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Old 22nd February 2009, 07:55 PM   #1
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Default Triode strapping a 6V6 (or any tetrode/pentode, really)

If you look up 6V6GTA specs, look towards the bottom of the page "vertical deflection amplifier" , it lists the MAX plate dissapation as 10 watts. I believe this is due to the screen's lower power handling. Should I use 10 watts for my calculations, or the more typical 13 watts ? : http://www.triodeel.com/6v6_p3.gif

Another question I have is , should we really use a resistor to tie the plate to the grid ? Isn't that forming a local feedback loop ? Wouldn't a direct connection be better ?


If a resistor is the best way to Triode tie a pent/tetrode , then why isn't it listed in ANY spec sheets ? All of them I've ever seen just say "grid connected to plate" . No mention of a resistor , or any resistor value to use.

I originally had a 100 ohm resistor tying the plate to the grid. Sure sounds cleaner with a wire in it's place. Perhaps it was just a crappy resistor muddying up the sound ?

Thanks for your help and time..........................Blake
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Old 22nd February 2009, 09:30 PM   #2
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Quote:
If you look up 6V6GTA specs, look towards the bottom of the page "vertical deflection amplifier" , it lists the MAX plate dissapation as 10 watts. I believe this is due to the screen's lower power handling.
Screen should read "grid" . Sorry for any confusion.





....................Blake
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Old 22nd February 2009, 09:48 PM   #3
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Hi Blake,

Your use of the term 'screen' is actually correct. Do notice that a vertical deflection amplifier is a special application: The applied pulse is for a very brief time, during which all of 1,2KV can be on plate + G2! For audio work the normal specs of Pa=14W and Pg2=2,2W is valid, i.e. 16,6W in total (well, in practice one typically keeps a little shy of maximum).

No grid stoppers are indicated in tube data sheets simply because it is not an inherent part of tube specs; it depends on the application/circuit design. Where a specific resistor is generally required for optimal tube behaviour it will be specified, e.g. where it is indicated that for UL operation of p.p. EL34s, a G2 series resistor of 1 k.ohm is advised.

Such resistors are often piously added without a real need - but the designer should check his design, and then under worst case conditions. This is often quite difficult to guess, so we put in stoppers anyway because they do no harm within limits.
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Old 22nd February 2009, 10:09 PM   #4
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Default Re: Triode strapping a 6V6 (or any tetrode/pentode, really)

Quote:
Originally posted by Nihilist
I originally had a 100 ohm resistor tying the plate to the grid. Sure sounds cleaner with a wire in it's place. Perhaps it was just a crappy resistor muddying up the sound ?
I used a 100R 2W Kiwame to triode my EL34 PP, sounds fine.

Jeff
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Old 22nd February 2009, 10:15 PM   #5
45 is offline 45  Italy
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The resistor to tie the grid to the plate is for protection and for preventing too much current flowing into the grid.
Given the tolerances in tube manufacturing it is highly recommendable.
In a data sheet you find a tube without manufacturing defects.....

Also, for the above reason it is always recommendable to stay within the sole plate dissipation for triode strapped pentodes and do not consider the sum of plate and screen grid dissipation.

For the 6V6, I would stay within 10-11W for long and reliable service. 100-220 ohms for the resistor is usually enough.

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Old 22nd February 2009, 11:51 PM   #6
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45,

Er - from the data sheet the maximum current drawn by the screen is of the order of 10 - 12mA. That through a resistor of say 220 ohm gives only <3V drop - not enough to limit any current to the safe region. The purpose of 'stoppers' here (both in G1 and G2, and occasionally in the anode circuit) is really to quench h.f. oscillations, which might occur under certain conditions in the super audio - MHz region, depending on the circuit.

Triode operation (g2 to anode) is relatively docile compared to pentode operation, where the anode can go to -200V with respect to the screen on low wave cycles. If the tube is made for such conditions, it is not really on that Ig2 (or the screen dissipation) will be a problem under triode conditions (within maximum specs, of course). Similarly instability caused by g2 where it is always within a few volt of the anode and at 0 phase angle (as in triode), is almost impossible, especially with a 'beam' tube.
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Old 23rd February 2009, 09:54 AM   #7
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Quote:
Originally posted by Johan Potgieter
45,

Er - from the data sheet the maximum current drawn by the screen is of the order of 10 - 12mA. That through a resistor of say 220 ohm gives only <3V drop - not enough to limit any current to the safe region. The purpose of 'stoppers' here (both in G1 and G2, and occasionally in the anode circuit) is really to quench h.f. oscillations, which might occur under certain conditions in the super audio - MHz region, depending on the circuit.
Those 10-12 mA are referred to 250V plate & screen grid voltages.
The G2 current is a function of the anode and screen supplies and bias.

For a triode-connected 6V6GTA only, you don't need any stopper if you stay within 350V /10W.

If you want to go for 12W you need stoppers.
Otherwise, like a more common 6V6GT with max ratings of 300V/12.5W, you have to drop the voltage.


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