6505+ Bias Mod-So easy !!

eddie7314

eddie7314

New member
I just did the bias mod on mt 6505+, 40 cents and about 30 min of time, just wanted to post this in-case any body was wondering how to do this mod,

If you can use a soldering iron You can do this :)

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Well since the 6505+ already has adjustable bias, I'm going to take a guess and say he changed the resistor to a different value to get a different range of the control.

P.S. to the OP- That high wattage resistor is a bit over kill in that spot ;)
 
311splawndude":3d2iinrc said:
Maybe it is real obvious to the amp builders here but what did you do exactly?


Also, welcome to RT!
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Hey Thanks ! This looks like a cool place to hang,

find the 15k resistor on the small board that the bias trim pot is connected to (r68) remove it,

I used a 4.7k 1 watt resistor, but it is recommended to use 5-7k resistors,

Bias to about -40 or above, to me below -40 was to hot,
 
It's cool. I was just curious. I don't even own a 6505+. All I knew is that the variable resistor Peavey uses did not affect the tone all that much for users and that the amps were biased extremely cold out of the factory. I was just suggesting to add a bit more to your OP.

Cheers,
 
Big improvement in tone?
I've been contemplating a couple mods in my 6505+, but I'm lazy! LOL! :D
 
Is the bias mod for an older 5150 pretty much just as easy? I know different components and board I think, but still... is it pretty easy you think?
 
311splawndude":2wkrt2ny said:
It's cool. I was just curious. I don't even own a 6505+. All I knew is that the variable resistor Peavey uses did not affect the tone all that much for users and that the amps were biased extremely cold out of the factory. I was just suggesting to add a bit more to your OP.

Cheers,
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Yeah man :LOL: :LOL: I kinda forgot to explain, :)
 
Vrad":3rin3pu5 said:
Big improvement in tone?
I've been contemplating a couple mods in my 6505+, but I'm lazy! LOL! :D
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You get a better clean channel with more head room , overall better tone I'd say :)
 
:rock: Hi guys. I just did the bias mod to my 6505 plus with the 4.7k resistor. Im superstoked at how much my amps sound improved. I do have a question. What are you guys Biasing your amp to using the 4.7 K resistor? Am i right beleiving that the bias will change if another resister is used with another value? Right now I have mine to 55 DCV. Thanks in advanced for any help!
 
bias voltage is not equivalent to the bias current unless you re-design the amplifier to accommodate ohms law. In all honesty this entire topic is incorrect and should be torn down as the taps on the back are not made to reference bias current to bias voltage. the circuit is not designed to pass that current linearly as well therefore voltages read will not correlate correctly to the bias reading you think you have. in actuality it will be much lower.

also the wattage on that first picture of the resistor is entirely wrong, all you are doing is modifying the sweep of the bias potentiometer. this is a grade A example of what not to do.
 
glpg80":19vxu0rc said:
bias voltage is not equivalent to the bias current unless you re-design the amplifier to accommodate ohms law. In all honesty this entire topic is incorrect and should be torn down as the taps on the back are not made to reference bias current to bias voltage. the circuit is not designed to pass that current linearly as well therefore voltages read will not correlate correctly to the bias reading you think you have. in actuality it will be much lower.

also the wattage on that first picture of the resistor is entirely wrong, all you are doing is modifying the sweep of the bias potentiometer. this is a grade A example of what not to do.
Click to expand...
so what about the link to jerrys bias mod.. is that incorrect??? i wouldnt think jerry of all people would post something that would be false or hurt anyones amp...
 
kaeli0430":3ldkg3yj said:
glpg80":3ldkg3yj said:
bias voltage is not equivalent to the bias current unless you re-design the amplifier to accommodate ohms law. In all honesty this entire topic is incorrect and should be torn down as the taps on the back are not made to reference bias current to bias voltage. the circuit is not designed to pass that current linearly as well therefore voltages read will not correlate correctly to the bias reading you think you have. in actuality it will be much lower.

also the wattage on that first picture of the resistor is entirely wrong, all you are doing is modifying the sweep of the bias potentiometer. this is a grade A example of what not to do.
Click to expand...
so what about the link to jerrys bias mod.. is that incorrect??? i wouldnt think jerry of all people would post something that would be false or hurt anyones amp...
Click to expand...

Jerry and myself are good friends - the link is dead as he tore it down. Jerry knows everything i just mentioned in the above post and will tell you the same thing. If you go to his site you will see a specific quoted page where he mentions modifying the bias circuit to allow current to be read linearly from the amp probes in the back of the amplifier. This is listed as a modification - they do not come with this functionality and this topic is doing nothing to fix it.
 
glpg80":1dex0jix said:
kaeli0430":1dex0jix said:
glpg80":1dex0jix said:
bias voltage is not equivalent to the bias current unless you re-design the amplifier to accommodate ohms law. In all honesty this entire topic is incorrect and should be torn down as the taps on the back are not made to reference bias current to bias voltage. the circuit is not designed to pass that current linearly as well therefore voltages read will not correlate correctly to the bias reading you think you have. in actuality it will be much lower.

also the wattage on that first picture of the resistor is entirely wrong, all you are doing is modifying the sweep of the bias potentiometer. this is a grade A example of what not to do.
Click to expand...
so what about the link to jerrys bias mod.. is that incorrect??? i wouldnt think jerry of all people would post something that would be false or hurt anyones amp...
Click to expand...

Jerry and myself are good friends - the link is dead as he tore it down. Jerry knows everything i just mentioned in the above post and will tell you the same thing. If you go to his site you will see a specific quoted page where he mentions modifying the bias circuit to allow current to be read linearly from the amp probes in the back of the amplifier. This is listed as a modification - they do not come with this functionality and this topic is doing nothing to fix it.
Click to expand...
so your not talking about like 5150/6505 where you have to actually use a bias probe to get the correct reading for each tube..
 
with 5150's you do have to use bias probes, or modify the amplifier. there is no way to measure anything valuable with the modifications depicted in this topic.
 
Ok so if I were using a bios probe to what would I set the bias to taking in consideration the mod done to my 6505 plus with the 4.7k resistor?
 
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