Changing bias range resistor to increase current range?

DonaldDemon

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9,279
So I got a new set of tubes that are rated lower and the dealer is giving me problems with swapping so it looks like I have to change my bias resistor again. The amp is a Splawn Pro Mod w/ Ruby KT88s. I'd like to get closer to 60-65% range of dissipation which would be around 50-52 mA but I can only get up to about 38mA right now.

In the past I replaced the resistor to match the tube set (is it normal to have to keep doing this?). Now there is a 10k resistor for the bias range resistor. What value should I add to get 10mA more range? Do I need to piggyback this on the existing resistor?

Thanks in advance
 
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WaltC

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not enough info (B+ voltage, kind of tubes, bias circuit setup, etc.) but.... I'd bet a cool five bucks that you could't find anybody that could tell the difference in a blind listening test between 47ma and 50ma.
 

DonaldDemon

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9,279
not enough info (B+ voltage, kind of tubes, bias circuit setup, etc.) but.... I'd bet a cool five bucks that you could't find anybody that could tell the difference in a blind listening test between 47ma and 50ma.
B+ voltage was around 485v, Ruby Kt88s (formerly Penta Lab in there) bias circuit is resistor + bias pot. I'm not tech savy, I know just enough to do a bias job with a bias meter. I was wrong on the 47ma, the average was down to 38ma.
 
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mark norwine

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any talk of "the resistor" is silly...

What to change is entirely dependent on the layout of the circuit... Got a schematic?
 

DonaldDemon

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any talk of "the resistor" is silly...

What to change is entirely dependent on the layout of the circuit... Got a schematic?
Can you explain why it is silly? In my extremely limited knowledge, I thought I understood that changing the value of a bias range resistor changes the sweep range of the bias pot, no? I know I am oversimplifying it but I thought that was the simple fix?

I have no schematic, Splawn is protective of the circuit.
 

mark norwine

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Can you explain why it is silly? In my extremely limited knowledge, I thought I understood that changing the value of a bias range resistor changes the sweep range of the bias pot, no? I know I am oversimplifying it but I thought that was the simple fix?

I have no schematic, Splawn is protective of the circuit.
Fixed bias (which is what we're talking about here) is typically accomplished with what is called a "voltage divider", where Vout = (Vin * R2) / (R1 + R2), and in the case where adjustability is designed in, one or more of the resistors involved can be potentiometer. Moreover, either R1 or R2 can be a resistor AND a pot.

To complicate matters, there are several different ways to configure the divider (compare Fender's & Marshall's circuits, for example).

Vout is adjusted so that, when applied to the grid, bias current is controlled; the "higher" Vout is, the "lower" the bias current is (i.e. "cold bias"). The opposite is also true..."Lowering Vout causes higher bias current (i.e. "hot bias").

Without knowing the circuit in question, I have know idea what "resistor" (singular) you're talking about. So for me to tell you to simply change "it" to "value x" would be pretty irresponsible of me (or any other internet tech).
 
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WaltC

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2,126
if it's a '76 or so (like mine) then this is probably the circuit:

http://www.drtube.com/schematics/vox/ac501976.gif

and it's the 10K between the two pots (ganged in mine) and ground

if it's an earlier model then it may be like this:

http://www.drtube.com/schematics/vox/ac501965.gif

and you'll have to play with the value of R23

or it might look like this:

http://www.drtube.com/schematics/vox/ac50jp.gif

in which case you'd probably have to adjust the value of the 100K resistor to ground in the bias circuit.

That said, I'd not be confident of recommending any of that without looking at the actual amp became Vox did some unusual things and none more unusual that the different versions of the AC50 and AC100 amps.

And, as Mark said, there is more than one way to adjust what you get for minus (-) voltage for the grid. You can adjust either side of the voltage divider, change the value of the pot to change the total range, etc.

For me, the way I usually do it is to sub a resistance box (lots of resistance values selectable by knobs) to determine the value of the resistor or resistors that makes the most sense and then solder them/it into the circuit.
 

DonaldDemon

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9,279
Fixed bias (which is what we're talking about here) is typically accomplished with what is called a "voltage divider", where Vout = (Vin * R2) / (R1 + R2), and in the case where adjustability is designed in, one or more of the resistors involved can be potentiometer. Moreover, either R1 or R2 can be a resistor AND a pot.

To complicate matters, there are several different ways to configure the divider (compare Fender's & Marshall's circuits, for example).

Vout is adjusted so that, when applied to the grid, bias current is controlled; the "higher" Vout is, the "lower" the bias current is (i.e. "cold bias"). The opposite is also true..."Lowering Vout causes higher bias current (i.e. "hot bias").

Without knowing the circuit in question, I have know idea what "resistor" (singular) you're talking about. So for me to tell you to simply change "it" to "value x" would be pretty irresponsible of me (or any other internet tech).
Well the Splawn is basically a take on the Marshall JCM800 as I understands it. All I can tell you is that it does have a bias resistor (R30) and a bias pot (VR1). Here is a picture of the pot and 16k resistor before I changed it to the 10k.



I should have prefaced this by saying I have been through this before, as instructed by the builder. So I am 100% positive that the tubes are rated to low for the bias circuit and that the bias range resistor needs to be changed. Up or down is what I am unsure of. Maybe I should just change it to the next lower value (assuming that is the direction I need to go) and go from there. What I am trying to figure out is if I need to completely swap it out or can I piggyback off the one there and what values should I be shooting for?
 

drbob1

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The problem is that you need to slightly decrease the bias voltage. But we're not clear if the resistor in question is in series between hot and pot (you'd need a smaller value) or between pot and ground (you'd need a larger value, I think). Why not just ask Splawn? Conversly, you could make a very slight change (add a 40k in parallel, which gives you an 8k total) and see what that does. It'll either make things slightly hotter or slightly colder, then you know which way to go.

BTW, do you remember the problem you were fixing when you went from 16k -10k? Too hot or too cold?
 

DonaldDemon

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9,279
The problem is that you need to slightly decrease the bias voltage. But we're not clear if the resistor in question is in series between hot and pot (you'd need a smaller value) or between pot and ground (you'd need a larger value, I think). Why not just ask Splawn? Conversly, you could make a very slight change (add a 40k in parallel, which gives you an 8k total) and see what that does. It'll either make things slightly hotter or slightly colder, then you know which way to go.

BTW, do you remember the problem you were fixing when you went from 16k -10k? Too hot or too cold?
Ah ok, that makes more sense to me. I guess it will ultimately warrant a call to Splawn. I try not to bother him too much because he is building away like a madman but I will call when I get chance.

How do I add resistor is parallel or in series? :anon :hide2

I wish I could remember but that was several years ago. I want to say I was trying to get colder/decrease bias range but I just not sure.
 

drbob1

Gold Supporting Member
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28,530
In parallel you could just solder another resistor to the leads of the one that's in there. In series, you'd have to remove the one you've got and just sub in another one of the correct value.
 

DonaldDemon

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9,279
Ok, talked to Scott Splawn and I have go up in value (which is the opposite of what I thought) to get it biased hotter. So in the case of my amp, lower value bias range resistor to go colder, higher for hotter. He said to go back to the 16k value to be in teh safe range for most tubes, which where I originally was!! I still think its odd that I am having to mod teh resistor for simple tube changes but I guess that's the nature of the beast if tube dealers are not willing to swap for ones that fit better.
 




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