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is the shunt method dangerous

4/2/1997 3:56 AM
Ariel Pozzo
is the shunt method dangerous
Some time ago I asked how to bias my amp without a scope. Some people told me that the best way (tone-wise) of do that is the "shunt" method ( 1-ohm resistors between pins 7/8 and ground) and while I was putting some groove tubes EL34R in my marshall 6100 I sent an e-mail to Groove tubes asking for the proper voltage for those tubes. The fact is that they answered me telling that they "DON'T" recommend that method and that the only way to properly bias an amp is with a scope. What gives?
4/2/1997 7:40 PM
The shunt method is when you parallel half the primary winding of the output transformer with an ammeter,to measure the plate current of the tube.If you don't know what you're doing it is dangerous.A good way to bias tubes would be a combination of several methods,and knowing what you are doing.
4/4/1997 2:43 AM
Zoltan, I meant the shunt method being harmful for the amp, not for me. I know what I'm doing, I just wanted to know what people think of it and why some say it's good and some (the guys at Groove Tubes) say it won't work.
4/4/1997 7:05 AM
I like using the shunt method because it's easier than inserting 1 ohm resistors onto every output tube socket of every amp i'm biasing or checking the plate current of.I always check with a scope after as I burn the tubes in,connected to a dummy load and a signal generator into the input.This lets me see if there's anything wierd going on.None of the common methods of setting or checking the bias is harmful to the amp,but,some of the points where the person adjusting the bias to might be.When I started working on tube amps,there was virtually no info on biasing available,and I used the scope method.Now there's lots of info and just as much confusion. :-)
4/6/1997 7:05 PM
Andy Ruhl

I think the shunt message is being confused here in the original post. The transformer shunt method is when you shunt the meter across the output transformer, from one leg to the center. Set your DVM to milliamps, and you're set. EL34's like to see about 40 ma, usually. Another method, the cathode resistor method, is a good one. You need to install a 1 ohm (exactly one ohm) resistor to the cathodes of the power tubes. To make a long story short, by reading the voltage in millivolts across the resistor by ohm's law is the same as plate current. And there are no voltages in the range of 500 to scare you! You can't install these resistors in every amp, though, only do it in one you plan to keep and mess with a lot. The transformer shunt method is best for general bias setting, but you have to be careful, you can really do some damage if you slip off the socket! Andy
4/7/1997 3:51 AM
j krogh

the impression that i had of the cathode resistor method is that the resistors should only be used for the bias setting.  
is it ok to leave these resistors in permanently?
4/7/1997 6:49 AM
Yes,leave them in,they have no effect on the circuit.
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