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No sure if that helps…
I have read some bad things about some bias probes. If the amplifier is of a certain design (cough!) bias can be checked in complete safety without one if you follow some simple rules. We need the make and model.
Yes you need a basic digital multimeter but also some insulated croc to croc leads. Your local Maplins should have both.
Dave.
As you're handy, instead of buying a probe, desolder the earth connection at pins 1&8 on the output valves and solder a high quality 1 ohm resistor in there instead.
Then measure away, do the maths etc. Usual safety rules apply.
At least with the cathode current method the error is in the 'safe' direction - the valve is running cooler than you think.
I do use the anode method - it's safe if you use insulated clip leads and check that everything is tight and can't jump off, before you turn on the power. It's the most accurate method.
NB - by measuring voltage drop across the measured resistance of the OT primary, not the 'shunt' method which is extremely dangerous and also inaccurate, in the 'unsafe' direction.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Basically I would never feed current outside the amp, even at the cathode - the right method is always to measure voltage using a resistance inside the amp, either a resistor or the transformer primary.
(Counting valve-socket bias probes with a built-in resistor as 'inside the amp'.)
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
EVH5150 MKIII 100w rebiased by a tech 18 months ago, down from a hot 120mA to a pretty cold 80mA, it's now developed a small fuzzy sound on all channels but loudest on the clean and is headed back to the tech. A quick chat with the main engineer at Fender for this amp reveals a factory bias of 110mA and he says 80 is too cold.
Therefore 1) What difference does too cold or too hot make? 2) Would 95mA literally be a good middle ground 3) Any known issues on these from you guys in the know?
Cheers all and apologies again.
The yard is nothing but a fence, the sun just hurts my eyes...
The "correct" biasing depends on a number of factors including valve type, HT voltage, class of operation and loading.
Generally in a 100 W amp using 4 6L6s or EL34s in class A/B push pull a la 5150 the HT will be around 460-500, and the bias will be adjusted so that the valves are drawing between 25 mA and 35 mA which will result in around 50-60% of maximum recommended dissipation at idle.
If the bias is set too cold, then one side of the push pull output arrangement will be driven into cut off before the other side can take up the slack so you end up with distortion in signal. Harmonics generated by crossover distortion are generally regarded as not pleasant.
Biasing the amp too hot can imperil the output valves as they will dissipate too much heat at idle.
Essentially any biasing value below the notional maximum permitted that sounds good is fine, so if you like the sound of your amp biased at 95 mA, then that's perfectly fine as this will not be over dissipating the valves.
The yard is nothing but a fence, the sun just hurts my eyes...
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
The yard is nothing but a fence, the sun just hurts my eyes...
All B's fixed biased amps were biased to 25mA cathode current per valve and I never had a punter ask for it to be made hotter.
The "fashion" for biasing very close to the limit seems to have passed? Not only did it put the valves on the edge but HEAT is the enemy of all things electronic so traffs and caps suffer as well, needlessly IMHO. Then, an amp could be biased to 95% of max Pa but with mains in at 230-240V and it CAN get to 253 and still be legal!
Dave.
The yard is nothing but a fence, the sun just hurts my eyes...
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
"I recommend a new set of power tubes if they have 40 or more playing hours on them."
I don't want to embarrass the guy as he's been really helpful, but even I in my limited knowledge had to scoff at that............
The yard is nothing but a fence, the sun just hurts my eyes...
Of course Blackstar are the last word in valve amplification, so of course that's the way to do it..............