a bit of bias confusion

bandmaster188

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!

Misterg

Your interpretation of bias *current* is correct. However bias *voltage* readings closer to zero mean that an amp will be running hotter.

(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.

bandmaster188

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.

Misterg

bandmaster188 said:

...25mA will be running cooler and 40mA will be running hotter ....

Yep.

All will be well as long as you don't go too 'hot' 

ICBM

For a plate voltage of around 470V, you don't want to go hotter than 35mA and preferably not much over 30.

bandmaster188

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.

 

ICBM

Try swapping them to the opposite sockets and see if it's the valves or the amp. Valves is most likely, but it's just possible you could have a very slight leaking coupling cap which will disturb the bias on one side. (Even if the other valves seem to match! They could could actually be out of match the other way round .)

6mA is a bit far out, but there's no reason it will be a problem if the amp sounds good and you don't get too much hum. If it is the valves, personally I would put the lower-drawing valve in the side nearest the PI, that one is under marginally more stress when the amp is driven hard due to the way the PI distorts. (Worse in Marshalls than Fenders.) I think it sounds better too.

There is a reason CBS put in the balance circuit . Although taking out the overall bias at the same time was a bit of a two-steps-forward, one-step-back solution. They didn't get it completely right until the red-knob 'The Twin' (one of the good things about that amp).

bandmaster188

cheers ICBM, will try that and see what happens.

 

bandmaster188

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.

:-/

ecc83

bandmaster188 said:

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.

bandmaster188

Can this be measured by jamming my multimeter probes into a plug socket *-:) Cheers Dave, yet another thing I hadn't considered on my biasing trip of discovery.

ICBM

bandmaster188 said:

Can this be measured by jamming my multimeter probes into a plug socket *-:)

Sort-of, but that's not very safe!

Personally I wouldn't worry about it - the variation in mains voltage is mostly relatively small and as long as you're not trying to bias right up to the hot limit, it won't affect it too much. My experience is that mains voltage variation is more geographic than time-based, so if you set it correctly where you are, it will stay more or less that way unless you move somewhere different.

It does sound like those two valves are a bit unmatched - which is good, because it means the amp is OK - but I still wouldn't worry too much if it sounds good and doesn't hum a lot.

bandmaster188

I should probably stop being such a tight arse and stick the new ones in and keep the other 2 as spares! Cheers people!

ecc83

ICBM said:

bandmaster188 said:

Can this be measured by jamming my multimeter probes into a plug socket *-:)

Sort-of, but that's not very safe!

Personally I wouldn't worry about it - the variation in mains voltage is mostly relatively small and as long as you're not trying to bias right up to the hot limit, it won't affect it too much. My experience is that mains voltage variation is more geographic than time-based, so if you set it correctly where you are, it will stay more or less that way unless you move somewhere different.

It does sound like those two valves are a bit unmatched - which is good, because it means the amp is OK - but I still wouldn't worry too much if it sounds good and doesn't hum a lot.

No, don't worry about it but if you are looking into biasing it is as well to know all the wrinkles.
Here's another one. New valves can have a fairly high grid current (measured across the stopper R) . This "fights" the bias a bit and is yet another cause of drift. This tends to drop by 100 to 200% over the first couple of hours or so, even faster if you soak test the amp at 50% full welly as I used to do for an hour.

I used to have 100 valves at a time to run bias and stress tests on and got into the minutia of the testing a bit. But for general working set ups don't mind me!

And yes! FFS! If you don't have a proper mains meter, leave it!


Dave.

bandmaster188

So when putting my new tooobs in, it's probably a good idea to leave the amp running for an hour or 2 and re test. I was jesting about the mains testing. I usually have tin foil sausages squashed into the plug so I can access readings easily. This is quite safe as the accompanying note clearly tells the children not to chew the foil! 8-X