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vibrochamp bias

12/1/1997 5:36 PM
marc vibrochamp bias
The question really is if an OP transfomer over heats enough to melt out some of it's wax will it change its resistance/impeadance?  
The Vibro-Champ(AA764-production 2- says the tube chart) I am working on burnt up it's power tube cathode resistor-2W 470 ohm- when I replaced it with a 489 Ohm unit it was pulling 44.5ma and the plate was 374V. I upped the resistor to 766 ohms and the cathode now pulls 36.5ma Plate 426V. This is still above the plate watts of 14 for a 6V6. that is a 60% increase in cathode resistance ...does this sound right? did I miss something?
12/1/1997 8:24 PM

FWIW, I've seen two Champs recently, both of which had bad cathode bypass caps, but 470ohm 5 watt resistors. On both, the cap was doing around .5uf basically waaaaay off. After fixup, they both did about 45ma thu the cathode.  
One thing to remember is that the plate voltage for calculation, should be in reference to the cathode. On those amps, the cathode ran about 24 volts above ground. This also prompted me to use 25ma@50volt caps instead of 25@25 as the drawing shows. It's also easy to get the cap a safer distance away from that hot resistor (like right about over the mounting screw for the board).
12/2/1997 5:31 AM
How are you guys measuring your cathode current? With the meter in series? If so, what about the internal resistance of your meter. Is this taken in consiteration?  
-just trying to learn.
12/2/1997 9:33 AM
Jim S.

Measuring current draw through a cathode resisitor is easy: Set your meter to measure DC volts, connect the black probe to chassis ground and connect the red probe to the cathode (pin 8 on the 6V6 socket). The DC volts you measure is the voltage drop across the cathode resistor. Using Ohm's law, divide that number by the value (in ohms) of the cathode resistor. The result is the current (in amps) being drawn through the cathode. For example, if you measured 23.5 volts across a 470 ohm resistor, the current draw would be .050 amp (or 50 milliamps).
12/2/1997 6:27 PM

>>How are you guys measuring your cathode current? With the meter in series? <<  
I use a probe that routes the cathode thru a ammeter. In that manner, yes, the cathode and meter are in series. No correction for the meter is needed. You could also do the resistor/voltage drop method. I believe it's easier to just read the milliamps directly.  
12/2/1997 9:25 AM
Melting wax out should not affect the resistance or AC impedance (at least not to low audio frequencies, although the high frequency self capacitance can change) to any significant degree.  
UNLESS some windings have shorted together, bypassing some of the copper wire turns.
12/2/1997 12:16 PM

This doesn't address your two questions directly, but it looked like an appropriate place to share some related information.  
First of all, it seems everyone refers to plate voltage (actually plate to cathode voltage) as being the major factor in gauging the performance of a tube circuit. It is, along with the plate current, used to calculate the power dissipation flowing through the plate, and has to do with ultimate power output/headroom available from a particular tube type, but in a pentode (which is the tube of choice in guitar amp output stages) the plate voltage doesn't directly influence the plate current as it does in a triode. The plate is the anode, where the main electron stream is collected, but it is the screen grid which is the accelerating electrode and it's voltage directly influences the plate current flow.  
Getting back to silverface champs, Fender kept increasing the power supply high voltage every few years to get more power & volume out of their smallest amp which pushed the 6V6 beyond the tube manufacturer's intended durability limits. I have encountered some champs and vibrochamps that worked but had worn down US made 6V6's, which would fry a russian replacement tube. It's just due to the conservative design of the real 6V6's that allowed them to run in an excessive dissipation state and not self-destruct. The russian tube meets the textbook (tube manual) published ratings, but won't take much more.  
What I did to remedy this situation was to reduce the screen voltage down to about 350 volts, and left the plate voltage alone (which was usually over 400v). This brought the tube's current back down to where the 470 ohm cathode resistor was a suitable value. I did not attempt to reduce the maximum B+ of the power supply. The screen voltage was reduced using a voltage divider, along with the normal series grid current limiting resistor (absent on most champs). The shunt resistor needs to be large enough not to bleed too much power supply current.  
With this mod, the russian 6V6 could be used without turning it into a heating element and the amps actually sounded better than with the higher screen voltage.  
What I'm wondering is for all the interest in ways to select reduced power modes, why not vary the screen grid voltage (within certain setpoints) on the outout tubes with an adjustable voltage divider?  

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