I had a problem yesterday. All of a sudden, my tube amp stopped working. The fuse blow.
I changed fuse and the problem came again. I checked with a amp meter and I blow the 10 amp fuse on that one also. I disconnected the heaters, and started again, same thing.
The transfo is a hammond 207HX with B+ and heater voltagte on the same unit. The B+ and heaters are on separate switches. All of the time, the B+ was off.
I'm thinking I may have a short on the primary side of the transfo. How can I test that? Is that a common problem?
Thanks
F
I changed fuse and the problem came again. I checked with a amp meter and I blow the 10 amp fuse on that one also. I disconnected the heaters, and started again, same thing.
The transfo is a hammond 207HX with B+ and heater voltagte on the same unit. The B+ and heaters are on separate switches. All of the time, the B+ was off.
I'm thinking I may have a short on the primary side of the transfo. How can I test that? Is that a common problem?
Thanks
F
Shorted primaries are fairly uncommon in my experience, but not unheard of, shorted high voltage secondaries are not at all uncommon particularly if the transformer is sized a little on the small side for the load current.
Measure the primary winding resistance with a multi-meter. Hopefully the Hammond Spec sheet includes this information.
The other possibility is that there is a filament wiring short. Check this carefully.
For trouble shooting purposes a lamp in series with the power to the amplifier is a good idea. It will prevent damage to the transformer if the transformer is not the culprit, although from your description it most likely is. For cheap and cheerful, not to mention really quick just use a 100W bulb or more and a cheap extension cord, cut just one of the wires in the cord and connect a lamp socket in series.
If the transformer failed it is quite likely that it is slightly to significantly overloaded, and incidentally most shorts in modern power transformers seem to occur in the high voltage secondary which has the highest voltage stress and is also often running hot due to the large non-sinusoidal currents in this winding. Even though the B+ was turned off if the transformer is energized a short in the winding will result in large currents flowing in that winding.
It might not be a bad idea to replace the existing transformer with one having a higher current rating if one of the windings shorted.
Measure the primary winding resistance with a multi-meter. Hopefully the Hammond Spec sheet includes this information.
The other possibility is that there is a filament wiring short. Check this carefully.
For trouble shooting purposes a lamp in series with the power to the amplifier is a good idea. It will prevent damage to the transformer if the transformer is not the culprit, although from your description it most likely is. For cheap and cheerful, not to mention really quick just use a 100W bulb or more and a cheap extension cord, cut just one of the wires in the cord and connect a lamp socket in series.
If the transformer failed it is quite likely that it is slightly to significantly overloaded, and incidentally most shorts in modern power transformers seem to occur in the high voltage secondary which has the highest voltage stress and is also often running hot due to the large non-sinusoidal currents in this winding. Even though the B+ was turned off if the transformer is energized a short in the winding will result in large currents flowing in that winding.
It might not be a bad idea to replace the existing transformer with one having a higher current rating if one of the windings shorted.
Thanks Kevin
A secondary short is also possible, I will check resistance of windings and post, but the Hammond specsheets do not specify the resistance of windings.
I doubt that I have a filament short (defect tubes) because I did test with filaments unconnected and it was the same problem.
I will completely remove the transfo and test alone. Thanks for the Light bulb tip.
It is possible that the transfo was a little bit too small. I had some slightly audible hum from it and it was getting hot. Is that a symptom?
Anyway, thanks for all the information.
I may replace with a bigger unit, if it fits...
F
A secondary short is also possible, I will check resistance of windings and post, but the Hammond specsheets do not specify the resistance of windings.
I doubt that I have a filament short (defect tubes) because I did test with filaments unconnected and it was the same problem.
I will completely remove the transfo and test alone. Thanks for the Light bulb tip.
It is possible that the transfo was a little bit too small. I had some slightly audible hum from it and it was getting hot. Is that a symptom?
Anyway, thanks for all the information.
I may replace with a bigger unit, if it fits...
F
Hi,
I'll gues you already done it, but you did pull the tubes and then check? Not so uncommon that the output tubes devolpe a short, or the rectifier tube might do it. I know that EZ35(6X5), EZ90(6X4), EZ80(6V4) and EZ81 are famous for doing that...
Good luck, let's hope that the mains tranny is not shot.
Best regards,
Magnus
I'll gues you already done it, but you did pull the tubes and then check? Not so uncommon that the output tubes devolpe a short, or the rectifier tube might do it. I know that EZ35(6X5), EZ90(6X4), EZ80(6V4) and EZ81 are famous for doing that...
Good luck, let's hope that the mains tranny is not shot.
Best regards,
Magnus
I did not try without tubes. But I ran out of fuses.
I will buy some more fuses and try without the tubes. Good idea.
I didn't think it was the tubes since I did not apply any B+ power to the tubes nor any heaters voltages, they were disconnected.
Only when I put power to the main transfo, fuse blow.
But still, I will test evevrything. Thanks
F
I will buy some more fuses and try without the tubes. Good idea.
I didn't think it was the tubes since I did not apply any B+ power to the tubes nor any heaters voltages, they were disconnected.
Only when I put power to the main transfo, fuse blow.
But still, I will test evevrything. Thanks
F
If the transformer buzzes loudly and gets hot enough that you can't touch it, this is usually a good indication of either a undersized power transformer or a circuit defect. Depending on the insulation class if the core reaches 60C this may be too hot as well. (Class A insulation)
The symptoms you describe are what you would expect in varying degrees from an undersized transformer, but might also be the result of poor quality control. If the transformer is bad, I seriously recommend trying to find one in the Hammond range with the right voltages and a significantly higher current rating.
The symptoms you describe are what you would expect in varying degrees from an undersized transformer, but might also be the result of poor quality control. If the transformer is bad, I seriously recommend trying to find one in the Hammond range with the right voltages and a significantly higher current rating.
Gary,
Hold off on the fuses. Perhaps some more testing without applying power.
I don't know the 207hx, perhaps 270hx? If so, there is a 5V winding that you might not be using. What is it doing? If you turn off B+ and heaters (6.3 only, I am guessing), a shorted 5V winding could easily blow fuses.
I don't know how you wired up the B+ switch. You could have >500v on that switch, which over time will give you trouble.
I am sure your amp is not spec'd for 10A fuses. Do not put in larger fuses. Between 1 and 2 amp slo-blo is probably all you need.
Hold off on the fuses. Perhaps some more testing without applying power.
I don't know the 207hx, perhaps 270hx? If so, there is a 5V winding that you might not be using. What is it doing? If you turn off B+ and heaters (6.3 only, I am guessing), a shorted 5V winding could easily blow fuses.
I don't know how you wired up the B+ switch. You could have >500v on that switch, which over time will give you trouble.
I am sure your amp is not spec'd for 10A fuses. Do not put in larger fuses. Between 1 and 2 amp slo-blo is probably all you need.
good guess, It is a 270HX. Sorry, i made a typo.
The 5 volt winding is curently not used and wires are insulated.
I have 2 amp slo-blo. My voltmeter has a 10 amp fuse for current measurements and I blew that one when trying to measure current draw from the amplifier.
This is the B+ power supply schematic. I have 275-0-275 secondary. The switch seems OK right now. A short in the Hi-voltage secondary or any secondary can cause this problem.
F
Attachments
With switch 2 open (B+), ohm out B+ to ground before the diodes. Should be pretty high if not infinite.
The orange drop before the diodes could be shorted. I doubt it, but
With the unit unplugged from the wall, ohm out the primary to ground. Should be pretty high, if not infinite.
Ohm out the heater taps to ground before the switch. Should be high if not infinite.
Double check the 5V are insulated and tied off.
Somewhere, juice is getting to ground and you need to find it.
The orange drop before the diodes could be shorted. I doubt it, but
With the unit unplugged from the wall, ohm out the primary to ground. Should be pretty high, if not infinite.
Ohm out the heater taps to ground before the switch. Should be high if not infinite.
Double check the 5V are insulated and tied off.
Somewhere, juice is getting to ground and you need to find it.
I had a similar fuse-blowing episode with an amp,and it took me a little while to track it down and figure it out.
One of the heater wires had rubbed-through the insulation right where it came out of the transformers end-bell... a few wraps of black-tape fixed it right up.. (I was too lazy to take apart the tranny and put new wires on it.. 🙄 ) It was tricky to find,cause you couldn't really see it,unless you looked close.
One of the heater wires had rubbed-through the insulation right where it came out of the transformers end-bell... a few wraps of black-tape fixed it right up.. (I was too lazy to take apart the tranny and put new wires on it.. 🙄 ) It was tricky to find,cause you couldn't really see it,unless you looked close.
Some results
OK guys, thanks for the help so far.
I made some resistance measurements. I tried to disconnect the windings as much as possible but not always since I don't want to disconnect everything right now.
270HX hammond transfo:
Primary coil resistance: 1.7 ohm
Primary vs gnd = inf.
275 volt secondary resistance: 58 ohm
275 volt sec. vs. gnd: inf.
6.3 volt secondary resistance: 0 ohm
6.3 volt sec. vs. gnd: inf.
5 volt sec. : 0 ohm
5 volt sec vs gnd: inf.
There is also a second transformer for 120 volt and 12.6 volt supply.
120 volt primary: 77 ohm
120 volt primary vs. gnd: inf.
120 volt secondary: 120 ohm
120 volt sec vs gnd: 5 Mega ohm (inf.)
12.6 volt secondary resistance: 3.6 ohm
12.6 volt vs gnd: 1.5 Mega ohm+
The 0 ohm value for the heaters windings is weird. But the 1.7 Ohm resistance for the main 120 volt primary seems small also. What do you think....
F
OK guys, thanks for the help so far.
I made some resistance measurements. I tried to disconnect the windings as much as possible but not always since I don't want to disconnect everything right now.
270HX hammond transfo:
Primary coil resistance: 1.7 ohm
Primary vs gnd = inf.
275 volt secondary resistance: 58 ohm
275 volt sec. vs. gnd: inf.
6.3 volt secondary resistance: 0 ohm
6.3 volt sec. vs. gnd: inf.
5 volt sec. : 0 ohm
5 volt sec vs gnd: inf.
There is also a second transformer for 120 volt and 12.6 volt supply.
120 volt primary: 77 ohm
120 volt primary vs. gnd: inf.
120 volt secondary: 120 ohm
120 volt sec vs gnd: 5 Mega ohm (inf.)
12.6 volt secondary resistance: 3.6 ohm
12.6 volt vs gnd: 1.5 Mega ohm+
The 0 ohm value for the heaters windings is weird. But the 1.7 Ohm resistance for the main 120 volt primary seems small also. What do you think....
F
Attachments
A shorted diode will cause the fuse to blow regardless of the position of the HV switch. Check them with ohm meter, or test again with them disconnected. If one is dead, replace them both. The other one got zapped with the full secondary current. It will die later.
I just tested a 270ax, and the 6.3v taps are 0.2 ohms, so no big worries on that. It all looks good to me. I'd go with Tubelab and check the diodes.
diodes
Silly question, but how do you measure diodes. With on ohmmeter, I get high values both ways (like 5 megaohm one direction, and 30 megaohm the other way).
anyway, i guess that shorted diodes will be 0 ohm, so it's ok if I get high values....
F
Silly question, but how do you measure diodes. With on ohmmeter, I get high values both ways (like 5 megaohm one direction, and 30 megaohm the other way).
anyway, i guess that shorted diodes will be 0 ohm, so it's ok if I get high values....
F
I just ohmed out a 270 FX which is slightly smaller and I got slightly higher readings. This shows that your readings are normal. This does not mean that the transformer is good (or bad) since the insulation could be breaking down under power. Power up the transformer with all secondaries disconnected. If the fuse blows, it is toast. If it doesn't, leave it on for about 1 hour. It should not get hot (slightly warm is OK) or make any smoke, or burnt smell. If it passes this test and puts out the correct voltage it is OK.
These things do fail, but it is uncommon if it has never been overheated. The only one that I have fried (without gross overloading or incorrect hookup) had unknowingly gotten wet during a hurricane. It lived for a month after that and then one day it smoked, then blew the fuse. The amp was playing normally even as the transformer was smoking! When I removed it from the amp there was rust under it. I took it apart, and it was full of rust. The window above the amp had leaked during the storm.
These things do fail, but it is uncommon if it has never been overheated. The only one that I have fried (without gross overloading or incorrect hookup) had unknowingly gotten wet during a hurricane. It lived for a month after that and then one day it smoked, then blew the fuse. The amp was playing normally even as the transformer was smoking! When I removed it from the amp there was rust under it. I took it apart, and it was full of rust. The window above the amp had leaked during the storm.
It appears your meter is not accurate enough to measure very low values of winding resistance...
Also measureing DC resistance will not show anything...since there is nothing symmetrical with respect to DC...
Just DISCONNECT all wires from the transformer....ONLY connect the primaries to a variac and bring it up slowly.... All other wires need to be floated in the air...
ALso make sure the insulation on the 5v windings has not been compromised...
You could also have a shorted cap somewhere....
Chris
Also measureing DC resistance will not show anything...since there is nothing symmetrical with respect to DC...
Just DISCONNECT all wires from the transformer....ONLY connect the primaries to a variac and bring it up slowly.... All other wires need to be floated in the air...
ALso make sure the insulation on the 5v windings has not been compromised...
You could also have a shorted cap somewhere....
Chris
An analog meter should read in the low ohm region in one direction, and open in the other, on the lowest ohm range, RX1.
Many digital meters do not always read a diode correctly on the ohm scale, they usually have a diode check function. A good diode will read somewhere near .600 in one direction, and maximum in the other.
A HV diode can test good on a meter that uses a 9 volt battery and still break down when the high voltage is applied. This is uncommon though.
Many digital meters do not always read a diode correctly on the ohm scale, they usually have a diode check function. A good diode will read somewhere near .600 in one direction, and maximum in the other.
A HV diode can test good on a meter that uses a 9 volt battery and still break down when the high voltage is applied. This is uncommon though.
It makes me remember that I drop some water accidentally on the amp when watering the plants about 3 weeks ago. But I did not care too much since I swapped it right away and it was not much...I do not remember if the main trans had some water on it.
Well see, I'm disconnecting it completely in the next few days and i'll put power on it. Thanks. enough for tonight.
F
Well see, I'm disconnecting it completely in the next few days and i'll put power on it. Thanks. enough for tonight.
F
A little water should not hurt much, unless it soaked into the transformer. In my case it was a lot of water. I drained about half a cup of water from my laptop computer which was on the desk next to the amp. That was almost 2 years ago, it still works! The battery was dead, and I let the computer dry in the sun for 2 days before powering it up. I didn't know that the amp got wet, until it died.
Main transfo OK
Hi, here is the follow up.
I disconnected all the secondaries on the main transfo, and power up. Fuse did not blow, so the mains transfo seems OK. Great news ($$$)
Now I reconnected only the B+ secondary, and the fuse went out. So I know that this part of the circuit is defective. I suspect the diodes, since I did heat them for soldering a few times, and one of them may have failed. I will rebuild the high voltage circuit all over again and check everything. I will improve some other things at the same time.
These diodes are 1000 volt 1 amp rating. The transfo is 275-0-275, so they have 550 volt RMS across them, that means 775 volt peak. I have maybe 125 volt max at the primary sometimes, so that put us at 837 volt peak. That's very close I guess. Is my calculation correct?
F
Hi, here is the follow up.
I disconnected all the secondaries on the main transfo, and power up. Fuse did not blow, so the mains transfo seems OK. Great news ($$$)
Now I reconnected only the B+ secondary, and the fuse went out. So I know that this part of the circuit is defective. I suspect the diodes, since I did heat them for soldering a few times, and one of them may have failed. I will rebuild the high voltage circuit all over again and check everything. I will improve some other things at the same time.
These diodes are 1000 volt 1 amp rating. The transfo is 275-0-275, so they have 550 volt RMS across them, that means 775 volt peak. I have maybe 125 volt max at the primary sometimes, so that put us at 837 volt peak. That's very close I guess. Is my calculation correct?
F
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