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Old 9th September 2009, 03:14 PM   #11
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Screen current is high when the anode voltage dips below it in pentode mode, This doesn't happen in triode (and in ultralinear, since screen voltage dips with plate). As long as screen stays within its dissipation rating...

Fritz Langford Smith (who was reputed to know a bit about valves...) used 807s in triode well above their screen rating in his version of the Williamson amplifier.
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Old 9th September 2009, 03:34 PM   #12
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Quote:
Originally Posted by barretter View Post
Wavebourn, I realise that you have some different ideas about how to emulate triode performance with a pentode but that is not what I am asking for. I want comments on Crowhurst's statement that screen grids can be supplied with the same high voltage as anodes when the valve is triode-connected. If you cannot address this issue, please do not comment.
Sure, no comments in your threads anymore! Ever! Sleep well!
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Old 9th September 2009, 04:06 PM   #13
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I will. Thank you for your kind wishes.
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Old 9th September 2009, 04:11 PM   #14
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by barretter View Post
Presumably this means that as long as we don't violate the maximum anode voltage, we can stop worrying about the maximum screen voltages quoted on spec sheets when we use pentodes as triodes.
I would propose a 'yes', but a qualified one:

A friend of mine did some tests on some ordinary small-signal pentodes and found that in triode mode, cathode current was often shared almost equally between anode and screen, or that screen current could even exceed anode current! (Perhaps because the anode is big, but the screen is closer to the cathode). This would vary somewhat with applied voltage. He managed to destroy at least one screen grid in the process of testing this!

Connecting the screen to anode through the usual 100 ohm stopper prevented this, so that screen current dropped to a much smaller fraction of the anode voltage, and I have been religious about using such stopper resistors ever since.
I haven't yet done my own tests to confirm his findings, but I don't have any reason to suspect he was mistaken.

By the way Baretter, where are you based?
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Old 9th September 2009, 04:44 PM   #15
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I think we're gradually converging on a "treat with caution" verdict on this statement. I started this thread because of contradictions in 6V6 spec sheets pointed out in another thread on this forum.
I live in Todmorden, on the Yorkshire/Lancashire border, but I don't have any divided loyalties as I'm originally from North Lincolnshire!
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Old 9th September 2009, 05:04 PM   #16
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Quote:
I would propose a 'yes', but a qualified one....Connecting the screen to anode through the usual 100 ohm stopper prevented this
I have also managed to blast the screen grid out of a 6EJ7 with the screen and plate connected directly together. I think oscillation might have been the cause. A resistor will fix this in either case.

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I think we're gradually converging on a "treat with caution" verdict on this statement.
Yes, some caution shound be advised when exploring the limits of a tube that has not been explored before. As pointed out, a resistor of at least 100 ohms should be placed in series with the screen grid as a current limiter and an oscillation stopper. 100 ohms is a good place to start with most tubes. Some tubes need a higher value, with 1000 ohms being the likely upper limit. The EL34 is one tube that wants more than 100 ohms, with 1000 ohms being mentioned in the original Mullard literature. If distortion measuring equipment is available test the distortion with several resistor values and pick the highest value that does not raise the distortion.

I have successfully operated several tubes well above their published screen ratings without issues in triode mode. In many cases you can not approach the plate voltage ratings. This is true with all sweep tubes and some RF tubes like the 807.

It has been stated that the usual 1/2 watt "stopper" resistor is sufficient since the dissipation in the resistor is low. I have found this not to be the case if the amp will ever see clipping. An EL34 can draw some fairly high screen grid current peaks when overdriven. I have fried a few 1/2 watt resistors, and now use a 1 or 2 watt resistor in all of my amps.
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Old 9th September 2009, 05:10 PM   #17
sajti is offline sajti  Hungary
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Quote:
Originally Posted by tubelab.com View Post
It has been stated that the usual 1/2 watt "stopper" resistor is sufficient since the dissipation in the resistor is low.
This is true especially for beam tetrode tubes, becaues of the tight grid aligment. I used 0.5W resistor for my 6550s without any problem. This is true for PL509s as well

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Old 9th September 2009, 05:11 PM   #18
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Quote:
Originally Posted by tubelab.com View Post
If distortion measuring equipment is available test the distortion with several resistor values and pick the highest value that does not raise the distortion.
Beat me to it. Inordinately high values can generate a large number of high distortion harmonics.
My understanding was the screen limitation is power, not simply voltage.
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Old 9th September 2009, 10:18 PM   #19
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Quote:
My understanding was the screen limitation is power, not simply voltage.
Some tubes (especially sweep tubes) tend toward runaway with high screen voltages. This is obvious in pentode mode because the screen voltage is constant. A similar thing happens in triode mode with no signal present, since the plate and screen are at nearly the same voltage.

I was playing with some triode wired sweep tubes at something close to twice the normal screen voltage rating and found an "avalanche" effect. The tube will make gobs of power when driven, but will go into runaway with no signal present. As the B+ (plate and screen) is increased the current will naturally rise as with most triodes, but will reach a point where it just takes off regardless of control grid voltage. I blasted a perfectly good 6LW6 into oblivion at 450 volts. I was getting over 100 watts out of a pair with no objection until I turned down the audio oscillator. I know that a 6CD6 will do the same thing, but a 6AV5 seems happy at over 300 volts in triode despite the 175 volt screen rating. Some 6AV5's will work at 350 volts.

When you go exploring outside the normal ratings you must realize that every tube is different and even two from identical batches may behave differently.
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Old 10th September 2009, 02:27 AM   #20
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barretter,
You may be on to something, I hope so! Much of the tube date we have to work with supports pentodes as radio transmitter tubes operating as class c amplifiers not class a audio. It would be a time trip to sit down Norman Crowhurst, buy him a cup of coffee and talk it over. My impression is that in pentode operation the diffrential voltages between the anode & screen can spike causeing lighthing in the bottle and an early death of the tube. I think that it was Crowhurst that called that flashover. If the anode and screen are tied together we leave the tube data behind. The anode and screen are at the same voltage potiential.
I am going to install the 100 ohm resisitor turn off the lights and crank up the plate/screen voltage untill the smoke comes out. Then turn it down.
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