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  1. #1
    Les Paul Forum Member moonweasel's Avatar
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    Do you have to rebias when you change preamp tubes?

    My '66 Vibrolux has been pissy for years, and I have just sort of sidelined it. Coming to my senses, I was told that the tube for the tremelo/reverb circuit was on the way out and a few weren't far behind by a trusted tech back a few years ago.

    Can I swap out the preamp tubes with identical spec replacements without any bias issues? I know next to nothing about tube amp issues, so please excuse my ignorance.

    I am under the impression that rebiasing is only when you mess with power or rectifier tubes?

    Thanks!

    PS - Was thinking about just replacing my entire set of smaller tubes to make sure my bases are covered. My Normal channel has no issues whatsoever, but the reverb channel tends to be staticy and exibit old tube-ness.

  2. #2
    Administrator MikeSlub's Avatar
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    Re: Do you have to rebias when you change preamp tubes?

    You only need to re-bias if you swap power tubes.

    If it's been a long time, you may want to also consider new power tubes and a re-bias. The power tubes make a difference in tone, along with the pre-amp tubes.
    Mike Slubowski

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    * "So many Gibsons to love, so little time..."

  3. #3
    Les Paul Forum Member Mitchell's Avatar
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    Re: Do you have to rebias when you change preamp tubes?

    You only need to re-bias if you swap power tubes.

    If it's been a long time, you may want to also consider new power tubes and a re-bias. The power tubes make a difference in tone, along with the pre-amp tubes.



    Today 10:03 AM
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  4. #4

  5. #5

    Re: Do you have to rebias when you change preamp tubes?

    I will tell you the BFVR's do sound so much better with NOS tubes vs the newer Russian and Chinese tubes. I had RCA's for the 12ax7's and some Mullards for the 12at7's. Good stuff. It might just kickstart and rekindle your interest in that wonderful amp.

    You can also pull the V1 preamp tube completely out for a little bit more gain into the second (Vibe) channel.
    "So I let the poison go, 'cause baby I always know it'll be there for me"

  6. #6
    Les Paul Forum Member
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    Re: Do you have to rebias when you change preamp tubes?

    MY observation since it hasn't been mentioned....if that amp hasn't been recapped, that mgiht do more for the overall tone of the amp than anything else.
    "As soon as man does not take his existence for granted, but beholds it as something unfathomably mysterious, thought begins." Albert Schweitzer

  7. #7
    Les Paul Forum Member moonweasel's Avatar
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    Re: Do you have to rebias when you change preamp tubes?

    Hey Wally, it was recapped in 2008, but my budget was super strapped and I couldn't do tubes at the time. Thanks for the heads up on that.

    Les strat, I will keep that in mind. What do tubes like that go for? EDIT: Just looked at tubedepot, they are ~$150 (12ax7)? That's going to be way high for me.

    What are the best, new production tubes for a BF VR? Thanks all.

  8. #8
    Les Paul Forum Member moonweasel's Avatar
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    Re: Do you have to rebias when you change preamp tubes?

    I woudl still love to know everyone's opinion on the best new production tubes, but I found these NOS which seem more reasonably priced. In fact, a memory just came back to me that I may have put a NOS Philips into this amp when I had it recapped, I think one of its tubes was truly dead at the time. (I found the amp at a garage sale in totally non-functioning condition).

    http://www.tubedepot.com/nos-12ax7-jange.html

    I think I will pop the back cover off and take some pictures of what is in there right now....

  9. #9
    Les Paul Forum Member moonweasel's Avatar
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    Re: Do you have to rebias when you change preamp tubes?

    Ok Left to right, starting with rectifier:

    RCA Gz34 USA
    Slyvania 6L6GC USA
    Slyvania 6L6GC USA
    12AT7 PhilipsECG USA (I think this was replaced in 2008)
    GE Elec___ 12AX7 USA (elec___ was illegible)
    Slyvania USA
    GE Elec___ USA (elec___ was illegible)
    Slyvania 12AX7A USA
    Sylvania USA.

    I couldn't read the type on some of them. Also, why would the Slyvania 12AX7A have the A on the end? I thought they were just 12AX7... Thanks for the patience.


    I believe the tech in 2008 made notes on the tubes in marker. One of the Sylvania's was marked "116" followed on the next line with "110B"... If that means anything to you guys, I can note the rest too...

    Kind of fun to get the ole' girl out of mothballs.

  10. #10
    Les Paul Forum Member Pat Boyack's Avatar
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    Re: Do you have to rebias when you change preamp tubes?

    I have read that if the tubes are a matched pair then a rebias is not necessary. Somebody correct me if I'm wrong.

  11. #11

    Re: Do you have to rebias when you change preamp tubes?

    I have used to Philips Jan preamp tubes. They were hardy tubes. Maybe not as warm as RCA's , but a good tube still. Maybe just buy and use a decent NOS preamp tube for your V2 (if that is the channel you plug into), and use something decent (JJ's, repro Mullards, Tungsol, etc) for the reverb slots, trem, and phase inverter.

    For new production power tubes, it's hard to beat the SED Winged C's. They sound great in a VR.
    "So I let the poison go, 'cause baby I always know it'll be there for me"

  12. #12

    Re: Do you have to rebias when you change preamp tubes?

    Quote Originally Posted by Pat Boyack View Post
    I have read that if the tubes are a matched pair then a rebias is not necessary. Somebody correct me if I'm wrong.
    No. The plate voltage can vary. Tube matching is just that... 2 (or 4) tubes that are very close, but that new matched pair of Mullards you bought might not run voltage similarly to the old set of matched Sovtechs that just came out of the amp. Unless it is a Class A cathode biased amp, new power tubes = rebias. There are exceptions, like with Mesa amps. If you use their brand tubes in their amps, you don't have to rebias as their tubes are specked the same.
    "So I let the poison go, 'cause baby I always know it'll be there for me"

  13. #13
    Les Paul Forum Member moonweasel's Avatar
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    Re: Do you have to rebias when you change preamp tubes?

    Thanks Les. I would be willing to spring for the Phillips NOS for that V2 spot.

    The Philips AT7's are very very affordable. Much more so than the AX7's.

    Do you guys shop at TubeDepot? Thanks again.

  14. #14

    Re: Do you have to rebias when you change preamp tubes?

    Quote Originally Posted by moonweasel View Post
    Thanks Les. I would be willing to spring for the Phillips NOS for that V2 spot.

    The Philips AT7's are very very affordable. Much more so than the AX7's.

    Do you guys shop at TubeDepot? Thanks again.
    I have a few times. Sometimes I have found good deals there. I bought my NOS Mullards from BluesforStevie here on the forum. (got my NOS Mullard GZ34 rectifier tube from him!)

    Make sure the V2 is a 12ax7. V3 and V6 are 12at7
    Last edited by les strat; 02-01-13 at 04:47 PM.
    "So I let the poison go, 'cause baby I always know it'll be there for me"

  15. #15
    Administrator MikeSlub's Avatar
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    Re: Do you have to rebias when you change preamp tubes?

    For new production tubes, I've had pretty good luck with Electro Harmonix brand. Have also used Tung Sols, but have had more failures than with the EVs. Lots of guys like JJ's, and they are okay, but they don't sound as harmonically rich to my ears as the EH tubes.
    Mike Slubowski

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  16. #16

    Re: Do you have to rebias when you change preamp tubes?

    Quote Originally Posted by MikeSlub View Post
    For new production tubes, I've had pretty good luck with Electro Harmonix brand. Have also used Tung Sols, but have had more failures than with the EVs. Lots of guys like JJ's, and they are okay, but they don't sound as harmonically rich to my ears as the EH tubes.
    Hey Mike, next time you re-tube that BFVR, try the SED Winged C 6L6's. They were very close to the old RCA's I had. Warm and glassy, but still had punch. I think you will likey.
    "So I let the poison go, 'cause baby I always know it'll be there for me"

  17. #17
    Les Paul Forum Member Pat Boyack's Avatar
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    Re: Do you have to rebias when you change preamp tubes?

    Quote Originally Posted by les strat View Post
    No. The plate voltage can vary. Tube matching is just that... 2 (or 4) tubes that are very close, but that new matched pair of Mullards you bought might not run voltage similarly to the old set of matched Sovtechs that just came out of the amp. Unless it is a Class A cathode biased amp, new power tubes = rebias. There are exceptions, like with Mesa amps. If you use their brand tubes in their amps, you don't have to rebias as their tubes are specked the same.
    Thanks Les!!

    Sometimes I might put stupid stuff out there and if anything I can get the real scoop. Appreciate it.

  18. #18
    Administrator MikeSlub's Avatar
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    Re: Do you have to rebias when you change preamp tubes?

    Quote Originally Posted by les strat View Post
    Hey Mike, next time you re-tube that BFVR, try the SED Winged C 6L6's. They were very close to the old RCA's I had. Warm and glassy, but still had punch. I think you will likey.
    Les, thanks for the tip!
    Mike Slubowski

    * "Gibson guitars are like potato chips - you can't have just one!"

    * "So many Gibsons to love, so little time..."

  19. #19
    Administrator MikeSlub's Avatar
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    Re: Do you have to rebias when you change preamp tubes?

    Quote Originally Posted by Pat Boyack View Post
    I have read that if the tubes are a matched pair then a rebias is not necessary. Somebody correct me if I'm wrong.
    Les gave the right answer (yes you should rebias), but I must confess that I have changed power tubes in several of my vintage Fenders with matched sets and have not taken them to a tech to rebias them in many cases. I will probably pay for it some day.

    But hey, I'm the same naive guy that in my youth thought there was no difference between a guitar cable and a speaker cable because they had the same kind of phone plugs on the end! Ran a 2 x 12 Bassman bottom with my Fender Twin for years that way, and thank God it never blew up!
    Mike Slubowski

    * "Gibson guitars are like potato chips - you can't have just one!"

    * "So many Gibsons to love, so little time..."

  20. #20

    Re: Do you have to rebias when you change preamp tubes?

    Quote Originally Posted by MikeSlub View Post
    But hey, I'm the same naive guy that in my youth thought there was no difference between a guitar cable and a speaker cable because they had the same kind of phone plugs on the end! Ran a 2 x 12 Bassman bottom with my Fender Twin for years that way, and thank God it never blew up!
    Me too! JCM 800 driving a 4x12 with a guitar cable! For 10 years.

    Glad that Drake tranny was tough.
    "So I let the poison go, 'cause baby I always know it'll be there for me"

  21. #21
    Les Paul Forum Member sonar's Avatar
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    Re: Do you have to rebias when you change preamp tubes?

    Quote Originally Posted by moonweasel View Post
    Ok Left to right, starting with rectifier:

    RCA Gz34 USA
    Slyvania 6L6GC USA
    Slyvania 6L6GC USA
    12AT7 PhilipsECG USA (I think this was replaced in 2008)
    GE Elec___ 12AX7 USA (elec___ was illegible)
    Slyvania USA
    GE Elec___ USA (elec___ was illegible)
    Slyvania 12AX7A USA
    Sylvania USA.

    I couldn't read the type on some of them. Also, why would the Slyvania 12AX7A have the A on the end? I thought they were just 12AX7... Thanks for the patience.


    I believe the tech in 2008 made notes on the tubes in marker. One of the Sylvania's was marked "116" followed on the next line with "110B"... If that means anything to you guys, I can note the rest too...

    Kind of fun to get the ole' girl out of mothballs.
    Before my internet tube rant I'd like to add to make sure your tube sockets are clean and tight.

    I doubt you'll need to replace all your pre's and whatever you do keep the GZ34. Try to locate some old AX7's and AT7's on ebay. I'd start with one or two of each type (AT7 and AX7) and swap around different positions with the tubes currently in your amp. Mix and match to try and locate the offending tube, if there is one. I'd go for ebay auctions and not the $150 buy it now tube that looks pretty. It might a little riskier this way, but you'll save a bunch of money. Of the dozen or so old pre's I've bought on ebay only two were too weak compared to new glass.

    I think new power tubes are pretty decent, but I'm still on the fence with current pre's. Most sound decent to good (if not spectacular) and the price is right. The problem I have had has more to do with microphonic's and reliability. If I need to go through a few new pre tubes to find one decent one I might as well do the same thing with old pullers or NOS preamp tubes.

    Another option is to buy both old and new. Then report back to us.

  22. #22
    Les Paul Forum Member roadrunner's Avatar
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    Re: Do you have to rebias when you change preamp tubes?

    Preamp tubes: no.
    Power tubes: always, no exceptions.


    See, this stuff's easy
    Achieve the Possible!


  23. #23
    Les Paul Forum Member moonweasel's Avatar
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    Re: Do you have to rebias when you change preamp tubes?

    Quote Originally Posted by roadrunner View Post
    Preamp tubes: no.
    Power tubes: always, no exceptions.


    See, this stuff's easy
    Thanks.

  24. #24
    Les Paul Forum Member moonweasel's Avatar
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    Re: Do you have to rebias when you change preamp tubes?

    Quote Originally Posted by sonar View Post
    Before my internet tube rant I'd like to add to make sure your tube sockets are clean and tight.

    I doubt you'll need to replace all your pre's and whatever you do keep the GZ34. Try to locate some old AX7's and AT7's on ebay. I'd start with one or two of each type (AT7 and AX7) and swap around different positions with the tubes currently in your amp. Mix and match to try and locate the offending tube, if there is one. I'd go for ebay auctions and not the $150 buy it now tube that looks pretty. It might a little riskier this way, but you'll save a bunch of money. Of the dozen or so old pre's I've bought on ebay only two were too weak compared to new glass.

    I think new power tubes are pretty decent, but I'm still on the fence with current pre's. Most sound decent to good (if not spectacular) and the price is right. The problem I have had has more to do with microphonic's and reliability. If I need to go through a few new pre tubes to find one decent one I might as well do the same thing with old pullers or NOS preamp tubes.

    Another option is to buy both old and new. Then report back to us.
    Thanks Sonar, I will keep this in mind. Might pick up one or two new tubes (a NOS Philips AT7 for like $11 and a EH AX7) and try swapping them in the various pre-amp positions. I seem to remember my tech (same guy Mapleflame uses) saying that the tubes in my reverb/tremelo circuit in particular were on the way out. I know when I kick on the trememlo with the pedal, it takes it a second or two to kick in. And it is nearly undetectable with the intensity less than 4 and then all of the sudden, bam, it's there in spades...

  25. #25
    Les Paul Forum Member bigtomrodney's Avatar
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    Re: Do you have to rebias when you change preamp tubes?

    Although it's already been covered, just to put it on record - the reason you don't have to bias preamp valves is because they're cathode-biased. It's not of huge consequence to know that, but I once heard a guy say it was because "they're not big enough to need biasing".

    Hilarious as that was it was quickly followed by the dark realisation that he wasn't joking.

  26. #26

  27. #27

    Re: Do you have to rebias when you change preamp tubes?

    Quote Originally Posted by les strat View Post
    No. The plate voltage can vary. Tube matching is just that... 2 (or 4) tubes that are very close, but that new matched pair of Mullards you bought might not run voltage similarly to the old set of matched Sovtechs that just came out of the amp. Unless it is a Class A cathode biased amp, new power tubes = rebias. There are exceptions, like with Mesa amps. If you use their brand tubes in their amps, you don't have to rebias as their tubes are specked the same.

    hey there, sorry to bring up an old post but I have a Bogner Atma which is a Class A cathode biased amp. bogner is saying that it needs to be re biased or i could damage the amp. i just replaced the two power tubes with a matching set and the same as the originals (jj el84). the new tubes solved the issue i was having but now I'm worried about damaging it if i don't get it re biased. i have been reading about class a cathodes not needing to be re biased but am confused why bogners amp guy was so adamant about it be re biased.

  28. #28
    Les Paul Forum Member
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    Re: Do you have to rebias when you change preamp tubes?

    Clearly Bogner's "amp guy" does not know very much about amps! Cathode biased amps do not need to be re-biased when changing valves, the bias point is set by a fixed value resistor between the cathode and ground and there is no adjustment. The only way to change the bias point on a cathode biased amp would be to replace the cathode resistors with ones of different values and it is not necessary to do this when replacing valves.

    Incidently despite what Bognor's marketing hype might say your amplifier is not class A, it is Class A/B. It would be impossible for 2 x EL84's to produce 18 watts in true class A. The vast majority of guitar amps that are claimed to be class A are not, it is just marketing bullshit. Unfortunately class A seems to have developed into some sort of mythical status as though it is somehow better than class A/B which is not the case.

  29. #29

    Re: Do you have to rebias when you change preamp tubes?

    Quote Originally Posted by PaulD View Post
    Clearly Bogner's "amp guy" does not know very much about amps! Cathode biased amps do not need to be re-biased when changing valves, the bias point is set by a fixed value resistor between the cathode and ground and there is no adjustment. The only way to change the bias point on a cathode biased amp would be to replace the cathode resistors with ones of different values and it is not necessary to do this when replacing valves.

    Incidently despite what Bognor's marketing hype might say your amplifier is not class A, it is Class A/B. It would be impossible for 2 x EL84's to produce 18 watts in true class A. The vast majority of guitar amps that are claimed to be class A are not, it is just marketing bullshit. Unfortunately class A seems to have developed into some sort of mythical status as though it is somehow better than class A/B which is not the case.
    thanks so much for your input. i posted this issue on the rig-talk forum under there tech section also. i did get a different response from somebody. I'm gonna quote what they said and I'm curious what your thoughts are on it. i have zero knowledge or experience so i just wanna make sure I'm doing the right thing.

    "Class A means linear, but that doesn’t imply fixed bias which makes the cathode more positive than the grid, or grid bias which uses an adjustable negative voltage to make the grid more negative than the cathode. Two different biasing methods but both can be used for class A.

    I would say traditionally grid bias is used more commonly in class AB amplifiers and cathode bias in class A, but I’m not familiar with your amplifier to know for sure. Just know the class isn’t as important as knowing the two biasing techniques which is why you’re dabbling in technician territory.

    A class A cathode biased amplifier can still be biased and need biasing - you do so by changing out the cathode resistors. A grid biased amplifier you adjust a grid bias voltage. What bogner said is correct as just because you swapped tubes and it’s cathode biased doesn’t mean you shouldn’t bias it especially for class A."

  30. #30
    Les Paul Forum Member
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    Re: Do you have to rebias when you change preamp tubes?

    All of the information there is technically correct and is essentially the same thing I was saying except that the last paragraph is somewhat misleading. It is technically correct that the bias point on a cathode biased amp "can" be changed by swapping the cathode resistor but it is not necessary to do so when changing valves. The nature of cathode biasing is such that it is essentially self-biasing and different valves will find their own bias point. Provided that the amp was designed correctly in the first place there will be no issues.

    I couldn't find a circuit diagram for your amp but the description of the circuit as a cathode biased 18 watt amplifier with 2 x EL84 output valves makes it perfectly clear what it is. The same basic circuit with a few variations has been used in many amplifiers over the years (Marshall 18 watt, Wem Dominator, Vox AC15 etc. etc.....) and you can happily change valves without any need to change the pre-set bias pint that the manufacturer has chosen.

    If you are really concerned a quick visual check that the valves are not red plating will confirm that all is well. Switch on the amp and let it warm up for 10-15 mins, look at the power amp valves in a dimly lit room and make sure that the plates (the dull grey metal section in the middle of the valve) is not glowing red hot. There will be a red glow from the heaters which is normal but the metal plate itself should not be glowing red hot.

  31. #31

    Re: Do you have to rebias when you change preamp tubes?

    Quote Originally Posted by PaulD View Post
    All of the information there is technically correct and is essentially the same thing I was saying except that the last paragraph is somewhat misleading. It is technically correct that the bias point on a cathode biased amp "can" be changed by swapping the cathode resistor but it is not necessary to do so when changing valves. The nature of cathode biasing is such that it is essentially self-biasing and different valves will find their own bias point. Provided that the amp was designed correctly in the first place there will be no issues.

    I couldn't find a circuit diagram for your amp but the description of the circuit as a cathode biased 18 watt amplifier with 2 x EL84 output valves makes it perfectly clear what it is. The same basic circuit with a few variations has been used in many amplifiers over the years (Marshall 18 watt, Wem Dominator, Vox AC15 etc. etc.....) and you can happily change valves without any need to change the pre-set bias pint that the manufacturer has chosen.

    If you are really concerned a quick visual check that the valves are not red plating will confirm that all is well. Switch on the amp and let it warm up for 10-15 mins, look at the power amp valves in a dimly lit room and make sure that the plates (the dull grey metal section in the middle of the valve) is not glowing red hot. There will be a red glow from the heaters which is normal but the metal plate itself should not be glowing red hot.
    thanks so much, i will try that out. can't understand why bogners amp guy was saying that then. i spoke to a different guy before when the amp was sent out to them last year for a separate issue. i wish i could talk to that guy cause he was really nice. this guys was very short, irritated i was asking questions, and a bit aggressive lol.

  32. #32
    Les Paul Forum Member
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    Re: Do you have to rebias when you change preamp tubes?

    Just for reference here is a picture of what to look for, the valve with the blue circle around it is red plating, the others are OK.


  33. #33

    Re: Do you have to rebias when you change preamp tubes?

    Quote Originally Posted by PaulD View Post
    Just for reference here is a picture of what to look for, the valve with the blue circle around it is red plating, the others are OK.

    ok thanks, i definitely don't see that red plating coming from my tubes. thanks again.

  34. #34
    Les Paul Forum Member
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    Re: Do you have to rebias when you change preamp tubes?

    Random, FYI, every time you install a new set of power tubes that amp is in effect rebiased. The tubes’ performance specs determine what that bias is. Imho, it is of interest to know what the biasing n7mbers are so that one can 1) find tubes that yield the biasing that gives one what they like from the amp and 2) one can ma8ntain that level of operation...and therefor the Sonics...when going from one set of tubes to the next. One does this by buying tubes with the same performance specs....matched tubes from one set to the next. Buying the same brand of tube does not achieve this. One has to buy the same ‘grade’ from set to set. Most people do not pay attention to this, but this is how to maintain consistent Sonics from one set of power tubes to the next.
    Preamp tubes....,these are cathode biased in our amps.
    "As soon as man does not take his existence for granted, but beholds it as something unfathomably mysterious, thought begins." Albert Schweitzer

  35. #35
    Les Paul Forum Member
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    Re: Do you have to rebias when you change preamp tubes?

    Caveat: This diatribe is the result of 4 beers and spray painting in my enclosed garage whilst "working from home"

    PaulD is correct. If your Atma is really cathode biased A/B then you don't need to rebias unless the tubes are really fucked up and off spec or so mismatched that one is pulling way more current than the other.

    There was a time when EL84 amps were considered "Class A" in the marketing hype/mythology due primarily because they were the Vox AC30 power tube, and of course those were associated with "Class A."

    The only true Class A guitar amps I'm familiar with are the single tube designs (Fender Champ, Silvertone 1481, old Gibsons, etc.) and some of the small build boutique amps. Any single-ended (i.e., one power tube) by default must be Class A.

    Fact: Cathode biased Marshall 1987 loaded with Sylvania 6CA7s is a religious experience we should all experience at least once in life.
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  36. #36
    Les Paul Forum Member
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    Re: Do you have to rebias when you change preamp tubes?

    Quote Originally Posted by thejaf View Post
    There was a time when EL84 amps were considered "Class A" in the marketing hype/mythology due primarily because they were the Vox AC30 power tube, and of course those were associated with "Class A."

    The only true Class A guitar amps I'm familiar with are the single tube designs (Fender Champ, Silvertone 1481, old Gibsons, etc.) and some of the small build boutique amps. Any single-ended (i.e., one power tube) by default must be Class A.
    Very true, and of course the AC30 is not class A either! Although It is technically possible for any push-pull amplifier to be biased as class A regardless of whether it is cathode biased of fixed biased I personally don't know of any guitar amps that are. The maximum power you could get from 2 x EL84's in true class A would be about 10 watts (or 20 watts from 4 x EL84's). Any more than this and the valves would be likely to self destruct in fairly short order.

    For the technically minded there is lots of good information on the subject here:


    http://www.aikenamps.com/index.php/t...ord-on-class-a

    http://www.aikenamps.com/index.php/i...really-class-a

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