Ok....
Been thinking about one of these so I can bias my own amps.
Now I'm far from a wizz with all things valve but can grasp the concept of plate volts and idle mA etc.
What I don't fancy is the whole resistor here/there and poking around pins with a multimeter when the amp is running!!
Is the TAD is good thing for simple biasing i.e just that and a screwdriver to adjust bias pot? (With one hand in your back pocket)
Am I missing something here, will it be more involved than that - provided I get the plate voltages calculation right?
Comments
I was looking at something similar to Bias my PRRI, but then looking at the schematic all I needed to do was measure the mV's between to points on the circuit. The amp has a bias pot so it didn't seem worth it to but something just to bias one amp occasionally.
If you have a decent digital multi meter and follow a few simple rules (draining filter caps, clip ground to chassis and use one hand etc) it is fairly simple. I think the fact you have reservations about doing it probably makes you the kind of careful person that would be sensible if you open the amp up.
IF your amplifier has a "sense" resistor in the cathode circuit of the OPValves then you really don't need anything more than a digital voltmeter and a couple of insulated croc leads and so long as you follow the procedure of...Cold amp, amp diss'ed, clip in meter. Hands out! Switch on amp, check for smoke. No? Go and make a cuppa. Check bias volts. If around 25-30mV per valve(octals) I would leave it. If outside those limits adjust with one hand in pocket but really, you would have to have a death wish to hurt yourself*
If the amp does NOT have a sense R, get a tech to fit one but not before checking in here with ICBM or other top tech (I do not have their vast experience of all the models out there) . I doubt a tech would charge the cost of the bias probe to do the job and if part of a valve change/service probably next to FA.
The other methods of measuring anode current are potentially more dangerous but again if you only ever connect "cold and off" not really.
*But, best to have a responsible adult around. NO kids or pets and Stone Cold Sober!
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
Meant to say! Even tho Wales is closer to US than the rest of we, them's STILL anodes!
Plates are what we eat us dinners off and TOOOBS are what they roll bog paper on!
Then, you don't really need to measure Va, just go by the amp makers specc (all B'satrs are 25mA per valve except the HT-20, ~11V and the HT-5 46mV. Newer stuff might be different but I doubt it).
Again, if you want to experiment with different currents check in first with ICBM&Co.
Dave.
So, another question.."warming up the amp before biasing" giving it 10-15 minutes to warm through off standby etc - with new or old valves?
This Shiva needs to be biased at idle when it's hot, this could be bullshit but I have read a lot about the bias drift on these amps relative to its operating temp, I've even read that some have experienced a rise when they simply put the chassis back in and the temp rises!!!
So, I'm thinking, chassis out, play it for 10/15 mins or so at gig volume, then slap on the bias meter, and go from there - the only bit I can't fathom out is new or old valves? Common sense tells me new ones warmed up...but another part of me screams damage by running a pair of unbiased valves at medium throttle...
Bias drift is always with us! This is just one reason I do not like the practice of super hot biasing. Do not forget that not only do valves change as they heat up, the mains voltage* can be + or -1 0% of 230V as well!
My MO (on say an A100) was to bias a new set of bottles to 100mV (total) with mains in set to 230V via a Variac. I would then check the clipping power point (was always a decent bit over 100W) back off the drive and check again. If the figure seemed stable I would box up the amp and give it the mandatory soak test at 1/2 power for at least an hour on heavy rock music. Hardly ever had a problem and hardly ever saw the amp again!
If you don't have a safe way to check mains volts don't try! Just assume it is 240volts.
*Yes, transformer derived bias volts should "track" the mains voltage and it does, but not perfectly! I dare say some amps are better for this than others?
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