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Post by rcrecelius on Jun 2, 2006 9:25:10 GMT -7
Just curious...I'd really like to learn how to check the bias in my Z28 but I'm not in a position to spend the money for the GT bias tool which comes with a nice book. I see some testers/probes on eBay in the $25 range...would one of these work just as well?
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Post by benttop (Steve) on Jun 2, 2006 9:37:22 GMT -7
I've never tried one of the probes. I usually buy two 1 ohm 5 watt resistors and put them in the cathode circuit (ground to cathode) so I can measure milliamps directly in millivolts with my DVM. I've put them in and left them with no descernable negative effect. In fact, I'm curious why this isn't common practice for all amp makers - maybe the $.28 or so is too hard to get past? I don't know.
Maybe Mike or Myles can tell me why this isn't common practice...
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Post by myles on Jun 2, 2006 9:49:36 GMT -7
I've never tried one of the probes. I usually buy two 1 ohm 5 watt resistors and put them in the cathode circuit (ground to cathode) so I can measure milliamps directly in millivolts with my DVM. I've put them in and left them with no descernable negative effect. In fact, I'm curious why this isn't common practice for all amp makers - maybe the $.28 or so is too hard to get past? I don't know. Maybe Mike or Myles can tell me why this isn't common practice... The reason that this is not common practice is because most folks would rather buy a pre-packaged tool. It is that simple. The precision 1 watt resistor is the heart of any of the tools and no tool is any more accurate than your method. I personally dislike tools that have an integral meter. If the meter breaks you are out of business. They are more prone to breakage and take up more space in a tool box. Plus, any decent toolbox should already have a meter. Many amps today do what you do and have external test points or a test point marked somewhere in the amp such as the Fender Hot Rod Deluxe and Deville, Bassman LTD reissue, all the Fender Pro Tube series amps, Marshall DSL/TSL/JCM2000 and some more. But ... your method ... you have to go to an electronics parts store and buy resistors and know what to do with them .... versus .... buy a $250 box with a meter and some cool switches and cool probes coming out so in some you can even look at all the tubes at the same time. Then again, that is one reason we should use matched tubes and even matched tubes when at idle can show a 10-15% difference when they are really nicely matched while running.
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Post by benttop (Steve) on Jun 2, 2006 9:56:44 GMT -7
The reason that this is not common practice is because most folks would rather buy a pre-packaged tool. It is that simple. The precision 1 watt resistor is the heart of any of the tools and no tool is any more accurate than your method. I personally dislike tools that have an integral meter. If the meter breaks you are out of business. They are more prone to breakage and take up more space in a tool box. Plus, any decent toolbox should already have a meter. Many amps today do what you do and have external test points or a test point marked somewhere in the amp such as the Fender Hot Rod Deluxe and Deville, Bassman LTD reissue, all the Fender Pro Tube series amps, Marshall DSL/TSL/JCM2000 and some more. But ... your method ... you have to go to an electronics parts store and buy resistors and know what to do with them .... versus .... buy a $250 box with a meter and some cool switches and cool probes coming out so in some you can even look at all the tubes at the same time. Then again, that is one reason we should use matched tubes and even matched tubes when at idle can show a 10-15% difference when they are really nicely matched while running. Thanks for that - I had a feeling it was so. Yeah, a lot of amps do have test points. But you still have to pull the chassis to do the bias. That's one of the greatest features of the Flexi - not only are there test points on the back, each power tube has its own adjustment pot, so you aren't required to buy matched power tubes. In fact, a lot of guys run the Flexi with two completely different power tubes - one of the most popular setups is with a 6550 on one side and an EL34 on the other side. I'd love to see a scope waveform of that, but owing to the separate bias, there is no crossover distortion anyway. I'll bet the top and bottom halves of the waveform are different though. Thanks for your comments Myles.
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Post by myles on Jun 2, 2006 11:01:09 GMT -7
The reason that this is not common practice is because most folks would rather buy a pre-packaged tool. It is that simple. The precision 1 watt resistor is the heart of any of the tools and no tool is any more accurate than your method. I personally dislike tools that have an integral meter. If the meter breaks you are out of business. They are more prone to breakage and take up more space in a tool box. Plus, any decent toolbox should already have a meter. Many amps today do what you do and have external test points or a test point marked somewhere in the amp such as the Fender Hot Rod Deluxe and Deville, Bassman LTD reissue, all the Fender Pro Tube series amps, Marshall DSL/TSL/JCM2000 and some more. But ... your method ... you have to go to an electronics parts store and buy resistors and know what to do with them .... versus .... buy a $250 box with a meter and some cool switches and cool probes coming out so in some you can even look at all the tubes at the same time. Then again, that is one reason we should use matched tubes and even matched tubes when at idle can show a 10-15% difference when they are really nicely matched while running. Thanks for that - I had a feeling it was so. Yeah, a lot of amps do have test points. But you still have to pull the chassis to do the bias. That's one of the greatest features of the Flexi - not only are there test points on the back, each power tube has its own adjustment pot, so you aren't required to buy matched power tubes. In fact, a lot of guys run the Flexi with two completely different power tubes - one of the most popular setups is with a 6550 on one side and an EL34 on the other side. I'd love to see a scope waveform of that, but owing to the separate bias, there is no crossover distortion anyway. I'll bet the top and bottom halves of the waveform are different though. Thanks for your comments Myles. Actually, in the Flexi 50 there is crossover notch ... as in any push pull amp .... and you never want to make it all go away otherwise the amp sounds sterile. This is one of the biggest issues I see in Marshall amps that are biased up at 40-45mA per rube ... harsh and sterile and they lost the characteristic plexi era marshall crunch as notch distortion was a part of the character of these amps. You just do not want too much x-over notch distortion.
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Post by benttop (Steve) on Jun 2, 2006 11:04:33 GMT -7
Actually, in the Flexi 50 there is crossover notch ... as in any push pull amp .... and you never want to make it all go away otherwise the amp sounds sterile. This is one of the biggest issues I see in Marshall amps that are biased up at 40-45mA per rube ... harsh and sterile and they lost the characteristic plexi era marshall crunch as notch distortion was a part of the character of these amps. You just do not want too much x-over notch distortion. Some day I'm going to scope that amp and try a few different power tube combinations so I understand this a little better.
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Post by myles on Jun 2, 2006 11:11:42 GMT -7
Actually, in the Flexi 50 there is crossover notch ... as in any push pull amp .... and you never want to make it all go away otherwise the amp sounds sterile. This is one of the biggest issues I see in Marshall amps that are biased up at 40-45mA per rube ... harsh and sterile and they lost the characteristic plexi era marshall crunch as notch distortion was a part of the character of these amps. You just do not want too much x-over notch distortion. Most manufacturers today do not use or recommend the scope method anymore. It is not repeatable or precise and is subjective to what is a proper "notch" on a scope screen. Try to actually play more and tweek less once you like how an amp sounds. 80% of the result is obtained with 20% of the effort .... that last 20% of the result takes 80% or more. Maybe that is why I was always a "C" student with a college GPA of 2.7 Some day I'm going to scope that amp and try a few different power tube combinations so I understand this a little better.
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Post by benttop (Steve) on Jun 2, 2006 11:25:37 GMT -7
I wouldn't try to bias using the scope method, but it sure does help in the "enlightenment" portion of the excercise. Whelp... that makes 1K!
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Post by rcrecelius on Jun 2, 2006 11:54:01 GMT -7
What about the different probes on the market? I have a meter, I need to know if a probe like this will work or should I spend the extra money to get the GT tool/book?
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Post by rcrecelius on Jun 2, 2006 11:54:44 GMT -7
Congrats, you Z-natic!
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Post by benttop (Steve) on Jun 2, 2006 12:00:31 GMT -7
What about the different probes on the market? I have a meter, I need to know if a probe like this will work or should I spend the extra money to get the GT tool/book? From the description and pictures, I would say that would work just as well as anything if you already have a DMM and understand what you're looking at/doing. The thing is, you are going to want to look at both tubes, even with a matched pair, because they are still going to be different by some amount. If they are different by more than about 10% you might want to consider swaping around with another tube to get closer, but most matched sets are that close. Anyway, to look at both without having to go through shutting down, pulling the tube, reseating it, moving the adapter to the other tube, etc. you might consider two so you can get to each more quickly. That still doesn't help if you have four output tubes, but you get my drift. One will do, two is easier to work with... But if you don't know what you're doing, spend the extra money to get the instructions!
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Post by benttop (Steve) on Jun 2, 2006 12:02:59 GMT -7
Thanks! 1000 messages in six months - I need to spend more time playing!
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Post by rcrecelius on Jun 2, 2006 12:04:21 GMT -7
Heres another one...neat idea but someone already bought it.
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Post by benttop (Steve) on Jun 2, 2006 12:08:15 GMT -7
Heres another one...neat idea but someone already bought it. There you go, that is a good idea. Better than fumbling around with probes - set it up, switch it on, do the adjust, switch the switch for a cross check, and you're done. Cool!
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SG123
Full Member
Posts: 221
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Post by SG123 on Jun 2, 2006 12:47:22 GMT -7
Not to confuse things, but I have a set of the old swAmp probes and they don't use a resistor. The milliammeter portion of your multimeter completes the cathode circuit of the output tube. Reading is directly in mA. - otherwise they look exactly like the other "socket with two wire" style probes.
No advantage to this type that I can think of. They were just the first ones I ever saw and I bought a pair. You just connect the red to black with a double female banana/jumper on the tube you're not measuring - or use two meters...
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Post by johnnyl on Jun 2, 2006 14:37:45 GMT -7
Hey Ronnie, here's the one I have which is pretty much the same as the one you saw on ebay. They have good customer service and ship fast as hell too. It works great.
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Post by Telemanic on Jun 2, 2006 19:54:05 GMT -7
Hey guys, Ive got a set from e-bay, i think they were SRS, if i recall. They work great, and use the resistor method. I seem to recall that the direct read probes (no resistor) work great in some amps and not so recomended in others, Marshall for example. Something to do with the lead length in the direct read type, with the marshall circuit ??
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Post by myles on Jun 4, 2006 12:22:46 GMT -7
What about the different probes on the market? I have a meter, I need to know if a probe like this will work or should I spend the extra money to get the GT tool/book? This will work fine and dandy. As far as the book .... if you just email me with what amp you want to do I will direct you to a link I have where you can find the bias for most any amp type and tube type. .... or ... I will be happy to answer questions directly.
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Post by ke4unr on Jun 5, 2006 16:04:48 GMT -7
I have built probes in the past, but only for octal tubes, using old tube bases and sockets. The only purchase was the 1 ohm resistors.
I was going to build some 9-pin probes to check EL84's in my Maz Jr and other amps. Ordered new sockets thinking I would have no problem finding bases. Wrong! I've looked everywhere that I know to look without luck.
Just for future reference, does anyone have a source?
I've since ordered a set of probes from Amp-Head. Hated to spend the money knowing what they're made of, but oh well.
Thanks, Ray K.
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Post by prowler on Jun 5, 2006 18:25:10 GMT -7
I use a Weber Bias Rite. Works just fine for my needs. If I were supporting players that change tubes often I would most likely go with a Fluke meter & a bias probe that has banana plugs.
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Post by rcrecelius on Jun 8, 2006 8:37:09 GMT -7
What about the different probes on the market? I have a meter, I need to know if a probe like this will work or should I spend the extra money to get the GT tool/book? This will work fine and dandy. As far as the book .... if you just email me with what amp you want to do I will direct you to a link I have where you can find the bias for most any amp type and tube type. .... or ... I will be happy to answer questions directly. Thanks...I'll get w/ya when I get the probe.
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