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The attached schem is for my version of willie's 26/26/45 amp, at least I think it is. I drew it up back when I breadboarded the amp. It was working at that time and sounding good. I am now building the final version and I have run into a snag.
The cap on the cathode resitor of the 45 on the right channel blew up on me when I was bring up the B+ voltage after having checked the filament voltages and adjusting them. I looked everything over and could not see any errors so I thought just maybe it was a freak bad cap. Put in another and kept a close eye on it this time and sure enough it started getting hot and the end bulged a tad before I shut it down.I am guessing it was on it's way to blowing again.
So my first question is a general one. Does anyone see any thing in the schem that would make this a non-functional circuit? I might have made a mistake copying it down as I built the breadboard.
My second question, if the circuit appears alright, what would be likely to cause problems with the cathode cap? I checked the hum pot and the resistor and they seem ok. Iam puzzled once again.Correction!!
Just noticed that I put in the operating points for the 45 and said they were for the 26. The 26 points are actuallyplate volts 105vdc
plate current 6mA
grid volts -4.5
cathode resistor 820 ohms
bypass cap is 100uf/35vdc
Edits: 04/18/16 04/18/16 04/18/16Follow Ups:
This can be a few things, cap voltage rating too low, shorted 45 drawing too much current, thus over voltage on cap, cap installed backwards.. don't like how cap is so close to resistor either, better to not get it so hot.
As a troubleshooting exercise cut cap out of circuit and measure voltage across bias resistor. It should bias up exactly the same, under static (no signal) conditions.
Tested the tube, no shorts. Cap direction is correct. cap voltage rating is higher than the original schem called for, and none of the other caps are having a problem and they are all the same. Cap and resistor are already out. I will try to jumper the resistor in and see what I get across it.
Edits: 04/18/16 04/18/16
There will be no voltage across the resistor if you jumper across it. It will also overheat and maybe blow the tube, assuming that is the cathode resistor.
I should have said jumper to it instead of across it.
I think he meant jumper leads to put the resistor back into the circuit, for testing purposes.. this layout isn't bad, I really like the use of bus wire as a ground down the middle, it's very RCA.
Thanks for the kind words. I have grown found of a central ground bus, very handy. A stripped secftion of #12 house wire works great.
Vinnie,
Q1 : Have you DCed three stages, OR, is there a cap couple between Stage 1 and 2, not drawn in?
Q2 : HOW can you be coming into the 125ESE output at 375 VDC and OUT ( on the plate of the 45 ) at 260 VDC ????
You "normally" can not use electrolytics as an Rk bypass, as they are IN the SET's signal path. Needs to be all film caps, and since NO cap plays all the ranges well, it surely needs to be MULTIPLE bypassed in my experience.
Jeff Medwin
Opps! There should be a 0.1uf silver mica cap shown between both the first and 2nd and the 2nd and third stages. both are between the 22k and 470 k resistors.
Not sure about the 260 volts out of the opt. That must be an error. I thought it was a measured voltage, but I guess one of the two is incorrect. can't check it until I get the cap to behave.
Edits: 04/18/16
Is that a hum pot in the cathode circuit?
It almost appears like you have the cap in series with one of the filaments.
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.
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.
--------------------------
Buy Chinese. Bury freedom.
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