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Old 2nd April 2007, 02:41 PM   #1
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Question Pentodes without cathodes?

I have some subminiature pentodes type 5672 (CK5672) on the way. I want to use four of them as output stage for a REALLY low watt guitar amp

When I started reading about them more closely, I found out that they don't have - or so it seems - cathodes
In the pdf files they are refered to as "filament type pentode", where the filament must be DC driven (1.25vdc). they have five leads:

1. Plate
2. Screen
3. Filament possitive
4. Grid
5. Filament negative and Suppressor together

All pdf files say that max cathode current should be 5mA... but where is it?
How do I bias such a tube?
Can it be "cathode" biased?
Any idea what grid leak/ground resistor should I use? there is no mention of that spec on any pdf I found.

Any tips will be kindly appriciated
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Old 2nd April 2007, 04:04 PM   #2
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The filament in a direct heated tube whether it's a triode or pentode is the cathode.

Cathode bias is possible, but may be a bit more complicated requiring separate filament batteries or floating filament supplies for the output tubes.

You didn't mention whether this amp was to be battery operated or run off of the ac line.
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Old 2nd April 2007, 04:12 PM   #3
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AC line.

Though the thought of a battery driven tube guitar amp does sound cool
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Old 2nd April 2007, 09:18 PM   #4
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"Pentodes without cathodes?"

a.k.a. MOSFET
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Old 2nd April 2007, 09:51 PM   #5
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Quote:
Originally posted by Miles Prower
"Pentodes without cathodes?"

a.k.a. MOSFET

That's a pentode w/o a heater.
cheers,
Douglas
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Old 2nd April 2007, 10:27 PM   #6
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Default Re: Pentodes without cathodes?

Quote:
Originally posted by Hendrixon Can it be "cathode" biased?
Any idea what grid leak/ground resistor should I use? there is no mention of that spec on any pdf I found.

Any tips will be kindly appriciated [/B]

it seems to me that most of the spec sheets assume grid leak bias from what i have found in these teely DHP's. i have tinkered with them, and found a little difference between 1M-10M on the grid leak, but i have not tried that specific tube. if you want more control, use a fixed negative bias on the grid. You could use a small DC switcher chip to generate a negative power supply from the battery... or just use another battery for lowest noise. Cathode bias on those is a pain in the butt, but you can do it.
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Old 2nd April 2007, 11:53 PM   #7
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"grid leak bias"??
I know cathode bias and fixed bias...

<hitting google>

Bingo!
Looks perfect for my needs!

I like using cathode bias mostly because it is best suited for post PI master volume, something that doesn't work as good with fixed bias (these are the only bias types I met so far from tinkering with guitar amps)

So I use a DC+ on one filament leg, DC- on the other and 10M grid leak resistor?
How would I calculate cathode current in this setup?
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Old 3rd April 2007, 12:35 AM   #8
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Quote:
Originally posted by Hendrixon
"grid leak bias"??
I know cathode bias and fixed bias...

<hitting google>

Bingo!
Looks perfect for my needs!

I like using cathode bias mostly because it is best suited for post PI master volume, something that doesn't work as good with fixed bias (these are the only bias types I met so far from tinkering with guitar amps)

So I use a DC+ on one filament leg, DC- on the other and 10M grid leak resistor?
How would I calculate cathode current in this setup?

You can most certainly use a PPIV with a grid biased amp, you just need to block the bias DC with a cap.

If you are using grid leak bias, there is no need for negative voltage, the tube will develop it's own bias across the huge grid leak resistor.
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Old 3rd April 2007, 12:38 AM   #9
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Oh wait... I see what you mean. No, you can ground the - terminal of the filament and apply the positive filament voltage to the + terminal. As far as measuring the current, just measure the voltage drop across the plate resistor and then use ohm's law... or put a 1 ohm resistor in series with the plate resistor and measure the drop across it in millivolts, and that will be your current in milliamps.
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Old 3rd April 2007, 01:06 AM   #10
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I know PPIMV works fine with fixed bias amps, but sound wise, it is not transparent. with cathode biased power tubes, I use a pot as grid leak resistance (grid to lug 1 and ground lug 3). incoming signal - from last coupling cap - goes to the pot wiper


So negative filament terminal strait to ground?
Makes more sense now

Thanks mate!
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