Hi cogitech, you're talking about installing the rectifier but leaving the other tubes out, right? If so that sounds like a plan (well, it's what I'd do anyway).
Yeah, but it was all in the name of, er, science. Besides, I think most of us here would be disappointed to learn otherwise!
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Yes. That's what my gut is telling me, but it is based on no real knowledge. Glad to know someone such as yourself would do the same. I am guessing there are some voltages I should test before I run the full 60 minute "smoke test" in this state.
I also think I should prop it (securely) up on its side while doing so, so if/when it starts to smoke I can pinpoint the problem.
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Absolutely. The first thing I'd do is verify all of the voltages are correct. I'd start with the plate pins on all of the tube sockets, and yes, even the heater pins (you never know).
Something else that just occurred to me: I know you said there were grommets in all of the output transformer holes, but they look a bit "tight" to me - that is, the wires appear to be a squeeze fit. That sometimes makes it tough to pull the OPT wires completely through the holes during installation, so you might verify that there's not a loop of wire sandwiched between the transformer and top plate. I did that once, and the transformer end bell "bit" into the wire, causing a short to appear. Those things sometimes lurk as latent defects, only to mysteriously (and disastrously) appear later.
Something else that just occurred to me: I know you said there were grommets in all of the output transformer holes, but they look a bit "tight" to me - that is, the wires appear to be a squeeze fit. That sometimes makes it tough to pull the OPT wires completely through the holes during installation, so you might verify that there's not a loop of wire sandwiched between the transformer and top plate. I did that once, and the transformer end bell "bit" into the wire, causing a short to appear. Those things sometimes lurk as latent defects, only to mysteriously (and disastrously) appear later.
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Would it be too much to ask for specific pin numbers? I can look at a tube pinout diagram but I don't trust myself to get it right. I also have no idea what voltages I should expect to see on any of those pins.
I am assuming my negative alligator clip would go to ground and then I would use a positive probe to touch the solder solder points of each pin (with the other hand in my pocket).
I am assuming my negative alligator clip would go to ground and then I would use a positive probe to touch the solder solder points of each pin (with the other hand in my pocket).
First off, since this has not been discussed, the SSE should have a 2 amp slow blow fuse in 120 volt countries, and a 1 amp slow blow fuse in 230 or 240 volt countries. For testing without output tubes a smaller fuse can be used to lessen the chances of collateral damage.
If I were working on this I would connect a meter to the B+ using clip leads so that the voltage can be watched. Install just the rectifier tube, no other tubes. Speakers or an input are not needed since no current should flow except for the bleeder resistors.
Set the amp on its side such that both top and bottom can be watched. Plug the amp into a power strip or other cord such that it can be turned on and off remotely.
In cases where an "event" is likely, I do these kind of tests outdoors, in the garage, or other places where good ventilation is available and the possibility of damage from smoke or flaming resistors is low. Having a suitable fire extinguisher nearby is a good idea. Note, that the probability of these events are very low, but the consequences of a small fire or even a smoke cloud in the house are not pretty....the wife really doesn't like my exploding parts, especially now when it's 35 degrees F here and opening the windows is not an option.
Power up the amp with just a rectifier tube. Watch the rectifier tube for internal sparking. If it's bad, it will usually be obvious. After a few seconds the B+ voltage should rise, then jump to nearly 500 volts. It should stay there for the duration of the test. A dropping voltage is an indication of a problem. Look for smoke, steam or other indications of a problem.
If the B+ voltage begins to drop, leave it powered for a few seconds, then shut it off. If it holds steady leave it on for a few minutes.
Wait for the B+ to drop to zero and look for hot parts. All parts should be at or near room temp except for the rectifier tube, R2 and R4. The parts near the rectifier may be slightly heated from the radiated IR emitted by the tube.
If I were working on this I would connect a meter to the B+ using clip leads so that the voltage can be watched. Install just the rectifier tube, no other tubes. Speakers or an input are not needed since no current should flow except for the bleeder resistors.
Set the amp on its side such that both top and bottom can be watched. Plug the amp into a power strip or other cord such that it can be turned on and off remotely.
In cases where an "event" is likely, I do these kind of tests outdoors, in the garage, or other places where good ventilation is available and the possibility of damage from smoke or flaming resistors is low. Having a suitable fire extinguisher nearby is a good idea. Note, that the probability of these events are very low, but the consequences of a small fire or even a smoke cloud in the house are not pretty....the wife really doesn't like my exploding parts, especially now when it's 35 degrees F here and opening the windows is not an option.
Power up the amp with just a rectifier tube. Watch the rectifier tube for internal sparking. If it's bad, it will usually be obvious. After a few seconds the B+ voltage should rise, then jump to nearly 500 volts. It should stay there for the duration of the test. A dropping voltage is an indication of a problem. Look for smoke, steam or other indications of a problem.
If the B+ voltage begins to drop, leave it powered for a few seconds, then shut it off. If it holds steady leave it on for a few minutes.
Wait for the B+ to drop to zero and look for hot parts. All parts should be at or near room temp except for the rectifier tube, R2 and R4. The parts near the rectifier may be slightly heated from the radiated IR emitted by the tube.
Sorry George I was typing my last response to Mr. Zenith as you were posting.
I will take your advice and follow it precisely. I'll be back in a while (maybe a couple of hours) with the results.
By the way, the only fuses I have available in this size are these 0233002.MXP Littelfuse | Mouser Canada
They are 2A medium/normal blow 125v. I bought them before I understood that 250v fuses are actually what I really needed.
I will take your advice and follow it precisely. I'll be back in a while (maybe a couple of hours) with the results.
By the way, the only fuses I have available in this size are these 0233002.MXP Littelfuse | Mouser Canada
They are 2A medium/normal blow 125v. I bought them before I understood that 250v fuses are actually what I really needed.
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For the B+ test George described above you have two options. The first is to test the B+ at the positive lead of C1, or failing that, the B+ binding post screw to one of the output transformers. The other option is to take the reading from pin 3 of either output tube. Someone please correct me if I'm wrong.
In either case, you should just use alligator clips for both leads so you can monitor the voltages hands-free during testing.
I know it's frustrating, but hang in there. You'll get it figured out.
In either case, you should just use alligator clips for both leads so you can monitor the voltages hands-free during testing.
I know it's frustrating, but hang in there. You'll get it figured out.
Those who have followed my antics for 15 years know that I'm not afraid of blowing stull up. I started playing with electricity at age 4 or 5 when the paper clip met the wall outlet, and haven't given it up yet. My childhood adventures centered on making guitar amps out of discarded TV's and radios.....parts were free, and even the smoke clouds provide knowledge....of how not to do something. This was not the case when I started blowing up car engines, or with your $800 amplifier.
Now nearly 60 years later, I understand the risks in all of my experiments, and the likelihood of things going wrong, and take the necessary precautions to mitigate any risk of serious damage, injury or expensive toasted parts. In the case of 1500 volts and cheap Radio Shack clip leads, there was 1/4 inch of Lexan between me and the amp experiment and a fire extinguisher between my feet.
At this point, with only the rectifier tube present, the only measurement of value is the B+ voltage. Since there were no obvious signs of fried parts, I expect this to go as expected, a steady voltage near 500. If it remains there for several minutes, all is OK. Power off and watch the meter. It should drop to near zero in a minute or two. Remember how long it took to reach a safe value, and ALWAYS wait that long before messing with the amp, and ALWAYS unplug the amp before messing with it. I make it a point to remove the power cord and set it on the chair. That way I have to make a conscious two step effort to turn it back on.
The next step would be to introduce the 12AT7 and rerun the test. The two easiest voltages to get here are on the cases of the CCS chips. The voltage on the heat sink or screw will tell us how much current is flowing in the 12AT7 tube. It should be about 100 volts less than the B+ and should also hold steady once the amp has warmed up.
Yes, or you can clip the negative lead to ground, and use another clip lead on the positive lead to the screw or heat sink, power up, take the reading, wait a couple minutes for the voltages to discharge, move the clip to the other chip and repeat.
By the way, the ground scheme baffles me. George's page/diagram states that the board itself is grounded through a ground wire that connects to one of the input signal grounds. This is exactly the way I have it configured.
Does this mean the DC current is all dumping to ground via that single connection? I find this shocking, but it must be true because I can measure B+ voltage (450v) between star ground and R4 of the board. I am using a single strand of CAT-5E for this. How can this be?
Does this mean the DC current is all dumping to ground via that single connection? I find this shocking, but it must be true because I can measure B+ voltage (450v) between star ground and R4 of the board. I am using a single strand of CAT-5E for this. How can this be?
All of the heavy current stuff is handled ON the board itself. The ground wiring caries zero current in normal operation. If say an OPT would internally short out and attempt to put B+ on the chassis or speakers, THEN this FAULT current would go through your wiring and blow the fuse. Even a single strand of CAT-5 wire can safely blow the fuse.....so your wiring is good.
In all of my years of messing with tube stuff, some of it dating back to the 1920's, I have seen exactly two such failures, and they were both in old stuff that lived in the Florida humidity. Most transformers then user ordinary paper for insulation....some still do.
Sorry to be such a pain in the butt, but I just have to try to understand this. How can my voltmeter measure 450v between R4 and star ground if there is no current?
Wait a second... asking the question again just made it click. I think.
The voltmeter itself is creating a circuit between R4 and ground. Duh.
I will follow the steps as you have outlined them above, then again with a sacrificial 12AT7 installed, and report back later today.
Thanks so much for everyone's help! I am feeling much better about this today and realize now that it is an opportunity to learn.
Haha... you're not being a pain, you're learning! So am I now that I think about it; it's a win for all of us.
Actually, I believe that current flows through R4 (the "bleeder resistor for C2) and R3, which is there to "elevate" the tube heaters to prevent violating the heater-to-cathode voltage ratings for the power and input tubes. If my calculations are correct, 450 V / (150,000 ohm + 10,000 ohm) comes out to about ~2.8 mA. Of course there's also some current flowing through R2 (the C1 bleeder), but we're not concerned about that for this test.
The current drawn by your DMM should be miniscule, otherwise it would load down the measured circuit and skew your results.
Actually, I believe that current flows through R4 (the "bleeder resistor for C2) and R3, which is there to "elevate" the tube heaters to prevent violating the heater-to-cathode voltage ratings for the power and input tubes. If my calculations are correct, 450 V / (150,000 ohm + 10,000 ohm) comes out to about ~2.8 mA. Of course there's also some current flowing through R2 (the C1 bleeder), but we're not concerned about that for this test.
The current drawn by your DMM should be miniscule, otherwise it would load down the measured circuit and skew your results.
Right, the two resistor paths (R3 + R4) and R2 are the only wanted complete circuits for current to flow at this time.
Technically when the meter is connected, it IS a complete circuit, and a few microamps does flow through it. It is a resistor in the megohm range and the current through this resistor is what is being measured, and then converted to a voltage by the chip in the meter.
This does not ordinarily affect the circuit being tested, but it can, and will if the circuits being tested normally operate in the microamp range.
This was the case in the Fender guitar amp circuit that I played with as a kid. I had built a nice amp that only worked when my meter was connected into it. There was no internet in the early 60's, only library books, and the older brother of a schoolmate who was a ham radio guy. He explained that my 1950's vintage meter was really a 200K or 2 megohm resistor depending on what scale I was using. I just needed to stick a similar sized resistor into my amp.....and it worked.
Technically when the meter is connected, it IS a complete circuit, and a few microamps does flow through it. It is a resistor in the megohm range and the current through this resistor is what is being measured, and then converted to a voltage by the chip in the meter.
This does not ordinarily affect the circuit being tested, but it can, and will if the circuits being tested normally operate in the microamp range.
This was the case in the Fender guitar amp circuit that I played with as a kid. I had built a nice amp that only worked when my meter was connected into it. There was no internet in the early 60's, only library books, and the older brother of a schoolmate who was a ham radio guy. He explained that my 1950's vintage meter was really a 200K or 2 megohm resistor depending on what scale I was using. I just needed to stick a similar sized resistor into my amp.....and it worked.
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