Older Randall Warhead..

[livewire]

Anyone have any thoughts on biasing an older Randall Warhead? Talking to US Music there is noone at the factory anymore when these amps were produced and serviced in house. There are no provisions in the chassis anywhere to make an adjustment. No bias pot, nothing. Many thanks....

[J M Fahey]

Maybe because nobody should be messing with bias.
Why do you want to?

[livewire]

What kind of an answer is that? I am a repair center. This thing had 3 shorted outputs which were replaced, and several bias resistors, which warrants a possible bias adjust in my book. Hows that for an answer? I understand you do not know who I am, or that I should be in there in the first place. I thought everyone was here to help one another, maybe not?

[eschertron]

What kind of an answer is that? I am a repair center. This thing had 3 shorted outputs which were replaced, and several bias resistors, which warrants a possible bias adjust in my book. Hows that for an answer? I understand you do not know who I am, or that I should be in there in the first place. I thought everyone was here to help one another, maybe not?

Please forgive JM, he is getting older and may get into one of his moods
But seriously, he is also a world-class expert in SS amps. If anyone can shoot straight in that realm, he can. My guess is that a specific to-the-point question gets a better and more accurate response on the forum than a generic, vague and hypothetical-sounding one. Now that you've provided some specifics, keep it coming. You will get help here, absolutely!

[livewire]

Thanks a lot!. I do not mean to be rude also, just trying to ask a question. Possibly he will somewhat understand what and why I am asking the question now. Thanks again....

[g1]

There are many SS amps that do not have provisions for setting bias. There is a range they feel is acceptable if all components are working properly. If not, the defective component needs to be located and replaced. Often they do not even state what to expect for bias current. Just to throw out an example, I'll say Fender Deluxe 112 Plus.

All that being said, if you post the idle DC voltage across the emitter resistors of the power transistors, someone will probably have an idea if there is still a problem.

[The Dude]

Yes, that ^^^^^^. It's also important, when servicing these amps without a bias adjustment, to use original part numbers. Substitutes or cross-referenced parts may not bias the same as originally intended. If you use the "correct" parts, a bias adjustment is generally not necessary.

[Enzo]

Welcome to the forum.

But really, you have very few posts, and we don't know you. This forum attracts everything from 14 year old kids messing with their amps up to 70 ear old industry veterans like myself. From people who first looked at a circuit yesterday to folks like Fahey who have been in the engineering and manufacturing side for decades. Some want to "bias" their amps because they read something on the internet. And some want to cure a problem. SO it is a legit question asking you WHY. We ARE here to help, but we need to qualify the response to the situation.

Solid state amps all tend to be similar, especially in terms of bias. The bias keeps the opposing transistor bases apart enough to prevent crossover distortion. Not all are made adjustable. Very few Peavey amps have an adjustment for example.

Have you a schematic we could see?

Many amps don;t really need biasing. The circuit is designed cold enough that most examples of new outputs won;t put it out of whack. SOme, again like Peavey, have simple bias circuits like a string of diodes. If a number of diodes is not close enough, we can parallel one with a resistor to drop the voltage a little. If ther is a transistor, we can determine its own biasing resistors and make one a trimmer, or just change a part.

You want the bias set so crossover distortion is minimal, but also cool enough that the amp won't run hot.

[livewire]

Ok, TY very much for the ideas and suggestions, I wil try them, and make some measurements and get back. And yeh Dude, I did use original part numbers.. Thanks again.