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Old 18th December 2004, 05:33 AM   #1
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Default A better triode strap?

Hi all,

I was doing a little cleaning up today and came across a article that I printed out a while back concerning an improved triode strap for tetrodes and pentodes. I drew up a diagram which I think is correct and I think I'm going to try it out. If anyone spots any mistakes please let me know. I will report back as to the results.
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Old 18th December 2004, 07:46 AM   #2
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Are the zeners backwards?
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Old 18th December 2004, 08:26 AM   #3
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...Ya.

Good way to ruin perveance there (i.e. current capacity, having the effect of increasing ideal RL and reducing power output).

Tim
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Old 18th December 2004, 01:54 PM   #4
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Is the motivation here a way to get around a limiting Vg2(max) spec?
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Old 18th December 2004, 02:27 PM   #5
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Quote:
Originally posted by SY
Is the motivation here a way to get around a limiting Vg2(max) spec?
The idea is to, and I quote, "to level- shift the screen to a lower DC level, thus limiting it's dissipation, and to bypass the level shifting circuitry with a low impedance ie. a *very* good, high value, low voltage cap of the BlackGate, Cerafine, Oscon variety.
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Old 18th December 2004, 02:45 PM   #6
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It might be interesting to see how the zener string will stay turned on at the low extreme of signal swing. FWIW, Audio Research did a trick a bit like this, though using the zeners to bias up the gate of a MOSFET, and had the MOSFET's source drive the screen.
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Old 18th December 2004, 02:47 PM   #7
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It seems that I have posted this in the past. I had forgotten. Here is where I got the circuit:

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

and here is the thread from the last time I asked about it:

A better triode strap?
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Old 18th December 2004, 09:26 PM   #8
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Well, the capacitor will keep the string "alive" for short periods (depending on the dynamic resistance of the diodes). For long periods... well, do you know of any tube whose screen can draw negative current?
If nothing else, you can add a few ohms to the zeners to increase dynamic resistance. Then you have a simple ohmic circuit with an RC time constant. Oh, and keep in mind that the capacitor charges through the screen grid, so 220uF will charge slow as nuts as the tube heats up, giving it an extra incentive of high screen voltage as things settle.

Also, do you know of any EL34 which can draw appreciable plate current, with Vs = 0V and Vg < 0V (not drawing grid current)? Point being, you'll never get plate to zero because current drops well before screen reaches zero. Which happens faster than the plate, with the diodes in place. And if the plate were somehow forced below string voltage, the screen would go negative, drawing no current and

Tim
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Old 19th December 2004, 12:28 AM   #9
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He did say "limiting it's dissipation". Not eliminating it. Let's use some good old fashioned logic here. It must work and give a measurable (and audible) difference when compared to using a simple resistor strap or there is no way Mr. Pearl would use it. It is not a cheap "trick" if you are going to use BlackGate caps. Rather than talk the idea to death I will sacrifice a pair of (awful) JJ EL34s and find out how effective the "trick" is. I guess I should turn the Zeners around huh? Although I was sure that I drew up the idea as he described it. Oh well.
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Old 19th December 2004, 10:44 AM   #10
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Quote:
Originally posted by G
Rather than talk the idea to death I will sacrifice a pair of (awful) JJ EL34s and find out how effective the "trick" is.
Be interested in your listening results, and some measurements if you can do them, compared to the baseline series resistor.
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