Ok, I think the amplifier has a problem. I put new 6P14P tubes in all four output sockets. I used the probes on my meter, set to the milliamps range, to measure from pin 7 to pin 7. On the right channel I get 0 milliamps. On the left channel, I get 26 milliamps, with a visible spark as the probes touch the pins. This is the same result that I saw when I first powered up the amplifier with the Chinese 6P14 tubes, and tube P6 blew after 48 hours of use.
If I pull out the driver tubes, I get the same problem. If I put a pair of the still-good Chinese output tubes in that channel, I get the same problem. As the tubes warm up over the course of 30 seconds or so, the current increases steadily to 26 milliamps. I tested and matched all of the resistors and capacitors before installing them. All tested good and within 1% or less. I cleaned all of the flux off of the board with alcohol, so it's nice and clean. all four output transformer primary halves measured about 152 ohms.
What is causing this problem? This is really frustrating, and I don't want to blow a new set of output tubes (again). 🙁
Here are the actual component values and the actual test voltages:
If I pull out the driver tubes, I get the same problem. If I put a pair of the still-good Chinese output tubes in that channel, I get the same problem. As the tubes warm up over the course of 30 seconds or so, the current increases steadily to 26 milliamps. I tested and matched all of the resistors and capacitors before installing them. All tested good and within 1% or less. I cleaned all of the flux off of the board with alcohol, so it's nice and clean. all four output transformer primary halves measured about 152 ohms.
What is causing this problem? This is really frustrating, and I don't want to blow a new set of output tubes (again). 🙁
Here are the actual component values and the actual test voltages:
Last edited:
Can you record all the voltages on each of the pins to ground. Can also be useful to measure resistance to ground with the tubes out and no power.
All voltage measurements with tubes in place:
All resistance measurements with tubes out:
All resistance measurements with tubes out:
Last edited:
The plate voltage for pin 7 for P6 on the schematic is not correct. The voltage for P7 is high, but might be linked to the other issue.
I presume one channel is ok?
Assuming you built it with the PCB, then I suppose 2 scenarios are possible: an error with the wiring from the board to peripherals, or something due to not using the magic eyes.
If the other channel is ok, can you compare measurements between the channels and spot a wiring mistake?
I presume one channel is ok?
Assuming you built it with the PCB, then I suppose 2 scenarios are possible: an error with the wiring from the board to peripherals, or something due to not using the magic eyes.
If the other channel is ok, can you compare measurements between the channels and spot a wiring mistake?
- I haven't played music with it with the new output tubes that I put in today. I was afraid to try it when I saw the same difference between pins 7 on the channel that developed the discolored output tube (left channel). So, all of this is being tested with large 8 ohm power resistors across the speaker outputs.
- The amplifier played music and sounded pretty good for the first 48 hours of continuous operation before the P6 tube (left channel) became discolored and the sound started popping. Both channels were still playing music at that point, but popping.
- The right channel hasn't ever showed any problems and it still played music just fine with the two left channel output tubes removed once P6 on the left channel discolored.
Here is the schematic with the magic eye tubes. None of those components have been installed.
If you really have no anode voltage on P6, that explains the melt-down. Don't run this with valves installed until that problem is solved. Might want to redo your OPT primary resistance measurements as a starting point.
All good fortune,
Chris
All good fortune,
Chris
Confirmed. Left output transformer is bad maybe? Which came first - the bad tube or the bad transformer? If that is the problem, which one killed the other? New Chinese output transformers cost about $140 per pair for an amplifier kit that cost about $350 plus $50 of parts - I asked and a seller will not sell just one transformer, only a pair.
Too late. I had to test the voltages.
I'll have to do that with the transformers in place (power off, tubes removed, speaker outputs shorted), but I'll use the directions someone posted earlier to do that now.
I have a sneaking suspicion that if I connect speakers I will get no sound on the left channel this time. I'm afraid to try it.
This amplifier played music just fine for 48 hours until this problem started. No idea what went wrong.
There's not much to go wrong with a transformer to cause a winding to open - it's just a piece of enameled wire, but might have internal connections between sections, if fancy enough. If you can get a look inside the covers, you might see something. Factory QC is bound to have been spotty at best.
But first, redo the solder joint at the valve socket. Just in case.
All good fortune,
Chris
But first, redo the solder joint at the valve socket. Just in case.
All good fortune,
Chris
Confirmed: bad output transformer on left channel, tube P6.
Apparently, it did anyway. The question now is did the tube blow the transformer or did the transformer blow the tube? I checked, and the primary wire is still connected to the terminal on the transformer, so no problem there. The amplifier was on overnight and not playing music when this happened. In the morning I heard popping. I tried it, and the amplifier played music in both channels, but with a whole lot of nasty popping.
I guess this $400 brick is headed for the garbage dumpster. The AliExpress seller is the absolute worst I have ever seen, so chances of getting a replacement from them are zero. Another seller will only sell me a pair of the 10k output transformers for $140, and I'm not doing that. I'm not throwing good money after bad. Expensive mistake, both time and money, I guess. 🙁
Apparently, it did anyway. The question now is did the tube blow the transformer or did the transformer blow the tube? I checked, and the primary wire is still connected to the terminal on the transformer, so no problem there. The amplifier was on overnight and not playing music when this happened. In the morning I heard popping. I tried it, and the amplifier played music in both channels, but with a whole lot of nasty popping.
I guess this $400 brick is headed for the garbage dumpster. The AliExpress seller is the absolute worst I have ever seen, so chances of getting a replacement from them are zero. Another seller will only sell me a pair of the 10k output transformers for $140, and I'm not doing that. I'm not throwing good money after bad. Expensive mistake, both time and money, I guess. 🙁
Last edited:
If the tube took the transformer out, it likely would have red plated first. Or R22 would have gotten blazing hot. The B+ would have been dragged down. Nasty smell. There would have been some telltale sign something was up before the winding opened up.
Sounds to me like one side of that trafo was open from the get-go. Of course with no plate connected the screen was going to take all the current, producing the blast pattern.
Rather than brick it I might be inclined to find a cheap transformer (or two) if you don’t want to pay the $140. Something in this power range might actually work with a $20 power toroid as the OPT. Voltage is a little high, which only means LF response would be limited to 60 or 80 Hz. It might not be better than that with the originals anyway. If you can find where the original went open circuit without tearing it to pieces I’d try that first.
Sounds to me like one side of that trafo was open from the get-go. Of course with no plate connected the screen was going to take all the current, producing the blast pattern.
Rather than brick it I might be inclined to find a cheap transformer (or two) if you don’t want to pay the $140. Something in this power range might actually work with a $20 power toroid as the OPT. Voltage is a little high, which only means LF response would be limited to 60 or 80 Hz. It might not be better than that with the originals anyway. If you can find where the original went open circuit without tearing it to pieces I’d try that first.
The amplifier worked and sounded great for 48 hours before this happened, so the left transformer was at least functional, and I measured the resistance values before assembly as shown in this picture. However, I did note the ~26mA difference problem between pins 7 and 7 on the left channel immediately from the start. I don't know if that was originally caused by the tube or by the transformer. The tube I assume blew the transformer somehow. DC resistances for the halves seemed fine before assembly:
When I woke up and heard the popping noise, the tube was not red plated. It looked normal except for the discoloration inside the middle of the tube shown earlier.
I am absolutely baffled. I have had a number of tubes red plate in my Dynaco ST-70 series ii, and it never has blown an output transformer since 1992 with hard, daily use for 20 of those years.
As you can see in the picture, any replacement transformer would have to be an identical replacement. There is exactly zero room for anything else. I have no tolerance for an amplifier that blows an output transformer every time a tube goes bad. It's not worth it. So if that's the problem ... into the garbage it goes as an expensive lesson in both time and money.
I am directly in touch with the manufacturer via another seller tonight to see if they can provide a single replacement transformer. they wanted to know the DC resistances. However, as I said, I have no tolerance for an amplifier that blows an output transformer every time a tube goes bad. It's not worth it.
When I woke up and heard the popping noise, the tube was not red plated. It looked normal except for the discoloration inside the middle of the tube shown earlier.
I am absolutely baffled. I have had a number of tubes red plate in my Dynaco ST-70 series ii, and it never has blown an output transformer since 1992 with hard, daily use for 20 of those years.
As you can see in the picture, any replacement transformer would have to be an identical replacement. There is exactly zero room for anything else. I have no tolerance for an amplifier that blows an output transformer every time a tube goes bad. It's not worth it. So if that's the problem ... into the garbage it goes as an expensive lesson in both time and money.
I am directly in touch with the manufacturer via another seller tonight to see if they can provide a single replacement transformer. they wanted to know the DC resistances. However, as I said, I have no tolerance for an amplifier that blows an output transformer every time a tube goes bad. It's not worth it.
You’d have to short cathode to plate to kill an output transformer instantly. Even if you red plated it would take TIME to burn out the transformer and you would have noticed before it did. Fuse the cathode circuit and its protected, both against long and short term overload. A quarter amp fuse and you‘re good to go - never have to worry about frying transformers. Open the cathode, and no plate or screen current.
Many push pull amps will run, ie, play music with one tube pulled. Not at full power, but at low enough power you might not have noticed. It would act the same with one side of the trafo open. When it was popping the tube was already beyond hope from too much screen current caused by the disconnected plate.
I had an old Fisher that had to be scrapped because red-plating Philips ECG 7591’s eventually took out the irreplaceable power trafo. Both OPTs were fine, and ended up being used in a build fairly recently. Fusible cathode resistors this time, of course.
Many push pull amps will run, ie, play music with one tube pulled. Not at full power, but at low enough power you might not have noticed. It would act the same with one side of the trafo open. When it was popping the tube was already beyond hope from too much screen current caused by the disconnected plate.
I had an old Fisher that had to be scrapped because red-plating Philips ECG 7591’s eventually took out the irreplaceable power trafo. Both OPTs were fine, and ended up being used in a build fairly recently. Fusible cathode resistors this time, of course.
So which part went wrong first given that I knew there was a ~26mA difference problem between pins 7 and 7 on the left channel immediately from the start? Which is to blame? The transformer or the tube? If it likely was a bad transformer, I can deal with replacing it once (and only once). If this amplifier is going to blow a transformer every time a tube goes bad, I'll just put it in the trash. If I should have known not to operate the amplifier with that ~26mA difference problem between pins 7 and 7 on the left channel, then MY BAD and I should learn why I shouldn't do that again (tell me why please because I'm a novice). I can test for that issue when replacing tubes every time if needed.
Note: I was asleep at the time this happened. The amp was left on overnight to let the tubes burn in (seems now literally). No red plate when I woke up. No smell anywhere then or now sniffing the transformer, and I have a sensitive nose! No visible signs. The only clues I have are that pin 7-7 difference and the discolored tube.
Note: I was asleep at the time this happened. The amp was left on overnight to let the tubes burn in (seems now literally). No red plate when I woke up. No smell anywhere then or now sniffing the transformer, and I have a sensitive nose! No visible signs. The only clues I have are that pin 7-7 difference and the discolored tube.
I'd say that wg_ski's diagnosis is spot on. A pentode with no anode voltage becomes a triode with a very small anode (its G2), which then melts. As observed on the inside of its envelope. It will not "red-plate" because there's no current through the anode. Just melt the G2. "Red-screen" maybe.
The photos aren't forensic enough to be very certain, but I'd be surprised if the cause of this open primary weren't visible on the exposed terminals. Only takes a flakey solder joint to cause this misery. Preferably re-solder all terminals on both transformers.
All good fortune,
Chris
The photos aren't forensic enough to be very certain, but I'd be surprised if the cause of this open primary weren't visible on the exposed terminals. Only takes a flakey solder joint to cause this misery. Preferably re-solder all terminals on both transformers.
All good fortune,
Chris
I see absolutely nothing on the output transformer terminals. When I remove it, probably tomorrow, I'll take photos.
If I carefully break apart the discolored tube, can I see this? I have nothing to lose.
Well, that's not an option for the dead transformer as it has to be removed forever. The wires were well wrapped around the transformer terminals and well soldered in place. Could a bad solder joint where the circuit board connects, via wire, to the output transformer cause this? How on Earth would I ever know? I was very careful, and I see no visible problems with any of the solder joints. They all look perfect. The solder flowed neatly to both sides of each solder point on the amplifier and power supply boards with no problems.
I hate to let unknown "gremlins" destroy what otherwise could have been a great little amplifier. I also can't let this become a money pit, throwing good money after bad. If there isn't a way to determine a definite problem and definite resolution, I may just toss it out.
Last edited:
We're not there and can't know what you're measuring or where. This can all be resolved with an Ohmmeter. Somewhere between B+ and the anode there's an open. If it's hidden somewhere deep within the transformer's windings, you're toast. If not, it's easily fixable.
With one multimeter connection to B+, walk away stepwise towards the valve socket's anode connection. Where do you first see the open? The problem is immediately before that point.
All good fortune,
Chris
edit: obviously, powered down, unplugged and cold
With one multimeter connection to B+, walk away stepwise towards the valve socket's anode connection. Where do you first see the open? The problem is immediately before that point.
All good fortune,
Chris
edit: obviously, powered down, unplugged and cold
Last edited:
After some cursing and swearing because this amplifier is extremely cramped inside, I have removed what I believe to be the offending component. The P1 connection used to measure about 152 ohms:
P1 was connected to the bad tube in this photo:
A very nice online seller actually has contacted the factory in China that manufactures these transformers. Tomorrow (their time) a person in the factory is going to see if he can locate a matching transformer. Apparently, they DO test and match them into pairs, which is why they don't really want to sell just one, only a matched pair. There is no way to know how long ago these were manufactured or if a match is available now. They immediately asked for all four DC resistance values before the problem happened. However, there is no point in proceeding if this is just going to happen again for some reason that cannot be identified. Unknown gremilns running around = bottomless money pit and endless frustration.
Yes, I AM deep into this, now having communication with the transformer factory itself in China.
All advice welcome.
Last edited:
Unsolder and remove the wiring from the P1 terminal. Scrape or sandpaper the end of the internal wire to remove any possible residual enameling that might be causing the open. Resolder, say a Hail Mary, and retest. If still open, you've done all that can be done to save it.
All good fortune,
Chris
All good fortune,
Chris