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I have an SE amp that uses a common cathode bias for the output tubes. If I can use two different output tubes in the amp, but need to change the cathode resistor to do so. Having done this, i like the sound of both tubes (1626 and 12V6). I am looking at making the bias adjustable, between 500 and 150 ohms.
What are the pitfalls in this situation, if any? Do I parallel the pot with the existing 500 cathode resistor? A setting of 250 Ohms would get me to 150 ohms. Or, do i simply replace the existing cathode resistor with a pot that i can vary between 500 and 150 ohms?
I am not a DIY-type. I learn as I go by doing small things to existing amps.
Follow Ups:
I recently built a 842/10 amp that needed either 1600ohms or 3000ohms in the cathode. using a switch proved to be the best option. It also worked nicely as a tie point (solder lug) inside the amp. Using a 25W pot is good for experimenting, dialing in the best values, but not much for the final design.
As mentioned, a switch is the best way to do this. The resistor (pot) would need to be a biggish variac-style variable resistor with heavy duty windings, and it's not going to be as quiet as a "real" switched resistor solution.
I have done this in one of my SE amps so I can use a 6v6 or 6l6 tube. I am also using a LM317t as a voltage regulator/ CCS for setting the cathode bias. I switch out the voltage setting resistor which is only 1/2 watt. I suppose you could install a pot in its location if you want to adjust the bias continuously but that means installing a test point and measuring the voltage at the cathode. cheers, Dak
Not knowing too much about electronics, I only know that variable resistors can get quite hot. An SPDT switch, I guess, wouldn't have that problem. I could add a 200-215 ohm, 3W resistor in parallel with the 500 ohm 3W one and use the switch to cut off the smaller resistor, right?
If you are going to use just the large resistors then you need to wire the cathode to the center pole of the switch and run one resistor from the outside pole to ground and again the second resistor on the other outside pole to ground. And don't forget your bypass cap. cheers, Dak
My suggestion was to leave the 500 ohm resistor in place and use a switch to place a 220 ohm resistor in parallel with that 500 ohm resistor when 150 ohms are wanted and out of circuit when the 500 ohms are wanted.The 220 ohm resistor could be permanently connect to ground and the other end could be switched to connect to the cathode or disconnected from the cathode.
He uses one shared cathode resistor for both output tubes so it would only take one single pole single throw switch to get the job done for his stereo amplifier.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 11/03/15
nt
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You might think about using an active cathode bias regulator for this. The LM337 can set the cathode to whatever voltage is required. You could adjust current not only for different tube types, but for an exact value within each family.
Here's the basic idea (not all parts are shown):
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Buy Chinese. Bury freedom.
Thanks, a little advanced for me; i'll have to find out what an LM337 is.
I have a 25w resistor that is adjustable. It is a wirewound ceramic tube. The windings are exposed along a narrow strip. It has a sliding contact that lets one adjust for any ohm value desired. This is great for dialing in the value you like. Ideal for your situation.
I don't know if such is still manufactured anymore but surely they can be found on eBay, etc.
Those adjustable resistors are absolutely still available.
Dissipation may prevent you from using a pot successfully.
The switch is a very good suggestion.
this is true. I have definitely blown several trying to adjust CCS on large tubes.
Yes, I was worried about heattoo and thinking about that a little.
You might think about a 220 ohm resistor and a switch.The switch would switch the 220 ohm resistor in/out of parallel with the 500 ohm cathode resistor.
220//500 is 152.7 ohms
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 11/02/15 11/02/15
nt
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