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barretter 9th September 2009 08:47 AM

Allowable screen voltages in triode-connected pentodes
 
I found the following statement in Norman Crowhurst's High fidelity sound engineering (Newnes, 1961, p. 124):

"Many valves, operated as pentodes, require usage of a screen voltage lower than the anode supply voltage. This is because the screen grid gets much more than its share of current in the upswing (anode voltage downswing), so that its voltage must be reduced. Working as triodes, the same valves can use a higher screen voltage, the same as the anode, because the high current upswing is avoided by signal voltage drop. The same thing, happening to a lesser extent, permits the full anode supply voltage to be used also on the screen in ultra-linear working." [my emphasis]

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. What do our gurus think?

Wavebourn 9th September 2009 09:09 AM

I think that fixed well regulated screen grid voltage allows to make better (more linear) triode from pentode applying parallel feedback from anode to the 1'st grid.

barretter 9th September 2009 09:17 AM

I'm talking about triode-connected pentodes where the anode (plate) is directly connected to the screen-grid and they are both supplied from the same voltage source.

Yvesm 9th September 2009 09:26 AM

The problem with screen is that its current sky rockets when plate voltage goes low while screen voltage remain "hi", its dissipation rise to dangerous values.
This does occurs when screen and anode voltages are equal.

Wavebourn 9th September 2009 09:26 AM

Quote:

Originally Posted by barretter (Post 1919428)
I'm talking about triode-connected pentodes where the anode (plate) is directly connected to the screen-grid and they are both supplied from the same voltage source.

It is not the only way to get triode characteristics by connecting screen grid to anode. Another way is to supply AC from anode to the control grid, this way is more flexible and more linear than better known one, "screen to anode".

However, input resistance is lower such a way, but considering Miller capacitance anyway similar driving current is needed for better than average performances.

oshifis 9th September 2009 10:22 AM

I use 807 type tubes triode-connected @ 425V anode voltage. No problem so far. The RCA data sheet permits 300V Ug2, perhaps the life expentancy is worse at higher voltage.

barretter 9th September 2009 10:34 AM

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.

sajti 9th September 2009 11:07 AM

I use 6550s connected as triode, with 450V, without any problem. I tried 500V some years ago. It could be stable too, but the bias should be less than 50mA, and direct coupled cathode follower recommended as driver, to keep the grid resistance as low as possible

Sajti

Yvesm 9th September 2009 11:54 AM

Quote:

Originally Posted by Yvesm (Post 1919433)
The problem with screen is that its current sky rockets when plate voltage goes low while screen voltage remain "hi", its dissipation rise to dangerous values.
This does occurs when screen and anode voltages are equal.

Ooops!
Should read:
"This does NOT occurs ..."

Rod Coleman 9th September 2009 12:32 PM

When you first build an amplifier this way, be sure to include a resistor between anode and screen - then you can be certain that the anode voltage is (at least a little) higher than screen.

The KT66 data sheet INSISTS you do this anyway, to prevent oscillation, using 100 to 300 ohms fitted right on the socket.

The more you want to violate the spec, the bigger the resistor you should use!

Test the amp in the dark when you first put the power on it. Look out for glowing screen, if you can see in there. Some valves can do this better than others, I believe.


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