Why don't more amps have external bias pots and test points?

[VacuumVoodoo]

Originally Posted by '58Bassman

How is it done where the voltage is so low?

1 Ohm (10 Ohm in some amps) resistor on each power tube socket between cathode and ground. Voltage on that resistor in mV is numerically equal to plate + screen grid current in mA.
Nitpickers can subtract 3-5 from the reading to get closer to true plate current value. BTW making plate voltage accessible on external test terminals would never pass CE. By same token some types of US fuse holders don't get CE approval, they allow live part to make skin contact while replacing fuse.

[jpage]

Originally Posted by '58Bassman

You don't think someone would sue the crap out of a manufacturer? Really?

No, in fact I find it to be a foolish assumption as far as external bias points are concerned. The alternative is so much more dangerous it is ridiculous. Aside from a desire to keep amp techs in business there is no reasonable reason for refusing to include this feature in a tube amp. Other than cost cutting, of course.

[scottl]

Exactly. A million percent in agreement.

Originally Posted by jpage

No, in fact I find it to be a foolish assumption as far as external bias points are concerned. The alternative is so much more dangerous it is ridiculous. Aside from a desire to keep amp techs in business there is no reasonable reason for refusing to include this feature in a tube amp. Other than cost cutting, of course.

[teemuk]

Aside from a desire to keep amp techs in business there is no reasonable reason for refusing to include this feature in a tube amp.

Based on how ridiculously clueless some people seem to be with amps I wouldn't even marvel going to extremities like refusing to include impedance matching sockets and effects loops. Many people are technologically impaired and stupid. That's enough reasonable reason for me.

[jpage]

It's like only letting electricians replace light bulbs.

[VacuumVoodoo]

Originally Posted by teemuk

Based on how ridiculously clueless some people seem to be with amps I wouldn't even marvel going to extremities like refusing to include impedance matching sockets and effects loops. Many people are technologically impaired and stupid. That's enough reasonable reason for me.

You certainly sound like you want to make Darwin awards redundant, hm?

But I certainly wouldn't sell even a flashlight to these geniuses:

[teemuk]

The bias pot is not a tone control, there's no need to adjust its setting continuously. Power tubes last a good deal and the control is really needed only when they are changed plus a few times more to compesate drift due to aging. If you want to go overboard with the latter cause or always make sure the tubes are at their optimal bias then a manual adjustment, IMO, isn't even an option; the amp should do it automatically because you just can't be there monitoring the bias every second.

On the other hand, if the sole reason for wanting an external biasing interface is that you're the type of person who just needs to swap power tubes every hour then I think it's on your shoulders to live with a bit of inconvenience while doing that.

Normal people just shut up and play the damn guitar.

Personally I see a point to touch the bias settings few times a year. max. I surely can live with opening the chassis during those occasions. Who knows, I might even vacuum some dust piles away and spray a little contact cleaner to critical points while I'm at it... I'm sure someone soon complains that there should be an external interface to do those things as well because it's terribly inconvenient otherwise.

It's like only letting electricians replace light bulbs.

Not even remotedly comparable. And speaking of that, I actually do know a few people who would easily f*ck up such a simple task, or would have to consult an "expert" for help. Biasing needs some expertise, not all people can be expected to have it. The ones who know how to do it are the ones who know how to open up the chassis and do it. Then there's the bunch who just tweak up the mystery pot at the rear panel and wonder why it doesn't seem to do anything, even at most extreme settings.

[tonyhay]

Originally Posted by scottl

There is zero downside of external bias. It is safer. It is easy to use. It is better for the tubes as you can much more readily keep them in range.

There is no rational argument I have heard from anyone on why it is bad, other than the additional cost and complexity.

Completely agree with these statements. External bias points are safer and easier to use. There are no rational arguments on why external points are a bad idea, other than cost.

[teemuk]

Why don't cars have an ignition timing adjustment in the consol...? It's so irrational because one would be so much safer and easier to use. Must be because of cost cutting.

[scottl]

Ludicrous post.

Designing in external bias points is safe and responsible design. There is no negative. Even if as you say, you only check the bias a couple times a year. Why open a chassis if you don't have to.

My car has the battery in the trunk. BMW installed jumper terminals under the hood. I could pop the trunk, remove the battery cover and do it directly, but it is safer and easier with the remote jump points.

Why is this any different?

External bias is undeniably safer, easier, and more convenient. The trimmer is recessed and there are recessed test points. Any moron with a $20 meter can get the mv reading. 30mv is 30mv. With the bias range on correctly implemented trimmers, there is little chance of an idiot hurting the amp.

So I ask, other than the $20 it adds to the amp cost, what is the downside? What is the upside? Clearly there is MASSIVE upside and almost no downside.

Btw, I am one who knows how to open amps. People bring me amps to tube as well. THe ones with external bias are my favorite to work on as the designer wisely incorporated this feature that makes it safer and easier. Even for experts and pros.

Originally Posted by teemuk

The bias pot is not a tone control, there's no need to adjust its setting continuously. Power tubes last a good deal and the control is really needed only when they are changed plus a few times more to compesate drift due to aging. If you want to go overboard with the latter cause or always make sure the tubes are at their optimal bias then a manual adjustment, IMO, isn't even an option; the amp should do it automatically because you just can't be there monitoring the bias every second.

On the other hand, if the sole reason for wanting an external biasing interface is that you're the type of person who just needs to swap power tubes every hour then I think it's on your shoulders to live with a bit of inconvenience while doing that.

Normal people just shut up and play the damn guitar.

Personally I see a point to touch the bias settings few times a year. max. I surely can live with opening the chassis during those occasions. Who knows, I might even vacuum some dust piles away and spray a little contact cleaner to critical points while I'm at it... I'm sure someone soon complains that there should be an external interface to do those things as well because it's terribly inconvenient otherwise.



Not even remotedly comparable. And speaking of that, I actually do know a few people who would easily f*ck up such a simple task, or would have to consult an "expert" for help. Biasing needs some expertise, not all people can be expected to have it. The ones who know how to do it are the ones who know how to open up the chassis and do it. Then there's the bunch who just tweak up the mystery pot at the rear panel and wonder why it doesn't seem to do anything, even at most extreme settings.

[teemuk]

With the bias range on correctly implemented trimmers, there is little chance of an idiot hurting the amp.

Any usable range must correspond to overall tolerance of the tubes, otherwise you need an additional master control. Can there actually be enough "safe" range? Can the interface indicate a faulty tube, or perhaps a screen resistor going down because of one....? Can it be a safe adjustment for a monkey who only knows how to turn a potentiometer and read a specific number from a screen? Say, the user fits in tube, the bias adjustment doesn't seem to have enough range to bias the tube properly, does the user know what to do next?

Yep. Thought as much.

Oh, need I remind, the talk is not about the technically oriented 1%, but we are talking about people who weekly chime in with questions about speaker impedances, usage of standby switches or effects loops, or about plugging additional amps into speaker outputs.


Of course an external interface is easier and more convenient. The question is if you really need those features for a bias setting. I sure don't since I don't need to bias my amp all the frigging time.

People bring me amps to tube as well. THe ones with external bias are my favorite to work on...

Of course they are. ...But the benefit of amp techs having to work less for x amount of money isn't still enough reason for me to see why amps should have external bias interfaces.

Even if as you say, you only check the bias a couple times a year. Why open a chassis if you don't have to.

Because I don't mind if I have to when I have to bias. I mean, when I really, really have to. While the chassis is open I can actually make some annual preventive maintance - if I choose so - plus an overall visual inspection, which is a nice feature if the set of replacement tubes decides not to play along nicely but to scorch the internals of my amp (or at least attempt so). Or if the set of tubes is ok but simply fails to be in the proper range and calls for tampering of the bias circuit (e.g. swapping of a fixed resistor). Basically, I'm more in control and if everything goes down nicely it's an inconvenience that I may not have to suffer for another year or two.... perhaps more. Some output tubes may last decades.

[tonyhay]

Originally Posted by teemuk

Any usable range must correspond to overall tolerance of the tubes, otherwise you need an additional master control. Can there actually be enough safe range? Can the interface indicate a faulty tube, or perhaps a screen resistor going down because of one....? Can it be a safe adjustment for a monkey who only knows how to turn a potentiometer and read a specific number from a screen? Say, the user fits in tube, the bias adjustment doesn't seem to have enough range to bias the tube properly, does the user know what to do next?

Yep. Thought as much.

Oh, need I remind, the talk is not about the technically oriented 1%, but we are talking about people who weekly chime in with questions about speaker impedances, using effects loops, or plugging amps into speaker outputs.


Of course an external interface is easier and more convenient. The question is if you really need those features for a bias setting. I sure don't since I don't need to bias my amp all the frigging time.


Of course they are. ...But the benefit of amp techs having to work less for x amount of money isn't still enough reason for me to see why amps should have external bias interfaces.

Your entire first paragraph pertains whether or not there are external bias points. You could get the chassis out, attempt a bias, and maybe screw it up and cost yourself a tube. The difference is that the whole thing would have been safer and easier if the bias points were external.

As for your paragraph about the "techically oriented 1%"... well thanks. I am definitely in the lower 99% according to your illustrious standards... However (along with most amp owners) I am not a blithering idiot. I can read instructions. I understand how to use a digital meter, and I somehow managed to figure out how to turn a bias pot. But as I've said before, I'd much rather do it without having to take the chassis out. It's safer and quicker. There is no downside at all.

Frankly, you seem annoyed about the idea that someone who is not as 'technically competent' might be able to bias their own amp when they feel like it.

[teemuk]

No, I'm annoyed about the idea that anyone might be able to "bias" their own amp when they feel like it.

If you can do it, fine. If you can't, I don't see why a feature to easily screw things up should be included in an amp.

Your entire first paragraph pertains whether or not there are external bias points. You could get the chassis out, attempt a bias, and maybe screw it up and cost yourself a tube.

Of course you can, but the point is that doing so isn't deliberately made extremely easy. It's like fitting fuses inside the chassis instead of using an external fuse holder: It isn't literally "fool proof" (some people will always tear things apart despite the "no user serviceable parts inside" warning) but at least it eliminates a portion of those fools who should not be swapping fuses in the first place.

If you know how to swap fuse properly then opening up the amp and doing so isn't a major headache or problem. In those very remote instances you do actually suffer a fuse failure.

[scottl]

I would make a huge bet, really big, that exponentially more amps and tubes are harmed by people replacing tubes without biasing than amps/tubes damaged on amps with external bias adjustment.

All this talk is speculative. There are thousands of instances of people replacing tubes without biasing. This is has way more potential risk than changing tubes on an amp with external adjustments. There are many many stories about how tubes redplated/failed, trannies killed, etc. Find me ONE instance of someone screwing up something on an amp with external bias. Maybe it has happened, I have never seen it, nor do I believe it happens at any where near the same net statistical rate of incidents on amps with no external control. Probably because it is next to impossible to screw up setting the bias on one of these.

So, in the real world now, the amp with external adjustment and CLEAR instructions is far less risky and/or a liability for the manufacturer. Hands down.

But, it is more complex and costly to implement. < The real reason.

[Blue Strat]

Quite possible, but Mesas with Mesa rated tubes have the lowest risk.

The bottom line, and only line, is what amp builders believe and there will ALWAYS be differences of opinion. Whether the complexity/cost to implement is the issue is an OPINION.

Originally Posted by scottl

I would make a huge bet, really big, that exponentially more amps and tubes are harmed by people replacing tubes without biasing than amps/tubes damaged on amps with external bias adjustment.

All this talk is speculative. There are thousands of instances of people replacing tubes without biasing. This is has way more potential risk than changing tubes on an amp with external adjustments. There are many many stories about how tubes redplated/failed, trannies killed, etc. Find me ONE instance of someone screwing up something on an amp with external bias. Maybe it has happened, I have never seen it, nor do I believe it happens at any where near the same net statistical rate of incidents on amps with no external control. Probably because it is next to impossible to screw up setting the bias on one of these.

So, in the real world now, the amp with external adjustment and CLEAR instructions is far less risky and/or a liability for the manufacturer. Hands down.

But, it is more complex and costly to implement. < The real reason.