Calculating Bias Resistance

[C Moore]

This is for power tubes. I did a search but could not quite find what I am looking for. In the schematic below, if you were going to add a 50k bias pot to the circuit, how do you estimate the value for R30 ?
I am assuming I add the pot in series with and after R30, so that there will always be some resistance to ground (via R30), even if the pot is Zero Resistance. Is it safe to assume if the fixed resistor is 56k, that a 50k pot and a 10k-15k resistor would be OK ?
Thank You

http://www.informatik.uni-bremen.de/~da ... r_yba1.pdf

[Jana]

I would start with a 33K in series with the 50k pot. test it and see what that gives you for an adjustment range. Ideally, when I choose these values I like to find a value for the fixed resistor that puts the pot right in the middle of its adjustment range for an average set of tubes.

If the 50K gives too much range and is too finicky when adjusting, you may want to go to a 25K pot and a 47K fixed resistor.

[tubeswell]

The resistor to ground is the leg of a voltage divider. The formula for calculating a voltage divider is:

Voltage out (at the knee) = (lower resistor)/(upper resistor + lower resistor) x Voltage in (at the top)

In your case

Voltage out = 15k/(15k + 56k) x voltage in

= 15/71 x voltage in.

Once you know the input voltage you can work out what you will get at the knee of the divider.

Same principle applies to calculating adjustable bias, but you work out 2 limits - one for each end of the pot setting. If you want to make the bias variable then aim for a pot and tail resistor combination that gives you a pot sweep where the resistance is about the same as the fixed resistor divider is (i.e.: 56k-60k), when the pot is at its mid-point. So 30k would be fine. Don't forget to short one side of the pot to the pot wiper

[Structo]

Don't forget to short one side of the pot to the pot wiper

Wha wha what?

[Andy Le Blanc]

That doesn't make sense

This is your amp? Make sure measure the bias V before you disassemble.

You can scab up something quick with bits and get a range around that point.

You might want to replace that 15k or parallel it with a 100k or 220k get a
little more V to play with.

[ChrisM]

Don't forget to short one side of the pot to the pot wiper

Wha wha what?

Just in case the pot fails, rather have some resistance than an open circuit. Reliability thing, has been brought up in other topics recently.

[greenbottle]

For more information regarding Traymor Bassmaster amps see Kevin O'Connor's TUT 3 Chapter 13. (TUT 3 is the construction volume)
Chapter 14 & 15 deal with other Traynor amps. Good amps, good reading good luck.

[tubeswell]

Don't forget to short one side of the pot to the pot wiper

Wha wha what?

Just in case the pot fails, rather have some resistance than an open circuit. Reliability thing, has been brought up in other topics recently.

yep if the pot wiper contact fails, you still have a bias, albeit a bit cooler.

[C Moore]

That doesn't make sense

This is your amp? Make sure measure the bias V before you disassemble.

You can scab up something quick with bits and get a range around that point.

You might want to replace that 15k or parallel it with a 100k or 220k get a
little more V to play with.

What is it that does not make sense ?
Yes, this is my amp.

[martin manning]

Some options here from a short while ago:
https://tubeamparchive.com/viewtopic.ph ... 310#105310

[Andy Le Blanc]

I agree with struco... not accustomed to seeing that.
Usually when you go after references, its best to use at least three.
Remember that those who can't do something, teach.

Haven't really seen more than two or three bad bias pots in 25 years.
Saw one last week but it was inside a carvin POS, crap trimpot.

The time honored solution for fixed bias worries is cathode bias.

[FunkyE9th]

Huh? What are you not accustomed to? What is it that does not make sense? What is it that was stated above that is incorrect?

[Jana]

"Remember that those who can't do something, teach."

Thanks for educating us.

[M Fowler]

Heck I am getting an education on word phrases alone, great stuff here

[Andy Le Blanc]

Ouch..... most of the representations of a fixed bias on schematics and most
of the old reference don't use that approach, ChisM , and Tubeswell
have already clarified the point in the discussion.