Fender DR Bias Mod?

Heady Jam Fan

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9,009
I started this thread discussing how I found my amp is smoother at higher volumes when I drop the bias about 10ma.

Its not to big of a deal to drop the bias a bit if I'm going from playing at home to a gig, but... hypothetically...

Would it be possible to install a switch that drops the bias by about 10ma?

My first thought was that it could adjust the range of the bias pot, but then I was thinking maybe it would make more sense if it worked like a cathode bias (in addition to the bias pot) that switches in and out a resistor to ground.

Do you think this is possible?
If so, a schematic / how it could be done?

I think that could be a really cool option for amps that has as setting ideal for playing at home and a setting ideal for gigs.
 

Blue Strat

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Both are possible but I like your first idea better. Cathode bias changes the feel of the amp from what you already know you like.
 

smolder

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This article is about modding a fender balance pot to a bias pot, but has some good info explaining the fundamentals.

http://www.el34world.com/charts/bias_conversions.htm

You could probably, through trial and error, find a resistor pairing that comes close... or even more complicated... put a pair of bias pots in parallel and switch between them. Not sure it would be worth the effort. I'd probably use a relay to keep the voltage off of the toggle.

There are after all circuits that switch between solid state and tube rectification.
 

Heady Jam Fan

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9,009
This article is about modding a fender balance pot to a bias pot, but has some good info explaining the fundamentals.

http://www.el34world.com/charts/bias_conversions.htm

You could probably, through trial and error, find a resistor pairing that comes close... or even more complicated... put a pair of bias pots in parallel and switch between them. Not sure it would be worth the effort. I'd probably use a relay to keep the voltage off of the toggle.

There are after all circuits that switch between solid state and tube rectification.
Nice call - I remember reading that article when I had a SFTR with a Bias Balance and I wanted a regular bias pot.

If using a resistor and a switch, the question is what value resistor and where in the circuit?

Instead of parallel switchable pots, would it be possible to do a 2nd one in series that goes from normal functioning of a single pot to decreasing the bias by approximately 10ma?
 

Heady Jam Fan

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9,009
Both are possible but I like your first idea better. Cathode bias changes the feel of the amp from what you already know you like.
Yeah, someone had suggested switching (completely) to a cathode bias, which is where I got the idea, but I'm trying to change the amp as little as possible.
 

smolder

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Nice call - I remember reading that article when I had a SFTR with a Bias Balance and I wanted a regular bias pot.

If using a resistor and a switch, the question is what value resistor and where in the circuit?

Instead of parallel switchable pots, would it be possible to do a 2nd one in series that goes from normal functioning of a single pot to decreasing the bias by approximately 10ma?
You could take the resistor value and divide it putting two in series and switch between access points, but it would probably be simple to use the resistor that's there, and another to drop it further in parallel to the switch. What other resistor to use is the "trial and error" part.
 

Heady Jam Fan

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9,009
You could take the resistor value and divide it putting two in series and switch between access points, but it would probably be simple to use the resistor that's there, and another to drop it further in parallel to the switch. What other resistor to use is the "trial and error" part.
Simpler is better IMO, so the latter seems like a better plan.

Basically, this involves adding a switch before or after the resistor thats already there and a resistor parallel to the switch that is part of the circuit when the switch is engaged?
 

Blue Strat

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You could take the resistor value and divide it putting two in series and switch between access points, but it would probably be simple to use the resistor that's there, and another to drop it further in parallel to the switch. What other resistor to use is the "trial and error" part.

This, plus using another pot, as the OP asked about, would allow him to dial in the "trial and error" part.
 

Heady Jam Fan

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9,009
This, plus using another pot, as the OP asked about, would allow him to dial in the "trial and error" part.
Thanks!

I'm a novice at this though and have no idea what this would look like / how I would wire it in the amp. I've modded bias circuitry, but with a clear schematic to follow.

This might be a ridiculous question, but your talking about the 10k resistor to ground?
 

smolder

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Mike is right... setting the bias to the higher point, then adding a 10k pot (variable resistor) in series to the existing resistor would allow you to dial in the difference in mA you want, and then you can read the pot with a multimeter and determine exactly what fixed value resistor you need. Just be careful handling that pot in the circuit with the amp on. I'd stick it somewhere with double stick tape on the back and use a plastic knob.
 

Blue Strat

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+1. HJF, do you know how to connect a pot in series with the range resistor, the one connected between the bias pot and ground?
 

Heady Jam Fan

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9,009
Mike is right... setting the bias to the higher point, then adding a 10k pot (variable resistor) in series to the existing resistor would allow you to dial in the difference in mA you want, and then you can read the pot with a multimeter and determine exactly what fixed value resistor you need.
+1. HJF, do you know how to connect a pot in series with the range resistor, the one connected between the bias pot and ground?
Yes! I believe so:

If I un-solder the lead of that 10k resistor connected to the terminal of the bias pot (leaving the resistor soldered to ground), then solder a wire from that terminal of the bias pot to the wiper of a new potentiometer, and the terminal of the new potentiometer to the free lead of the 10k resistor.

I have a 25k pot that isn't used... not sure if that value is too high.

 
Last edited:

neteraser

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In these amps you SHOULD NOT TOUCH the 10k pot and the 10k resistor to ground. 100% guarantee this is the way to kill the tone. You can however adjust the 470R input resistor (i.e. resistor before the diode) without any significant effect on the tone. So the right way is to set the bias lower with a pot, then adjust the input resistor to drop more voltage for a higher bias setting. Then switch these two resistors.
 

Heady Jam Fan

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9,009
In these amps you SHOULD NOT TOUCH the 10k pot and the 10k resistor to ground. 100% guarantee this is the way to kill the tone. You can however adjust the 470R input resistor (i.e. resistor before the diode) without any significant effect on the tone. So the right way is to set the bias lower with a pot, then adjust the input resistor to drop more voltage for a higher bias setting. Then switch these two resistors.
hmm

The only 470 resistor I see is connected to pin 8 of the PI
 

Blue Strat

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30,194
In these amps you SHOULD NOT TOUCH the 10k pot and the 10k resistor to ground. 100% guarantee this is the way to kill the tone. You can however adjust the 470R input resistor (i.e. resistor before the diode) without any significant effect on the tone. So the right way is to set the bias lower with a pot, then adjust the input resistor to drop more voltage for a higher bias setting. Then switch these two resistors.

I disagree 100%.

If you can adjust the bias pot (and you CAN), there's no downside to increasing the range resistor value.
 

Heady Jam Fan

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9,009
Ok, it didn't work the way I thought, but...
1) not sure I wired it correctly.
2) think the only pot I had laying around is not functioning correctly anyway.

I'd think this would be a popular switch for amps - kinda solves, or improves, the dilemma of how to get and amp to sound good at bedroom levels and gig volume. So I wonder why amps techs/builders don't use a switch like this: does it have side-effects or is it just overlooked?
Anyway, deciding if I'm gonna get another pot so I can keep experimenting because biasing cooler for gigs is a fairly easy solution itself (since I don't know exactly what I'm doing, lol).
 

Blue Strat

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30,194
No downsides other than another part to pay for and another component to fail.

Not everyone will agree that it sounds better for low level playing, and few even care.
 




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