TAD Biasmaster

welshboyo

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?

menamestom

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.

ecc83

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.

ICBM

What Dave said.

The TAD or a similar bias kit does make it a lot easier and quicker though, and usually safer, especially if the amp doesn't have sensing resistors. It's true that you still have to work inside a live chassis if you want to adjust it without taking a day over it, but turning a trimmer with a screwdriver is an order of magnitude less dangerous (to you, the meter and the amp) than poking around near the valve sockets because you should be nowhere near the high voltages.

I generally don't bother with sensing resistors unless a customer wants them, because I use one of the more dangerous methods... which is a fact not a recommendation! Although I did fit my own amp with them, plus an external test jack and a trimmer.

Just for the record - never use the 'transformer shunt' method which is sometimes mentioned because it is not only by far the most dangerous, it's not even accurate. If you want to use direct measurement on the OT, use the resistance measurement and voltage drop method, like wot I do.

ecc83

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.

bandmaster188

i bought one last year and all though its not cheap, its very simple to use. also you can switch between the 2 or 4 power valves (depending on your amp) which gives you an indication of  how closely matched your set of valves are, or if there is a problem with the amp if its showing a big difference between valves on the reading.

welshboyo

Cheers all.

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...

ecc83

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.

jpfamps

A cheaper kit can be had from Tube Town.

http://www.tube-town.net/ttstore/Tube-Accessories/Bias-Adjustment/Bias-Probe-Octal-assembled::3132.html

This doesn't allow HT measurement, but usually this doesn't matter too much as this can be estimated.

The bias doesn't generally need to be that accurate.

welshboyo

cheers guys, so warm up on new valves - give them a rough bias before playing in/warming up and then rebias as required with amp at idle?

jpfamps

With new valves \i like to leave the amp running for at least 30 minutes to see if the current draw is stable; longer is preferable.

ICBM

jpfamps said:

With new valves \i like to leave the amp running for at least 30 minutes to see if the current draw is stable; longer is preferable.

Same here, and I check again after the 'thrash test' - 3 minutes or so of full-on, trying-to-kill-it distortion into a dummy load. The idea is not to deliberately wreck it so I will be monitoring it carefully, but if something - particularly a valve - is going to die, I'd rather it was on my bench than at the customer's next gig… so it needs to be able to at least get through a song at that sort of power output.

welshboyo

oh well, ordered a Bias Meter from HotRox.

Wish me luck......apparently 29ma (ish) per valve at warmed up idle is good and what is recommended by Bogner..