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PostPosted: 05 Jul 2018, 21:32 
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Joined: 26 Jun 2018, 16:27
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Hello, I am relatively new to tubes and this is my first post on the forum. When i saw 350 used tubes for 20$ at a garage sale I decided I would try using some tubes in my projects, and Im finally designing a simple preamp out of a 6gu7 dual triode (There were about 20 or so in there so I figure one has to be good) and im running into issues with biasing.

Firstly, Im kind of curious why you rarely see protoboard and pcb constructed amps, why is ptp used so often? It seems really messy and and inefficient use of space.
Anyways, when I looked at all the datasheets I could find for the 6gu7 none of them had the Max cathode current listed, and from what i read on wikipedia the formula for finding a bias resistor is:

Cathode bias = |grid voltage|/Cathode current
Anyone know what units to use here?

But I was also confused when wiki said Cathode current could be found by Plate Current + Screen Current, I am working with a triode, not a tetrode or pentode so there is no screen only grid (Is that correct?)

Since this build is possibly gonna go portable, I would like to power it off of at most 12 volts. Is this feasible with minimal distortion? Would led biasing be a good option?

Here is a link to the wiki page I read: https://en.wikipedia.org/wiki/Cathode_bias
And here is a copy of the 6gu7 tube datasheet: http://www.nj7p.info/Tubes/PDFs/Frank/135-GE/6GU7.pdf

Any help is appreicated, Thanks!


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PostPosted: 08 Jul 2018, 13:32 
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Joined: 06 Apr 2009, 10:08
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Location: US Pacific Northwest
Wow. Lets start small and work up.
vieau wrote:
Im kind of curious why you rarely see protoboard and pcb constructed amps, why is ptp used so often? It seems really messy and and inefficient use of space.
There are several reasons for this but first off, it's due to flexibility. Making a change on an amp which it point-to-point wired is much simpler than making a change to a printed circuit board.
vieau wrote:
Anyways, when I looked at all the datasheets I could find for the 6gu7 none of them had the Max cathode current listed, ...
This is actually quite common. In most small triodes like this, the limiting design factor is actually the heat dissipation capacity of the plate and not the current emission capacity of the cathode. This means that maximum cathode current depends on the plate voltage and current at the selected bias condition.

The attached following figure is a copy of the plate characteristics curves from the 6GU7 data sheet. I have added a red line curve that shows where the 3W maximum plate dissipation limits would be for a class-A amplifier.
Attachment:
6GU7_Current_Limit.jpg
So the maximum plate current (which is also max cathode current because triodes have no other grids and the control grid operates at zero current in class A1 operation) depends on where you want to put the bias point. For example: at a plate voltage of 200v, the maximum plate current (also cathode current) is 15ma. And at this point, the plate would de dissipating 3W of heat (its maximum rating).

Let me know if this makes sense so far. After you have this down, we can talk about choosing a good bias point.


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PostPosted: 20 Sep 2018, 16:25 
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Joined: 26 Jun 2018, 16:27
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Hey, sorry this reply is grossly late, Ive been busy recently and my hobbies got pushed to the side as they do.

Either way, thanks for the help on this!

I understand the way the heat dissipation limit works, and I looked into biasing a bit. I think I want to run them as cold as possible without sacrificing a good sound, but how can I find the bias voltage and what is the purpose of biasing in a simple explanation?
Also forget the whole portable 12 volt thing, id like to add a toggled power amp stage for some speakers. But like you said, start small.

I found some circuits for preamps in the backs of RCA tube manuals, most use a 12au7 or 12ax7 as the preamp tube. Could I build that circuit and use a 6gu7 instead? (Circuit attached)


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