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The zero bias valve
The zero bias valve
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Old 12th January 2016, 09:57 AM   #1
nicohfda is offline nicohfda  Spain
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Default The zero bias valve

dear All
I like to remind a deveopment in the years 1932 / 1933 that is mighty interesting. Most valves at this moment could produce approx 20% efficiency of undistorted audio. Especially for battery equipment, there were investigations to improve this figure and prof. Barton in the USA did design a valve that was really diffent, the zero bias valve 46. This valve has 2 grids, but these are connected in parallel. The anode current at zero bias is than quite low. The idea was to drive these valves in positive grid voltages in order to increase the efficiency and drive the valve to high currents at low anode voltages. An other effect is quite clear and may be more interesting for us.
A valve has in the negative part of the transconductance characteristics a square law . More to the zero volts grid voltage, the characteristics become quite linear upto a big part in the positive region of the grid voltage, so the valve characteristics are surprisingly linear. But the disadvantage is that the grid start taking current, so there is a driving power required.
The results are quite amazing. A pair of simple 46 valves driven by a small power stage non biased without any resistor in the final, delivers 20 Watt at 70% efficiency, quite a difference with the 20% of a 45 Valve.
Further devolpments resulted in double triodes like the 6N6 wich can deliver 10 watts!!. In order to avoid the high driving power, valves were developped where a cathode follower wass included in the enveloppe directly connected to the grid of the zero bias valve. The valves were called tripple twin valves, delivered 3 times more power than a 45 and two times more that a 47.
AND...
The first amplifier for guitar amplification made by Fender used these triple twin valves.

---This sound made Fender famous!!---

I have a pair of 46 valves and a nice and big speaker from 1935 with electromagnetic bias. In future for sure will try to reproduce the fender amp
Friends, I think that it is worth experimenting with zero bias valves and listning how these will behave in audio amps. Must be quite different.
I hope to have contributed to many hours of nice experimantation
Regards
Nico
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Old 12th January 2016, 11:43 AM   #2
ballpencil is offline ballpencil  Indonesia
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Since this has to be Class B amp, i wonder how audible will the crossover distortion be since we have an OPT to consider, unlike direct coupled solid state class B amps. Perhaps the OPT will make it less harsh? Anyway, i guess it won't qualify as hi-fi?
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Old 12th January 2016, 12:19 PM   #3
jazbo8 is offline jazbo8
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The zero bias valve
Quote:
Originally Posted by nicohfda View Post
The first amplifier for guitar amplification made by Fender used these triple twin valves.
Which model is that?
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Old 12th January 2016, 02:10 PM   #4
nicohfda is offline nicohfda  Spain
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A zero bias does take quite some current at zero bias, so it is a class AB with a considerable A part in the operation. And the characteristics are quite linear. I can't tell you about the results, never tried sofar. I consider it worth trying and testing.
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Old 12th January 2016, 02:13 PM   #5
nicohfda is offline nicohfda  Spain
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Quote:
Originally Posted by jazbo8 View Post
Which model is that?
It was in a Dutch magazine about old radio's called 'the historic radio club'. I am planning to move to Costa Rica, so unfortunately I have all books already packed. I think that the amp was 1935
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Old 12th January 2016, 02:50 PM   #6
billshurv is offline billshurv  United Kingdom
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The zero bias valve
VinylSavor: Tube of the Month: The 46
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Old 12th January 2016, 03:09 PM   #7
jazbo8 is offline jazbo8
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The zero bias valve
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Originally Posted by nicohfda View Post
It was in a Dutch magazine about old radio's called 'the historic radio club'. I am planning to move to Costa Rica, so unfortunately I have all books already packed. I think that the amp was 1935
Have a great move, hopefully you can post the article once you unpack and settle down. My interest was piqued because I could not find a reference on any Fender amplifier that used the triple twin power tube...
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Old 17th January 2016, 09:03 PM   #8
Miles Prower is offline Miles Prower  United States
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Originally Posted by ballpencil View Post
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Since this has to be Class B amp, i wonder how audible will the crossover distortion be since we have an OPT to consider, unlike direct coupled solid state class B amps. Perhaps the OPT will make it less harsh? Anyway, i guess it won't qualify as hi-fi?
In the first place, you don't have to work it in Class B. You can always bias into Class AB. Also, VTs don't have so wicked x-over behaviour as transistors (BJTs are better in this regard than power MOSFETs that can have really nasty x-over). These days, making use of types like this, and the RF zero bias triode finals, or the near zero bias power pentodes like the 814 or 1624, is a good deal easier than back in "the day" since you can use a source follower as the grid driver.

If these don't have that audiophool premium, and/or they're not unobtainium, might be worth looking into.
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Old 17th January 2016, 10:01 PM   #9
pcan is online now pcan  Italy
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The zero bias valve
The model EH-150, one of the first Gibson guitar amplifiers, has a push-pull of 6n6 in the output stage. I don't remember any Fender with 6b5/6n6.

This line of composite power tubes started in 1935 with the Triadyne 6B5. Here is the original manufacturer documentation, with very detailed data: http://frank.pocnet.net/sheets/201/6/6B5.pdf ; the evolution were the 6n6 (same tube but octal base, either glass or metal can: http://frank.pocnet.net/sheets/201/6/6N6MG.pdf ) and 6AB6G (automotive applications, http://frank.pocnet.net/sheets/201/6/6AB6G.pdf ). The optimal SE output transformer load impedence is 7Kohm; a 15V RMS input signal is required to get the full 5W rated output for SE operation.
I've done a bit of research about this tube in the past because it seems to require a minimal number of components to assemble a complete SE triode amplifier; I then put the project aside because it looks good on paper but there must be some flaw in practice: this tube family with the dual-grid output triode was never widely deployed and fell of out fashion very rapidly, in a matter of 5-6 years only. On my research, I only found a few small PA amplifiers and the Gibson guitar amp.

Last edited by pcan; 17th January 2016 at 10:04 PM.
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Old 18th January 2016, 04:16 AM   #10
Wavebourn is offline Wavebourn  United States
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The zero bias valve
We recently discussed "right handed" triode in another thread, about magnificent TV tubes.
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