Current crop of KT66 tubes

[petertub]

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 Maybe this is the reason why KT66's have been replaced by those 'modern' tubes in more recent amplifier designs?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!

Nothing demands pinch type for KT66. JJ uses short leads through the bottom, just like EL34.
pinch types was simply an older method of making the tube base.

 

[mbrennwa]

What do you mean by "soft" vacuum? Higher residual gas pressure?

 

[Adolf Corkscrew]

Who says that EL34's have softer vacuum? I'd regard this as a fairy tale, 'cause the characteristics and performance become more unpredictable, the more air molecules remain in the envelope.


Best regards!

For the life in me I can't remember where I read this claim. And of course I couldn't find a source for this anymore. So it should be considered hearsay and tube lore for now. But anyhow I remember reading somewhere, that the original EL34 was left with a slightly softer vacuum compared to some other power tubes.

 

[6A3sUMMER]

Try and compare the EL34 to the KT77. The transconductance, and G1 to G2 u, are very similar. The dimensions are also similar.
The largest difference of these types is the Wire Suppressor grid versus the Beam Forming plate.
Listen.

Then, compare the EL34 to KT66, but the tubes are not close to equal.
The transconductance; andG1 - G2 u; Wire Suppressor grid versus Beam Forming plates are all differences.
Listen.

The True EL84 and True 6BQ5 have similar specs.
The difference is the Beam Forming plate, versus the Wire Suppressor grid.
Yes, there are many mis-labled or dual-labeled EL84 and 6BQ5, but they are not Truly what they claim to be (the original tubes as designed and introduced).
Listen.

'Just saying'.

 

[Adolf Corkscrew]

Are there any true 6BQ5's made these days..? All the other tubes I'm quite familiar with. I've built SE's and push-pulls with EL84's, KT88's and EL34's. And a couple of SE amps accepting KT66 and 6L6. No push-pulls with the latter, though, before this current prototype. I've tried both the JJ and Gold Lion KT77's in all my EL34 amps, and they're both nice.


Now that I'm back at my lab, I took a THD measurement with the TAD-branded KT66's at 30 watts in my prototype. Result was 0,13% whereas the GL-branded russian ones give a number around half of that.

 

[6A3sUMMER]

I am pretty sure that it is less expensive to build a 'fake' 6BQ5 with a Wire Suppressor grid,
instead building a real 6BQ5 with Beam Forming plate.
Lower cost "wins" out over building real 6BQ5 tubes, regardless of whether there are any performance and/or sound differences or not.