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5E7 Plate resistor

6/4/1999 7:54 AM
Ron Orlando
5E7 Plate resistor
I have recently built a 3-10 5E7 Bandmaster clone.Nice amp !! I've checked all the voltages on the amp and compared them to the original schematic..everything is reading within spec or dead nuts balls on..except for the second pre-amp tube..12AX7. The print calls for +140v and I'm seeing about 200v.. which is within spec but not by much. I'm curious to know why this stage is reading so much higher then the others..Yes, I've swapped tubes. Should I increase the value of the Plate resistor to bring the voltage down to spec? And how has this effected tone?
6/4/1999 8:47 AM

Instead of increasing the tube's load resistor, take the voltage drop with another R-C decoupling stage between the plate resistor and it's present DC source.  
You need to know the current through the tube to determine the dropping resistor size. Measure the cathode resistor (disconnect any bypass cap for the reading), then, knowing the voltage drop across it during power-on, you have the current. You'd like to get rid of (200-140)=60 volts. So multiply the current by 60, and that's the required series dropping resistor size. Put the resistor in series with the +supply and the plate resistor. At the resistor junction (outer end of plate resistor) connect a new 10uf/250v electrolytic and ground the negative end.
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6/4/1999 9:03 AM
Ron Orlando

Doc, I did find that if I increased that 100k resistor to 200k the voltage was right on. I like your idea of adding the additional resistance in series with voltage source. The source is coming off of the final filter cap (8uf 450v). Now, in the 'if it ain't broke don't fix it department' does the higher voltage here effect the tone or headroom?
6/4/1999 11:45 AM

The higher voltages place the tube in a more linear operating region. The sound will be cleaner, more accurate. The circuit may also have greater headroom, so ultimate clean voltage gain will be higher. If you prefer the sound with the 200k load resister in there, it's fine to go that route.  
I guess I kind of read into your question that you intended to have both the stage's component values and electrode voltages as originally specified. I didn't mean that it's "better" to try and keep the 100k. Many circuits use 200/220k plate resistors. There's nothing sacred about the 100k that fender used over and over. Hey, it's your amp. Experiment a little to personalize it.
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