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biasing question

4/23/2000 10:37 PM
JC	biasing question Is there any url where I could learn to bias an amp with no bias trim pot/pots, please? A book maybe? I think it has something to do with changing the value of some resistors, am I right? The tubes are 6L6's. Don't know even from wich tube pin number I have to do the measurement. I know I shouldn't be here asking this, but maybe someone could help me. Months ago you tought me how to bias my Orange, and I learned that pretty well. Hand in my back pocket included Thanks, Again. JC

4/23/2000 10:37 PM

biasing question

Is there any url where I could learn to bias an amp with no bias trim pot/pots, please?

A book maybe?

I think it has something to do with changing the value of some resistors, am I right?

The tubes are 6L6's.

Don't know even from wich tube pin number I have to do the measurement. I know I shouldn't be here asking this, but maybe someone could help me.

Months ago you tought me how to bias my Orange, and I learned that pretty well. Hand in my back pocket included

Thanks,

Again.

JC

4/24/2000 12:03 AM
Steve Slick	Unless you enjoy this as a hobby, I don't recommend that you do it yourself. It's cheaper to pay somebody to do it. I've found that amp techs are like musicians. You can usually find a pretty good one around (in the big cities) that will work for cheap. If you really want to get into it, it'll cost a little up front both in money and time. The best book is in my opinion The Ultimate Tone by Kevin O'Connor. But at $55 it's more expensive than a tech. Plus you have to have the meter and a few accessories. For a bias probe, I'd recommend the SwAMP probe at http://www.swampamp.com/ And you have to learn, i.e., spend some time learning how not to hurt yourself or your equipment. If you really want to do this, I'd start with R.G. Keens FAQ at http://www.geofex.com/tubeampfaq/taffram.htm There are some suggestions there and an excellent reading list. Good Luck, Steve Slick Oh, and let's be especially careful out there!

4/24/2000 12:03 AM

Steve Slick

Unless you enjoy this as a hobby, I don't recommend that you do it yourself. It's cheaper to pay somebody to do it.

I've found that amp techs are like musicians. You can usually find a pretty good one around (in the big cities) that will work for cheap.

If you really want to get into it, it'll cost a little up front both in money and time. The best book is in my opinion The Ultimate Tone by Kevin O'Connor. But at $55 it's more expensive than a tech.

Plus you have to have the meter and a few accessories. For a bias probe, I'd recommend the SwAMP probe at http://www.swampamp.com/ And you have to learn, i.e., spend some time learning how not to hurt yourself or your equipment.

If you really want to do this, I'd start with R.G. Keens FAQ at http://www.geofex.com/tubeampfaq/taffram.htm There are some suggestions there and an excellent reading list.

Good Luck,

Steve Slick

Oh, and let's be especially careful out there!

4/24/2000 11:34 PM
JC	Yes I know all that. What I don't know is how to rebias my Ampeg that has no variable resistor to adjust the bias, like my Orange head. Thanks for your answer, anyway. And of course, RG's site is great and valuable, I have (need) to visit it from time to time. A nice cane for someone like me that doesn't wan't to take away work to the real technicians, just a few easy things that don't need a deep knowlwdge of electronics. JC

4/24/2000 11:34 PM

Yes I know all that.

What I don't know is how to rebias my Ampeg that has no variable resistor to adjust the bias, like my Orange head.

Thanks for your answer, anyway.

And of course, RG's site is great and valuable, I have (need) to visit it from time to time.
A nice cane for someone like me that doesn't wan't to take away work to the real technicians, just a few easy things that don't need a deep knowlwdge of electronics.

JC

4/25/2000 2:56 AM
Steve Slick	First, taking work away from real techs is not the issue w/me. This is a hobby with me. It's just that it's like... well like learning a computer programming language. You have to learn a bunch of stuff--I call it a lump--before you can do some simple things. Figuring out what to measure, replace, etc. is not necessarily easy if you don't know more than a little. But, I understand what your saying. If I was you, here's what I'd do. 1st I'd get a regular tech to mod the amp to put variable resistors on the amp to adjust the bias for each tube. And make him put it in a place you can get to easily without disassembling the amp. 2nd, I'd get a SwAMP probe ($33) and a good multimeter ($25-50). The probe is a socket you put between a tube and the socket that allows you to measure the current flow with the multimeter. You don't need a fluke or a real expensive one. There are some recommendations at other places here that address the multimeter. A good $25 Radio Shack would be okay for your needs. I wouldn't buy any books right now. None of the ones I have seen go into a lot of detail about biasing. You can get the biasing stuff free off the Internet. For example, I think there is stuff at R.G.s site. There is also a white paper at the Svetlana site, I think there is some stuff at Randall Aiken's site, some stuff at the London Power site, and at the SwAMP site. If you have more questions, call Charles at SwAMP when you buy the probe and talk to him. No, do it anyway. HE'S HARD TO GET AHOLD OF. But, once you do he'll talk your ear off. He will gladly answer questions. Be careful. I'm no novice, albeit at times an idiot, but a couple of weekends ago I hit the wrong pin on my amp. ZAP! Thank God for fast blow fuses! Thank God I didn't crap my drawers too! It was a close call. Finally, when in doubt, ask questions here. One reason I love this place is because there are some pretty heavy tech guys that are very tolerant of idiots like me. So speak up. I'm going to pull out some of my books later this week. One of them has some stuff on Ampeg amps. If I find it, I'll add a post. Hope That Helps, Steve Slick

4/25/2000 2:56 AM

Steve Slick

First, taking work away from real techs is not the issue w/me. This is a hobby with me. It's just that it's like... well like learning a computer programming language. You have to learn a bunch of stuff--I call it a lump--before you can do some simple things. Figuring out what to measure, replace, etc. is not necessarily easy if you don't know more than a little. But, I understand what your saying.

If I was you, here's what I'd do. 1st I'd get a regular tech to mod the amp to put variable resistors on the amp to adjust the bias for each tube. And make him put it in a place you can get to easily without disassembling the amp.

2nd, I'd get a SwAMP probe ($33) and a good multimeter ($25-50). The probe is a socket you put between a tube and the socket that allows you to measure the current flow with the multimeter. You don't need a fluke or a real expensive one. There are some recommendations at other places here that address the multimeter. A good $25 Radio Shack would be okay for your needs.

I wouldn't buy any books right now. None of the ones I have seen go into a lot of detail about biasing. You can get the biasing stuff free off the Internet.

For example, I think there is stuff at R.G.s site. There is also a white paper at the Svetlana site, I think there is some stuff at Randall Aiken's site, some stuff at the London Power site, and at the SwAMP site.

If you have more questions, call Charles at SwAMP when you buy the probe and talk to him. No, do it anyway. HE'S HARD TO GET AHOLD OF. But, once you do he'll talk your ear off. He will gladly answer questions.

Be careful. I'm no novice, albeit at times an idiot, but a couple of weekends ago I hit the wrong pin on my amp. ZAP! Thank God for fast blow fuses! Thank God I didn't crap my drawers too! It was a close call.

Finally, when in doubt, ask questions here. One reason I love this place is because there are some pretty heavy tech guys that are very tolerant of idiots like me. So speak up.

I'm going to pull out some of my books later this week. One of them has some stuff on Ampeg amps. If I find it, I'll add a post.

Hope That Helps,

Steve Slick

4/25/2000 1:29 PM
JC	Steve: I should have said at first that my multimeter is 25 years old, and I bought it new. But in all these years I have managed to go by without knowing how a simple transistor works really. So, I have done a lot of simple things, and even complicated as constructing ss hifi amplifiers and that kind of things. You know, ther's no time to learn everything I would like to know. Another problem is that lots of parts are difficult to find and sometimes much more expencive here where I live. I learned to discharge the big caps before any attempt to do anything inside an amp. I like the SwAMP idea, as it looks pretty safe. I wouln't mind to put a variable resistor for each tube, it seams to be the best solution. I would like to do it myself, but I have to know how before I can Thanks for your help, I'll revisit all those urls to see if I can find something. It helped. JC

4/25/2000 1:29 PM

Steve:

I should have said at first that my multimeter is 25 years old, and I bought it new. But in all these years I have managed to go by without knowing how a simple transistor works really.

So, I have done a lot of simple things, and even complicated as constructing ss hifi amplifiers and that kind of things. You know, ther's no time to learn everything I would like to know.

Another problem is that lots of parts are difficult to find and sometimes much more expencive here where I live.

I learned to discharge the big caps before any attempt to do anything inside an amp.

I like the SwAMP idea, as it looks pretty safe.

I wouln't mind to put a variable resistor for each tube, it seams to be the best solution. I would like to do it myself, but I have to know how before I can

Thanks for your help, I'll revisit all those urls to see if I can find something.

It helped.

JC

4/25/2000 2:00 PM
Ray Ivers	JC, Which Ampeg do you have? Some Ampegs, and also Marshall JCM900 amps, and probably a bunch of others I don't know about, take their AC bias source thru a capacitor off the HV transformer secondary. Since this is always done with a bridge rectifier (with a regular full-wave circuit, the center tap is grounded), the AC must be ground-referenced, rectified, filtered, and adjusted before being sent to the output tube grids. I personally think this is a cheesy system used to save the cost of doing it the right way - separate, dedicated bias winding on the transformer - but since the current draw under "normal" (i.e. non-overdrive) conditions is so small, it works OK until the cap fails - then your tubes go to Hell (literally). However, the voltages are still set by a resistor, the same as any other amp. In the case of the Ampeg V-4, this resistor is R50, a 56K resistor, although R49 (75K) will also have a more limited effect. Increase these resistors to raise bias voltage and reduce bias current thru the tubes, or vice versa. It takes me almost an hour to bias an amp, using three different techniques, and even then, with new tubes I know it's going to change within 20 hours of use (I try to take this into account, but I always wonder what current those tubes are drawing NOW?). If you do know a good tech, it wouldn't be a bad idea to have him do it... but it's also a good idea to go there informed, and to state your preferences on bias technique used with your amp, if any - it's your amp, after all. www.aikenamps.com - definitely check it out (I've PRINTED it out - over an inch of tube tech talk!) Ray Ivers R.A.G.E. Electronics

4/25/2000 2:00 PM

Ray Ivers

JC,

Which Ampeg do you have?

Some Ampegs, and also Marshall JCM900 amps, and probably a bunch of others I don't know about, take their AC bias source thru a capacitor off the HV transformer secondary. Since this is always done with a bridge rectifier (with a regular full-wave circuit, the center tap is grounded), the AC must be ground-referenced, rectified, filtered, and adjusted before being sent to the output tube grids. I personally think this is a cheesy system used to save the cost of doing it the right way - separate, dedicated bias winding on the transformer - but since the current draw under "normal" (i.e. non-overdrive) conditions is so small, it works OK until the cap fails - then your tubes go to Hell (literally).

However, the voltages are still set by a resistor, the same as any other amp. In the case of the Ampeg V-4, this resistor is R50, a 56K resistor, although R49 (75K) will also have a more limited effect. Increase these resistors to raise bias voltage and reduce bias current thru the tubes, or vice versa.

It takes me almost an hour to bias an amp, using three different techniques, and even then, with new tubes I know it's going to change within 20 hours of use (I try to take this into account, but I always wonder what current those tubes are drawing NOW?). If you do know a good tech, it wouldn't be a bad idea to have him do it... but it's also a good idea to go there informed, and to state your preferences on bias technique used with your amp, if any - it's your amp, after all.

www.aikenamps.com - definitely check it out (I've PRINTED it out - over an inch of tube tech talk!)

Ray Ivers
R.A.G.E. Electronics

4/25/2000 5:04 PM
Steve Slick	Two big thumbs up on the Aiken stuff Two big thumbs up on the Aiken stuff! There are a few guys out there that I don’t understand why they don’t organize their stuff into a good, tube amp how-to book. Randall is one of them. I printed that stuff out too and was shocked at the size of the pile. Why doesn’t he organize it into a book? I got a big pile of excellent information... FOR FREE??? Thanks, man. RANDALL AIKEN! WHY? Another guy is R.G. Keen. That FAQ of his is like a cult favorite. I see people stealing his stuff all over the place. Why doesn’t he organize his stuff into a good book and get paid? Who know? He seems to want to just play around with stomp-boxes. R.G. KEEN! WHAT’S A MATTER WITH YOU? I tell all the engineers that work for me that you should set as your goal in life to get paid for what you know, not what you do. Write a book, man. Then you’ll get money for nothing and chicks for free. Kevin O’Connor is the man when it comes to books. He gets $55 for TUT and it looks like a xerox of a typewritten manuscript with hand drawn schematics. But I love that book and would probably pay twice that if I had to get it. I just got TUT2 ($40) and feel the same about it. My problems with most of the good books out there are that they are a compilations of articles for some publication or a thinly disguised sales pitch. Or both. I put Gerald Weber’s, Aspen Pittman’s and Dan Torres’s books in this category. While I like these books, especially Gerald’s, they don’t... flow. So for now, it looks like Kevin O’Connor is the only want with enough brains and ambition to profit from his knowledge. Cheers, Steve Slick

4/25/2000 5:04 PM

Steve Slick

Two big thumbs up on the Aiken stuff

Two big thumbs up on the Aiken stuff!

There are a few guys out there that I don’t understand why they don’t organize their stuff into a good, tube amp how-to book. Randall is one of them. I printed that stuff out too and was shocked at the size of the pile. Why doesn’t he organize it into a book? I got a big pile of excellent information... FOR FREE??? Thanks, man.

RANDALL AIKEN! WHY?

Another guy is R.G. Keen. That FAQ of his is like a cult favorite. I see people stealing his stuff all over the place. Why doesn’t he organize his stuff into a good book and get paid? Who know? He seems to want to just play around with stomp-boxes.

R.G. KEEN! WHAT’S A MATTER WITH YOU?

I tell all the engineers that work for me that you should set as your goal in life to get paid for what you know, not what you do. Write a book, man. Then you’ll get money for nothing and chicks for free.

Kevin O’Connor is the man when it comes to books. He gets $55 for TUT and it looks like a xerox of a typewritten manuscript with hand drawn schematics. But I love that book and would probably pay twice that if I had to get it. I just got TUT2 ($40)
and feel the same about it.

My problems with most of the good books out there are that they are a compilations of articles for some publication or a thinly disguised sales pitch. Or both. I put Gerald Weber’s, Aspen Pittman’s and Dan Torres’s books in this
category. While I like these books, especially Gerald’s, they don’t... flow.

So for now, it looks like Kevin O’Connor is the only want with enough brains and ambition to profit from his knowledge.

Cheers,

Steve Slick

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