6B4G dedicated heater winding...

[Greg]

Good afternoon, morning or night everybody.
 
I would like to build an old style amplifier using a pair of 6B4G in push-pull with a fixed-bias arrangement driven by a 6C5 into a 10k plate to push pull grids transformer inverter. That sort of thing... However, in the the 1947 «Pratical Amplifiers» manual by Robin & Lipman, they used a separate 6VAC winding in fixed bias for the 6B4G and I'm wondering why. I tried to search online and most of the designs that I've come across uses DC filaments with IC regulators and all that shite. Now, I understand that a separate heater would maybe be necessary in cathode bias or with 2.5V tubes but my question is, can I use the same 6V for the whole amp with a 50 ohms humdingy pot? Thanks!   

[shooter]

no expert or qualified 

https://frank.pocnet.net/sheets/127/6/6B4G.pdf

from the datasheet filaments can be either AC or DC, tube can be biased either fixed or self so i'll say a qualified yes, someone actually qualified should check in soon 

[Greg]

Hi Shooter,

Well my question wasn't clear... or too elaborate maybe. I wasn't wondering about using AC or DC but whether or not using the same supply for the whole amp since 6B4G tubes are directly heated (the filament IS the cathode). For an example, in those R&L schematics (not my plans), there's always a dedicated filament for the preamp when using 6B4G as output devices. Not when using 6L6's etc... Since Robin & Lipman were engineers, there must have been a good reason for it. Or maybe not? There must have been a reason why they invented the 6B4G... (circled in red in the schematics are the «separate» preamp/radio 6V winding).

[shooter]

guessing the use of 2 filament windings is to keep PA fluctuations from preamp tubes.
other than that I don't see an electrical reason, looks to me nothing goes up in smoke, sound like crap, no clue

[shooter]

fwiw
from the audiophile world;
http://diyaudioprojects.com/Tubes/300B-SET-Amplifier/

just skimmed for key-words, author might gleam filament logic or ideas

[Greg]

Roger that. Well in that case (the schematic you provided) the 300B need a separate winding since it operates on 5V. They choose DC for the least amount of hum possible with the sand state bugs regulators I was talking about on my first post (because they chose a 6.3vac transformer). Since it's SE, they wanted to bias each tube independently. I need an elegant and era-friendly practical solution. AC heating is fine for me and it could be balanced by a humdinger pot. Single coil pick-ups will probably hum more than AC filament hum.

[shooter]

believe in the write-up he opted to just use the preamp filament winding for both but DC'd it for clean filtered.  That tells me just do it, make sure the windings can handle the current, walla done

[Greg]

Love the «walla» hehe... Anyways, thanks for chiming in. I appreciate. I have a pair of Sylvania 6B4G's with excellent readings on a test meter and the Sovtek replacement is kind of pricey. But I agree, I won't start a fire trying this. I wanted to save on an auxiliary transformer, voilà.

[Greg]

Ok... I realize I don't know shit when I paralyse for that kind of stuff. I have to ask a thermionic expert to validate or invalidate my fear to destroy a 80 years old device. I admit this is maybe a different hobby than when I had the opportunity to experiment empirically (and joyously) with a couple of buck's 12AX7.

[shooter]

ask a thermionic expert

it's the internet, 10 like replies is a minimum for validation, then it probably still lacks credibility 

[Greg]

The internet  really got me into this stuff and it started in 2002 with a great informative site like FunWithTubes (No joke) It's the name of the site. Mr Robinson, the gentleman who constructed his great site studied tube technology in the sixties. It's still operating and overlooked I think. He's been really kind to respond to my questions by e-mail at the time. I got many credible answers through searches in this forum I have to say. Lots of stuff. Great information and reasonable members, meaning phool-free in general... Thanks to Mr. Hoffman.

[shooter]

yup, if you can't figure it out from here, couple others like here, you probably need to change hobbies 

here's a few for you favorites tab if you don't already have them;

http://www.valvewizard.co.uk/
http://www.aikenamps.com/index.php/what-is-biasing
https://frank.pocnet.net/
http://www.angelfire.com/electronic/funwithtubes/Amp_Design.html
https://www.tubesandmore.com/tech-corner

[PRR]

Both the plans in #2 are "self-bias" (tho wired as back-bias) with the large resistor from cathode(filament CT).

The 6B4 is the older 2A3 but 6.3V filament.

We "always" float the filament of 2A3/300B/6B4 on a dedicated winding. I'm not sure why. In an intermediate age the 2A3 would be the only 2.5V tubes in the amp so that makes sense. So why the 6B4? There will be some (rectified!) audio on the cathode and you don't want that coming into the driver stages via H-K leakage. I can't see harm in a "power amp", though bringing several volts of distorted audio back to a guitar input preamp cathode does seem a poor idea.

Lemme think.

You do know that a 2A3/6B4 guitar amplifier will be super tame and mellow. It aint rock-n-roll. and it aint light for the Watts.

[Greg]

Thanks PRR. I looked at the schematic (the one with the driver transformer) I was suprised to notice that the circuit is indeed not «fixed bias» and dependent on the total load of the amp (mainly the power cathodes) I never saw a power resistor in the grid circuit. Always through the cathodes!

Got you about the filament theory ... So all the 6B4G current is obviously going to get through the 50 ohms center tap humdingy pot and wreak havroc on the sound in a sensible 6J7 input stage like an oscillation? Not a good idea...   

I know my thing is a gamble... And not very practical indeed. It just came as a blind wish... Those 6B4G were given to me, although I am going to pay 50$ a piece for new Sovtek in the future... This wasn't for rock'n'roll though... More jazz...

I was thinking of a hot-biased pentode input, to maybe compensate for some of quirkiness that might be lost using power triodes. To try and get well defined chords that often contain minor seconds intervals without turning everything into mush. Crispiness with a certain incision in the sound.

Usually, the plan I see with a pentode input is to make every tube in the chain fall apart except the tremolo oscillator. And on top of that, it is usually with a paraphase inv. and class A (almost) beam tetrodes. For me, it is an indispensable sound and part of the palette but I already went that route before. Surely triodes will need lots of drive... Never tried triode on any sort of amp before. I was thinking about a 6J7 into a 6C5 which is transformer coupled to real fixed bias at -68V  PP grids. Not even a treble cut, just a volume control with a treble bypass.

I have a nice transformer 3k P-P/8 ohms. Will follow manufacturer's chart. Ok I kind of «know» this could be accomplished with 6L6's... Dang.

[PRR]

Rather than think too hard, specially for jazz, I'd run your 6.3V winding to the 6B4s, get a 12VDC switcher to power the small heaters hum/buzz-free.

[Greg]

Ha ha thanks...Great advice! I had moderate success on another project with a small plastic wall mount unit that I just epoxy-glued on the bottom of the chassis using the AC pins as terminal strips, just filed them a little. The only problem was it took longer for the filament to light up than the HV ramp-up time of rectifier (a 6CA4) which was the only one of the other two devices to run on ac filament. I had to install a switch on the dc side to turn it off after one second and back on while the rectifier was ramping up on HVDC. Being a small SE amp with a small transformer it would go higher than the WDC of the caps. I used a 6 volter at the time. In the new project (the 6B4 project, two preamp tubes) is the choice of a 12VDC in series sharing 300 milliamps will prevent this problem? The heater circuit being higher in resistance? Or is it that the 12VDC is just more common?