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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

g2 driven tubes

Hi Fuling,

Fuling said:


The first link shows a complete project, the second goes to a set of measured plate curves for EL509.

Actually I made the plot (2nd link) exactly for the conditions shown in the 1st link, beacuse I couldn´t believe the plot shown in the 1st link. As it turned out, the plot shown in the first link is not based on read measurements, but a simulation, using an oversimplified model. So much simplified, that it doesn´t make any sense anymore.

To the original poster:

1) Note the very narrow "channel" between the extremely instable area of (even) negative anode impedance and the max. Pd curve: You will have to put the loadline into that narrow channel or else you will experience either danger of Barkhausen oscillations or thermal runaway of the valve (E/PL509 is very prone to it)

2) The g2 current demand when driving g2 is very non-linear, anything else but a linear function. Forget about driving it with anything else than a CF driver using a valve of very high gm, or you will introduce lots of distortion by the current-demand-nonlinearities.

3) There are some more g2-driven plots available on my homepage, see overview at http://www.tubes.mynetcologne.de/roehren/daten/as_triode_e.html You will notice that the general problems 1) and 2) just remain with other valves and operation points.

All in all, using screen drive is not recommendable for quite some reasons, since the method it is "broken" by pentode/BPT working principles you can´t work arround. This was mentioned already by Professor Barkhausen (this name should ring a bell) about 70 years ago...



P.S.: Here is another project (German) using screen drive: http://www.jogis-roehrenbude.de/Leserbriefe/Synola-509/SYNOLA_SE509.htm
It would seem to me that the g1 grid bias is what's eating up the plate stability space headroom. When the plate drops below the g2 voltage is when trouble starts. Biasing g1 negative pushes up g2's range, causing it to eat into plate voltage range.

A low screen voltage horizontal output tube should be preferred I would think, with maybe even a little +bias on g1. Just theorizing here.

Hi Stuart,

SY said:
Tom, why such a high pre-bias on g1?

You should ask Bob D. about that :)

Well, using considerably lower pre-bias, statical Eg2 must be much lower, too, limiting the possible Eg2 swing range and upping Ig2 and Pg2. Also, the more (negativer) Eg1, the higher is statical stability due to automatic bias self-regulation, just as with any other grounded cathode auto-bias arrangement. This makes sense with such high gm & perveance tubes. You may remember, at ETF06, Tim de Paravincini mentioned, that at least _some_ auto bias is required to keep xL5y9 from runnaway even in plain old BPT mode.

Also, the negativer the pre-bias, the wider the "channel" between max Pd hyperbole and the area of spoiled impedance behaviour you can lay the loadline into, as you can see from this series of plots for SD EL36:




As you can see, it doesn´t make sense to use less than, say, -20V Eg1 with SD EL36, since the spoiled impedance area extends so much.

There is no reason why xL5x9 should behave differently in this regard: Below a certain Eg1 prebias, the spoiled impedance area will overlap the Pd hyperbole, so there is no chance whatsoever to get stable and safe operation for _any_ loadline.

Hence practice confirms that Professor Barkhausen was right.



I respectfully cannot agree with your analysis of the g2 input range being decreased with less neg. g1 bias. You can always turn the tube off with lower g2 signal, so that end is not impacted. With less neg. g1 bias, you get more plate current for the same max. g2 signal voltage. So the upper end is extended, not compressed by less g1 neg. bias.

I do see you point about cathode biasing for stable idle current. But for a required idle current, this can be achieved with less neg. g1 bias and less g2 pos. bias. Plate instability sets in sooner if the g2 voltage is higher however. All theorizing admittedly, but I don't see anything wrong with my analysis.