I'm finishing up an Ampeg B18N that is getting a 3 prong cord, new filter caps, new 7027A tubes and one new 6SL7 tube, plus a new speaker. I had no issues with the power cord changeout or the filter caps. I love that they include the schematics on the inside plate of the amp.
I'm now checking the bias on it. I am getting 536V on the plate, with -56.4VDC on pin 5. I'm using a socket bias probe to measure the cathode current thru the two power tubes and each is 57mA. With the plate voltage of 536V, that's 30.5W per power tube. If I increase the 47k resistor to make it more negative on pin 5 (-62V), I get about 40mA and plate voltage increases (as expected) to 547V. Excluding the initial 600V cap on the first stage, the cap can is only rated to 525V per leg. Got any suggestions to lower the initial voltage, or how to bring the bias a little cooler while keeping the cap can happy? Thanks!
I'm now checking the bias on it. I am getting 536V on the plate, with -56.4VDC on pin 5. I'm using a socket bias probe to measure the cathode current thru the two power tubes and each is 57mA. With the plate voltage of 536V, that's 30.5W per power tube. If I increase the 47k resistor to make it more negative on pin 5 (-62V), I get about 40mA and plate voltage increases (as expected) to 547V. Excluding the initial 600V cap on the first stage, the cap can is only rated to 525V per leg. Got any suggestions to lower the initial voltage, or how to bring the bias a little cooler while keeping the cap can happy? Thanks!
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