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So I am changing out some stock tubes in this Bugera 6260 to JJ 6L6's and just have a quick question. This is a matched quad pair, but I have different bias readings on each one. That seems like its normal and trying to get them exact would be impossible nearly. Anyhow my question is in relation to matched pairs of V5-V8 bias readings. Meaning V5= 42ma V6= 43ma V7= 39ma V8= 37ma. So should any of the four tubes be switched around to match each other's repective socket? I am not sure this matters in a case like this one, but sometimes it does. Couldn't find a schematic a while back about this amp, but just seeing if anyone can offer some feedback, thanks.
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When the going gets weird... The weird turn pro! -
Schematic? Four power tubes with a bias supply.
What if this was an old Twin Reverb Fender? No reason to treat this amp any diferent.
But to adress your question: pull the power tubes, pick one socket, now install just one of the tubes in the socket and measure its current. Now pull the tube and install the next tube in the same socket. DO NOT change any adjustments. Then the next and the next. BY testing each tube in the same socket with no adjustments, you will find if the tubes are in fact all matched, or if they just naturally have differing currents.
Now pick a tube, verify in the same socket. Now, changing nothing, move the tube to the next socket and measure it, then the next socket and the next socket. BY changing nothing and using the same tube in each socket, we find if the sockets all perform the same or if them have inherent differences.
BY now you will know whether the tubes vary or the amp varies socket to socket to explain the higher and lower currents you got. In other words, does that 42ma follow the tube, or stay at the socket?
Having said all that, you have four tubes whose average current under your testing is 40ma. one is 3ma lower and one is 3ma higher. the other two are closer. 3ma out of 40ma is a 7.5% variance. How close do tubes need to be to be matched? if we say 10%, there they are. If we say 5%, well, we are not off much.
Does it matter? Only if you want it to. The amp certainly won;t care. Matched power tubes is a relatively recent thing. That old Twin Reverb I mentioned never left the factory with matched tubes. But today it is a matter of choice, or if nothing else, a matter of feeling good about one's amp.
The test point appears to be Peavey style, bias voltage, so if nothing else, you could do it the Peavey way and just set it for -55v and be on your way.Education is what you're left with after you have forgotten what you have learned. -
Being a quad setup, I would do as Enzo pointed out & verify the separate tubes & sockets.
Then I would place the two highest tubes on either end of the output transformer & then put the lower ones next to them.
So you end up with a high/ low on each side of the OT.Comment
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OOoooOOooooooo.... just think how good life would be if one has a Bugera 6260 Infinium!
"INFINIUM Tube Life Multiplier Technology
Our revolutionary INFINIUM Tube Life Multiplier technology is like having your very own technician inside your amp constantly monitoring and tweaking tube performance for the ultimate in tonal integrity and reliability. This revolutionary circuit not only optimizes the performance of each output tube, it dynamically balances the drive between them to more evenly distribute the load. With INFINIUM, your tubes last up to 20 times longer, saving you substantial money over the lifespan of your amplifier. INFINIUM also compensates for the effects of aging, automatically adjusting current levels to keep your tubes sounding as good as the day they were installed.
If and when a tube does reach the end of its life, the system even tells you which one to replace. All you need do is replace the faulty tube and let INFINIUM take care of the rest for you. This all adds up to more consistent sound quality and longer life for your tubes. Maintenance for tube amps has never been this easy!"Comment
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Sounds like you're getting a commission!Originally posted by Fragger View Post
"If Hitler invaded Hell, I would make at least a favourable reference of the Devil in the House of Commons." Winston Churchill
TerryComment
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Well, changing the sockets definitely changed the bias reading just a tiny bit but nothing tremendously. Truely the tube was the inherent difference as a matter of numbers. So by the end I have 42 | 39.3 | 38 | 42.4 as my reading at the end of the day. I am quite happy with these final readings and understand better the balance involved with the push/pull operation of the amp. I see now that when the matched pairs are in the right sockets how they react when biasing the amp. I know that I could have also had the tubes in the order of 39.3 | 42 | 38 | 42.4. (Note: That those are not my actual numbers for that arrangement and just an example) I understand now that there are two different ways that you can set up the order of matched pair tubes in relation to the OT. Thanks for the help!
When the going gets weird... The weird turn pro!Comment
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I would put the higher idle tubes on one side and the low on the other. There will be a slight increase in second harmonic which could sound warmer and fuller. A more ragged sound will come from two conducting tubes pulling different current on a half wave. If they have a Bugera they probably want a harder less warm sound so ignore what I wrote;>)
The auto bias circuit works fine in the Bugera, but "20 times" longer life....hardly. A brand (Yerasov )here has been using a similar system for years and it works well for them so maybe Behringer borrowed the idea..
Considering the light weight components used, the Bugera's get good reviews from players, particularly metal players.
The idle current matching is normally done a low anode potential and a resistive load monitoring some value with some scaling factor and they right that down on a sticker and toss it into a bin with values in a modest range of +5% or so and call them matched. That does not mean much when those tubes are transferred to an amp with the screens not tied together and running higher anode voltage. Does that 5% stay 5% when operating at twice or three times the voltage and using the screens in pentode mode instead of triode mode? Tubes are by nature non-linear of course so you are on your own in determining the match in an actual amp.
But matching is highly overrated, if a tube is way off and effects the sound, it is defective, not simply not perfectly matched.
If you buy a lot of tubes, get them unmatched and do it yourself in a real amp and label them in a way that makes sense to you. Two results, you learn what the tubes really behave like, and you save a buck or so per tube.Comment
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