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Old 13th July 2003, 03:25 AM   #1
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Default low voltage tubes (6GM8 and 12DL8)

I have two 6GM8 that I would like to put into use. Finding another similar one may prove to be difficult, so I would need to order quite a few to get matching tubes. I can order 12DL8 for cheap, and would like to do so.

Attached is a circuit I threw together with 1/2 6GM8 driving a 12DL8 (space charge tetrode). I am guessing you just attach the space charge grid to B+ through a resistor, correct? Does my topology look ok? Values good enough? Anything to change?

Suggestions and comments are welcome. Thanks!
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Old 13th July 2003, 01:36 PM   #2
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I think you have your control grid and screen grid reversed.

If you want it to act like a tetrode, you need to put a constant DC voltage (with respect to the cathode) on that screen grid. This can be done by running a resistor or a divider from B+ to the screen, then bypassing with a cap from screen to cathode. You'll also need some sort of grid leak from the control grid to ground. Not having my tube manual to hand, I can't comment on the choice of values or B+.

If you're using the tetrode as a CF, what's the function of that plate resistor?
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Old 13th July 2003, 01:46 PM   #3
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Hi,

Quote:
I think you have your control grid and screen grid reversed.
That's what I thought at first as well.
With the 12DL8 G2 is the control grid though.

The datasheet from Sylvania has some typical operation points for this tube:

12DL8

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Old 13th July 2003, 02:52 PM   #4
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I have a pair of 12DL8s, and they're yours for the postage... about $1.50 to US or Canada. I'm sure I have some other LV tubes if anyone is looking for them.
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Old 13th July 2003, 03:05 PM   #5
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Bizarre. Cool. Thanks, Frank!
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Old 13th July 2003, 03:11 PM   #6
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Tom Bavis- Well that is cheaper than the price I was going to pay before.


I did notice the odd position of the screen grid, which is why I drew it like that. So, I can get rid of the plate resistor on the 12DL8? Any value that you think would work well on the screen grid? So, I do B+ to screen grid via resistor, then screen grid to cathode with cap? Ok... do-able. Any suggested values?

Thanks
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Old 13th July 2003, 04:59 PM   #7
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Well, what you need to do is look at the expected screen current and scale the resistor accordingly to give you the desired screen voltage. The bypass cap ought to be chosen to give a rolloff a decade or so lower than the pole from the coupling cap. If you want to be REAL fancy, you can set up a floating regulator between cathode and screen, but that's fanaticism above and beyond the call of duty.
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Old 13th July 2003, 05:45 PM   #8
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Hi,

Quote:
If you want to be REAL fancy, you can set up a floating regulator between cathode and screen, but that's fanaticism above and beyond the call of duty.
In this case, quite probable.

But:

Let's not forget that since, in a penthode, the plate operates as little more than an electron collector, there is little reason to regulate it.

OTOH, regulating the screen is akin to regulating the anode in a triode.
A neat trick, since the penthode screen draws much less signal current than does the triode plate. By regulating the "surrogate plate" of the penthode (i.e., the screen) you prevent the screen grid from swinging with signal level and thereby you eliminate the conflict with the control grid for control over plate current.

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Old 13th July 2003, 06:36 PM   #9
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If you look at the 1958 Beitman's under Motorola, you'll find a number of radios that use this type of tube - 12K5 and 12DL8 - as 1st adudio, driving a power transistor. Generally a 10M grid leak is used, which you'd connect grid- to- cathode in a cathode follower. Probably could go a bit lower... Generally no screen resistor or cathode resistor was used.

Beitman's on line: http://www.eskimo.com/~p0lez1/beitman/beitmanhome.htm
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Old 13th July 2003, 10:37 PM   #10
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I now have one half of a 6GM8 set up for voltage gain no my bench... I don't have a noval socket laying around, so I just wrapped resistor legs and wires around the tube... Well, in short, it works, just not all that well. After a little warming, it sounds fine, but who can hear it?! It is way too quiet. Somewhere, I don't have enough gain. The mu (14) should provide mroe than enough voltage gain, and it is hooked to a BUF634 for current gain. Catode res is 470, plate res 4.7k, input res 100k. What have I done wrong? This little thing gets pretty darn warm too, considering I only have 24V going to it. Oh, in case it is important- the tube is running on +/-12V rails like the BUF. Any suggestions?
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