When you measure bias current of an amp without the 1 Ohm cathode resistors or external test points, do you use the shunt method from OT center tap to plate?? I personally thought this was a great way to measure bias current on older amps but I get the feeling from a lot of other tube amp builders that this is way to riskey due to opprotunity to create shorts but I think if you are extreamly carefull using insulated alligator clips, take your time observing all connections, it can be done rather safely.
Some said it would be better to temporarily solder in some 1 Ohm resistors using the Mv setting for testing, test and removed them when done.
That's the reason I bought the Fluke is to accuratly perform the shunt method when needed on vintage amps.
What's your take on this? Thanks, Platefire
When I measure current without 1 ohm resistors I am using bias adapters that I have built specifically for setting bias. I see 5-10 amps per month come across my bench and I need to be efficient with my time otherwise I will get behind.
I bought the parts to make the adapter from Doug. http://www.el34world.com/charts/BiasChecker3.htm
They are inexpensive, easy to assemble and make the process super quick to measure bias current using my flukes. That is why I have two fluke 85's...one meter for each side of the OT plate leads. I simply install the adapter into the sockets and install the tubes into the adapter socket, turn the amp on and read mA on each meter. I then take my other fluke (117) and connect that to the plate and monitor B+ vdc. I go even further sometimes and connect more meters to a hot amp and then play the amp and monitor all the voltages while I'm jamming out. It makes me feel like I can see how the amp behaves to the way I play. It's just interesting to me as I'm trying to teach myself how these amps really work....long process!
The transformer shunt method can be done safely. I've done it a few times but it kind of freaks me out. I always use tight meter leads. Installing a 1 ohm resistor is pretty quick too.
Either way works... just don't let the lead slip in the transformer shunt method!