TRIODE OPERATION USING THE SCREEN GRIDS AS THE CONTROL GRID - this work ? - diyAudio
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Old 18th May 2010, 04:12 AM   #1
aldovan is offline aldovan  Brazil
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Default TRIODE OPERATION USING THE SCREEN GRIDS AS THE CONTROL GRID - this work ?

Friends

I see this in a article.
This is possible ? Work fine ?

" TRIODE OPERATION USING THE SCREEN GRIDS AS THE CONTROL GRID

This mode of operation offers yet another alternative for the application of Pentodes, Tetrodes or Beam Power Tubes as Triodes.

The method is simple.

Connect the Control Grid #1 to the Cathode

Use Grid #2 as the Control Grid.

This approach requires substantially higher DC negative bias voltage and considerable AC drive signal input voltage than as per the standard configuration, but offers the benefit of allowing full rated Plate Voltage to be applied.

This is because the Screen Grid does not attract electrons - in fact it is negatively charged so cannot.

Power output should thus be similar to that obtainable in Tetrode or Pentode mode - but with the tonal characteristics of Triodes. This is because the tube is now a real triode - ie only 3 effective elements (Cathode, Control Grid and Plate).

I am unable to provide details of typical operating conditions but this method has been used in professional broadcast standard equipment and in theory should be more reliable than the standard approach.

Class AB2 or B with fixed bias is essential for high Plate circuit efficiency and dynamic performance. "

Thanks,

Aldovan
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Old 18th May 2010, 04:35 AM   #2
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Where have you been? This has been discussed (and done) frequently here. The only place it's generally worthwhile is with sweep tubes, which have more sensitive screen grids than your average pentode. Use search to find relevant threads - there are quite a few.
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Old 18th May 2010, 11:40 AM   #3
aldovan is offline aldovan  Brazil
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Default OK

Quote:
Originally Posted by wrenchone View Post
Where have you been? This has been discussed (and done) frequently here. The only place it's generally worthwhile is with sweep tubes, which have more sensitive screen grids than your average pentode. Use search to find relevant threads - there are quite a few.
Ok, but I will use a more sensitive screen tube, a small power "real" tetrode and no pentode or "beam tetrodes"...but the idea is the same, thanks for the patient !!!

Aldovan
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Old 18th May 2010, 03:25 PM   #4
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Quote:
This approach requires substantially higher DC negative bias voltage and considerable AC drive signal input voltage than as per the standard configuration, but offers the benefit of allowing full rated Plate Voltage to be applied.
You will also find that for most tubes the screen grid will require a small POSITIVE bias voltage, with considerable signal voltage applied to it. The plate resistance is much higher than the same tube operated with conventional G1 drive, often requiring NFB.

A few people have used this technique in SE amps, but the real advantage comes in P-P operation where the output tubes can be operated very close to class B with excellent linearity providing higher overall efficiency. Many P-P amps, especially those with sweep tubes are limited by output tube dissipation, so increasing the efficiency affords higher power output.

A few of us are beginning to experiment with applying drive signal to both G1 and G2. Preliminary results show this to be viable too.
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Old 18th May 2010, 06:02 PM   #5
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Quote:
Originally Posted by tubelab.com View Post
A few of us are beginning to experiment with applying drive signal to both G1 and G2. Preliminary results show this to be viable too.
Driving both grids still results in a "high impedance triode" mode of operation but requires roughly 1/2 the drive voltage of g2-only drive.

Tubes that are good candidates for this mode have relatively low g2/g1 mu factors (3-4) and high-ish gm. I guess that's why the 6L6 types etc are less suitable and sweep tubes and transmitting tetrodes are more suitable.

A typical sweep tube might need 200V peak drive signal or more in g2 drive, but maybe only 100V in g2/g1 drive.

Driving both grids, probably optimally at the ratio of the tube's g2/g1 mu, should result in good linearity + sharp cutoff as in g2 drive, allowing a lower current idle point. Output power, distortion, and tube life in normal service are all improved.

Peak screen voltage should be less than in the fixed-supply case, perhaps lowering the minimum Vak "knee" voltage (diode line).

There is a thread with plate curves of several tube types tested in this mode on a curve tracer (done by smoking-amp)

The regulated g2 supply is not needed, but the required driver circuit is more complex. I prefer MOSFET followers for driving power grids, but high-gm cathode followers should work about as well.

Here's a work in progress that shows how I am implementing the scaled drive. Screen drive could be done in a similar fashion without the voltage divider, but would need a higher driver B+ for a given tube type.
Attached Images
File Type: png darkside-scaled-drive.png (19.2 KB, 298 views)
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Old 19th May 2010, 02:43 AM   #6
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For more information on driving both G1 and G2 see this thread:

G1=G2/mu Scaled Drive Strawman Design

I made a simple test amp starting in post #28. No further work was done on this design but it told me what I needed to know. A much bigger version is planned. Of course finding 45 NOS 35LR6's for $3 each at a hamfest didn't hurt either.
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Old 19th May 2010, 06:36 AM   #7
jfitz57 is offline jfitz57  United States
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Quote:
Originally Posted by tubelab.com View Post
The plate resistance is much higher than the same tube operated with conventional G1 drive
Hi Tubelab

How much higher? 2:1? (see where I'm going?).
But I'm looking to build something with NOS tubes and no iron, so it would be just for reference info. I had the idea for a sub-woofer amp using 2 6LR6's in grounded grid screen drive about 30 years ago. 10 watt PP OPT in reverse driving the G2's, 600-800 volts B+, wind my own OPT using a big a$$ '60's era TV power xformer. I never got it all together though.

Jim
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Old 20th May 2010, 05:40 AM   #8
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Any reason why an interstage transformer couldn't be used for screen drive?
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Old 20th May 2010, 06:30 AM   #9
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Not at all - I'm trying an interstage autotransformer phase splitter driven by a source follower for a class AB2 amp using 6P3S-E output tubes (check out "The Bursar" thread). If this works out ok, I'll be trying the same principle with screen-driven sweep tubes.
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Old 20th May 2010, 09:23 PM   #10
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The idea of G1 & G2 drive is interesting, but it seems to me likely that any unique advantages in efficiency, at least, would require that G1 be driven into the grid current region.
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