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Question regarding 5763 datasheet.
Question regarding 5763 datasheet.
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Old 14th January 2021, 08:16 AM   #1
Meehah is offline Meehah
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Default [Resolved] Question regarding 5763 datasheet.

Hello all. I am relatively new to tube electronics, and designing a guitar amp using a 5763 tetrode in the output stage. On page 4 of the datasheet (available here) it says, regarding the screen voltage, that "The screen voltage, where lower than the anode voltage, should be obtained from a potentiometer across the HT line to chassis adequately by-passed to AF signals, and not by means of a series resistance." I understand the general idea here, and what should not be done, but am struggling to visualise how the potentiometer is arranged in the circuit. Is it asking for a voltage divider, like a volume pot, with a capacitor across it to allow AF signals through? Or am I totally off track? I'd very much appreciate the help of anyone willing to provide it, as I'm rather stumped. Thank you all in advance.

Last edited by Meehah; 29th January 2021 at 09:54 AM.
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Old 14th January 2021, 09:49 AM   #2
6A3sUMMER is offline 6A3sUMMER  United States
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Welcome to the Tubes / Valves Threads of this Forum.
These are generally for Hi Fi Stereo tube amplifiers.

There are the Instruments & Amps Threads on this same Forum.
These are generally for Guitar amplifiers.
You might get some answers there too.

The 5763 data sheet you called out is suggesting a potentiometer to act as a Resistive Divider for the Screen, plus a bypass capacitor from the divider junction to ground.
The potentiometer seems to me that someone did not want to calculate the resistance of the series resistors needed from the B+ voltage and the desired screen voltage and screen current, to end up with a specific screen voltage.

I do not use Pentode mode, and do not use Beam Tetrode mode / Beam Power mode with those types of tubes. Instead, I use either Ultra Linear mode or Triode modes with such tube types.
I never had experience with a 5763 Beam Tetrode, and have not even seen one.

Many circuits just use a single resistor from B+ to the screen.
This tube has a sensitive screen, and most applications want no more than 225V, and 250V maximum. Just a series resistor might cause the screen to cook, under quiescent or under signal conditions.

Two Resistors in series from B+ to ground, can divide the voltage.
Keep in mind, the screen will draw current too, and will reduce the voltage versus just a resistor divider by itself.
I think the data sheet may list the screen current for certain plate voltages, and certain screen voltages (and with certain bias voltages).
You have to combine the current of the screen, and the current of the bottom resistor in the divider, in order to determine the top resistor, to get the screen voltage you want.

There are other applications of Beam Power Tetrodes (Beam Power tubes), that use a regulated screen voltage.
For example, a resistor from B+ to both the screen and to the plate of an 0D3 gas regulator tube (with the cathode of the 0D3 connected to ground) will regulate the screen voltage to 150V.
Some will series 2 regulators, a 150V one, and a 105V one, and get 255V, for example.
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Old 14th January 2021, 11:46 AM   #3
Meehah is offline Meehah
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6A3sUMMER, thank you very much for your response. That is very helpful, and entirely clears my confusion.
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Old 20th January 2021, 06:37 AM   #4
Meehah is offline Meehah
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Hello everyone again. I'm very sorry, I should've thought of this at the time of my initial post, but I've realised that I'm not exactly sure how to calculate the value of the bypass capacitor. Should I follow a similar method as is used for cathode bypass capacitor calculations, setting cutoff frequency to 1Hz?
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Old 21st January 2021, 12:15 AM   #5
PRR is online now PRR  United States
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Question regarding 5763 datasheet.
> guitar amp using a 5763 tetrode in the output stage

Why?? This is a high-strung tube, prone to oscillate, with awkward screen bias.

Use 6V6, 6BQ5/EL84, etc.

I'm sure "potentiometer" here does not mean an adjustable volume pot but a voltage divider of fixed resistors. The screen current varies a LOT. Like 4mA to 17mA. If you get screen bias through a simple dropper, Vg2 varies a lot. If a divider's idle current is much larger than the variation of screen current, not so much. But that suggests 50-100mA of divider current, much more than plate current.
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Old 21st January 2021, 08:15 AM   #6
Meehah is offline Meehah
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PRR, I do recognise that the 5763 is not a good choice, but I am determined, probably foolishly, to use what I have. I am therefore also quite prepared for poor results, and it is really mainly the process of building and designing that interests me. I apologise if my mention of volume pots was confusing, I intended it as an example of a voltage divider that was familiar to myself. Thank you very much for explaining why a voltage divider is called for here, that confused me, but now makes a lot of sense.
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Old 21st January 2021, 12:35 PM   #7
Tubelab_com is offline Tubelab_com  United States
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The 5763 is an old (1949) VHF radio tube.

My RCA data sheet does not list any modes of linear operation, only class C RF transmitter specs. There is no mention of a pot or other variable resistance for obtaining screen voltage. My sheet says "Obtained from plate supply voltage of 300 volts through a series resistor of 12500 ohms." This was common for class C RF amplifier use since the level of the input signal does not change, and is always high enough to drive the tube into hard clipping. That's how class C works, but this will not work for class A or AB audio.

I have a box full of those tubes somewhere. I was given over 100,000 well used military tubes about 20 years ago. I saved them because I thought they may be useful, but never got around to playing with them.

I typically use a mosfet / zener diode circuit to develop a fixed screen voltage that's lower than the B+.

All of the audio applications shown in your linked data sheet show a load impedance over 10,000 ohms. These OPT's are rather hard to find, so use the highest load impedance you have.

Being an RF tube, the control grid can be driven positive (AB2) to get high power output, but the typical guitar amp driver circuit will not get there.
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Old 22nd January 2021, 11:23 AM   #8
Meehah is offline Meehah
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Thank you very much Tubelab_com, it is truly wonderful to have such experienced people as yourself and others who have already replied so generously share their knowledge. I'll keep your info in mind, that is very interesting and helpful in further understanding this.
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Old 29th January 2021, 09:53 AM   #9
Meehah is offline Meehah
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Hello again to all who replied to this thread. I have just completed and tested my design, and it works! I ended up using the voltage divider solution, and I will post more in the way of documentation in the Instruments and Amps section in the next few days. Thank you again to all those who replied, it was really very helpful for me.
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