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Old 31st May 2004, 05:35 AM   #1
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Default A better triode strap?

I followed a link while reading another thread and came across this article or rather copy of a email:

http://ken-gilbert.com/techstuff/tri...ntodesbts.html

This is of great interest to me. Can anyone explain the advantage of this method over that of using a simple resistor. Also, what is the advantage of biasing the "screen grid to -5v? Why not connect it to the cathode in the traditional method? I would like to try this on my new EL34 amp but I would like some opinions first. Will this method make a strapped EL34 sound more like a "real" triode?
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Old 31st May 2004, 06:40 AM   #2
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Stopped reading about 2 pages down. AHHHH THE WHITE ON BLACK IS THE HURTING OF EYES NOW! ...Fading... still got lines across my eyes... ugh... anyways.

The very logic this guy has is bad, a screen's potential for acceleration has nothing to do with how far it is inside the electron beam. Shadowing it by the grid is fine AFAIK. Although... I suppose the density of electrons at that point would tend to produce a generally negative field there (space charge) meaning that putting a screen wire in the middle of this charge would tend to assist things. But then, the repulsion of this charge on the electrons from any given point to another would be negative closer to the cathode (tending to repel backwards) and positive closer to the plate (pushing them into it). So I say it averages out and you're better off with screens in the grid's shadow to reduce dissipation.

Don't know what else he's going on about. Reducing screen voltage will ALWAYS reduce plate current, so you'll certainly get less power with the zener modification. Thus, being closer to max. output, it could be argued that distortion will be higher for a given signal level.

Typically, using a screen voltage lower than plate reduces distortion, but as I recall it's mostly odd order. SE will always have 2nd H distortion.

Tim
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Old 31st May 2004, 01:51 PM   #3
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I'd be interested to see if there are any actual (as opposed to claimed) performance differences from using a zener to stand off the screen from the plate by a few volts. The poster in this reproduced exchange doesn't cite any. I've got a guess, but would rather see data.

It's interesting to consider the magnitude and relative phase of the voltage variation with signal caused by the strapping resistor.
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Old 31st May 2004, 05:26 PM   #4
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Oh yeah, the strapping resistor. That does basically nothing in normal operation with normal values. Even if screen peaks to 50mA, it'll drop only 100 * .05 = 5V. A 1k resistor will soften things up a bit more, but it's important to note the voltage across such a resistor varies wildly with operating point, making the tube wheze and cough as a signal swings near Vg=0V.

Tim
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Old 31st May 2004, 08:00 PM   #5
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Quote:
Even if screen peaks to 50mA, it'll drop only 100 * .05 = 5V.
You're being too severe, even with those conservative numbers. Let's consider some tubes that might actually be likely to be strapped and see the difference between zero signal and max screen current per tube (tube manual numbers):

6L6GC: 5-8 ma
7591A: 5-10 ma
7027A: 5-10 ma
8417: 12-15 ma
6550: 15-18 ma

So, in your worst case, the screen fluctuation due to a strapping resistor is something like 1 or 2 volts out of a 300 volt swing.
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Old 31st May 2004, 08:20 PM   #6
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Quote:
Originally posted by Sch3mat1c
Stopped reading about 2 pages down. AHHHH THE WHITE ON BLACK IS THE HURTING OF EYES NOW! ...Fading... still got lines across my eyes... ugh... anyways.
In my browser it is Black on Black, makes it even harder to read, but at least i'm not cross-eyed. Here is the original email that i clipped:

Quote:
Hi All:
This post is one that has been sitting in my "Drafts" folder for months.
It relates to a discussion back then on triode connecting pentodes and
tetrodes.
----------------

Kurt Th. Steffensen wrote:

> I do not know how Bill does the trick , and I respect that he wants to
> keep that to himself.
> However I described such a circuit on this list , maybe12-18 months ago.
==================
Hi Kurt, Joes:
I don't care to keep this any great secret . . I just worked hard to
get the theory into my head - much digging thru old texts on tetrode
theory - and I didn't feel like giving it away . . . just wanted to have
some fun is all, and maybe stimulate a little thinking and re-evaluation of
some tired old "standard procedures."

The way I do this little trick - as a couple of folks already knew - is
to bypass a zener with a cap. But, not just any zener and not just any cap.
My favorite zener is a 1N821, a 15cent, 1/2 watt, 6.2 volt part that is
both very quiet and of nearly zero tempco. I forget why 6.2V is the "magic"
voltage for zeners but it is, the noise falls thru the floor and the low
tempco yields a very stable voltage source . Maybe someone could post on
this 6.2V business . . .
To build this:

- string 5 of the 1N821s in series and set them aside.
- take two, 16V, 470uF Sanyo OSCon caps and put a 10k resistor across
each one, then hook the two RC combos in series; then connect that
assy. across the zener string being sure to observe the polarity.
- then put a ten ohm, one quarter watt resistor on the cathode end of
the string and a resistive ferrite bead on the other.
- connect the other end of the ten ohm resistor to the plate of the
tube to be "triode connected" and the ferrite end of the assembly
to the screen, and there you have it.

Note that if you are using true, three grid EL34s that the suppressor
is best connected to about -5V or so. Just voltage divide off your bias
supply.
Biasing that grid slightly negative reduces its electron interception to
essentially zero.

Have fun discovering what a triode connected tetrode/pentode really
sounds like.

Bill - PEARL, Inc.

Quote:
Originally posted by Sch3mat1c The very logic this guy has is bad
I never dismiss anything Bill says lightly (at least about electronics)... i haven't tried this yet, but i have seen Bill install it into an amp for a client.

dave
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Old 31st May 2004, 09:27 PM   #7
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Quote:
Originally posted by SY


You're being too severe, even with those conservative numbers. Let's consider some tubes that might actually be likely to be strapped and see the difference between zero signal and max screen current per tube (tube manual numbers):

6L6GC: 5-8 ma
7591A: 5-10 ma
7027A: 5-10 ma
8417: 12-15 ma
6550: 15-18 ma

So, in your worst case, the screen fluctuation due to a strapping resistor is something like 1 or 2 volts out of a 300 volt swing.
That would be with a suitable capacitor bypassing the resistor. I mean instantaneous figures you find on the left side of the loadline.

Tim
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Old 1st June 2004, 01:40 AM   #8
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Default huh?

From reading Planet10's copy of the email, it seems to me that even just 5 mA of screen current will charge the zener limited capacitor string to the 31V (5*6.2V). The two 10K equalizing resistors in series with 5mA, would give .005 * 20,000 = 100V, so no problem charging up to the 31V limit. The 10 Ohm resistor and ferrite bead is probably just to prevent parasitic oscillation like a grid stopper?
Having 31 V less on the screen grid than the plate will drop the max. intercepted plate current (= screen current) down on the left, or high current, side of the plate characteristics nicely, causing the screen to run cool. Seems like a nice idea for the tube's longevity. But it will limit max plate current and, hence power output a bit, since the screen voltage determines the max plate current. If one could boost the B+ voltage by 31V as well, then it would be a "wash". If you don't care about the tube life, then Tim is probably right about more headroom giving less distortion, so skip the mod. The -5V on the Suppressor grid may be a little overkill, since it usually is a pretty open spaced grid. My 2 cts. I could be wrong.

Don
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Old 1st June 2004, 01:55 AM   #9
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Hi,

Quote:
The -5V on the Suppressor grid may be a little overkill, since it usually is a pretty open spaced grid.
I don't think the voltage as such matters but the polarity does. It does seem rather far fetched, something I'd like to see on a plotter first.

BTW, this will only be of use on the Euro/Russian EL34s.
Not the current production Ei EL34s which are tetrodes like the US 6CA7s with the suppressor tied to the cathode.

Quote:
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I'm putting my on you...

Cheers,
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Old 1st June 2004, 11:48 PM   #10
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I'm so glad "G" asked this question, since I was puzzling over it myself and was going to offer it up to the group for discussion.

I'm still not sure I understand it its circuit effects yet, but I'm extrapolating from everyone's comments that:

1. No-one here has yet tried it, and so can't yet comment on it sonically or electrically (ie its actual measured performance in a circuit)

2. Despite the pedigree of the original designer; the theoretical foundations that appear to support his mod are either not clear or unsound, thus negating the desire for someone to actually want to try it.

<goes back to trying to think it all through...>
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