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In Reply to: RE: hot time in the old cap tonight.... posted by vinnie2 on April 18, 2016 at 05:58:10
But in your case, you have the electrolytic capacitor hooked up backwards. Black band goes to ground. Black band with arrows pointing to a terminal indicate negative which goes to ground. Positive goes to the hum pot.
What is going on in your circuit is that the cathode resistor is passing some current from the 45 which causes the resistor to develop voltage across it. Let's call it 40 volts for now depending on your bias. The cap is there just to eliminate the AC signal from the voltage on the cathode to get rid of the feedback. It just has to go in so that the positive goes to the tube and the negative goes to ground. Put a meter across the cathode resistor to see for yourself. The amp will work without this cap, but not sound very good.
Follow Ups:
I wish it wee that simple Chip, but I already checked for that and the caps are installed with the negative end connected to ground. All the other caps seem to be ok. It's just this one 45 cap that has the problem.
Here are two pics of the underside. More crowded than I had hoped, but was trying to repurpose the chassis from my old preamp.
One close up of the 45 hum pot with resistor and cap (left channel) that is behaving. I removed the right channel cap and resistor.
One photo of the whole bottom. You can see the six filament trans and all the wiring it took to hook everything up. Not my best effort, but I may try to clean it up after I get it operational.
All the other tubes, hum pots and resistor caps combos seem to be fine. It's just the right channel 45 that is the problem so far.
Note that the white tube resistors that go from the coupling caps to the plates are not connected in the photos. I had them tied in with clip leads when I was trying to bring it up.
Specs for the bypass cap are 100uf/63vdc. That is twice the voltage rating Willie had in his unit.
Edits: 04/18/16 04/18/16 04/18/16 04/18/16
Aluminum housed resistors are designed to be mounted to heatsinks. Your resistor is using the cap as a heat sink. Get it a few inches away.
If I see it right, the cap is a 100uF 63V? Use at least a 100V cap and keep it cool.
I think I will just go with the dog bones on all the cathode resitors. They seem to be doing fine. There was a 1/4" gap between the aluminum resistor and the cap, I thought that might be enough to allow for suffiient air cooling.
They are like the heating element in your electric oven. The radiant heat will kill, not just ambient hot air. Try 105C rated caps as well for longer life.
Thing is, if that amplifier is working at the shown operating point, the resistor should only be dissipating 1.5W. (41Vx0.036A), which shouldn't kill the capacitor right away even mounted like that.
Definitely something weird is going on here, I wonder if the B+ is way high
I have been checking voltages back and forth between the different sections and I am not happy with the amount of variance I seem to be getting. Need to search some more.
Some variation is normal but shouldn't be drastic. Begin by drawing a really accurate schematic, make a photocopy for the second channel and then record the DC voltage to ground at each of the tube terminals, the power supply points, etc. It can really help to zoom in on a Problem. One possibility is that those silver mica caps leak, causing the 45 on that side to conduct way too much and then putting a big voltage across the tube's bias reaistor and frying the capacitor. Some voltage checks with a digital multi meter or vacuum tube volt meter will get it all sorted out. The grid oh the 45 should be 0VDC.
Caps blow if polarity is incorrect or too low a voltage rating. Try a 100 + VDC cap.
Are both channels doing the same thing? What are the specs for the cap? A photo of the wiring would also help.
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