'68 Twin bias question...Help!

[strat-rock]

Hi, thanks for reading.

I'm trying to bias one of my Twins and I know there is a way to record the current of an individual tube, but I don't remember what it is. The way I'm biasing is by connecting the positive lead of my VOM to the red output transformer wire and the negative lead to pin 3 on my tube(s).

I've heard from an amp tech that I should be running idle between 40mv and 80mv so I think I'm within range?

I think the reading I'm getting (60mv) is the combined current, and what I'd like to know is what each individual tube is running at.

Is this possible? Am I totally lost?

[Blue Strat]

What you're using is the "transformer shunt method" which measures actual current in mA (milli amps), not mV (milli volts). Your meter should be set for current, not voltage.

To measure each tube individually using that method, you'll have to pull the "sharing tube" (each left/right pair of tubes share current through the output transformers primary) for each pair. Otherwise you're getting a reading for both tubes combined.

30 to 40mA per tube is the correct ballpark.

[strat-rock]

Thanks so much Mike! I meant to say ma, and that's what I was testing, milli amps.

How long can I run it with only one tube in a pair?

Now on to my meter, I have a note to myself that it's 10 ma off...I think I may pick up a new one, just in case.

Thank you!

[John Phillips]

DO NOT DO THIS.

The transformer shunt is dangerous and inaccurate. (And with respect, should not be recommended.) If you accidentally touch something you shouldn't with the meter probes, you will short the amp's B+ supply to whatever it is, which will likely damage the amp, the meter, or if it's you that it discharges to, possibly kill yourself.


But with a simple change, it can be made much safer and totally accurate...

You do want to be measuring millivolts (or more likely volts), not milliamps.

Measure the voltage drop across the PT primary in the same places you have been doing (it will likely be around 1 to 2 volts or so), and then - with the amp fully off and cold - measure the resistance of the OT primaries too - you will usually find they're slightly different, measure both sides and use each exact figure to calculate the current, which is simply the voltage divided by the resistance.

(You can obviously aim for a particular current by multiplying it by the resistance, which will give you a target voltage drop.)

This is much safer because the meter has a very high resistance when set to read voltage, so although you can still slip and short something with a probe, it won't be through the meter and so it's less likely to get anywhere near you. It's also far more accurate because it does not rely (as the shunt method does) on the meter resistance being effectively zero (which it isn't, quite).

You can run the amp with only one tube in each pair as long as you like, in a fixed-bias amp - it's perfectly safe, certainly at idle.

[Blue Strat]

John's right about the warning. Consider my response "correct in theory".

[JDJ]

A Bias Rite is your friend: http://taweber.powweb.com/biasrite/br_page.htm

[John Phillips]

I learnt this one the hard way, BTW.

After a few years of using the shunt method with apparently no problems despite the safety warnings I'd read, I made a mistake with one meter probe and touched something I shouldn't have... pin 2 on the power tube socket, while the other one was already on the CT. (On a Twin, too.)

There was a huge bang and flash, and smoke started pouring out of my old AVO Multi-Minor multimeter, as well as the Hum Balance pot on the amp. I was lucky... I only destroyed a nice multimeter and a pot, and the only shock I gave myself was psychological (although it was bad enough).

I've never done it again that way since.

If it had been set to voltage the worst it would have done is tripped the cut-out on the meter.

[Blue Strat]

I've done the same thing several times. Only blew meter fuses and one meter, but realized that it wasn't worth the risk.

[phsyconoodler]

I bought a box of fuses for my meter and then gave up and added one ohm resistors from the cathode(pin 8) of each power tube to ground.Then you simply measure the millivolts across the resistor and it converts directly to miliamps.
But for the uneducated a bias meter will likely save you a lot of blown parts and even a shock.

[strat-rock]

Funny you all mention this...I went through three multi-meter fuses last night, and shorted out my wall socket breaker once (lucky I had one of those). Each time I shorted to the chasis (ground). That said, this method is easy...

John I'm not sure I understand the way you're telling me to do it. What does this mean "with the amp fully off and cold - measure the resistance of the OT primaries too?"

Thanks for the heads up!

[John Phillips]

Please, stop now and learn more about the risks you're taking.

You've been very lucky so far not to seriously damage something or yourself - with respect, you aren't experienced enough to be working in there, and you don't know enough to be even aware of the danger you're running.

Casually describing going through fuses in your meter and tripping the breaker is not funny. A wall breaker will NOT save you if you short the amp's B+ supply through yourself - in fact, the amp wouldn't even need to be plugged in for it to kill you if there was enough charge in the caps.

I PM'd you as well in case you didn't see this.

[strat-rock]

Thanks John, I know it wasn't a great night. I did discharge the caps when I was done (need to change two filters out, one is still a Mallory!).

I'll be more careful in the future. I've been hacking at stuff like this for a long time and while I admit I don't know much, I haven't done myself in yet.

[John Phillips]

OK, in response to your PM, and for anyone else who wants to know, this is how to do it by the safer voltage-drop method.

WARNING: DON'T EVEN TRY THIS UNLESS YOU ARE CONFIDENT OF WHAT YOU'RE DOING.


FIRST, with the amp off and unplugged from the wall, and the standby switch to on (up/play), wait two minutes for the caps to discharge (they will on this amp, you don't have to do anything else - but be careful with other amps and make sure you know how to check that the caps are discharged).

SECOND, measure the resistance from the transformer center tap (red lead) to each of the power tube connections (blue and brown leads). Write down the numbers - on the amp chassis next to the tube sockets is as good as anywhere since it never changes and you will need it again one day. For a Twin you should be expecting about 50 ohms per side, and the numbers will differ by about two to four ohms.

THIRD, get some meter leads with clips on if you can - at the very least for the center tap end, it's much safer if you don't have to concentrate on holding two in position at once. Clip this one on, and set the meter to VOLTS (20V range). If it won't clip securely there, try one of the terminals on the standby switch itself (it's the same place electrically). Keep the other probe somewhere safe away from the amp. If you have two clip leads, clip the other to the power tube/side you want to measure.

FOURTH, now power up the amp fully and wait for it to stabilize (1 minute or so). Measure the voltage to each power tube transformer connection. You should get around 2 to 4V per side with all four tubes in. Write down the numbers. You should work with one hand holding the meter probe only and the other one nowhere near the amp - this is important, it's your best chance of avoiding a fatal shock if something bad goes wrong. Do NOT under any circumstances hold the probe with one hand and the amp chassis with the other.

FIFTH, switch off the amp (power switch, leave the standby in the play position) and unplug it, wait two minutes, and disconnect the meter lead.

LAST, divide your voltage figures by your resistance figures for each side, which will give you the current (in Amps, multiply by 1000 for milliamps) for both tubes.

eg, if you measured 3.2V and 50 ohms on one side, that's 3.2/50*1000=64mA.


Good luck, and be careful. As far as I am aware I cannot be held responsible for anything that happens to you or your amp :).

[strat-rock]

Just wanted to say thanks publicly John!