I don't suggest we move away from cathode bias, but there are a few circuits kicking around that allow adjustable cathode bias.
I'm thinking it might be a better option than what we have now and if it doesn't effect tone why not.
The other change I'm doing to my Rocket is to put 1 ohm resistors on the cathodes, it is so helpful for little effort. The only issue is the screen rail and the new earth rail, they would have to be sufficiently far away from each other.
Adjustable bias for the Rocket.
Moderators: pompeiisneaks, Colossal
Adjustable bias for the Rocket.
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Yours Sincerely
Mark Abbott
Mark Abbott
Re: Adjustable bias for the Rocket.
That's a great idea Mark.
The early Vox AC30's were prone to overheating - hard on the transformers & output tubes. High voltage biased hot. Lotsa smoke.
The cathode biasing on the Rocket is also aggressive on the tubes.
Works - but I'm not convinced they need to be pushed so hard.
Interesting idea.
The early Vox AC30's were prone to overheating - hard on the transformers & output tubes. High voltage biased hot. Lotsa smoke.
The cathode biasing on the Rocket is also aggressive on the tubes.
Works - but I'm not convinced they need to be pushed so hard.
Interesting idea.
Why Aye Man
Re: Adjustable bias for the Rocket.
The circuit came from Dutch at 18watt.com
http://18watt.com/storage/BiasIdeas.gif
Changing resistors to change bias is just silly, this would appear to be a better idea. I don't think the 5 volt zener really needs to be bypassed by the capacitor. I say this as the zener could be mounted where the output stage earth is, then it is a matter of running wire to a pot and then to the 220K resistors.
Is it me or is the zener diode the wrong was around?
I think it would have to be the other way around to develop a positive voltage.
http://18watt.com/storage/BiasIdeas.gif
Changing resistors to change bias is just silly, this would appear to be a better idea. I don't think the 5 volt zener really needs to be bypassed by the capacitor. I say this as the zener could be mounted where the output stage earth is, then it is a matter of running wire to a pot and then to the 220K resistors.
Is it me or is the zener diode the wrong was around?
I think it would have to be the other way around to develop a positive voltage.
Yours Sincerely
Mark Abbott
Mark Abbott
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martin manning
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Re: Adjustable bias for the Rocket.
Yes, the Zener is the wrong way 'round in the schematic.
First, why do this? If you want to lower idle plate dissipation, just put a larger cathode resistor in. If you want to be able to adjust the bias point for different tubes, this would help, but cathode bias is already self-adjusting to a large degree. Granted it would make it easy to experiment with different bias levels.
Where I think this concept would be useful is to balance output tubes, provided it were reconfigured for separate cathode resistors. It could also be simplified, as I see no reason to split the cathode resistor or use a Zener. An adjustable voltage divider connected from the top of the cathode resistor (either shared or separate) to ground would work fine.
Suitable values for the components would be:
- Increase Rk by ~10% from nominal to reduce plate dissipation (by roughly 10%, order of magnitude)
- Parallel each cathode resistor with a total resistance of ~100x Rk, made up of a pot (connected to ground) that is ~20x Rk and a fixed resistor (from the top of the pot to the top of Rk) that is ~80x Rk.
- connect the existing grid leak resistors to the pot wiper(s)
- There will be little current flowing in this branch, so low-wattage resistors and trimmer pots can be used.
- Setting the pot at mid-range should put the idle dissipation at approximately the nominal level.
Cathode voltage at the same idle dissipation will increase by a few volts using the 10% larger Rk, using up more HT, but this is probably insignificant.
First, why do this? If you want to lower idle plate dissipation, just put a larger cathode resistor in. If you want to be able to adjust the bias point for different tubes, this would help, but cathode bias is already self-adjusting to a large degree. Granted it would make it easy to experiment with different bias levels.
Where I think this concept would be useful is to balance output tubes, provided it were reconfigured for separate cathode resistors. It could also be simplified, as I see no reason to split the cathode resistor or use a Zener. An adjustable voltage divider connected from the top of the cathode resistor (either shared or separate) to ground would work fine.
Suitable values for the components would be:
- Increase Rk by ~10% from nominal to reduce plate dissipation (by roughly 10%, order of magnitude)
- Parallel each cathode resistor with a total resistance of ~100x Rk, made up of a pot (connected to ground) that is ~20x Rk and a fixed resistor (from the top of the pot to the top of Rk) that is ~80x Rk.
- connect the existing grid leak resistors to the pot wiper(s)
- There will be little current flowing in this branch, so low-wattage resistors and trimmer pots can be used.
- Setting the pot at mid-range should put the idle dissipation at approximately the nominal level.
Cathode voltage at the same idle dissipation will increase by a few volts using the 10% larger Rk, using up more HT, but this is probably insignificant.