6BX7 Outputs

[lpd]

Anyone here try a 6BX7 push pull triode amp? Just wondering how these tubes are for output stage?

[Geoff H]

Hi lpd, I haven't tried them, I just checked the specs. Could get 5w in class A. Plate resistance of 1300 ohms could make it hard to find a suitable transformer. You can use the taps on an ultralinear xformer. Leave the full winding open, or drive an ESL.
Plate current of 42mA. Max plate dis 12 W - both plates. 250 v on the plates.

HTH,
Geoff

[refference]

Hi lpd ,

Ten years ago , I had built a project with a 6BX7 in the
output stage , wired in push-pull way .

The original design was published by Ziff Davis Publishing Co.
in the 50’s , and the author was Paul Popenoe Jr . ( W6IWM )

The input stage is a 12AX7 tube , which one half is working
like voltage amplifier being Rk = 3300 ohms , Rp = 220 K ohms
and + B = 265 Volts , the 12AX7’s second half is working as
concertina ( split load ) phase inverter , being Rk = 1800 ohms
for bias purpose , and Rk = Rp = 47 K ohms .

The + B value is 270 Volts ( maximum value ) and the total
current consumption is around 50 mA .

The 6BX7, total plate to plate load is 8000 ohms and the NFB
is taken from the 16 ohms secondary winding , feeding the
resistor Rk of the voltage amplifier ( 3300 ohms ) through a
27 K ohms resistor .

The frequency response is between 20 – 100000 Hz , within 1 dB
with an output level of 1 Watt and between 20 – 30000 Hz , within
1 dB with an output level of 4 Watts , these values are for the
Peerless S-510-F output tranny , or another high-quality unit .

If you want to get the maximum power output ( 4 Watts ) , you
should use a signal source with 1.5 Volts RMS .

Talking about performance , I can say to you without any doubt ,
that the final result was very , very great .

The sound was soft , clear , without any sibilance , or bass excess .

The only advice is “ This amp , must be used with high efficiency
loudspeakers , due to its low power output “

And this was my experience with 6BX7 tubes in the output stage .
Now , the decision is yours .

Regards ,

Carlos

[Jaime]

30 years ago I build a PP ampli. One tube per chanel. Very good sound.
best regards Jaime

[Eusebius]

I built a parallel PP amp and the sound was very good - I dismantled it because it was a bit too complex - the halves of the 6BX7s never matched, so I biased each individually with fixed bias and used a coupling cap seperately to each grid. Sounded very good though. Be warned that the halves very rarely match with these tubes, so be prepared to compensate in some way.

[kevinkr]

Standard PP output transformers for the 6BQ5 in the range of 6.6K - 8K pp work great with the 6BX7 and are in the right range impedance wise. 6BX7 section to section matching is often not that great and either fixed bias independently adjustable for each section or individual cathode bias possibly with some provision for adjustment seems warranted.

About 5Wrms is attainable with a single tube.

Another great tube for these sorts of applications is the 6CK4 with about 1.2K plate resistance IIRC. > 10Wrms in pp with an 8K transformer and fixed bias. Sounds very good too.

Low impedance driver stages are preferable due to significant miller capacitance.

I've designed and built with both, prefer the 6CK4..

[Eusebius]

I've designed and built with both, prefer the 6CK4..>>

As Kevin says, another good triode that can be used for outputs. I'd go a step further and say that the 6AH4 is superior to the 6K4 - more delicate treble, and a really nice sound. Another fun tube is the 1626 - now that one rocks, boogies and stays up all night on the razzle. I had an amp with that in and when I played Jimmy Smith you couldn't stay in your seat. I intend to use it in a bass stage amp for inputs into a MOSfet output. That should keep the joint jumpin' Plus don't forget the 12b4 as an output tube - seriously good in combinations.