6L6 vs KT66

[Tdale]

How similar are 6L6 and KT66 tubes? The pins are similar, and the ratings are pretty close. Still, on the duncanamps tube data page, they are not listed as close or similar.

What are the differences between them? Heater current is one, and bias is probably another, but are there more differences?

Could I try KT66 tubes in my 6L6 amp, as long as the heater wires can handle the ectra current?

Tommy

[mhuss]

They are very similar, specs-wise, with the exception of heater current.

As long as they physically fit into the amp (and the heater current is not a problem) they'll work fine (maybe better )

--mark

[Tdale]

what about component values? grid resistors etc?

Tommy

[dartanion]

They should be a direct swap, save for the aforementioned filament current draw.

[mhuss]

Well, now that you mention it, the Chinese KT66s have been known to 'sing solo' in Marshall circuits, so it's a good idea to put 5k6 grid stoppers right on the tube pins in these amps to prevent oscillation if you want to use KT66s.

--mark

[David Root]

Main differences come down to two in my mind.
1) KT66 doesn't have the midrange scoop in its frequency response that the 6L6GC has. Kt stands for Kinkless Tetrode, i.e. no "kink" in the response.

2) KT66 can handle significantly higher currents than the 6L6GC, so can be run hotter with less risk than 6L6GC.

Having said that, I once unwittingly misbiased and redplated a pair of black plate RCA 6L6GCs at 265mA cathode current each for several minutes and they survived, so it does depend on the tube quality. I wouldn't expect some lower quality current production 6L6GCs to match that standard.

I almost forgot;

3) KT66s RULE in JTM 45s!

[dragonbat13]

I put two kt66s in my twin. I had four 5881s. The kt66s seem to have a more "Airy" sound to them if that makes any sense. More punch in the lows and mids. I blackfaced the amp and swapped caps. The Kt66's do seem to sustain alot more. I am using the new tung-sol's. I had tung sol 5881s.

[David Root]

Some recent developments in both types--the new "Tungsol" KT66 appears to be a (decent) Russian 6L6GC repackaged in bigger glass w/metal base, and not a true kinkless tetrode design. Pity. Sources one of the JTM 45 forums and Lord Valve.

The new "Tungsol" 6L6GC-STR, however, looks VERY much like a short bottle RCA black plate (graphited plates & all). Thick glass, no voids, looks right. I just received a pair today and will be trying them out next week in my last amp, which currently has a very nice sounding pair of original RCA blackplates in it. I'll test the Tungsols for transconductance, a good measure of quality viz a viz original specs, and let you know how they sound in a Brown Fender circuit.

[mhuss]

I'm curious -- how is a "kinkless tetrode" different from a "beam power tube?" As far as I know, both are tetrodes which use beam-forming plates instead of suppressor grids to get rid of the "kink" in the characteristic curve that a true tetrode produces (in a way that got around Mullards patent on the suppressor grid ).

--mark

[David Root]

Good question. Not too clear on that myself either, so I googled it. The parent co. of MOV, called HMV, had a technical agreement with RCA, who had developed the 6L6 years before. Mullard had a patent in the UK on the pentode, so MOV got the 6L6GC beam tetrode design from RCA, reworked it and called it the "kinkless tetrode".

So essentially the beam tetrode is the "kinkless tetrode".

BUT many people say the KT66 and 6L6GC are emphatically not the same design or performance. They do sound different, so what was it that MOV did with their KT66 that goes beyond the 6L6GC?

In with this I also found out that beam tetrodes produce less third order harmonics than true pentodes eg EL34, which I didn't find difficult to believe, but they also produce more IM distortion than pentodes which ain't too good as it sounds BAD.

[Zippy]

+1 regarding David's comment regarding current-handling ability for the 66. My understanding is that the 66 was designed to have the same power as the 6L6 albeit at a lower voltage.

[rfgordon]

I recently got some Groove Tubes 6L6s, some to put in an amp going to a client, and I was surprised to see them labelled also as KT66.

Since the true KT66 draws more current, it can't be true for the tube to be both...but to the unsuspecting...

[David Root]

I have a pair of the Tungsol 6L6GC-STR and some Shuguang KT66s in my shop. These are real KT66s I got from Lord Valve, and the only criticism he made of them was "they don't last as long as I would like". I will put them on my power tube tester at 400V plate and measure the current draw and the transconductance and report back.

[David Root]

OK, I put a matched pair of 6L6GC-STR and a matched pair of Shuguang KT66, and a third single Shuguang KT66, on my MaxiMatcher. All measurements at 400 V plate.

At -36V bias, :
6L6GC-STRs ran 50.0 and 49.8 mA and 4.55 and 4.45 transconductance respectively. So about 67% of Pa at 20W dissipation.

KT66s ran 77.6, 78.0 and 76.5 mA and 5.49, 5.55 and 5.44 transconductance respectively. Averaging almost 31W dissipation.

At the tester recommended bias voltage of -48V:
6L6GC-STRs both 16.1 mA, 2.40 transconductance
KT66s 36.3, 36.4, 34.5 mA and 3.56, 3.58 and 3.45 transconductance.

So there's a big enough difference in both current draw and transconductance to confirm the oft-heard claim that a true KT66 is like a 6L6GC on steroids. I would guess about -44V bias would put the KT66 at 70% Pa, at 400V plate.

The KT66 plates are bigger than the Tungsol 6L6GC-STR's. The Tungsol is very similar in construction to the RCA blackplate. I compared it with a 1960 RCA blackplate, the plate is the same width as the RCA on the long side, but much wider on the short side.

[Phazor]

I played the Jim Hendrix Marshall with KT 66s. I own a 1987 50 watt. The difference was stunning. It made me research some old marshalls on Marstran. A 1987 uses a 3400 ohm Primary for 2 EL34 or 6550s. The same transformer for a KT66 from Marstran uses an 8K primary so the tube load is very different. I have an old large trans from an EICO tube amp with a 6600 ohm primary I want to try with KT 66's. Seriously the Jimi Hendrix Marshall with the new speakers are quite amazing to my ears. Is it the KT 66's I dont know but there cleaner than my 1987 60 watt. I bought my 1987 in 1970 used and a new 1959 marshall at the same time. I sold the 1959 cause it was to loud for most clubs but always hung onto the 50 watt.