As written before, the AC filament (if not balanced enough) modulating output signal via the cathode.
This is the worst scenario: AC feed 801a filament and only one side of filament "grounded" (in this case with very small resistor).
If you compensate it, the phenomenon can be reduced:
This is the worst scenario: AC feed 801a filament and only one side of filament "grounded" (in this case with very small resistor).
If you compensate it, the phenomenon can be reduced:
- hum potentiometer slider as virtual cathode to ground -or cathode resistor-;
- or 22-47R resistors from each filament leg to ground -or cathode resistor-.
toshiba_nz
1. The 801A was designed to be a Power Output tube.
Decades later . . . some decided it is a good preamp tube, and some decided it also is a good driver tube.
All the above are correct.
However, the designers decided that you need lots of Plate voltage on an 801A to get any appreciable power out; Thus, 425V, 500V, 600V.
The designers who wrote the tube data sheet many decades ago, never considered using an 801A as a preamp tube.
I am sure all those designers and marketeers status is RIP (Rest in Peace).
I hope that explains it.
2. There is an interesting case of a tube that had a major addition to its original data sheet.
The original 807 Beam Power tube data sheet listed the maximum screen voltage as 300V.
Soon after that, some designers looked at the rugged and robust 807, and designed them into their audio amplifiers.
Some designers even decided to operate the 807 beam power tube in Triode Wired mode.
Then Marketing saw a new sales opportunity, so . . .
The 807 data sheet had a new specification Added: Maximum Screen voltage in Triode Mode, 400V.
Ultra Linear, anybody? With a 40% UL tap, I figure the 807 Screen is good for 340V maximum; but it is too late for any data sheet revisions.
In support of my estimate, consider that a Triode Wired 807 screen is working from a 100% tap.
History often gives surprises.
3. As to AC operation of an 801A filament, a hum pot can reduce the hum.
However, for all AC filament DHTs, including the 801A, there is another cause of hum:
With AC filament current, each filament wire creates a varying magnetic field.
If any filament wire is closer to one flat side of the plate, than it is to the other flat side of the plate, the wire will be attracted to the closest flat side of the plate.
That creates vibrating motion at the rate of 2 X the power mains AC frequency.
Since the filament wire moves with respect to the non-moving grid, the tube parameters vary at that 2 X rate, causing varying plate current at that rate . . . we also call that hum. The hum pot can not reduce that 2 X frequency hum.
The magnetic aspects of this phenomena . . . 50Hz mains gives 100Hz hum; 60Hz mains gives 120Hz hum.
Have fun with 801A tubes!
1. The 801A was designed to be a Power Output tube.
Decades later . . . some decided it is a good preamp tube, and some decided it also is a good driver tube.
All the above are correct.
However, the designers decided that you need lots of Plate voltage on an 801A to get any appreciable power out; Thus, 425V, 500V, 600V.
The designers who wrote the tube data sheet many decades ago, never considered using an 801A as a preamp tube.
I am sure all those designers and marketeers status is RIP (Rest in Peace).
I hope that explains it.
2. There is an interesting case of a tube that had a major addition to its original data sheet.
The original 807 Beam Power tube data sheet listed the maximum screen voltage as 300V.
Soon after that, some designers looked at the rugged and robust 807, and designed them into their audio amplifiers.
Some designers even decided to operate the 807 beam power tube in Triode Wired mode.
Then Marketing saw a new sales opportunity, so . . .
The 807 data sheet had a new specification Added: Maximum Screen voltage in Triode Mode, 400V.
Ultra Linear, anybody? With a 40% UL tap, I figure the 807 Screen is good for 340V maximum; but it is too late for any data sheet revisions.
In support of my estimate, consider that a Triode Wired 807 screen is working from a 100% tap.
History often gives surprises.
3. As to AC operation of an 801A filament, a hum pot can reduce the hum.
However, for all AC filament DHTs, including the 801A, there is another cause of hum:
With AC filament current, each filament wire creates a varying magnetic field.
If any filament wire is closer to one flat side of the plate, than it is to the other flat side of the plate, the wire will be attracted to the closest flat side of the plate.
That creates vibrating motion at the rate of 2 X the power mains AC frequency.
Since the filament wire moves with respect to the non-moving grid, the tube parameters vary at that 2 X rate, causing varying plate current at that rate . . . we also call that hum. The hum pot can not reduce that 2 X frequency hum.
The magnetic aspects of this phenomena . . . 50Hz mains gives 100Hz hum; 60Hz mains gives 120Hz hum.
Have fun with 801A tubes!
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Choosing of operating point depends of method of use.
If you want the tube as power device, must to select such op. point, where the desired power of given loadline within the maximum limits (mainly dissipation), but provide as large as possible power with acceptable distortion.
If you use it as preamplifier, the power demand is negligible, the grid swing is relatively small (only few Volt). The goal is small distortion and manageable output impedance. These are requiring relatively large anode current.
If you see the 801a curves in datasheet, the the optional operating points at about -10V grid curve.
This is measured by L0rdGwyn:
There is enough large optional current range, the anode voltage is manageable, the bias can be produced easily.
If you want to use filament biasing, too large bias voltage not optimal, larger filament bias resistor (due to the 1.25A filament current) dissipation would be enormous.
If you use cathode biasing, the 25...40mA current requiring 250...400R cathode resistor ... and enough large bypass capacitor.
So, for preamplifier at about 300V anode voltage, 30mA anode current, -10V bias can be made easily.
If you want the tube as power device, must to select such op. point, where the desired power of given loadline within the maximum limits (mainly dissipation), but provide as large as possible power with acceptable distortion.
If you use it as preamplifier, the power demand is negligible, the grid swing is relatively small (only few Volt). The goal is small distortion and manageable output impedance. These are requiring relatively large anode current.
If you see the 801a curves in datasheet, the the optional operating points at about -10V grid curve.
This is measured by L0rdGwyn:
There is enough large optional current range, the anode voltage is manageable, the bias can be produced easily.
If you want to use filament biasing, too large bias voltage not optimal, larger filament bias resistor (due to the 1.25A filament current) dissipation would be enormous.
If you use cathode biasing, the 25...40mA current requiring 250...400R cathode resistor ... and enough large bypass capacitor.
So, for preamplifier at about 300V anode voltage, 30mA anode current, -10V bias can be made easily.
Am I reading the datasheet correctly when it says distortion goes down , with more anode voltage? E.g. distortion at 600v is ~half of what it is at 425v?
Go directly to Rod Coleman V9 filament regs. Do not pass GO, do not collect $200.
https://www.lyrima.co.uk/dhtreg/dhtRegIntro.html
https://www.lyrima.co.uk/dhtreg/dhtRegIntro.html
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Distortion is a mix of factors:
Bias voltage,
Plate voltage,
Plate current,
Plate load impedance,
And lastly, the peak to peak signal voltage to the grid, versus the grid bias voltage.
Also, the gain depends on the plate load impedance, and the operating plate impedance, rp.
At some point of peak to peak signal voltage at the grid . . . the combination of gain, plate voltage, and plate peak to peak swing, there may be a point where [plate] clipping occurs.
And, at some point of peak to peak signal voltage at the grid, given the bias voltage, the grid may draw grid current, or the grid voltage (bias plus peak negative signal voltage), may drive the tube into cut off.
All factors interact.
All things may appear to be simple, until we discover how complex they really are.
I have been studying tubes since 1956, and working with tubes since 1958, and I still make lots of mistakes.
Hang in there, Have Fun!
My general experience with tubes is there are brand to brand differences; And there are differences within any given brand.
Many production lines select tubes.
Many production lines select transistors.
That applies to Audio, Test and Measurement products, military equipment, etc.
Bias voltage,
Plate voltage,
Plate current,
Plate load impedance,
And lastly, the peak to peak signal voltage to the grid, versus the grid bias voltage.
Also, the gain depends on the plate load impedance, and the operating plate impedance, rp.
At some point of peak to peak signal voltage at the grid . . . the combination of gain, plate voltage, and plate peak to peak swing, there may be a point where [plate] clipping occurs.
And, at some point of peak to peak signal voltage at the grid, given the bias voltage, the grid may draw grid current, or the grid voltage (bias plus peak negative signal voltage), may drive the tube into cut off.
All factors interact.
All things may appear to be simple, until we discover how complex they really are.
I have been studying tubes since 1956, and working with tubes since 1958, and I still make lots of mistakes.
Hang in there, Have Fun!
My general experience with tubes is there are brand to brand differences; And there are differences within any given brand.
Many production lines select tubes.
Many production lines select transistors.
That applies to Audio, Test and Measurement products, military equipment, etc.
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Do you mean, that R loaded 801a connected to FET splitter?
In this case, the FET gate is very large impedance vs. anode resistor, so the loadline almost the R.
Thanks for the link.
On my other DHT's , for filament duties I used (DC) - an adjustable linear regulator LM317T , with Bipolar Transistors to boots the current. It sounds really good.
No.
At increasing anode current the plate impedance decreasing, the mutual conductance increasing, so with changeless load the distortion will be lesser (simply more optimal loading).
Well, this is the thing. I like the sound of R loaded. The one CCS i used in the past sounded like "artificially expended". And I also like the sound of choke loaded. I guess for the output swing I need , it will either be CCS or choke...