transformer shunt bias for fender concert

[Denny]

can some one please tell me what wires on the power tubes I connect the black to to read the mA? My techie told me either the blue or the black coming from the power tubes, but the freakin amp has all white wires - so - what pins do I use Thanks

D.

[John Phillips]

Do NOT use the shunt method - it's potentially dangerous, and possibly inaccurate.

Use the voltage drop method instead - which means finding the same points, but you need to measure the resistance of each side of the OT primary as well. It's still dangerous, but less so, and is the most accurate method.

The points you need are the power tube plate connections (pin 3 on the power tube sockets), and the main B+ feed, which is either connection on the standby switch (when set to play).

With the amp off, measure the resistance of each side of the OT, and write it down - they will be slightly different, this is normal. Then, with the amp on, measure the voltage drop across each side. Divide the voltage by the resistance to find the current.

BE CAREFUL. You are dealing with 450+ volts above ground, even though the voltage you are measuring will only be a couple of volts.


The shunt method is dangerous because the meter has almost zero resistance when set to read current, so if something goes wrong it will produce a dead short across the B+ supply. Count on serious damage to the meter or the amp - or you, if you happen to come into contact with one of the probes. But set to voltage, the meter has a high resistance, and although you can still short out something with the probe (the most common is pin 3 to pin 2 on the tube socket), you won't be able to draw a big current through the meter itself. You might still get a shock but it's less likely to be fatal.

It's inaccurate because it assumes that the resistance of the meter is exactly zero - it isn't, and in fact although it's very low, it can still be significant compared to the resistance of the OT primary, which is only in the order of 10 to 20 ohms on some amps. This means that some current will still go via the OT, and the reading you get will be low, which is not the way you want any unknown error to be. The voltage drop method avoids this problem too, because it doesn't actually affect the current passing through the OT.

[unklmickey]

hi Denny,

IMHO the tranformer shunt method is a bit inaccurate, and very dangerous.

the voltages there can KILL you............i won't have your blood on my hands.

you might think i'm being condescending, but i'll risk that. this is serious business we're talking here.

if you had experience, you would know where to find a schematic, and how to read it.

but you appear to be a newbie in that regard, so i suggest you read about installing 1 ohm resistors in series with the cathodes of your output tubes.

you can more safely and accurately measure the current there. just measure milivolts, and that will indicate miliamps.

maybe your "techie" might be willing to show you the ropes.

that would be one way to get some experience, and also survive.


good luck,

unk

[BLC]

What age Concert are we speaking of here ?? Is it a 4 x 10 Concert or the Rivera 1 x 12 Concert?

[Denny]

rivera concert. Thanks for the cautions - I appreciate it. I am NOT an electrical engineer but have been doing the shunt method on my amps for a while with great success. I'll just wait til my guy gets home and take it from there. I TOTALLY UNDERSTAND your concern.

D.

[Blue Strat]

You need to probe the center tap of the transformer (if you can't find this from the schematic and layout I don't think you should be using this method) and pin 3 of either power tube.

[RussB]

Try both methods, and compare

[Denny]

Thanks - my guy called me back last night - the red wire from the OT and pin 3 on the power tube - 37mA for 6L6's

I do appreciate the concern from everyone here. shows you all care. Honestly - I've been doing this method on all my amps for years, but (obviously) I don't do it often enough to remember - LOL. I've been zapped and thrown backwards once - back in the 70's (thank God because I was young and resielient) - and I can assure all of you - this will never happen again - Ouch!!! My tech -guy (I don't want to mention his name) is the guy who does all the mods on Trey Anastatio's Fenders and he might be a member of this forum too but I've never asked him his logon name. We've known each other for going on 20 years and I trust his expertice 100% (not to say that the good folks here don't know what they're talking about because they definately DO! - I'm near the bottom of the food chain when it comes to circuits, amps, etc...) and my friend has been there every and any time I need help.

I do have a question though - why are there so many conflicting opinions as to the "best" biasing methods? Seems there should be a standard or something? Thanks again everyone.

[John Phillips]

The various methods - except the OT voltage drop method - all contain inherent (and unknown) errors.

The transformer-shunt method may read low, which is potentially bad for the tubes. The proportionate error is dependent on the resistance of the OT primary vs that of the meter, and so is completely unknown.

The cathode-current method (whether done with a bias probe or an internal 1-ohm resistor) will read high, because the measurement also includes the screen current. This gives some safety margin, and so in that sense isn't bad, but the problem is that the screen current is unknown, and varies between tubes (even of the same type). It's never a large percentage of the plate current (unless the tube is faulty) but still large enough to cause a meaningful error.

All these methods - including the voltage-drop method - also could possibly cause the amp to become unstable due to coupling between the meter wires and the amp circuit, and either give spurious readings or even cause damage; the two worst for this are the transformer shunt method or any bias probe that feeds the current out of the amp and through the meter, rather than measuring the voltage across a resistor.

Basically, avoid ANY method that requires you to read a current, rather than a voltage. They're all inherently dangerous and inaccurate.

I would STRONGLY recommend to stop using the shunt method - sooner or later something will go wrong. Speaking from personal experience... which is the reason (apart from the accuracy) why I will never do it that way again.

[Denny]

Thanks for the info - I will certainly look into your recommendtions - sounds like experience is the best teacher.

D.