Sorry Paul. I'm in Stoney Creek and don't really know the crowd up your way. There was a guy in Peterborough but I think he's retired as I picked up one of his customers.PaulS said:That's a lot of good info there Wild Bill, do you know any good amp techs that could tune up my Ampeg SJ-12R, it has a horrific noise problem at the moment. :food-smiley-004:
Can't remember his name, Paul. I heard from WillieP, if you know him you can ask.PaulS said:Well when all knobs are at 0 there is no noise, when I turn the bass knob up to about 3 there is a loud hum in the speaker,above 5 and it increases a bit but peaks. When I tturn up the mid it does it also but not noticeable on the treble control. This is all with the volume at 0. I checked inside for loose or burnt stuff components but all looks good. I tried new tubes all the way around but the noise persists. Who was the guy in Peterborough, was his name tony?
Hi Craig!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.
Hate to be a stickler, but this isn't accurate. The idle current is not the minimum current that the tube will see. In fact, in the case of a class A design, the idle current will be significantly higher than the minimum current that a particularlar tube will conduct when the input signal (for that tube) is at it's maximum.Wild Bill said:It's like the idle speed in a car engine. Bias refers to a control voltage that adjusts the minimum current happening through the tube(s).
But if you are going to post something technical, it should be correct. There's already enough mis-information out there.Adicted to Tubes said:Ok.We are not here to educate the world on tube amps
This is the statement that I pointed out as misleading...Adicted to Tubes said:I don't think an analogy would classify as 'technical' information.
If he said bias your blackface reverb at 95ma and it burnt down,then he would be guilty of misinformation.
So a direct question like the actual acceptable bias would need an accurate answer.