hey guys,
as much as I've searched & read in the last 24 hrs., I still haven't found any "step by step" details for checking the bias on a vintage hiwatt. (ok, don't laugh ).
I'm new to this, but willing to jump in. I have a novice understanding of electronics, capable of soldering, minor troubleshooting, building cables, fixing FX, etc. but tube amps are new to me. I've read thru aspen pittman's book. watched weber's DVD. cruised all 3 hiwatt sites.
I have a pretty good idea about what to do to check the plate voltage & current, but wanted to know for sure before I begin...
I think most guys will tell you to remove the jumper between pin 8 to the chassis ground and soldier in a 1 ohm resistor. Then you measure the voltage drop across this resistor and this is equal to your current, V/R = I, e.g. 35 mV (voltage drop) / 1 ohm = 35 mA. The advantage of this method is that it's a relatively safe low voltage measurement. The annoying thing is you need matched 1 ohm resistors (not always easy to measure) or it will drive you crazy that the currents don't match for your measurements on each output tube. I like the following method but only do it if you're comfortable measuring high voltages, otherwise it's not worth it.
The NO MODs solution
If you don't want to add those resistors try measuring the plate current by measuring the resistance across each side of the output transformer primary, that is, between the center tap to each plate side. Then measure the DC voltage drop across each side of the OT. Your bias current is then that voltage drop divided by the OT resistance on that respective side, I = dV / dR . Simple and accurate and no mods necessary Safety note: to measure the dV across the OT, rather than measuring directly between the center tap and the plate, clip the ground probe to the chassis and measure the voltage at the center tap and on the OT plate side and then subtract the two values to find dV . This will allow you to only use one hand in the chassis at a time which is important for safety reasons. One hand in the chassis, one in the pocket... always Also, remember to measure the resistance with the power off.
Good luck
"The faster you play, the fewer woman will be at your shows" ~Scott Henderson
GigawattCustom121 wrote:I think most guys will tell you to remove the jumper between pin 8 to the chassis ground and soldier in a 1 ohm resistor. Then you measure the voltage drop across this resistor and this is equal to your current, V/R = I, e.g. 35 mV (voltage drop) / 1 ohm = 35 mA. The annoying thing is you need matched 1 ohm resistors (not always easy to measure) or it will drive you crazy that the currents don't match for your measurements on you output tubes.
I like this method for DIY builds, using 1 ohm 1% wirewound resistors and external test points - easy and you don't have to open up the amp. But you have to subtract the screen current, since the voltage drop on the cathode includes both the plate and screen.
For really accurate measurement, or for nice Hylights that you don't want to be modding, yeah, I prefer the OT primary DC voltage drop/OT primary resistance method.
You can almost feel the current flowing
You can almost see the circuits blowing
I use a Bias Probe which is basically a power tube adaptor socket with a built-in 1 ohm resistor. Just stick the banana plug in your multimeter and that's it. No mod or soldering around in my vintage Hiwatt, no siree!
). 
Safety note: to measure the dV across the OT, rather than measuring directly between the center tap and the plate, clip the ground probe to the chassis and measure the voltage at the center tap and on the OT plate side and then subtract the two values to find dV . This will allow you to only use one hand in the chassis at a time which is important for safety reasons. One hand in the chassis, one in the pocket... always 
Also, remember to measure the resistance with the power off. 
