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Post by gotmojo on Jul 14, 2014 9:46:10 GMT -7
I'm putting this thread here, with the risk that Lefty will delete it for being in the wrong place, but this is where it gets mentioned several times, so it seems appropriate to the Therapy topic.
Hey Phil, you've mentioned several times about cathode biased amps vs fixed bias amps, and with some expectation of a tonal quality therein. What is the difference in sound/tone/attack (whatever) of these alternately biased amps, and why do they sound/respond differently? What are amplifier examples of each type of biasing in vintage amps (like tweed Bassman vs AC30, etc)?
Thanks, and hope I'm not in really, really BIG trouble here.
jj
help my ignorance, please! um ........... And I made that word up, deletment. maybe shoulda been deletion. no, I like deletment better.
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Post by Phil (aka Phil) on Jul 15, 2014 5:48:58 GMT -7
Okay, here it is…tube biasing explained for the non-electronic tech Tubes are wonderful inventions. They take a small signal and amplify it. They have a huge reserve of energy on one side just itching to get out and a “gate” that keeps it from all coming out at once. It’s like a dam with millions of gallons of water behind it…and a gate to let it out. You’d think that it would be as easy as inputting your signal into the tube and having a larger signal come out on the other side…much like the small movement of your hand on the lever that operates the water gate lets a proportionally greater amount of water out of the dam the more you move the lever. In practice it’s not that simple. Tubes need a way to know what is the “middle” of their operating range. The signal that is fed to them (in this case, the guitar signal) gyrates up and down as you play. The tubes have a “happy range” that they like to stay in kind of like your car’s engine. Running to high or too low is bad. You don’t want the signal you’re giving it to exceed the top of the operating range or go below the bottom of the range at idle. This insures that when the signal grows (like when you’re playing the guitar and hitting the strings) that the signal is “evenly” amplified. Tubes without biasing would be like a car with no mechanism to regulate the engine idle. You’d start it up and it would either stall or rev until it blew itself up. There’s two main ways that we can make sure the tube(s) is (are) happy and in the middle of their operating range. These are cathode biasing and fixed bias. Cathode biasing is also called ‘self biasing’. It uses a couple of relatively small and cheap components and a trick of physics/electronics to allow the tube to self-regulate. It’s kind of like a buoy bobbing on the ocean…you pull it down and it bobs back up. You raise it out of the water and gravity pulls it back down. It’s cheap and extremely effective. It’s called self biasing because as the tube tries to draw more current it actually “shuts off it’s own gate”, keeping it happy. Almost all guitar amp preamp tubes are cathode biased (with the notable exception of the Ghia’s phase inverter). This is why I said it’s “more common” than fixed biasing - there’s usually more preamp tubes in guitar amps than power tubes! Two disadvantages of cathode biasing are that they run hotter…which result in less output power being available from a given tube or tubes as that energy is wasted as heat. That's why you see cathode biased 6L6 amps like the THERAPY only developing 30ish watts while the same two tubes in the fixed biased EZG-50 make over 50 watts. When used to bias a power tube or sets of power tubes in a guitar amp the sound is said to have a spongier attack, great sustain, dynamics, harmonic content, and a “smoother” sound in general. The AC/30 is probably the most famous cathode biased amp. The Trainwreck EL84 amps are as well. Most Dr. Z’s are cathode biased. These include: THERAPY Carmen Ghia M12 Maz 18/Maz 18 NR Maz 38/Maz 38 NR Maz 8 Mini Z Monza Remedy Stang Ray Z Wreck Rx/Rx Jr Mazerati/Mazerati GT JAZ 20/40 Almost all single power tube guitar amps are cathode biased. The early Fender tweeds were cathode biased. Later the larger tweeds went to fixed biasing in an attempt to make the amps louder and less distorted. The two-6V6 tweed amps were almost all cathode biased (except the 5G9 narrow panel Tremolux). Cathode biasing is one reason why so many guys love those old Fender small and medium sized tweeds. EL84’s really lend themselves to cathode biasing…which is why almost all EL84 amps you’ll see are biased this way (with the notable exception of the Fender Pro Jr.) Fixed biasing uses a “fixed” or non-changing voltage input to tell the tube where the middle of the operating range is. This is set by the amp technician/designer. It’s kind of like the set screw on carburetors on older cars. You turn the screw until the car is idling at the right RPM. Unless something changes that setting is good with that engine. It’s the same with the tube. You turn up the bias setting until the tube is happy in that circuit and you’re done. If you change tubes and use a tube with the same characteristics as the previous tubes you don’t have to change the bias setting. The new ones will be just as happy in that circuit. This is why some tube vendors “rate” their tubes. They test them and rate the current they draw on a numerical scale so that consumers can just buy the same rating as the previous tubes and plug them in. When used in the output section of a guitar amp, they can develop more power than with cathode biasing, and have a crunchier, harder sound in general. Dr. Z amps with fixed biasing include: EZG 50 KT45 Route 66 SRZ 65 Z28 Galaxie Antidote Amps such as the Fender Tweed Bassman, blackface and silverface Twins and Super Reverbs and Marshall EL34 and 6550 amps are all also fixed bias. So are the Dumbles. Dumbles get their "rounder" sound from the preamp circuit, not the power output section. Having said this, when you put tubes into guitar amps it’s not so simple. Other design choices a designer makes can influence an amps sound and “blur the lines” between cathode and fixed biasing. For example, the Marshalls we all grew up with are good old fixed biased EL34 amps. The Remedy is a cathode biased 6V6 amp. Even though cathode biasing is generally said to have a spongier attack, design choices made with the transformers, internal voltages and other components give the Remedy more of a fixed biased crunchy “Marshall” feel. Certain fixed biased amps can have a softer attack and smoother sound like the original lower-filtered Route 66 or Delta 88. The Z-Wreck has the “comfort/speed” switch which changes voltages to allow for a harder or softer attack even though the amp is cathode biased. Amp designers like fixed biasing due to the higher wattages that are possible…back when Leo Fender was designing amps his goal was the loudest, cleanest amp he could make to let the guitar be heard above the rest of the band. Jim Marshall copied the Bassman and as the amps got more distorted the “harder” sound of the fixed biasing suited rock and hard rock very well. **** Denney and Tom Jennings at Vox favored the sweeter sound cathode biasing and the AC/15 and AC/30 were born. Gotta go now. Tendonitis killing me.
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Post by nmz on Jul 15, 2014 6:01:11 GMT -7
Phil that was a great class! Thank you for all if the info you have a great way of communicating what can be hard to grasp at times.
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Post by iluvpunz on Jul 15, 2014 6:09:10 GMT -7
Thanks Phil. I appreciate you putting this up here. I have a much better understanding after reading your post.
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Post by Phil (aka Phil) on Jul 15, 2014 6:11:42 GMT -7
Phil that was a great class! Thank you for all if the info you have a great way of communicating what can be hard to grasp at times. Thank you. That's pretty much why I try to pass on the knowledge I have acquired over the years...it's NOT so I can say "look how smart I am!". I hope it never comes across that way. Amy has told me that I have the ability to explain technical subjects like guitar and photography in an easier to understand way. Hopefully I do that. I've got one more aspect of guitar amps I'd love to go into, but I'm not sure where it fits. It's "Why does my 30W rated amp sound louder than my 50W rated amp" or "Why is my Z-Wreck so *@#$@* loud?" LOL. I was thinking about doing it in Pete's post about the Therapy.
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Post by jimiata on Jul 15, 2014 6:21:19 GMT -7
Thanks Phil for a very easy explanation to a very complicated (at least to me) process in amp design. I now understand how biasing works.
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Post by doctorice on Jul 15, 2014 6:36:38 GMT -7
Wonderful lesson, Phil. Thanks.
I think something that throws folks is that "fixed bias" amps have adjustment pots. I believe the link is that the pot(s) are there so that the bias can be set at the appropriate "fixed point" to ensure proper operation.
Minor update: the 6545 is fixed bias too.
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Post by Alan M (aka 'Milky') on Jul 15, 2014 6:56:00 GMT -7
Brilliant tutorial, Phil. Thanks. I think I've learnt more in the last 10 mins reading your post than in the previous 40+ years!!
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Post by markT on Jul 15, 2014 7:05:33 GMT -7
Awesome as always Phil! Thanks for the through, easy to understand lesson....... ...and even though I knew most of that, I could never explain it! Boy that guy is smart!!!... lol. Thanks for asking jj...
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Post by fishbeiner on Jul 15, 2014 8:37:02 GMT -7
Can't express my appreciation for this thread. Been reading the Dave Hunter book, The Guitar Amp Handbook, and I really love your analogies, Phil. So often, as human beings, we have these "aha" moments where certain things just make sense. You definitely just did that for me. I love learning about the technical aspects of things. And one of the great things about this forum, is all the knowledge and information. I love this place!!
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Post by DRZ on Jul 15, 2014 8:58:33 GMT -7
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Post by markT on Jul 15, 2014 9:05:33 GMT -7
The ultimate approval....
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Post by gplayer1965 on Jul 15, 2014 9:09:37 GMT -7
I have been schooled thanks Phil
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Post by lumco on Jul 15, 2014 9:13:42 GMT -7
That was very interesting. I enjoyed the lesson...
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Post by Deleted on Jul 15, 2014 9:19:11 GMT -7
Thanks Phil, I really appreciate your sharing your knowledge.
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Post by dock66 on Jul 15, 2014 9:20:33 GMT -7
Thank you Phil for posting. I learn a lot from reading your posts.
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Post by aufamily on Jul 15, 2014 11:16:13 GMT -7
Very good. Thanks to people like Phil...people like me never stop learning!
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Post by BritInvasion on Jul 15, 2014 11:48:56 GMT -7
Great post , Phil!
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Post by skipper on Jul 15, 2014 12:01:32 GMT -7
Wow this was a great thread thanks for the effort on this.It has been a great education.
Just so I am clear on the cathcode amps if I am replacing tubes with new ones I would not have to have to have a tech do a bias adjustment.Am i reading this correct?
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Post by Phil (aka Phil) on Jul 15, 2014 12:29:35 GMT -7
Wow this was a great thread thanks for the effort on this.It has been a great education. Just so I am clear on the cathcode amps if I am replacing tubes with new ones I would not have to have to have a tech do a bias adjustment.Am i reading this correct? I REALLY neglected to mention this in the post. I meant to get back to it. It should have come under the heading of "advantages of cathode biasing"...the ability to plug in new tubes without re-biasing...with one big caveat: Cathode biasing is commonly called "self biasing" but it's not magic. When vacuum tubes are made, no two are exactly alike. Due to manufacturing tolerances that are impossible to control all tubes have a certain amount of current the "like" to draw. If they were all identical we wouldn't need to ever adjust bias. This is why tube vendors like Groove Tubes and Eurotubes test and "grade" or "rate" power tubes by current draw. GT had a scale from 1-10. For fixed biased amps this meant that you could just buy another tube of the same rating as your old, used up one, pop it in and keep playing. The thing not many folks talk about is that cathode biased amps need to see a certain range of current draw to be happy. You know that cheap component I talked about earlier that cathode biases the amp? It's electrical value is set by the amp designer to work with a certain range of current draw...the "average" tube (in the case of Dr. Z amps, that was rating 5-6 or the "middle" of the Groove Tubes scale). If you buy a tube that "likes" to draw a lot more current or a lot less current (say 1-2 or 8-10 on the GT scale) then technically you need to change that biasing component in your cathode biased amp to a lesser or greater electrical value...or it's just as bad as turning the bias too high or too low in a fixed bias amp. The amp can no longer self-regulate the tube. If you buy from Eurotubes and tell them you've got a Dr. Z they will send you JJ tubes from their rating scale that match GT's "middle" tube (I think Eurotubes calls them #28). If you just buy random, unrated EL84's and put them in your Z's you're running the risk of the cathode biasing component not being able to "handle" the extreme low or high current draw and being biased too hot or too cold. Some amps are more tolerant of this too. I've found the Mazerati GT REALLY wants to see matched tubes in that #5-6 rating area...or else.
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Post by gotmojo on Jul 15, 2014 13:57:58 GMT -7
Phil, wait, what, will you say that again? Very nice discussion, man. I'm glad I asked. Thank you so much. And glad I'm not the only one that wanted to hear it. I find it interesting that I've let go of all the doc's amps that are fixed bias, and have held onto nearly all the ones that are cathode biased (except for the MAZ18 - I need to get another one of those and give it a try again). Wreck, Rem, Stang and Rx do it for me in a massive way. Of course, my BD fave amps (before Doc) were the matchless DC30 and '60's vox AC30. The only ones that don't fit that pattern in my tastes are my old tweed Bassman and wide panel twin! But those seem to sound different than the cathode biased ones. jj
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Post by Phil (aka Phil) on Jul 15, 2014 14:09:12 GMT -7
The only ones that don't fit the pattern in my tastes are my old tweed Bassman and wide panel twin! But those definitely sound different! jj The wide panel tweed Twin WAS cathode biased...and...you know when I said that the lines can be blurred between cathode and fixed biased? The tweed Bassman, while fixed biased was still pretty "compliant" due to the plate voltages, thin baffle board and sweet 5881 output tubes...so you're still consistent
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Post by Phil (aka Phil) on Jul 15, 2014 14:35:06 GMT -7
Clarification to above post: if it was a wide panel Bassman from the mid '50s it was cathode biased as well IIRC. The 58-60 narrow panel was fixed biased. You didn't say which era the Bassman was from
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Post by gotmojo on Jul 15, 2014 15:13:22 GMT -7
Yeah, the wide panel Twin is from '53. The Bassman is a '57. And I have a '59 Bassman that is different sounding still! I kinda like the '57 better. My son likes the '59 best. Either way, interesting points you make on all accounts, Phil. Thanks once again. Now to try the Therapy. Two days time if all goes well! A lil pumped, I am.
jj
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Post by jazzsnds on Jul 15, 2014 17:44:50 GMT -7
Great post Phil,I really enjoyed it.
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Post by benttop (Steve) on Jul 15, 2014 18:20:29 GMT -7
Nice job Phil!
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