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hi folks, i recently bought a TAD bias probe so i can bias my amps myself. on reading the instructions it gives an example where a pair of valves in a super reverb are reading 15mA which they describe as being too low and should be 25-40mA. i have interpreted it as running cold. i have just read (or misread) some information on the web somewhere that the closer the reading is to 0, the hotter the amp will be running.
can someone clarify this for me.
cheers!
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(Bias voltage is usually a negative number).
There is also a potential confusion lurking in that it's quite common to check bias current by measuring the voltage across a small (usually 1 ohm) resistor in the cathode. That is not the same as the bias voltage.
so if i stick within the suggested current variables i.e. 25mA will be running cooler and 40mA will be running hotter then all should be well hopefully.
All will be well as long as you don't go too 'hot'
"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
cheers for that.
just another quicky while we're on the subject, i've just had this particular amp (73 bandmaster reverb) changed to the black face bias circuit. previously it was only possible to balance the valves. i have just put back in the amp a pair of philips 6l6wgb that are a bit old but tested good on the techs equipment. trouble is they are reading around 6mA different from each other.
what kind of tolerances should i be looking for between the 2 valves?
these were a matched pair but they are probably around 8 or more years old now so have obviously drifted. i'm just trying to squeeze the last bit of life out of them. i do have a new set of these 6l6's which i put in and they tested within 0.4mA of each other.
"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
cheers ICBM, will try that and see what happens.
just performed a midnight test and as the vavles were positioned the next along from the PI read 31mA and the next 26mA.
i have swapped the valves over and they now read 25.5 and 31.5.
edit.
have just made some bias adjustments and as things have hotted up i'm now getting readings of 23 and 33. and after swapping them back over again 28 and 26.3. both readings are gradually creeping up. the second valve along from the PI seems to creep up a bit quicker than the other.
:-/Just a teensy point. You should also note mains voltage (IF you have a safe way to measure it!) since bias will jodrell about with that a bit.
Dave.
"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