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Old 23rd March 2015, 12:03 AM   #1
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Default Advice on triode connecting a 'JZ6 tube

I just got a good deal on a lot of 21JZ6 tubes. Spec is attached. I'd like to try triode strapping them. A couple questions come to mind:
  1. If I connect the beam forming electrodes to the plate, what does this do to the power rating?
  2. I've seen some examples where the screen grid is used to drive and G1 is connected to the cathode. Any advantage to this?
Any other ideas would be appreciated. Thanks!
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Old 23rd March 2015, 12:31 AM   #2
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In triode mode you will lose quite a bit of power.
You might consider pentode mode or ultra linear.
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Old 23rd March 2015, 12:46 AM   #3
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6JZ6/21JZ6 is equivalent to the 21HB5A. http://frank.pocnet.net/sheets/123/2/21HB5A.pdf

Some Triode and Schade curves for it here:
Schade and CFB exactly equivalent

Schade Fdbk mode will give you the most linear and good power out (nearly same as pentode).

1) Triode
2) Schade
3) B. Pentode

Connecting the beam forming plates won't give you any additional power capability. Anything inside the plate has poor heat radiating ability. The usual rule for the screen grid is: for every Watt of added screen grid dissipation, the plate dissipation gets reduced by 4 Watts.

Driving the screen grid as input gives a nice linear spaced pentode curve like set. It can be dangerous for the screen grid if overdriven though. It will require 4.8 X the voltage swing on g2 as on g1 from the driver stage (internal Mu of 6JZ6 is 4.8), as well as screen current loading on the driver stage.

Schade Fdbk mode requires a special driver stage capable of linear and significant current output (or a large voltage swing, if it simulates the current output mode by using a series resistor from a subsequent follower.)
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File Type: jpg rsz_21hb5a_t.jpg (44.9 KB, 125 views)
File Type: jpg rsz_21hb5a_s.jpg (41.9 KB, 125 views)
File Type: jpg rsz_21hb5a_p.jpg (50.3 KB, 122 views)

Last edited by smoking-amp; 23rd March 2015 at 01:16 AM.
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Old 23rd March 2015, 01:28 AM   #4
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The best (linearity) approach of all is to take Schade like Fdbk (resistive from plate or UL tap) back to the driver stage cathode(s). This puts the most gain in the local loop to linearize the output stage (like another 50 X), and makes for a relatively easy to design, and easy to drive, driver stage. For P-P amps, it also is VERY effective at removing crossover distortion (provided the driver stage is differential, CCS tailed). It may however be SO linear as to sound modern SS like.

The RCA handbook has a 50 Watt amplifier (P-P) schematic that uses both the above approach, and the more usual local Schade (R to the output tube grid from plate) approach, combined.

21HB5A/21JZ6 g2 or screen drive curves: (stop that drooling!!)
50 mA/div Vert., 50 V/div Horiz.

I suppose one could combine screen drive with Schade to the driver cathodes if you want to break the tube amp laws. ( super ultra mega linearity)
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File Type: jpg rsz_21hb5_g2.jpg (56.4 KB, 118 views)

Last edited by smoking-amp; 23rd March 2015 at 01:58 AM.
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Old 23rd March 2015, 04:27 AM   #5
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The Data for the 21JZ6 gives max screen voltage as only 220. Can you use it as a triode or UL? Surely only Screen drive or Pentode
Phil
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Old 23rd March 2015, 05:46 AM   #6
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The Data for the 21JZ6 gives max screen voltage as only 220. Can you use it as a triode or UL? Surely only Screen drive or Pentode
Phil

===

I was wondering about this. If I connect the screen (and the beam forming electrodes) to the plate does this limit still apply? I'd like to run it at ~300V for more power if it can be made to work.
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Old 23rd March 2015, 08:47 AM   #7
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Quote:
Originally Posted by geezertron View Post
I just got a good deal on a lot of 21JZ6 tubes. Spec is attached. I'd like to try triode strapping them. A couple questions come to mind:
  1. If I connect the beam forming electrodes to the plate, what does this do to the power rating?
  2. I've seen some examples where the screen grid is used to drive and G1 is connected to the cathode. Any advantage to this?
Any other ideas would be appreciated. Thanks!
1) Connecting the beam formers to the plate defeats the whole purpose for including them in the first place: preventing secondary electrons from increasing the screen current enough to push the screen characteristic into negative resistance. You'd get the same sort of nasty plate current "kinks" that you have with pure tetrodes. Sounds hideous, and makes for instability.

The spec sheet does advise some positive bias on the beam formers to prevent snivets.

2) Screen drive can get you some extra watts at the expense of requiring more driver input swing. You can also take the operation deeper into Class AB and not have the otherwise severe x-over distortions. These days, MOSFET source followers can drive the screens and their heavy current demands, making screen drive more viable than in the "good ol' days". Just be careful if you go this route. Overdrive can result in overheated screens, plate voltages that actually go negative, and the excessive screen currents that can encourage the poofage of driver transistors and finals.

Given that this is a HD final, it probably sounds quite good without needing any assistance from lNFB. If you do need lNFB, there's always parallel ("Schade") feedback, or cathode feedback if your OPT has secondary taps that can accommodate this. For UL, you'd need a source follower that supplies the screen voltage, and feedback voltage from the plates. This can be made adjustable (unlike UL taps) for the best performance. As a side benefit, the high current sourcing capability make for easier to design OPTs.

A PP pair looks like they could do some 60W, much more than you'd ever get trying to use these as triodes.
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Old 23rd March 2015, 09:54 AM   #8
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Quote:
Originally Posted by geezertron View Post
I was wondering about this. If I connect the screen (and the beam forming electrodes) to the plate does this limit still apply?
Yes.
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Old 23rd March 2015, 05:05 PM   #9
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"I'd like to run it at ~300V for more power if it can be made to work." "triode"

Well, first of all, you don't need high B+ with TV sweep tubes, because they can handle amazing plate currents. Which allows them to use a higher performing, low Z primary, OT.
And I don't know about the 21JZ6s you got, but the 21HB5s were on the $1 list for years. So just use two in parallel for more Watts out if that's an issue. Edcor's got a 1700 Ohm Zpri, 100 Watt, OT that would work for paralleled tubes.

But just for fun:

Looking at the datasheet curves versus screen grid voltages:

at 50V on screen and plate: max g2 current = 4 mA and max Ip = 110 mA
at 150V on screen and plate: max g2 current = 22.5 mA and max Ip = 610 mA

Determine a rough power law for g2 current: (150/50)^1.6 x 4 mA = ~ 22.5 mA
so about 1.6

So at 300V on screen and plate (triode configured): max g2 current = 68 mA and max Ip = 1849 mA,
that would give .068 x 300 = 20.4 Watts on the screen grid.

However, that's MAX Watts using 0V on g1.

For class A operation, average Ip current (and Ig2) would be 1/2 that, so .034 x 300 = 10.2 Watts on g2, exceeding the 3.5 Watt g2 rating.

For class AB operation, we can probably figure 1/2 that again average Ip current though, giving 1/2 the average screen current and Watt dissipation, so 5.1 Watts on g2. Still above the max g2 rating. Class B maybe could cut that by 1/2 again.

Then there is the 220V screen voltage limit, which would be rather exceeded in triode mode at a peak 600 V plate voltage. (I figure you can hit at least 2X the max g2 V safely in triode)

But just for fun (a $1 tube), I put a 21HB5A in triode mode on the curve tracer with 1150V plate peaks, and Da_n if it doesn't still work!! It took -260 V on g1 to cut it off. I'm surprised it didn't develop a g1 to K short at least. I don't think I would recommend this though, this was a cold tube. There's sure to be some inductive spike in class aB, or a hot grid under load, that would flash over.

Last edited by smoking-amp; 23rd March 2015 at 05:17 PM.
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Old 25th March 2015, 08:02 PM   #10
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I've been out a few days and am trying to catch up on all this.

To be clear I was interested in triode strapping mainly just to try it out, and because this tube does not connect the beam plate to the cathode, So I thought that connecting it to the plate might do something "interesting". I guess not.

Running at 300V was again just an idea based on something I read about triode connected tubes being able to handle higher voltages. I appreciate the complete analysis from "smoking-amp". Sounds like it's not a big win.

Can someone point me do a "Dummy's Guide" for Schade feedback. There's a whole lot about it out there, but mostly discussions between folks that already know what it means.

BTW - I paid <2$ a tube, 4 NOS for sure, 5 "test new" according to the seller. I'll get then tonight. I've found that 21V heater compactrons are often a LOT cheaper than the same 6V versions, and you can run them with a cheap-as-dirt 24V transformer and a small dropping resistor, or none if the transformer is wimpy...
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