832A as a Single Ended Tetrode Amp? - diyAudio
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Old 30th December 2012, 12:31 AM   #1
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Default 832A as a Single Ended Tetrode Amp?

Hi there,

I just finished my first tube project, building a Single Ended Triode amplifier based off a kit. It sounds wonderful, but I have a feeling I'm loving the time I put into the design of the amp rather than the resulting sound (I'm not an audiophile in any way).

When I picked up the amp, I was also offered an 832A tube. I immediately grabbed it for it's aesthetic value, but looking further into it, I noticed people have made amps based off of it! All these amps are Push Pull type amps, but I was wondering if it would be possible to make a "single ended tetrode" (got the terminology from wikipedia) type amplifier using only this tube. I wouldn't mind having only a mono output (would this tube being a double tetrode mean it can possibly have two inputs and two outputs?), and sound quality is not a large concern. Would this be possible?

Thanks,
Sonik

Last edited by supersonik319; 30th December 2012 at 01:08 AM.
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Old 30th December 2012, 02:32 AM   #2
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The only tetrode I would like to listen is a GMI-11 wired as Triode:
http://www.gstube.com/photo/2969.jpg

This jet radar tube works since 1200V on plate;
The socket is cheap and hi quality stuff:
''GSTube.com''. Tubes, sockets etc. Tube sockets of this type.
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Old 30th December 2012, 07:23 AM   #3
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You can make a stereo SE amp with one of these since there are indeed 2 inputs and outputs available. Use fixed bias, -22 volts on the grids, 250 volts on plate and screens, and 5k transformers as a starting point and tweek from there.
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Old 30th December 2012, 02:39 PM   #4
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I tried my hand at making a schematic, based off of existing 832A amps and my SET amp. Could anyone advise me on what is missing and suggest parts I should grab?

jerluwoo, I'm not sure what you meant by -22 Volts on the grids, could you explain that part?

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Old 31st December 2012, 03:44 AM   #5
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How much idle current at the -22 volt bias?

Looks like a fun amp.
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Old 31st December 2012, 03:37 PM   #6
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Pano: Are the resistors I added in this schematic appropriate for this tube?

I created another schematic to add in what I believe to be the appropriate application of bias voltage. I am, admittedly, just referencing others' schematics and adapting them to this design. I have a few questions though which may be helpful for understanding these components and deriving the most appropriate values. I would greatly appreciate anyone taking the time to answer them:

Is the value for the bleeder resistor R5 appropriate? Should there be any more bleeders throughout the circuit? Is R1 a bleeder?

What is the purpose of C1 and R1? Some circuits have pin four going directly to ground, while others have a resistor, a polarized capacitor and an unpolarized one!

Is the input stage overkill for an RCA signal? Or would the biasing stage assist in the signal getting through?

Should there be some sort of filtering between the plates and the HV supply (currently the signal goes straight through the transformers into HV) so that the signals from both the left and right side don't mix when they go back into pin 3? Currently there is only a resistor R2 there...

Thanks is advance for all your help in helping me understand the principles behind these wonderful technologies!


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Old 1st January 2013, 05:26 AM   #7
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Interesting Do you want to operate in tetrode mode or strap it as a triode?

It's something of a high voltage tube. Are you okay working with high voltage B+ power supply? Pretty easy to kill oneself with that kind of potential. Just sayin... It might be better to do this one on paper.

Either way, you will need about 20 volts pp to swing this tube so you will need an input stage.

It looks like a -10 volt bias to me. It's not clear to me why you can't just use cathode bias. Perhaps Jerluwoo can answer that.

Operating point could be in the neighborhood of 400v, 100ma, -10v with an 800v B+ and a 5k OT, assuming tetrode mode. LOAD LINES. (Ignore the lower line)

I couldn't find any data on triode connected operation.

Data sheet LINK




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Last edited by Captn Dave; 1st January 2013 at 05:30 AM.
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Old 1st January 2013, 01:25 PM   #8
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Quote:
Originally Posted by Captn Dave View Post
It's something of a high voltage tube. Are you okay working with high voltage B+ power supply? Pretty easy to kill oneself with that kind of potential. Just sayin... It might be better to do this one on paper.

800v B+
Never mind the high voltage comments. The B+ in this application will be just ten volts or so higher than the OP given the transformer load.

Too much wine.
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Old 1st January 2013, 01:55 PM   #9
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Captn Dave: Completely understandable, it's been a long new years that said, I am comfortable safely working around 300-400V Tube Amps, and have constructed, tested, and measured capacitor diode voltage multipliers in a lab for learning purposes.

Is there a way to create an input stage to swing the input 20Vpp from a typical stereo without using another tube? I cannot find a reliable source for RCA input specifications, but assuming it's 1Vpp at 0db, the gain would have to be 20 to make this tube start singing? I'll try building it as is in this schematic, and if this isn't enough, I'll try a simple CMOY or even transistor amp hardwired to give me a gain of 20?

I modified the schematic to incorporate the power supply I brought up in a separate thread so that I can use the same transformer from a different tube amp project I built.

My calculations show that the negative bias from this supply will be around -15 V, would this be enough?

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Old 1st January 2013, 03:49 PM   #10
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Here is a LINK for you. You can search for data sheets on just about any tube. I'd look at 6dj8 first. Its a twin triode so one will do both stereo channels. You could probably swing the power triode with a 1v pp signal. Print out the curves and draw the load lines to see.

Read about unbypassed cathode resistors. The negative feedback inherent to this mode will reduce your gain by 20% or so and make it about perfect.
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