Oh oh...techie differences of opinion!
KHINGPYNN said:
Nice read Wild Bill I cannot disagree with anything you've said... LOL
Tubes need to be warmed up before they recieve a full signal so place your amp on stanby for 2-3 minutes before turning the amp "on" and using it.
Never turn your amp off and then quickly turn it back on... the power tranny will hate you for it and is probably the single most expensive component in your tube amp.
Allways allow time for your amp to cool down after being turned off... 5 minutes minimum. Tubes when running have molten parts and need cooling time for max life expectancy.
Never use your ear to bias an amp.. ofcoarse you can hear it when it sounds right but read on...
I do not recommend you use a Multimeter to read plate voltage for the purpase of adjusting bias...
Your ear and a multimeter are a GUESS at best.
Amp biasing should be performed with an oscilloscope. This way the waveform can be seen and the bias adjusted until the waveform is as close to if not perfect.
I do all my own Scope biasing and after taking a few voltage measurements it is clear that the voltage can be different to result in the same wattage due to varying current draw even with the same tubes and same amp. Even a few 10'ths of a volt can make a difference to the waveform.
Tubes get gassy even when just laying around. Tubes change electrical characteristics and values throughout their lives and it is for these reasons that you should consider periodic tube biasing even if you are not changing the tubes.
At the very minimum bias you tubes at every change.
Purchase you own tubes. In matched sets from a quality supplier. Purchase from a tech only if you absolutly trust him.
If you are paying someone to bias your amp make sure it is done with a oscilloscope. Believe me you will pay the same for some guy to use a Fluke Multitmeter as you would for another guy to use a Fluke Oscilloscope so get the most for your money. Any good tech will have the right gear so call around and ask what is used.
I use a Tektronix 422 benchtop anolog oscilloscope to do all my amp biasing.
I only use a multimeter to record readings never to base my bias adjustment on.
Craig
Hi Craig!
For the lion's part of what you said we're on the same wavelength but I think it only fair and proper to point out that among techies there is some disagreement about meter vs. 'scope biasing. In fact, i would suggest that 'scope biasing is actually championed by a very small minority of tube amp gurus.
In practice I've found that it works out close either way. The problem is that you need a reasonably good scope to have sufficient trace definition AND you have to fully understand what the 'scope is showing you!
Randy the Tone Lizard out west is a 'scope method champion. He's actually the only one I've come across. Also the only one that seems to know what he's talking about! For those sufficiently interested there are excellent sites such as Mr. Aiken's
http://www.aikenamps.com that have FAQs on biasing.
I can 'scope bias if the customer insists but frankly I would immediately check it with my meter!

I use my 'scope pretty well only for audiophile amps, which are supposed to amplify an existing generated sound without changing it and therefore by definition you're shooting for the ultimate lowest distortion. Guitar amps are an extension of the guitar and further shape the sound, which is adding distortion, no matter if it's pleasing. How on earth could someone expect to sound like Van Halen playing through a Harmon Kardon Citation II tube hifi power amp?
I only point this out because I personally would never tell players to ONLY have their amp biased by the 'scope method and that metering is wrong. Metering IMHO is always more than close enough for properly designed tube amp circuits whereas a 'scope in the hands of a newbie will likely make him sorry.
Then again, if the tech is truly inept he will find a way to blow things up no matter what methods and tools he uses.
Warm up time is always a good idea but I differ about whether or not a lack of it damages the amp or tubes. The usual reason quoted is the phenomenon of "cathode stripping". While I saw lots of info on this when I used to sell Eimac power tubes to radio and tv stations I've never seen boo to indicate it applies to the tiny little cousins used as guitar amp output tubes. I would even quarrel about the need for a standby switch in any amp under 100 watts.
I absolutely agree with letting the amp cool down! Tubes are like light bulbs. When the filament is hot it's very fragile. Everyone has learned that if you knock over a lamp that's not switched on the bulb usually survives just fine but if the lamp is on at the time it ALWAYS kills the bulb!
I doubt if snapping the amp on and off will kill the power tranny. While it can cause a big current surge, if done fast enough it would actually be less harmful! Consider that the surge is caused by the filter caps being drained so they appear as a dead short at the moment of turn on and that the filaments also have a near-short resistance when they are cold. If the amp is operating when you snap the switch the caps and the tubes don't have time to completely cool and drain so they have a much higher effective load resistance.
To blow a trannie you need to have it passing excessive current long enough for the wire in the internal coils to overheat to the point where they melt. We're talking about millisecond time intervals of current spikes here when quickly and repeatedly snapping the switch. I doubt if a lab could measure any scary heat buildup in that kinda time frame.
That being said, I would not be surprised if it took out the power supply silicon diodes. Surge currents are much harder on them. A tube rectifier would likely not notice at all or just laugh at it!
I would agree that biasing by ear is just dumb showmanship.
As for the need for rebiasing, unless a tube is aging prematurely I've rarely found the need before a year or more. By that time with some brands and frequent playing you may need new tubes anyway! We both agree to always have new tubes biased.
I've learned not to be too anal about close matched output tubes. This seems to be a "hifi" audiophile thing. It does result in the lowest distortion figure but unless you're Mantovani who in their right mind would want that for electric guitar? An electric guitar is SUPPOSED to be distorted! I find that a few ma of difference will result in a bit more harmonics, that give a pleasing "swirl". More than 5 ma. or so can noticeably unbalance the output transformer and give extra hum or stress on the windings but even then I seriously doubt if this amount would be more than an annoyance rather than a catastrophe.
I suspect our differences here illustrate our personal tastes. You're probably more of a hifi guy or if a player you lean to jazz. Me, I'm straight up rockin' blues and classic geezer rock! Let me plug some humbuckers straight into a vintage non-master volume Marshall "Plexi" with a stack of either vintage Celestions or modern Eminence Red Coats and I couldn't care less if the other guy's EL 34's are biased 1/4 of a milliamp closer than mine!
This is fun! Might we end up with a Canadian site to rival Ampage?
Oh well, this was just my .02. Now that we've totally confused all our audience...
