• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Those Magnificent Television Tubes

That 38HE7 pentode curve set above (curve 5 ) looked like it had slightly more screen current kinks than I recalled being typical, so I checked a few more tubes. The others weren't that obvious on kinks, so I traced a good one. (probably some grid1 / grid2 mis-alignment in the original 38HE7 tube traced)

Below is a more typical pentode curve set for 38HE7. Looks more like the 42KN6 curves now too. Using 117V on grid2 to get the vertical current span of the bigger tubes (versus 76V for the 42KN6), so the screen current kinks are a little wider than the 42KN6. (when parallel'd, the 38HE7 screen V would only need to be 76V to match the vertical current span of the 42KN6 traced above)

38HE7 in pentode
 

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smoking-amp, just curious, how do tube manufacturers determine plate dissipation ratings? what process or procedures are involved..?
The fundamentals start with basically the cathode temperature and heater power, as the cathode has to be large enough to emit sufficient current for the tubes application, and the heater has to get the cathode surface temperature to the right goldilocks zone.

The anode radiates its heat away, but its temperature has to be kept below some upper level due to influence of it bouncing electronics back in to the valve, and to outgassing, and it being the go-between for internally higher temperature parts (cathode) needing to radiate heat away (which effectively transfers to the anode).

The anode radiates its heat out via the glass bulb - the longer wavelength portion of that heat transfer gets soaked up by the glass, and the glass has to cool itself using outside air convection, and not get above a certain temperature or it becomes a major outgasser.

The getter aims to soak up remnant gas after production, and what is outgassed over time. Extending the production effort can reduce remnant gas and outgassing, but the law of diminishing returns and economics means a compromise is reached.

The RCA 1962 technical manual is a great place to immerse yourself if you are keen to browse the very complex nature of all things 'valves'.

As an aside, it's not that easy to diy measure anode temperature - as you have to check what wavelength is being used by the sensor, to make sure it operates at sufficiently short wavelength where that thickness of glass is basically transparent, and without an internal thermocouple you don't have a reference.
 
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Technical books online Tube theory & circuit design

Electron Tube Design, RCA 1963 943 pages!

Vacuum Tube Design, RCA 1940 260 pages

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Or just use Tubelab's approach: heat until plate glows, then de-rate.

Which I would suggest maybe amending to:
de-rate a known similar (cheap) tube to find the de-rate ratio for getting the datasheet rating, then apply the same ratio to de-rating the unknown tube (providing you have a cheap one to test up to red-plate). Maybe adjust the de-rate ratio upward, the cheaper the tubes are. $1 tubes at 70% of red-plate. :D
 
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12HL7, 12HG7, 12GN7 get mentioned occasionally for a paralleled output P-P stage. Especially when the 12HL7 was on a $1 list for a while. The tiny frame grid wires would likely be touchy about over-driving them into positive grid territory, but could be protected by a Si diode from cathode to grid. I haven't heard of anyone actually building such a paralleled design though. (too many bias pots are the show-stopper I think)

The other concern is microphonics with such tiny grid wires. Someone mentioned the sound of 100 pianos. Although these frame grid wires are well tensioned on the support rods. Can always rubber suspend the (bank of) sockets somehow.

The 12HL7 has triode curves that rival a DHT (when curve tracer selected).
These frame grid type tubes may be rather sensitive to shipping shocks however, causing slightly off center grids (due to tight mechanical tolerances) and then poorish triode curves.

The 12HL7 (and others) work quite well for Crazy/Twin drive too. (2nd pic below)

I generally think of these tubes as a super driver stage for local N Fdbk, especially to the driver cathode or grid.

Another scheme is to use them as a tube Op Amp for correcting linearity of another TV Sweep tube. (rough concept diagram below)
 

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Those 10 Watt+ frame grid tubes (12HL7, 12GN7, 12HG7) could be used in some unique driver circuits due to the high quality triode between grid1 and grid2 or because of the high gm.

High gm:
Triode emulation via reverse N Fdbk thru a triode, enforced by a high gm stage (essentially a one tube Op Amp.). A Japanese site has been big on variations of this. This allows control of a big TV Sweep such that it exactly emulates an "emulant" triode. (V1 is the triode emulant below, V2 would best be a high gm tube, so pentode best, and M1 would be the power TV Sweep tube.)

a) rough emulation idea
b) $4 perfect DHP in triode mode to emulate (who needs 300Bs! )
c) or just use another 12HL7 etc in the diff stage as the triode emulant

Internal quality triode:
Jan Veiset pioneered a P-P pentode driver design where the local N Fdbks (crossed for phasing) come back to the driver pentode screen grids. By adjusting the amount of N Fdbk, the voltages on the screen grids can be made to operate at some constant % of the driver plate voltages, so they draw a constant % of plate current. Making the screens look like nice constant resistive loads for the N Fdbk R dividers. (otherwise Mosfet followers would be needed for the screen grids)

With a pentode driver containing a high quality triode (g2, g1) this would have the effect of making the output power tube emulate the internal driver triode directly. (similar to the emulation scheme above, except with N Fdbk sent through the internal triode in reverse, so high gm drive and "triode" emulant all in the same tube here. )

If the N Fdbks came back from CFB OT taps (instead of plates or UL taps), then they would not need to be crossed for phase, so could work for an emulated SET output design.

However, you don't really need 10 Watt+ driver tubes for these fancy driver schemes unless a whole bank of TV Sweeps follow them. Some 4 or 5 Watt frame grid 9DX base tubes would work just fine too. 6LY8, 6JT8, 6LQ8 and 6KV8 have very nice triode mode curves (of the frame grid pentode) for example. And these include an extra triode for the amplifier front end as well. Most 9DX base tubes are on the $1 list. Some 20+ 9DX types to roll through if you are into that.
 

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Here are some triode mode curves for the 9DX base pentodes: 6LQ8 and 6HZ8.

6LQ8: 50V/div Horiz., 10 mA/div Vert., 0.5V steps (frame grid, about 50 Mu)

6HZ8: 50V/div Horiz., 10 mA/div Vert., 1 V steps (non frame grid, about 25 Mu)

You will have a tough time finding "real" triodes this good!
 

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Here are some triode mode curves for the 9DX base pentodes: 6LQ8 and 6HZ8.

6LQ8: 50V/div Horiz., 10 mA/div Vert., 0.5V steps (frame grid, about 50 Mu)

6HZ8: 50V/div Horiz., 10 mA/div Vert., 1 V steps (non frame grid, about 25 Mu)

You will have a tough time finding "real" triodes this good!


this might put the tube off the dollar list, but nice to know anyway...
 
Here is the list (from the back of the GE manual) of the actual 9DX 9 pin mini base tubes:

6AU8, 6AW8, 6BA8, 6BH8, 6CX8, 6EB8, 6GN8, 6HF8, 6HZ8, 6JE8, 6JL8, 6JT8, 6JV8, 6KR8, 6KS8, 6KV8, 6LB8, 6LF8, 6LQ8, 6LY8, 6MV8, 10JA8, 10LW8, 10LZ8, and higher heater V versions of these

then (similar/compatible base) 6KT8, 6GN8, 6ET7, 6KU8

Keep in mind that getting a good triode curve does not guarantee reliable operation in triode mode over extended time. Some sweep tubes tend toward bias run-away in triode mode after long hours. But if you keep the B+ reasonable, they probably will work OK in triode mode. Usually 300V or 330V datasheet screen grid ratings for these. (versus some TV sweeps with just 250V ratings) These tubes were intended for low screen V (150V typical) in practical operation, so no need to push the B+ up too high anyway.

Good triode curves also mean low distortion pentode curves, due to the grid not being too close to the cathode. These can work great for the pentode driver scheme with N Fdbk to the screen grid.

Some of the other 9DX tubes need to be curve traced yet to see how good they are. I've just cherry picked the most likely ones so far. 6HZ8 was a real surprise, big enough to replace a 6BM8 for low power, low cost, two tube P-P amplifiers.
 
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I would guess that 400V peak in triode would work OK, but that's just a guess.
Maybe can go by some pentode tubes that do give triode ratings.

The screen grid support rod only has a small bit of mica between it and the grid1 rods. While typically a plate has a cut out air gap in the mica around the support rod. Any leakage over to grid 1 and away it goes...poof! Of course a low impedance drive that limits the (positive) excursion of grid1 would greatly help, so that probably should be a design emphasis. A SS diode limiter for grid1 (from cathode) would be a good idea. As well as a low Watt fusible cathode resistor. Or a CCS tail. A grid2 stopper resistor (to plate) would also be helpful for triode safety.
 
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Call me a fan of Crazy Drive (and sweep tubes in general). Having first read about crazy drive here, I wanted to explore its use in modulators for amplitude modulated (AM) amateur radio transmitters.

There is a relatively small, but very dedicated, community of radio amateurs that continue to use the AM mode and many strive for high fidelity audio in their transmitters. There are many modified commercial transmitters as well as completely scratch-built/DIY/home-brewed units on the air. Some of these require very high power audio amplifiers. TV sweep tubes have been used for this purpose for as long as I can remember. Many have used screen driven sweep tubes with 6DQ5's probably being the most popular choice. Crazy drive is a great refinement of screen drive and offers many benefits.

To help bring the crazy drive concept to that community, I have posted a link to this thread in a very popular AM community forum, AMfone.net. Here is a link to the thread which I started:
The RCA "807 in Special Triode Connection" gets a new life. With Sweep Tubes!

My hope is that the crazy drive method will gain traction with the many experimenters in that community and further refinements can be made.

I am very grateful to those in this forum who have obviously spent many hours of work exploring crazy drive. My hope is that the common interest in hollow-state technology and home construction that these two communities share will add to the knowledge and enjoyment of them both.

Respectfully submitted,
Don - KK4YY