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Question re 6BX7GT in 300B push pull designs

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Posted on July 21, 2016 at 15:07:28
David Loader
Audiophile

Posts: 3
Joined: July 21, 2016
Greetings Kevin et al. I'm presently researching circuits that operate 300B's in Class A push-pull for a potential forthcoming build, and Kevin's designs from back in the day interest me greatly - the ones with a 6BX7 driver and 6SN7 in the first stage.

So my first question is this: The only concern that I have about the 6BX7 is that it's not a valve being manufactured currently, and ideally I'd like to use 'current manufacture' valves so that I don't have to run the gauntlet of potentially unscrupulous eBay sellers.

Last night I was poring over some data sheets and I noticed that the KT66, when triode connected, has near identical figures to the 6BX7 for plate resistance, amplification factor and transconductance at 250V plate voltage. Just requires a little more current (60mA vs 42mA). It's also about as linear a device as you could want.

I've found almost no references to the use of KT66's as 300B drivers so am I missing something obvious?

 

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RE: Question re 6BX7GT in 300B push pull designs, posted on July 21, 2016 at 15:24:53
JKT
Audiophile

Posts: 454
Location: Midwest
Joined: November 26, 2002
Your concern is well founded. I have such amps, albite PPP KT-120 output stage, and finding 6BX7s with sections matched for bias current (required) in a differential gain stage will be problematic (fortunately I have a goodly stash of them). If you do build these beasts, I would suggest that you consider using CCS fed shunt regs for the input and driver stage B+. For the 6SN7 cathode CCS I would suggest using a cascode BJT current sink a la Morgen Jones. For the output stage use a Fairchild power MOSFET/TL431 cathode CCS. These changes from the original circuit have increased my enjoyment of these amps.


"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln

 

RE: Question re 6BX7GT in 300B push pull designs, posted on July 21, 2016 at 18:47:24
KevinC
Manufacturer

Posts: 2897
Location: NC
Joined: April 19, 2001
I have used EL34s for this purpose in two similar amp designs over the past 5 years with somewhat better results than I was able to get with 6BX7GTs. The voltage and current requirements are greater, so that is a disadvantage, but as you point out, the tubes are still available and since they are single element tubes, matching can be accomplished in a straightforward way, and they are arguably better drivers for high swing requiring tubes like 300Bs. When using EL34s I used the LL1671/PP as the interstage, as it's a better match and that eliminates the need to deal with high frequency out-of-band resonances.

I would say that we were missing the mark by not using a little more current to create better driver stages for challenging output tubes!

Kevin Carter
K&K Audio
www.kandkaudio.com

 

RE: Question re 6BX7GT in 300B push pull designs, posted on July 21, 2016 at 22:18:48
Boli46
Audiophile

Posts: 20
Location: Uppsala
Joined: March 6, 2015
JKT,

You should have a look at your tagline. The quote is incorrect and as it stands completely meaningless.

 

RE: Question re 6BX7GT in 300B push pull designs, posted on July 21, 2016 at 23:29:15
David Loader
Audiophile

Posts: 3
Joined: July 21, 2016
Thank you Kevin. I remember coming across some posts mentioning this resonance issue with the LL1660 and 6BX7 when I did a search before.

Forgive my further question but I presume this occurred as a result of the output capacitance/impedance of the valve and the inductance of the interstage? I'd like to confirm what I need to look at to calculate the projected behaviour with a triode connected KT66 and predict if the same issue will occur.

 

RE: Question re 6BX7GT in 300B push pull designs, posted on July 22, 2016 at 04:27:14
KevinC
Manufacturer

Posts: 2897
Location: NC
Joined: April 19, 2001
The resonance(s) are a result of the interaction of the tube's parameters and the leakage inductance/stray capacitance of the transformer. If you had all of the necessary parameters for both parties, you could probably simulate the behavior to some unknown degree of accuracy, but the simpler way, in my view, is to choose the interstage transformer that has just enough primary inductance to provide the desired low frequency cut-off. Shooting for low frequency cut-offs at very low frequencies, just because you can, puts you in a position of choosing interstage transformers with higher leakage inductance and stray capacitance to get bass response that is likely irrelevant to real in-room performance.

In any case, the resonances can usually be minimized with a carefully chosen RC network across the entire secondary.

Kevin Carter
K&K Audio
www.kandkaudio.com

 

RE: Question re 6BX7GT in 300B push pull designs, posted on August 14, 2016 at 11:07:35
nerdorama
Audiophile

Posts: 427
Location: Seattle
Joined: February 2, 2002
I remember Lynn Olson saying that 6W6 triode connected is a good driver. It's in the ballpark of 6BX7 for dissipation but closer to 6BL7 for plate resistance. Since it's a single tube the matching would be easier and it's quite inexpensive.
John
De gustibus non est disputandum

 

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