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Old 15th February 2004, 05:44 PM   #1
wfmali is offline wfmali  Europe
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Default G2-driven 6BM8

Hello everybody,
After some triode-amps I have developed a little circuit for a G2-driven amp which should be fine to develop approx. 15W before clipping using 6BM8 or PCL82. And in fact it does with good results - using pspice. Because of these promising results I decided to give it a try in reality and prepare some PCBs for it. But before I start this painful work every comment on the circuit is very welcome (some call it review...).
In general, I prefer circuits with no "over-all-feedback" but I have provided it here fearing some instability. However, dont take the value of the r and c in the feedback-path to serious, the exact values have to be developed in reality

Marcus
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Old 15th February 2004, 05:54 PM   #2
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I haven't tried this with a 6BM8, but I've had great results using G2 drive with sweep tubes. General comments: get rid of o/p stage cathode resistances (except for metering purposes); ground g1 directly, then use a variable voltage at the grids of the cf to set idle current (return the cf cathode resistor to a good negative rail). With sweep tubes, the optimal idle current will be shockingly low (3 ma or so for 6LF6s). I'm not crazy about using a 12AU7 as a cf in this application, but it will still probably work ok.
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Old 15th February 2004, 08:05 PM   #3
wfmali is offline wfmali  Europe
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Sy,
I have chosen the 12AU7 and the 8k-PP Output-Transformer because I have them available.
I trust both, but of course running the 12au7 with significant current. 6LF6s with only 3 mA sounds interesting, less heat, better load-situation for the power supply... have you driven 2 G2 in parallel with them ?
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Old 15th February 2004, 08:44 PM   #4
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Haven't had to- a single pair with a 1250 ohm p-p load and 600V B+ gives me close to 300 W in short bursts. The grid load of a single 6LF6 driven that way is probably a lot more than the two 6BM8s in parallel.

In a review in Audio some years ago (I think it was for a Berning power amp), Bascom King showed some curves for sweep tubes run this way, low idle current, high B+, cf screen drive. They were impressively linear, triode-like for the first few watts.
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Old 15th February 2004, 09:11 PM   #5
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I also like these big sweep tubes, have some 6DQ6 and 6DQ5 waiting for some experiments. However, I never heard a sweep tube amp. But hey, the year has just begun... I will first try these 6BM8s, and than the sweep tube time might come.... For the amp above: I am really considering to use "fixed bias" in the driver stage to set the idle current, its probably better.
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Old 15th February 2004, 10:15 PM   #6
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Default 6DQ6

I'll check the tube manual, but I think that g2 drive for those runs too high to be useful.
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Old 23rd March 2007, 04:16 PM   #7
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Hi SY

As relaxation therapy between classes, I spent some time looking at Berning's amplifiers (screen driven class B), and, therefore, I tried to do your homework on the 6DQ6 Looking at the curves for G1=0, the curves for G2 seem to meet your requirements:
- evenly spaced,
- not too high a voltage for a reasonable amount of current swing
(Actually they look much like the 6JN6 curves).

But, that's just an indication of suitability, as, if I understood well, the curves for actual operation at very low bias currents are different. (quoting you)

Quote:
In a review in Audio some years ago (I think it was for a Berning power amp), Bascom King showed some curves for sweep tubes run this way, low idle current, high B+, cf screen drive. They were impressively linear, triode-like for the first few watts.
Sure they must be different, as in Berning's amplifiers the screen grids are at negative potential, while, at the curves from the datasheets, they are at positive potential. Also, in Berning's amps the bias current is some wimpy mA, while the curves with positive screen grids show a standing current of several tens of mA. The curves for g1=0 as shown in the curves are actually suitable for a design such as the EL509 by Bob Danielak (I think).
http://www.geocities.com/bobdanielak/technoteNo33.html

As said, I am trying to understand it, for the moment I do not have plans to build something (but, as the driver stage of the CP is in the makes, I may try to use it to drive some 25DQ6B's I have on hand)

Thanks for your attention!

Erik
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Old 23rd March 2007, 04:30 PM   #8
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Tubelab has started doing some work in the direction. He's seeing the same thing I do, that there's not much linearity penalty for trickling the current in the output stage.

The 6DQ6 does indeed look pretty good.
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Old 23rd March 2007, 04:43 PM   #9
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Did you try 25BQ6?
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Old 23rd March 2007, 05:25 PM   #10
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Quote:
Tubelab has started doing some work in the direction. He's seeing the same thing I do
I experimented with a screen driven push pull amp that used 6AV5's a while ago. It made 80 watts at clip, and 2.5% distortion at 50 watts with zero feedback. This was an experiment to learn about screen drive. The amp had 3 power supplies hooked up so that I could adjust everything for maximum linearity and power output. I found that I got the best results with a slight negative bias on G1. This allowed a higher swing on G2 for more linear power output. Bias current was low, and the amp was highly efficient, allowing large power outputs without high tube dissipation. The output power was limited only by the maximum voltage from my power supply. Your driver circuit must be capable of supplying a lot of drive voltage and the screen current. I used mosfet followers.

I needed space for some new experiments, so I cleaned this design off of my breadboard just last weekend. At least this time I documented the design. I could post the schematic this weekend if anyone is interested.
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