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Do you have to rebias when you change preamp tubes?

sonar

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Jan 10, 2003
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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.
 

roadrunner

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Aug 25, 2001
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Preamp tubes: no.
Power tubes: always, no exceptions.


See, this stuff's easy:salude
 

moonweasel

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Jan 20, 2004
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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...
 

bigtomrodney

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1,448
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.
 

randomdude84

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Feb 7, 2015
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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.
 

PaulD

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Jun 25, 2007
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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.
 

randomdude84

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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.

[FONT=&quot]"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.[/FONT]

[FONT=&quot]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.
[/FONT]
[FONT=&quot]
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."[/FONT]
 

PaulD

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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.
 

randomdude84

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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.
 

PaulD

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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.

red-plate-jpg.410427
 

randomdude84

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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.

red-plate-jpg.410427

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

Wally

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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.
 

thejaf

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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.
 

PaulD

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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/the-last-word-on-class-a

http://www.aikenamps.com/index.php/is-the-vox-ac-30-really-class-a
 
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