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I just replaced the filter capacitors in my Princeton (stock 20+20+20+20 replaced with 40+20+20+20) and I am getting very high voltage readings on the plates. My meter first showed "OL" (500VDC range). I just checked again and got 470 and seems to be staying there. The schematic lists 420 and before I replaced the caps I measured 390 and then 360 when the capacitors acted up. The AC into the rectifier was at 325 or 340 (schemo says 340). Now it's at 355. Is this a problem? Will this increase the output and damage the O/T? No smoke or burning smells yet. What are the dangers of the extra voltage?
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the higher voltages are likely due to the fact that your old caps were leaky and were sucking voltage. you're not likely to have any problems due to the higher voltages, as long as everything is rated to handle the voltage, and as long as you make sure your power tubes are biased in a safe range. -
The voltages are considerably higher on the power tubes (474 to 420) and pin 1 of V1 has 220 (schematic says 190). No smoke yet. Right now the tubes are overbiased. I figured the old caps and old rectifier were losing me some voltage but now I'm worried that I might be overstressing parts. I'm paranoid because these modifications started with a minor adjusment to the tone stack and escalated. I'd hate to think I modified an excellent amp into a paperweight.Comment
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When you say the power tubes are "overbiased" are you saying the negative voltage is lets say -50 when it should be closer to -40?If so when you raise the current the volts will drop.There is no reason that a recap should raise the volts so much,but if your power tubes are not drawing enough current the volts will rise.Comment
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Voltages better!! After after lowering the bias (it was at -55) to 45, the plate voltages dropped (424. where they're supposed to be). Is this a normal function of bias or does it have to do with the Princeton design? The bias supply is tied into one of the legs of the PT secondary that go to the rectifier.
Oh, thanks for the quick reply Andy!Comment
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That's exactly what happened. I brought the bias to -45 volts and it sounds right! Thanks for the quick and informative replies, Andy and Stokes.Comment
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None of the power supplies in a guitar amp is regulated. The more powr you demand of it, the lower its voltage will drop. The higher the bias voltage, the LESS current the tube conducts, so less draw from the power supply. Turn the bias voltage down means the tube current goes up, which means more power is drawn from the supply and its voltage sinks. All very normal, and expected, and will work the same way on any model amp. And that is why if one ever powers up his amp with the tubes not in it, all the B+ voltages will be WAAAY high.Education is what you're left with after you have forgotten what you have learned.Comment
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Yes I bet your old caps were drawing current and you unloaded the PT a bit with the new caps. I'm assuming you replaced the bias filter cap at the same time too... another reason the bias voltage went more negative.
As Enzo mentioned, with the unregulated supply, the secondary Hi-V rose a bit too... thus increasing the Hi-V AC to the bias diode, which created an over bias condition.
This is common.Comment
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Yes, I did replace the bias filter cap as well as add resistance to get the bias more negative. It didn't occur to me that the idle current would drop the plate voltage significantly and I didn't want to install the new tubes underbiased. It also just occured to me that my old rectifier is probably still good.Comment
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Hey I've just got a question about this, I was doing a search on the topic because I'm experiencing a similar issue after replacing the caps in a jcm800 2203. Except before I had replaced them I was reading about 450-500v on the filter caps now I'm reading over 1000v after replacing them. It seems really likely that my caps were bad and drawing current as the amp was blowing brand new power tubes but double the voltage just seems too high? the caps are rated at 500v, they are totem poled though, should I be concerned?Comment
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You should not see your B+ double with new caps.Something aint right,reading over 1000v's is way too high.Better check the schem and make sure you wire it right.Comment
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yeah I'm going to have to do that, but I have already triple checked at this point, I even took pictures of the wiring before I disconnected anything so I could go by that if need be.
Can you think of anything in particular that might cause this? my only two thoughts are this:
the 5k6 resistors that go across the filter caps that are totem poled, their leads got a bent around a bit trying to get everything back into place when I re-soldered everything,
and also I am running the amp with no tubes, enzo said the b+ would be waaay high? is this beyond way high? or is it just way too high if I were running the tubes in there. since the caps are stacked like that the max would be 1000v no?Comment
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Or your meter is freaking out.Education is what you're left with after you have forgotten what you have learned.Comment
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Are you willing to put the old tubes back in and try it. As soon as I put the tubes in mine and brought the bias in range, the voltages went from out of range (500 volt meter) to 425 where the schemo says they should be. Have you checked the voltage on the PT secondary? The only causes of increased voltage I can think of are voltage leakage no longer present, removal of resistance that causes voltage drop, or increase in supply voltage. Could the PT have the wrong taps engaged?Comment
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