Fender Passport biasing

[Bob W.]

I have a Fender Passport that I have changed the outputs on one side and need to bias it. Is there a recommended procedure for doing that? Thanks for any help. Bob

[The Dude]

I don't know what the "actual" procedure is, but couldn't you simply match the repaired side to the non-repaired side? That should get you pretty close. Those things are a pain in the a$$ to work on. One of my least favorite repairs- possibly second only to Behringer powered mixers.

[Jazz P Bass]

I will have to assume that the unit is the PD-250.

The only mention of bias adjustment in the SM, is to 'minimize crossover distortion at low signal levels'.
Basically a Class B, barely A setup.
VR202 is the bias pot.

There is not any mention of VR201 but it appears to be a DC offset adjust.

[Enzo]

Exactly, the point of bias is to tune out the crossover notch. That is any amp.


I adjust both channels when I repair one, who knows how far a channel could have drifted since it was made, even a good one.


Set it cold, then run a sine wave through it and watch the output, see the notch? Now advance the bias until that notch JUST disappears, then back off just a hair. Now do the other one.


You want a dirty way? When the bias is cold, the V+ and V- sides take turns conducting. As you turn it up warmer, there comes a point when the V+ and V- sides are both conducting at once, The hotter you turn from there, the more they conduct. That path from V+ to V- is shunting current across the supply. That shows up as a rise in mains current draw. It just so happens that the point they both start to conduct is also the point the crossover notch disappears. The notch being the difference in time between the two sides turning on. So if you monitor the mains current, start with a cold setting, then advance the bias until the mains current just starts to rise, and back off that hair. Then do the other side.

[Jazz P Bass]

I love biasing the 'dirty way'.

(I check tube amps the same way)
(not set, check)

Good tip, Enzo.

[g1]

For the SS amps, are you biasing with or without load? (xover or dirty method)

[Jazz P Bass]

I do not use a load on Solid State dual rail voltage amps to set bias.

I will check that the load did not change anything though.

[Enzo]

No load. The bias point is the bias point. No value in having the load current obscuring your task.

Jazz is right though, check it afterwards.

[g1]

I don't use a load when biasing SS, and I know when emitter resistor measurement is used they usually specify cold, no load.
However, I've never tried the "dirty" line current method, and thought maybe the load was required to get current going.

[Enzo]

No, the idea is to find the point where the two sides start being on at the same time. That will thus cause current to rise.


In my opinion, the whole emitter current standard thing is the result of the amp maker determining that such and such a bias setting is good. Now we measure the current through the emitters under those conditions. Then we tell technicians to set it for that current, expecting then that the bias will be appropriately set for crossover. If we set it for a few milliamps, then the two sides must be just barely on together.


If you measure emitter current, starting from a cold setting, there will be no emitter current, until you reach the point they start cconducting across, then the emitter current will start to ramp up.

When I refer to cold, I have been talking about a cold setting of the bias. That is not the same as having a cold amp when adjusting. Even if the amp has been running hard for an hour and is quite warm, I still can have a "cold" bias setting. Or I can set the bias to the hot end, but not have the amp on for more than a few seconds so it never heats up.


I don;t recall offhand, I think Carvin is one, maybe Yorkville, but tube guitar amps will have fairly consistent draws for everything but plate current. The heater draw won;t vary much, the low voltage circuits, like bias or channel switching. won;t vary much, but setting the B+ current will vary. So they set up their amp the way they wanted then measured the mains draw. Then the spec is to adjust the power tube bias until X amounbt of mains current is drawn. The amp is then where it ought to be.

[g1]

That's the Carvin method (or one of). I understand what you mean about hot and cold bias and did not want to confuse the issue, but many of the specs for SS amps I've seen call for the amp itself to be cold.

[Enzo]

That is another reason I never liked the emitter current method. They spec the amp cold because they also know that the current will settle once it is hot. So the cold amp setting they provide SHOULD result in the desired numbers once it warms up. So the whole process is sorta roundabout.

[Jazz P Bass]

The Crate Blue Voodoo also uses the mains draw to check that the bias is good.

(4) 6L6 tubes: 120 watts mains draw.