Basic bias question

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bluesbear
Hey get your own solder
Posts: 362
(4/20/04 6:31 pm)
 Basic bias question
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 Hi. I need to double check my understanding of this. I get it in theory, but I need someone who will answer 2 questions straight with no explanation of current draw or negative and positive voltages and current, etc. Please help. these are hypothetical questions, meant to give an example of bias, NOT a real problem I'm trying to fix.

1. In a cathode bias amp with 2 6V6's and a 250 ohm cathode resistor, the tubes are too hot and cherry colored. To cool down this amp, do I INCREASE or DECREASE the ohmage of the cathode resistor?

2. In an amp with fixed adjustable bias (AA864 Bassman, for instance) given the same situation with glowing cherry 6L6 tubes, do I INCREASE or DECREASE the ohmage of the trimmer pot to cool down the amp?

Jerry, if you're reading this, I'm making sure of I get it, not checking up on you.

Thanks!
Dave
 
GroundhogKen
Forum Moderator
Posts: 2348
(4/20/04 8:12 pm)
 Re: Basic bias question
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 1. In a cathode bias amp with 2 6V6's and a 250 ohm cathode resistor, the tubes are too hot and cherry colored. To cool down this amp, do I INCREASE or DECREASE the ohmage of the cathode resistor?

INCREASE


2. In an amp with fixed adjustable bias (AA864 Bassman, for instance) given the same situation with glowing cherry 6L6 tubes, do I INCREASE or DECREASE the ohmage of the trimmer pot to cool down the amp?

This one is tougher to answer. You turn the pot in the direction that makes the voltage at the wiper MORE negative. But if the pot is already maxed out, I would simply change the 15K resistor to 22K. Here is the AA864 scheme, so you can see the resistor I'm talking about.

www.ampwares.com/ffg/sche..._schem.gif

Hope that helps.

Ken
 
Lucid Alice
Senior tube assistant
Posts: 195
(4/20/04 8:35 pm)
 Re: Basic bias question
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 I can answer the first question. You would increase the ohms on the cathode resistor. A higher resistance passes less current.

On the second question, the bias pot on a plexi is a variable voltage divider. More resistance increases the bias voltage. Would someone confirm for me that increased bias voltage lowers current flow through the tube?

 
GroundhogKen
Forum Moderator
Posts: 2349
(4/20/04 8:41 pm)
 Re: Basic bias question
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 When you increase the negative voltage on the grids ( make it a larger negative number ), the plate current decreases ( tube runs cooler).


Ken
 
bluesbear
Hey get your own solder
Posts: 363
(4/21/04 11:20 am)
 Re: Basic bias question
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 "When you increase the negative voltage on the grids ( make it a larger negative number ), the plate current decreases ( tube runs cooler)."

But would a smaller or a larger resistance increase the negative voltage to the grids? It's the whole "negative/positive" thing that's throwing me off.
Thanks,
Dave
 
GroundhogKen
Forum Moderator
Posts: 2351
(4/21/04 11:34 am)
 Re: Basic bias question
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 In the case of the AA864:

A larger resistance on the "ground side" of the bias pot wiper makes the bias voltage more negative (tubes cooler).

Increasing the resistance on the tranformer side of the bias pot wiper makes the bias voltage closer to 0V ( less negative--tubes hotter ).


Ken
 
Leevi
Junior tube assistant
Posts: 140
(4/21/04 1:32 pm)
 Re: Basic bias question
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 This is my understanding, please correct if I´m wrong:

1. If the tubes are running too hot you have to decrease the current which is going through the tube. In cathode biased amp you have to increase the voltage (minus voltage) between the cathode and grid (pin 5). It means that you have to increase the cathode resistor.

2. It depends on how your trimmer pot is connected. Instead of checking of the position of the trimmer pot measure the voltage towards ground at the grid (pin 5). The higher the minus voltage is the less current will pass through the tube i.e. the tube will run cooler.

/Leevi
 
bluesbear
Hey get your own solder
Posts: 364
(4/21/04 3:25 pm)
 Re: Basic bias question
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 Okay, I think I've got a handle on it now. Thanks, all. I'd better mull over this a bit.
Dave
 
pabloxyz
Hey get your own solder
Posts: 202
(4/21/04 5:21 pm)
 re: bias question
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 I too sometimes need things to be spelled out as simply as possible. It's like taking a math class - if there's one piece of information that you miss or don't understand, everything from that point on is muddy water.

I have been away from my amp tweaking bench for a few months, busy with other pursuits (new wife, getting settled etc.). But now I'm wanting to spend some time with the tube amps.

pab
 
 
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