How To Bias A Vintage Fender Champ Amp
A Quick Guide In Ohm's Law
When it comes to hip vintage guitar amps, the Fender Champ is still a favorite in our guitarist’s hearts. Yet, not all sound that great and in fact they can be hit and miss sometimes when it comes to expectations vs. reality when you forget listening is done by ears and not with the eyes.
The complaints usually are that they sound harsh, stay too clean and lack that forgiving fullness that the tweed versions are famous for. However, these can actually breath and sound as deep and sweet as you expect a vintage Fender amp to sound after correcting the increased incoming wall voltage with this simple BIAS calculation and often needed BIAS value correction. Here we go!
Ohm's Law
This is such a wonderful little tool to help get all the information we need from the amp. The steps for any cathode biassed amp are:
1. Measure the BIAS resistor. We get (R)
2. Measure the voltage accross it. We get (V1)
3. Divide (V1) by (R). We get (I)
4. Measure the Plate Voltage on pin3 of the 6v6. We get (V2)
5. Multiply (V2) by (I). We get the plate dissipation in Watts.
If that looks like a little too much to digest, let’s see it in perspective within the 1965 Fender Blackface Vibrochamp you see up here.
Calculation the Original Bias
Let’s collect all the info we need. Needless to say: Don’t do science experiments with high voltage equipment if you’re new to this. Leave it to the experienced and qualified if you’re not considering yourself as such.
The stock value of the BIAS resistor is 470 ohms. In this amp it measures 532, so we write that down. The voltage drop across that BIAS resistor is 21,4v or -21,4v if you prefer the negative voltage. Dividing the 21,4 by 532 gives us 0,040 amps. or 40 milliamps. That number is on the absolute max of the 6V6 already and it explains why the plate voltage we got here was only 350v. It was pulling too much current.
The number 350 can now be multiplied with the 0,040 and we get 350×0,040 = 14,1 Watts. And no surprise that’s also on the absolute max of the 6v6 tube. The BIAS is too hot. Despite the amp is technically original, the value of the components working with the wall voltage being higher than in the 1960s make it operate less optimal than these were intended to.
WHAT HAPPENS
A cathode biased single ended amp usually runs on a hotter bias then a class A/B amp. This overbiassing makes the amp a little louder and clean then intended, not necessarily a disaster, but we might also kill some of the Vibe of this amp. The warmth, sweeter rumbly overdrive at a lower level where these are supposed to be great at. We won’t get all of that goodness if we don’t give it the right wall voltage. And we can’t really, unless we bring out variacs and power conditioners. But this is something you just want to throw in the situation and sound great with not too much BS to bring along.
The Fix
By increasing the value of the BIAS resistor, we can mimic the incoming wall voltage hitting the BIAS section and then the 6v6. It’s the best we can do without structurally changing the amplifier. Yes, some other voltages will be too high still, but they won’t harm the response that much. I went a bit all the way to 1k in this, measuring 982 Ohms in the circuit and let’s calculate what that gives us.
The NEW Bias
The new voltage drop went down to -27v. Divided by 982 gives us 0,027 amps, immediately a much healthier number. The plate voltage went up to 372v and multiplying that by 0,027 gives us 10,22 Watts. That’s 0,73 % of the max of the Tube. You might consider this low for this amp but for now I think this is OK as it’s getting dark and I’d like to go eat something now. You can check out how it’s done and how it sounds in this video.