Once again~a question about tube biasing

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cellardweller

cellardweller

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So here I am in the armpit of hell where there is no one who does good work on amps. I've had two tube amps (Carvin and Ampeg) which were both never the same once I'd sent them to be tubed and biased.

So what's it take for a guy to bias his amp?
I may seem developmentally challenged by my soundcard quandaries, but I think I could probably learn to do this.

So there's this Bias King Pro thing. Has a socket that the tube goes into, which then goes into the socket of the amp.
Digital readout measure cathode current.
So how is "plate voltage" adjusted?



Okay, MAYBE the gist of this question is should I buy a "fixed bias" amp and pay out the arse (and then find out later that the cold bias of Mesa isn't for me), or buy a book/take an electronics course/sacrifice goats for knowledge?

Somebody hep' me! :eek: :D
 
If you need diagrams the article included a couple,..... I can put them up if youd like....

Steve
 
I have a "dual bias probe" that I got on e-bay. I use it with my Marshall, and an old Fender head. You have to supply your own Digital multimeter. It was cheaper...Anyway, My amps have a "bias pot" on the chassis to make the adjustment. Very easy once everything is hooked up. Also much safer than trying to hold meter leads on the pins of the tubes. It was a good investment.
You have to determine if you have a pot for this adjustment , or if you have to play "hit and miss" with resistors till you get it adjusted properly.

Also the tubes have a lot to do with the sound as well. I replaced the tubes in my Marshall with Svetlana Sed's , because I read they were supposed to be the real deal for it. ...They were lame. I stuck a cheap set of Eh 's in and it was much better.
 
Thatupstateguy said:
If you need diagrams the article included a couple,..... I can put them up if youd like....

Steve
Click to expand...
Please good sir, if you could.
After reading that several times, some of it started to make a little sense :confused: :D

It's a bad morning!

Did you see the blurb I "sent" you, some time ago?
 
OK, just go here,.... it will help more than I can....


http://www.diyguitaramp.com/bias.html



and the post by gold sounds interesting,....


I wish all amp makers had the forsight to include a bias pot somewhere on the chassis,... it would make it too easy tho,...


Steve
 
goldtopchas said:
I have a "dual bias probe" that I got on e-bay. I use it with my Marshall, and an old Fender head. You have to supply your own Digital multimeter. It was cheaper...Anyway, My amps have a "bias pot" on the chassis to make the adjustment. Very easy once everything is hooked up. Also much safer than trying to hold meter leads on the pins of the tubes. It was a good investment.
You have to determine if you have a pot for this adjustment , or if you have to play "hit and miss" with resistors till you get it adjusted properly.
Click to expand...
Please explain this "hit and miss" with resistors method. Are the resisitors adjustable, or just "replacable" :eek: ???
 
the 'replace' thing,..... hope you have some time on your hands,..... sort of the long way around,... but will work,.....


Steve
 
Well a pot is a variable resistor. Some amps only have a resistor, and you have to swap difference values until you get it right.
 
The pot in a bias circuit is nothing more than an adjustable resistor. In a fixed bias amp, there is a fixed value resistor in the circuit, no pot. You adjust the bias by replacing the resistor with one of a higher or lower value. Takes some experience to get right. Or you can replace the resistor with a pot and voila, adjustable bias. I did it to my 5150 and the results were amazing. I wouldn't recommend attempting to do it yourself though. It's about a $100.00 mod not including tubes.
 
Is it common for amps to not have an adjustable pot?
Geesh. Sort of a design flaw, isn't it?
 
cellardweller said:
Is it common for amps to not have an adjustable pot?
Geesh. Sort of a design flaw, isn't it?
Click to expand...

Not really. Tubes used to be readily available and pretty consistent, and it was easy to find the same tubes you already had, which basically meant you didn't have to change the bias. By readily available, I mean by them at 7/11 available. They were easy to find. Also, many smaller amps are cathode-biased, and self-adjust over a certain range. No pot necessary. And people used to be less into tweaking.

As to what gbdweller said, fixed bias means there is a bias supply providing a set amount of negative voltage for bias. The voltage stays the same, unlike in a cathode-bias design where the bias drifts a bit. It can be adjusted with a resistor or a pot. Fixed does not mean non-adjustable, nor does it mean the bias is set with a fixed-value resistor.

Got your PM, but I don't know what to tell you. Metal ain't my forte. :(
 
You can also slightly adjust the bias( current), by swapping the RECTIFIER tube with a different one,..... but unfortunately, most mashalls that are tube,... have a solid state rectifier ,......

If not,... and the rectifier tube is a 5Y3,..... you can swap it for one that is compatible,.... say a 5V4,..... which is a military spec rectifier,.....or a 5R4,or 5AR4,..... this only works for some amps,...... DO NOT,... swap if it is a 5AR4,.... that is a modern tube type,.. and is not compatible with others I mentioned,....and should only be replaced with the same number,

Usually, a cathode biased amp,... like a single ended Champ,.. this can be done without dire consequences......

I sort of like what the 5V4 does when combined with the others in that amp.....
I wish more amp builders would install a bias pot,... it would make it SOOOOO much easier for the average guitar player to just swap out the power tubes and adjust the amp himself.....

I guess they just want any adjusting to be done by the retailers.....

Kinda makes sense really,... kinda like a car dealer wanting you to come back to them for parts, and maintenence after you buy a car from them.....captive audience sort of thing.....


Steve
 
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