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previous: Mike T I just fired it up and something is... -- 12/27/1997 4:21 PM View Thread

Re: Cathode Bias Idling Current

12/27/1997 11:28 PM
StephenRe: Cathode Bias Idling Current
50mA per tube should be working _close_ to optimum - so that's unlikely to be the direct cause of your problem. Check screen volts. Check that pins 1 and 8 are linked on EL34 sockets.  
1 Ohm current monitoring resistors are NOT fitted or jumped from cathode(s) to ground, and NOT in parallel with 250 Ohm Rk. Either of these is a recipe for large tube currents and rapidly approaching meltdown :-)  
For "permanent" installation, diss each o/p tube cathode lead and insert a 1 Ohm resistor in each lead. So one end of each 1 Ohm goes to each pin8 (6L6) or pins 1+8 (EL34) Other ends of 1 Ohm R's join together and go to the top of 250 ohm shared Rk. Bottom of 250 Ohms goes to ground as usual. *Imagine a Y shape*....Any bypass cap remains across the 250 ohm.  
Additional biassing effect of the 1 Ohm R's is not usually significant - it's less than the normal tolerance of your 250 Ohm Rk. Measure separate tube currents _across_ 1 Ohm R's, NOT to ground.  
You can make temporary current sniffers (bias probes?) from octal plugs and sockets glued together, with all pins and sockets wired like to like, except for pin 8 and socket 8 which are linked by a 1 Ohm resistor.  
Extend thick wire stubs or tags out from pin 8 and socket 8 so you can clip on your DVM leads. Make preferably two of these adaptors. You can plug them between the o/p tubes and the sockets, and monitor individual currents without doing any mods to the amp. You can buy these things, but they are easy enough to make.  
The 1 Ohm R can go inside, but if it fails, replacement will be a pain. I use tiny 1/4 watt metal film 1 Ohm R's, and mounted them on the OUTSIDE of the adaptors, directly across the meter clip-on points. Makes a useful emergency fuse, and easily replaced if it blows or drifts off-spec.  
I am sure this must be in Keen's tube FAQ somewhere. If this is not included, I could do a drawing and a tidier explanation, if people think it would be a useful addition.  
For same tubes, in this kind of p-p setup, the no-signal volts across the shared Rk is _roughly_ constant. It's a balancing act - a kind of negative fedback which stabilises the total cathode current (as long as your tubes are similar and in good condition)  
As V g-k is approx constant, the value of Rk controls total cathode current. . Fender did apply a separate grid bias _and_ cathode resistor(s) on some of their later amps, but these seem to be generally disliked by amp fanciers - whether because of the mixed bias, or for other reasons, I don't know .  
Cheers, Stephen.  

Mike T Thanks, Stephen. Cheers to you too... -- 12/28/1997 7:30 AM
Doc Stephen:Thanks for the extr... -- 12/30/1997 5:02 PM