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A Princeton 65 Reissue came back from the Tonight Show, wouldn’t’ power up. Power Transformer primary open. Bought a Weber W022772 replacement, slightly taller core stack, wired up to match that for the Princeton, as it had an extra lower set of HV taps on the secondary, a C/T for the heater, and a 125V tap on the primary besides the 0 & 120V taps.
After getting it wired up, I powered it up, and found the plate current too high. Bias voltage level was -17V, and would no adjust to encompass -40V voltage (nominal) while trying to get a 23mA plate current reading (measuring across the 1 ohm cathode resistors of the 6V6GT tubes. The Intensity control was turned way up.
Turning up the Intensity control, I could get the plate current in excess of 110mA, and the AC Mains current doubled (500mA to 1A), while turning it fully CCW, it was still too high.
Removed the power tubes, unsoldered the bottom side of the bias pot (going to R2-27k to gnd), and verified the 56V zener was functioning. All the bias components measured nominal (d7 rectifier, R28 120K resistor, D4 56k Zener, 10k bias pot, R2-27k). Only by increasing the value of R2 could I get to the -40V range, which reduces the span of the bias adjustment range.
I was still getting the bias to increase substantially by turning up the Intensity control. I ended up lifting the main PCB, replaced C22 (being the coupling cap from the Tremolo circuit to the CW side of the Intensity pot). I had plenty of LF oscillator signal at R14, but with it running, I had to crank the Intensity all the way up to get any hint of modulation, while the plate current was now over 110mA So, no DC leakage thru C22 to the Intensity pot, to explain the bias going so high. I lowered the value of R14 all the way down to 20k, to get reasonable tremolo, but that made no sense.
After looking at another 65 Princeton Reverb re-issue from our inventory, and saw very little change in bias while adjusting Intensity control of the Tremolo (based on observing AC Mains wattage change from 56W to 68W), it was pointing to the Intensity control.
With the Princeton still on the test stand, I removed P18, and measured the DCR from the junction of R28 (120k), 56V zener and the bias pot string, reading about 55k (having added 20k in series with R2). When I plugged P18 back in, it dropped to around 37k. Unplugging it, and instead, jumpered the bias injection point of R18 & R19, I got no change in resistance. Leakage path on the control PCB.
Removed the control panel PCB, and measured the Intensity control terminals to ground. ~ 250k. There shouldn’t be ANY connection to ground on that pot. Pulled the pot, and the problem remained ON the pot itself. I took the pot apart, though didn’t find the connection. Put the pot back together, and now had no connection to the housing, which is at Ground potential. Put the pot back onto the board, and NOW, no change in resistance when I re-connected P18. Very good sign.
Plugged the power tubes back in, removed the added 20k resistor between R2 and the bias pot, re-connected for measuring DCV at the cathodes of the power tubes, and powered up. NOW I’m getting correct bias, and virtually NO change when adjusting the Intensity control. Giving a listen, now the tremolo function sounds like the other amp I checked out.
Bloody leakage path on the Intensity pot itself! Prior to finding the fault, with it turned up all the way, I was seeing more than twice the power consumption, and over a little while at that level, the new power transformer was so hot you couldn’t put your hand on it. Most have been what killed the power transformer!
65-princeton-reverb-schematic.pdf
After getting it wired up, I powered it up, and found the plate current too high. Bias voltage level was -17V, and would no adjust to encompass -40V voltage (nominal) while trying to get a 23mA plate current reading (measuring across the 1 ohm cathode resistors of the 6V6GT tubes. The Intensity control was turned way up.
Turning up the Intensity control, I could get the plate current in excess of 110mA, and the AC Mains current doubled (500mA to 1A), while turning it fully CCW, it was still too high.
Removed the power tubes, unsoldered the bottom side of the bias pot (going to R2-27k to gnd), and verified the 56V zener was functioning. All the bias components measured nominal (d7 rectifier, R28 120K resistor, D4 56k Zener, 10k bias pot, R2-27k). Only by increasing the value of R2 could I get to the -40V range, which reduces the span of the bias adjustment range.
I was still getting the bias to increase substantially by turning up the Intensity control. I ended up lifting the main PCB, replaced C22 (being the coupling cap from the Tremolo circuit to the CW side of the Intensity pot). I had plenty of LF oscillator signal at R14, but with it running, I had to crank the Intensity all the way up to get any hint of modulation, while the plate current was now over 110mA So, no DC leakage thru C22 to the Intensity pot, to explain the bias going so high. I lowered the value of R14 all the way down to 20k, to get reasonable tremolo, but that made no sense.
After looking at another 65 Princeton Reverb re-issue from our inventory, and saw very little change in bias while adjusting Intensity control of the Tremolo (based on observing AC Mains wattage change from 56W to 68W), it was pointing to the Intensity control.
With the Princeton still on the test stand, I removed P18, and measured the DCR from the junction of R28 (120k), 56V zener and the bias pot string, reading about 55k (having added 20k in series with R2). When I plugged P18 back in, it dropped to around 37k. Unplugging it, and instead, jumpered the bias injection point of R18 & R19, I got no change in resistance. Leakage path on the control PCB.
Removed the control panel PCB, and measured the Intensity control terminals to ground. ~ 250k. There shouldn’t be ANY connection to ground on that pot. Pulled the pot, and the problem remained ON the pot itself. I took the pot apart, though didn’t find the connection. Put the pot back together, and now had no connection to the housing, which is at Ground potential. Put the pot back onto the board, and NOW, no change in resistance when I re-connected P18. Very good sign.
Plugged the power tubes back in, removed the added 20k resistor between R2 and the bias pot, re-connected for measuring DCV at the cathodes of the power tubes, and powered up. NOW I’m getting correct bias, and virtually NO change when adjusting the Intensity control. Giving a listen, now the tremolo function sounds like the other amp I checked out.
Bloody leakage path on the Intensity pot itself! Prior to finding the fault, with it turned up all the way, I was seeing more than twice the power consumption, and over a little while at that level, the new power transformer was so hot you couldn’t put your hand on it. Most have been what killed the power transformer!
65-princeton-reverb-schematic.pdf
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