actually new, so you could buy 10 sets of say Valve Art compared to one set off GEC's, so what happens if you bias a cheap set of tubes a bit hotter ? do they sound better ? then you just change them out for another pair.
 

playloud

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The "Gold Lions" seem to be the most valuable - no idea if that translates to a better sound though.

I like the look of the grey ones best.
 

Kelia

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The "Gold Lions" seem to be the most valuable - no idea if that translates to a better sound though.

I like the look of the grey ones best.
I have GEC's in a JTM45 and it sounds stellar , also have a matched
quad of Gold Lions in a Ceriatone Experience 45/100 and it sounds stellar also.
 

playloud

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I have GEC's in a JTM45 and it sounds stellar , also have a matched
quad of Gold Lions in a Ceriatone Experience 45/100 and it sounds stellar also.

Are we talking originals or reissues here?

The originals - with the individual reference sheets - are what I was referring to. Never seen one in person, but they cost a small fortune! There's some more info here from Brent Jessee.

One thing I still don't understand about matched tubes is when people started caring about them. From what I can tell, Aspen Pittman (Groove Tubes) was the first person to start making a lot of noise on the subject. If so, were all those classic recordings prior to this made with "mismatched" power tubes, good fortune excepted?

The challenge with buying 'matching matched pairs' would seem to be the second pair. Once you've got a target idle current/transconductance, that'll really whittle down the market of remaining pairs! And that's if you can even trust the numbers provided by sellers online enough to make direct comparisons with said target...
 

Kelia

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Are we talking originals or reissues here?

The originals - with the individual reference sheets - are what I was referring to. Never seen one in person, but they cost a small fortune! There's some more info here from Brent Jessee.

One thing I still don't understand about matched tubes is when people started caring about them. From what I can tell, Aspen Pittman (Groove Tubes) was the first person to start making a lot of noise on the subject. If so, were all those classic recordings prior to this made with "mismatched" power tubes, good fortune excepted?

The challenge with buying 'matching matched pairs' would seem to be the second pair. Once you've got a target idle current/transconductance, that'll really whittle down the market of remaining pairs! And that's if you can even trust the numbers provided by sellers online enough to make direct comparisons with said target...
The Gold Lions in the 45/100 are reissues and they sound great in every way so
like mentioned above , don't loose sleep over this and have fun playing you amp.
 

Matthews Guitars

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One thing to understand about matched tubes is that back in the "golden days" of tube manufacturer, when Genelec, Mullard, Telefunken, Siemens, GE, RCA, Philips, Sylvania, Brimar, MO Valve, RFT, etc. were all active and making tubes by the truckload, tube quality was very high, with rigid quality and performance standards. Substandard tubes were simply crushed.

The tubes that came out of the shipping dock of those factories were of a uniformly high specification and you could just pick four new KT66s or EL34s out of the bin, put them in your Marshall, set the bias, and go. They'd work. They were well matched no matter which ones you pulled off the shelf due to the high quality standards they had to meet to be accepted for sale.

But after those factories shut down, and we got into the era of reissue tubes, made in Russia, China, or Yugoslavia, since quantities made are smaller than in the golden era, their attitude toward quality has been much more lax. "If it functions to a basic standard, ship it!" has been the way that the Russians and Chinese tube makers have been doing business, while the Yugo built tubes have retained somewhat higher quality standards. (So there's some good reason to look for the tubes made in Europe today.)

Since there is more variance among the tubes sent out from China and Russia, the need to match them arose. A badly out of balance pair of tubes can cause hum in the amplifier. We match them because of the need to do so. Nobody wants his amp to be humming.

Now, as for actual tube qualities, I'd say that the quality of the better new-made tubes today is very much on par with the qualities of NOS classics from the golden age. They'll last as long, and perform about the same. I don't really think that there is anything magical about the golden age tubes, they were simply ALL made to a high standard that is SOMETIMES met by new made tubes.

Tonally speaking, I don't think we're talking about much of an audible difference. Take the best of today's new manufactured Russian EL34s and compare them to a set of NOS Mullards in an amplifier. Will you hear a difference? Will you still hear it if you play the amp with first one set of tubes, then the other set, and then leave the room while someone chooses which set to put into that amp at random, then come back and play and try to identify which tubes are in there now? Want to repeat that test five times and bet on how many times you get it right?

I'm not going to say that there would be NO tonal difference. But is it significant enough that your memory is sufficient to always pick which is which? Probably not reliably.
 

neikeel

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I agree with Matthew’s Guitar post above. The tonal difference is subtle. The guitar, strings, cab and preamp are all important factors. I have GECs in my 45/100 JH clone and my 65 JTM45. The 65 Dual OT has RI Gold Lions. All sound great. I have spare NOS Marconi KT66 pair and quad of some sino treasure KT66 that I’ve tested and are also very good.
Only time I’ve been knock over by the improvement with different tubes was replacing some EH EL34s with Mullard xf2. The depth, girth and range of the mullards was spectacular in a JMP50.
 

StingRay85

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The tubes that came out of the shipping dock of those factories were of a uniformly high specification and you could just pick four new KT66s or EL34s out of the bin, put them in your Marshall, set the bias, and go. They'd work. They were well matched no matter which ones you pulled off the shelf due to the high quality standards they had to meet to be accepted for sale.

On this part I have to disagree, and I have more than enough data of "new in box" tubes to prove that. There could still be a considerable difference between the tubes. But I think matching has not that much to do with build quality.
 

neikeel

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Yes I think that as far as audiophile use they would still need to be curve matched, but for rock n roll probably ok. Up until 1970 or so most amps were truly fixed bias with no adjustment possible!!!
 

Ivan H

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On this part I have to disagree, and I have more than enough data of "new in box" tubes to prove that. There could still be a considerable difference between the tubes. But I think matching has not that much to do with build quality.
I agree with @StingRay85 on disagreeing with this point. Tubes were, as they are still, graded by the manufacturer (bell curve system), with those having characteristics too far from ideal being disposed of, but even back in the day, tubes did not all have to exhibit exactly the same electrical characteristics to make the grade. I (like I imagine quite a few here do) have several NOS/NIB matched duets of Blackburn Mullard xf2 EL34's that when plugged into the same amp, each duet has different bias requirements.
Large consumers of tubes could request that power tubes be supplied to them with specific characteristics, so that they could just plug the tubes into their circuit & expect them to bias up correctly. That does not mean that just any of this tube type from that manufacturer would be suitable for this though. The fact that consumers could buy factory matched duets etc points to not all tubes of a type all having the same characteristics (they wouldn't need to match them if this was the case). I have a few NOS/NIB factory matched duets, like this.
15970473471126474148917050932888.png
A factory "matched pair" of Amperex Globe branded single halo getter Blackburn Mullard xf2 EL34's. Cheers
 
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playloud

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On this part I have to disagree, and I have more than enough data of "new in box" tubes to prove that. There could still be a considerable difference between the tubes. But I think matching has not that much to do with build quality.

I was hoping you'd chime in @StingRay85! I remember your nice mu data a while back. Would be really interesting to see if there is in fact a difference in variance of Gm between (NIB) NOS and new-production brands of pentodes.
 

Matthews Guitars

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I'm not saying that tube matching only became a thing in recent years, but regarding the bell curve, let's just say that today, Russian and Chinese tube manufacturers will sell any tube that will score anything from 60 to 125 on a theoretical tube performance scale with 100 being "exactly the performance intended". But back in the golden days, manufacturers would not ship anything that was less than an 80 or more than a 110 on the same scale. Tubes that performed above 110 might even be rejected as they may result in unsatisfactory and unstable operation in some equipment.

You could try an experiment. Take a pair of tubes that are 10 percent different in gain and put them in your 50 watt amp. I bet the amp will run just fine. It might not even hum appreciably. In truth I've found larger differences between a pair in an amp that was working just fine.

But I've also found that it seems that amps with ultralinear transformers are more sensitive to tube variances. That probably is exactly why the Fender amps with ultralinear transformers all have hum balance controls. My Pro Reverb is one of those amps, and the hum balance is very useful. I've used it to match the tubes in it and get the amp running so utterly dead silent, you'd think it was turned off. If I swap out the pair of output tubes, I'll have to rebalance.

I stock a large number of lightly used power tubes that I've acquired, of all common types. They all work fine, and read fine on my calibrated B&K 707 tube tester. I catalog them by transconductance (gain) and have a range of values that allows me to provide a close match to virtually any single tube of that type that's still "good". Even if they're very different, say a Siemens pattern EL34 vs. a big bottle Ruby 6CA7, if they both read 105 on my meter, that's a pair that matches well in the amp, at least in the short term. I prefer to match by gain AND construction type both, but gain alone seems to be sufficient for a good match.
 

Ivan H

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I catalog them by transconductance (gain) and have a range of values that allows me to provide a close match to virtually any single tube of that type that's still "good". Even if they're very different, say a Siemens pattern EL34 vs. a big bottle Ruby 6CA7, if they both read 105 on my meter, that's a pair that matches well in the amp, at least in the short term. I prefer to match by gain AND construction type both, but gain alone seems to be sufficient for a good match.
Ok, let's just get a few things straight. Transconductance (aka mutual conductance) is NOT gain. It is actually expressed as the opposite of resistance, hence the unit of transconductance being expressed in mhos, the opposite spelling of ohms.
Transconductance is a ratio of the incremental change in plate current divided by the incremental change in the control grid voltage producing it (taken directly from my copy of The Radiotron Designers Handbook, 4th edition).
This brings us to "gain", usually associated with pre-amp "voltage amplifying" tubes, aka the "common cathode gain stage". The "amplification factor" of these type tubes is often INCORRECTLY referred to a the tube's "gain", but actual "gain" is calculated. Gain = (mu * RL)/(rp + RL + Rk (mu + 1))
Where;
mu = amplification factor of tube.
RL = Plate load resistance.
rp = internal plate resistance.
RK = Cathode resistance
(RK = 0 if cathode is fully bypassed).

We must remember, that when we post stuff on forums like this, that other people read it & take it in, often in an endeavour to further their knowledge. To that end, it is beneficial (or essential) that what we post actually be factual.

Ok, onto ultralinear amplifiers needing hum balance controls & being more sensitive to tube variances. It is a well accepted fact that tube guitar amplifiers are pretty basic in design when compared to their "audio" counterparts, whether ultralinear types or not. There are just soooo many ultralinear audio amplifier designs that don't utilise a hum balance control nor are they overly sensitive to power tube variances. Case in point, before it was stolen in a burglary, I owned a British made Leak "Point One" Stereo 60 Ultralinear Amplifier & Varislope 2 Pre-amp (called a "point one" as it produced less than 0.1 of 1% THD. Of note, American engineers tested this type amplifier & got an even lower figure). The power amplifier did not feature hum balance controls. Power tubes were individually cathode biased & the amp could be fitted with KT66's, 5881's or EL34's with absolutely nothing needing to be done to facilitate the use of the different tube types, simply plug them in & good to go. From the Leak literature, matched power tubes were not needed at all. Cheers
 
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