• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Current crop of KT66 tubes

I'm seeking wisdom and advice on the current production crop of KT66's. I'm working on a KT66 AB1 UL push-pull amplifier, and currently have just two sets of KT66's. The first set is a re-issue Gold Lion pair, sounds awesome and gives me around 42 watts at the onset of clipping in this circuit. The other one is a Chinese Valve Art pair, which can only squeeze out some 33 watts!

Now, I'm inclined to think that the VA's are just duds or tired. But I'd like to get some insight from people who've tried different KT66's and maybe even made some measurements with different brands. One of my gripes with internet tube reviews is that hardly anyone ever does even a basic set of measurements.

If someone has experience with the TAD selected KT66's I'd be especially interested to hear. Look identical to the Valve Arts, so probably Shuguang production also. But are they really any good?

Then there's JJ Electronics. Love their EL34's and EL84's, but the KT88 sounds rubbish. Don't know 'bout the 66.

So, which are the best performers of the current crop?
 
Good point there. Here I was more curious about performance when new. To be honest, I've no recollection as to how many hours these two pairs of tubes have worked. Nor do I have idea when I even bought them. They've been in and out of some SE amps during the years, but haven't really been the most used tubes in the house. If only I had a tube tester...

I suspect, that the VA's might be at the end of their useful life, but then again they might've been weak from the factory. Maybe I should give the TAD's a chance at least and test them on the bench.

Reason for all of this is that there's a possibility this amplifier will end up being offered in kit form, and TAD's tubes are quite reasonably priced. That tempts me to specify them as a part. Then again I haven't had much luck with chinese tubes. Some of them sound quite fine, but none of the bunch I've had have been long lived. For example the lovely sounding Psvane KT88's gave up the ghost in under a year where JJ's rubbish sounding KT88 ran for years on end and still were as strong as new.

Any ideas on the JJ KT66?
 

Jazid

Member
2009-12-15 7:02 pm
Personally I have no experience of their kt66 or KT88, but I can vouch that the JJ 300b is well made, fantastic sounding and durable in the circuits I've tried it in (SE, PSE, and PP) . I'm a fan of JJ tubes, their 12a*7 series have many standouts as well.
 
Well, the JJ KT88 is well made and rugged, that I can say. And apparently the KT66 is of the same litter, at least build-wise. And their EL34 and EL84 are excellent both build-wise and sound-wise. But to be honest, even JJ has had their share of quality woes. And some of their tubes are excellent, others just "meh"...


Sadly, my local supplier doesn't currently have the JJ in stock. Maybe I should give them a shout to order some, but at this time, the TAD is what there is of "true KT66" aside of the Gold Lion reissues. They also stock the Tung-Sol reissue KT66, but from what I understand it isn't a true KT66, more like a 6L6.
 

6A3sUMMER

Member
2016-06-07 6:50 am
I like my JJ KT66 tubes. I get mine at Eurotubes.com I just drive over and pick them up. Eurotubes does lots of extra testing on them when they get them from JJ in Slovakia. I like all the JJ tube types that I get from Eurotubes. The reliability and sound are great. I am lucky to have a good tube vendor.
 
Well, I just pulled the trigger and ordered some TAD KT66's from my local supplier. I'll almost certainly try the JJ's too once he gets those back in stock, but for now I just need these for testing the circuit itself. It just can't be, that one pair of tubes gives me 42 watts (consistently), and the other one barely makes 33 watts. I'm quite sure that my pair of VA tubes must be just tired, since they generally get quite good reviews.


I'll post my findings here once I get something measured.


Also ordered a quad of JJ EL34's. Need these for another idea...
 
Well, I just pulled the trigger and ordered some TAD KT66's from my local supplier. I'll almost certainly try the JJ's too once he gets those back in stock, but for now I just need these for testing the circuit itself. It just can't be, that one pair of tubes gives me 42 watts (consistently), and the other one barely makes 33 watts. I'm quite sure that my pair of VA tubes must be just tired, since they generally get quite good reviews.


I'll post my findings here once I get something measured.


Also ordered a quad of JJ EL34's. Need these for another idea...
Feel free to tell us about the idea and outcome!
 

kevinkr

Administrator
Paid Member
Not completely related, but my experience with VA 300B in the past was not good to put it kindly. They failed at operating voltages and currents considered normal in 300B circuits, and also produced less power at significantly more distortion than I expected. I stopped using them as a consequence about 14 years ago.

Could it be that the VA KT66 is just not a very good copy of the KT66?
 
Feel free to tell us about the idea and outcome!
Oh, it's nothing special. A pair of monoblocks I built years ago for a friend are now coming home to roost, and I thought that instead of just letting them gather dust on a shelf, I'd make some use of the chasses and the iron they bear.

They started life as KT88 push-pulls, triode mode and class A, and no GNFB. Very simple circuit I adapted from an article written by a local old gentleman over a decade ago. ECC81 configured as a differential pair with a simple JFET on the tail, and then the output stage. The original schematic used a different output transformer and EL34 tubes, and sounded better than my adaptation, to be honest. I always found the KT88 lacking in transparency. This wasn't helped by the fact that the combined input/phase splitter was barely enough to drive the output stage, with no gain to spare for feedback.

Now that these monoblocks are coming back home, I thought I'd give 'em a little remake. Since the output transformers are only rated for 25 watts (Hammond 1650F IIRC), I'm not aiming for much power over that. And since I don't want to drill any more holes to a chassis that's already painted, I thought I'd just slap an ECF82-based pentode input/cathodyne splitter to replace the original front end. Combine this with a UL-mode EL34 AB1 power stage, and I should be good for ~30 watts and above if the OPT's could handle that much. Gain should be adequate for around 12-14dB of carefully implemented GNFB; these are going to be separate power amps, so an active preamp is already assumed. For simplicitys sake I'll leave the power stage cathode biased, though I'd prefer fixed.

But this is just an idea I'm doing for the fun of experimentation. I'm not expecting it to be a circuit worth even publishing, though if it sounds good, I might. Just a side project in between experimenting with the KT66 PP, since that project still has a long way to go before completion, and I think I can modify these old monoblocks to a new schematic in under 24 beers.
 
Well. It seems I'll have to answer my own question: the chinese KT66's are indeed weaker than their russian counterparts. I just got a pair of new TAD-branded KT66's. Premium, even. And they're exactly the same to the Valve Arts. Same tube, same innards, different silkscreening on the bottle. And the most irritating thing: they both only make 33 watts in a circuit where the russian Gold Lion tubes push out a smidgen over 40 watts.

In the future I'll test the JJ's also, once I manage to get my dirty hands on a pair. As for now, I'm a bit disappointed.
 
Instead of 33 Watts versus 40 Watts, I would worry more about the quality of the sound, and the reliability of the tube.

33 Watts is only 0.84 dB less than 40 Watts (less than 1 dB). That is less than a single step (step to step) of the finest quality most expensive step attenuator volume control (either resistor or auto transformer models).

One characteristic is that to some extent, negative feedback "hides" a multitude of amplifier problems (but can create other problems).

When it comes to tubes, try to get recent production ones. I love my JJ KT66 tubes. Again, I get new production JJ tubes from eurotubes.com They may ship orders to overseas, I do not know. I have been using the KT66 tubes in SE. I am now working on a Push Pull design too.
 
Last edited:
Yeah, I know that the difference isn't much in terms of sound pressure. But I was aiming for a goal here, so I am a bit disappointed and slightly baffled at the same time. If I had a tube tester, I could measure the parameters and maybe find the reason for this difference in power output. I suspect the chinese ones might have slightly less transconductance.

To be honest, the TAD tubes don't sound all that bad. I didn't make a power vs THD measurement on them yet, though. And I still prefer the GL's for sound. Just for fun I popped a pair of JJ 6550's in to have a listen, and I prefer both the TAD and GL KT66's over those tubes. Never been a fan of the 6550/KT88 sound...

As for ordering the JJ's overseas or even overborders I have a reason not to, an economical one. I'm not in a hurry here, so I'll just wait for my local supplier to restock, and save some money on those instead of paying list prices.

It's a bit sad, that the KT66 has been sidelined so badly. Most amps in its power range use the EL34 wherever you look. Haven't seen many KT66 based designs anywhere, be it DIY or commercial. The latter even less... But then again, it's quite understandable. EL34 makes the same power, maybe even a smidgen more when pushed, and is a lot cheaper. And even the 15€ JJ EL34 compares so favourably to NOS tubes, that it seems almost a no brainer. KT66 production is more sparse, they cost more... But the oft overlooked thing is they have a sound of their own, which some might prefer, me included. And yes, this does come through even if the amplifier had 17dB of NFB, like mine.
 
I've used Shuguang Black Treasure KT66-Z's in a Quad II amp in the past wich sounded very good, as good as the Gold Lion KT66's and way better than the JJ KT66's that came with it when i bought it. I did sell the amp a few years ago (i do buy and sell a lot of stuff the last years) but the time i had it i used it with the Shuguangs.

Shuguang is also the factory where Tad buys their (with little adaptations and rebranding) to sell as their own brand. But they don't buy the top models (wich would be more pricey) Now i don't have a KT66 type amp anymore, but my Prima Luna Prologue 4 (my standard amp) also uses rebranded Shuguang tubes (selected, tested and rebranded by Prima Luna) and it's hard to find better (Psvane en Gold Lion are the only that are in the same league, and PSvane are also made in the same factory as Shuguang).

But with Shuguang you need to watch out, you get what you pay for. Their cheap tubes are the typical cheap chinese crap tubes. But if you pay the right price, you can get the best new production tubes that are availeble from them. And those Black Treasure KT66-Z's are among them and are not very expensive ( i paid 65€ per tube).
 
Last edited:

6A3sUMMER

Member
2016-06-07 6:50 am
It is hard to get a definitive answer as to why 'similar' tubes sound different.

The operating conditions of plate, screen, and bias voltages and currents; output transformer primary impedance, inductance, distributed capacitance and leakage reactance; the amplifier's input and driver circuit topology and drive capability, etc.

Then there are Transconductance differences, as was mentioned earlier.
There is control grid to screen grid mu (u), that changes both Ultra Linear and Triode Wired modes, but not Pentode mode.

The EL34 uses a true wire Suppressor Grid.
The KT66 uses Beam Forming plates, and has a much bigger glass envelope for cooling.

There are other differences, perhaps too many to list.
 
Last edited:
It's a bit sad, that the KT66 has been sidelined so badly. Most amps in its power range use the EL34 wherever you look. Haven't seen many KT66 based designs anywhere, be it DIY or commercial. The latter even less... But then again, it's quite understandable. EL34 makes the same power, maybe even a smidgen more when pushed, and is a lot cheaper.
Let's compare the original designs. The M-OV or G.E.C. KT66 is a rather old fashioned tube, due to it's pinch stem construction with lengthy internal wires. EL34, 6L6GC and almost any other tube of comparable power have envelopes with Bantam bases, hence very short wires. Maybe this is the reason why KT66's have been replaced by those 'modern' tubes in more recent amplifier designs?
Best regards!
 
I wouldn't think that the pinch construction would've been a concern with audio amplifier designers, even back then. RF work could've been a different story, but neither the EL34 nor the KT66 was never intended for that.


EL34 does have higher transconductance than the 66, and is cheaper to manufacture, which I believe was partly intentional. I remember reading, that the softer vacuum they had was specified just for this reason. They're also smaller. Cheap, small and plentiful - I assume - would have had much weight in considering an output valve for a design, especially in the post war decades. Back then vacuum tube amplification was a necessity, not a luxury as it is today...
 

6A3sUMMER

Member
2016-06-07 6:50 am
11 different EL34 tube prices averaged $20
5 different KT66 tube prices averaged $40

The old rule of manufacturing cost to customer dollar was $1 part, $5 customer.
Lets do a Push Pull Mono-Block
EL34 $20 x 5 x 2 = $200
KT66 $40 x 5 x 2 = $400
Want to do a $$$ Push Pull Stereo amp?

Of course the amp manufacturer does not pay the same prices for tubes that you and I do,
but he pays a lot more than $8 for the KT66.

Real Estate:
An EL34 diameter is 33.3mm
A KT66 diameter is 52mm
Now, put 4 of those on a Stereo amp, and provide enough space for proper and reliable air cooling.
Every square cm of the chassis costs the customer $$$.

The EL34 has higher Transconductance and higher u (relates to Ultra Linear mode, and to Triode Mode; for both gain, and plate resistance);
Versus the lower Transconductance and lower u of the KT66.
The EL34 can be easier to drive, for the same open loop/closed loop gains, and for the same power out.

You are a new company engineer who has been assigned to design a lower cost power amp, which tube do you use?

Oh, and in spite of cost cutting, just make the amp sound good enough to sell lots of them.
And make sure it will not need repair during the exclusive 3 year Warranty.
 
Last edited: