Strange bias

[Amp Kat]

Not only is the current sense method easy on some amps but it's safe and that's what I'm all about. Not only safe for you but safe for your OT. Rob I do agree it's easy on some amps to install 1 or 10 ohm whichever you prefer but on the 5150 it's not an easy chore because for one you have to take the board off because the traces are on the botton side and it is rivoted and each tube pin soldered so you either unsolder all of the pins or drill out the rivots. Your not done yet because theres other problems. You have to cut the traces which for me really isn't a biggie because I do it all the time when isolating circuits but then theres another deal. On the 5150 the last tube isn't directly tied to the other grounds so you have to jump that last tube back to the V3 ground and cut the trace. While your doing that you may as well just tie all pins 1 & 8 together and install 1k 5 watters so if you do want to try EL-34's at least you have the protection in there. Then you can rivot the board back in or use screws and then your done.

[stokes]

Rob,another way,and the way I learned to do the "shunt method"is to connect the red lead of the miliammeter to the ct or first filter cap and the black lead to the plate and just read the current that is shunted to the meter,because the internal resistance of the meter is lower than the internal resistance of the OT primary,current will take the path of least resistance..Most fenders will work fine this way,some Marshalls wont and I am sure there are other amps OT that wont work for the same reason the Marshall,I think because the dc resistance of the Marshall OT is lower than the Fender OT's and all the current wont go thru the meter on these OT's.

[Enzo]

I agree, to me "shunt" method means putting the CURRENT meter in parallel with the OT half winding. The current is SHUNTED through the much lower resistance of the meter around the relatively higher resistance of the OT winding. SO with probes set for ammeter, one goes to CT and the other to plate.

To me using the resistance of the actual OT winding and checking voltage across it is just a different way if saying sense resistor. Series current is the same throughout, so it doesn't matter if you do it at the plate or the cathode. Yes, correct for the couple ma the screen adds. Nothing wrong with it, I just wouldn't refer to it as a shunt measure.

[Rob Mercure]

Stokes, Enzo - yeah, you're right as I remember a long discussion on the subject on the old board a few years ago and both methods were referred to - I had forgotten the more direct approach. Probably cuz I've never, as best I recall, used it - guess I'm just conservative in my approach and spoiled cuz a few years ago I purchased several hundred 1 ohm, 1% 5W metal film mil-spec resistors at an army surplus store for less than five dollars so I really like to provide "permanent" bias test points for myself and other techs. Being in a small town it's likely that I'll see the same amps again and again over the years.

Rob