#102 died yesterday :( how,why?

[sluckey]

That's good. NEVER CHECK RESISTANCE WITH THE POWER TURNED ON! Unless you're shopping for a new meter.

[norburybrook]

That's good. NEVER CHECK RESISTANCE WITH THE POWER TURNED ON! Unless you're shopping for a new meter.

LOL, Ive a fairly decent meter with a good 10A fuse


so does that mean everything seems good for a power up now then? without tubes first.


M

[norburybrook]

replaced the resistors and the blown LED pilot light.

Fired up and bingo all working as intended


thanks for everyone's help here.

Its strange that I was getting those crazy readings.....oh well another little learning experience.


M

[pompeiisneaks]

That's good. NEVER CHECK RESISTANCE WITH THE POWER TURNED ON! Unless you're shopping for a new meter.

LOL, Ive a fairly decent meter with a good 10A fuse


so does that mean everything seems good for a power up now then? without tubes first.


M

The 10A fuse should only be for the Amps side, and a smaller one rated for the mA range, the non amps sides aren't fused I don't believe.

~Phil

[talbany]

The Carbon trace will now create a conductive path to gnd So you have to replace it or it will continue to short out under heavy loads and back EMF. Your Meg readings look OK? Replace the 120R fire it back up and see if the fuse blows and check your voltages on pin 2 and 7 and see if it comes back down to 6V AC. A couple reasons for this is either a tube went out (heater cathode short). Someone played the amp without a load or your 120 ohm resistor just failed. I've had a few cheap resistors go on me.
BTW You can sometimes scrape off the carbon on the socket but it's usually best to go ahead and replace the socket. After you get everything back up check your OPT and lets hope you didn't take that with it

Oh I just noticed you got it going cool!!
Tony

[norburybrook]

The Carbon trace will now create a conductive path to gnd So you have to replace it or it will continue to short out under heavy loads and back EMF. Your Meg readings look OK? Replace the 120R fire it back up and see if the fuse blows and check your voltages on pin 2 and 7 and see if it comes back down to 6V AC. A couple reasons for this is either a tube went out (heater cathode short). Someone played the amp without a load or your 120 ohm resistor just failed. I've had a few cheap resistors go on me.
BTW You can sometimes scrape off the carbon on the socket but it's usually best to go ahead and replace the socket. After you get everything back up check your OPT and lets hope you didn't take that with it

Oh I just noticed you got it going cool!!
Tony

Tony,

I think there was a combination of the OD tube going and speaker swapping causing an open speaker load for a moment, not sure what caused what but it took out the 120R resistor and the power LED and an arc across the 6L6 pins 2 and 3 .

all good now though and ready for Mr Clapton on Sunday


M

[sluckey]

what about the 175v AC heater readings?

I suspect that was a bogus reading. Were you maybe measuring the heater voltage with one meter lead on chassis? If so, that explains it. For sure that 175v was a bogus reading since your heater ground reference resistors were missing at the time. Some people call that a phantom voltage. To always get a true reading on the filaments put one probe on one socket filament pin and the other probe on the other socket filament pin, never from chassis to one socket pin..

[talbany]

The Carbon trace will now create a conductive path to gnd So you have to replace it or it will continue to short out under heavy loads and back EMF. Your Meg readings look OK? Replace the 120R fire it back up and see if the fuse blows and check your voltages on pin 2 and 7 and see if it comes back down to 6V AC. A couple reasons for this is either a tube went out (heater cathode short). Someone played the amp without a load or your 120 ohm resistor just failed. I've had a few cheap resistors go on me.
BTW You can sometimes scrape off the carbon on the socket but it's usually best to go ahead and replace the socket. After you get everything back up check your OPT and lets hope you didn't take that with it

Oh I just noticed you got it going cool!!
Tony

Tony,

I think there was a combination of the OD tube going and speaker swapping causing an open speaker load for a moment, not sure what caused what but it took out the 120R resistor and the power LED and an arc across the 6L6 pins 2 and 3 .

all good now though and ready for Mr Clapton on Sunday


M

I would put my money on speaker swapping or some kind of mis-matched load.BTW- A good way to prevent this from happening is a diode from pin3 OPT to ground. Dumble does this on his Music Man amps as well as Ken Fisher on his Expresses. This is a really good mod if you or anyone else runs the amp hard generating high current spikes on back EMF cycles. This will give those spikes a safer path to gnd saving your sockets
Tell Eric hello for me
Tony

[Guy77]

Glad to hear your Power transformer survived. Good old Hammonds take a licking and keep on ticking!

Its amazing how hi the AC voltages can spike on the speaker jacks. I was burning in an amp recently and had a dummy load on the speaker jack with a meter to measure the AC voltage and when I cranked the amp and strummed the guitar really hard the voltages would spike really high and maybe just for a a few seconds but i guess that is all it takes to damage the amp when there is no speaker load!

I like the idea of the diodes on the pin socket like Tony just said.

I believe using the 100R resistors instead of PT center tap actually saved the life of the core of your PT heater winding CT if it had one..

Out of curiosity what wattage were the 120R resistors you used Marcus?

Thanks for posting Marcus its posts like these that make things so informative for the rest of us.

Cheers
Guy

[Charlie Wilson]

Not really buying the fly back voltage theory but, if that were the case, you need to worry more about the output transformer than the power.
CW

[talbany]

Not really buying the fly back voltage theory but, if that were the case, you need to worry more about the output transformer than the power.
CW

CW
It's called a "flyback diode" Correct saves the OPT

https://ampgarage.com/forum/viewtopic.php?t=30328

T

[Charlie Wilson]

Thanks, yeah I got you Tony. It just seemed odd to me that a fly back voltage would only hit one side of the output transformer primary. Also, the few times I have seen it happen because of an open load, it usually kills the output transformer pretty quickly. I would suspect, as others have, that it was a cathode to plate short in one of the power tubes. The high voltage on the filaments might have killed the OD tube. I asked if it was a Belton socket because those seem to be occasionally failing(arching). However, as we know with amp repairs, anything is possible.
CW

[martin manning]

I agree most likely open secondary and the resulting flyback voltage. Once one side or the other arcs, then there is a path for the remaining energy to dissipate. I'm not a fan of flyback protection diodes. They can become damaged without your knowledge, and then you're left wondering why the amp sounds funny. Just tell the owner to put it on standby before unplugging the speaker.

The crazy heater voltage has to be due to capacitive coupling between the windings. That will be drained away once the filament winding has a ground reference as Steve says.

[mojotom]

Replace the sockets that show signs of arcing. May be a good idea to just replace both.

the arcing looks like its only from the metal pins at the top where the heater cable is wired in the top holes, the brown base is completely clear.

what about the 175v AC heater readings?

M

I could be wrong but once those two 120R resistors are open the heater winding as no reference for its middle point to ground and it will sit at a voltage different from 0V (usually positive) hence the reading and the noise.
Happened on old Ampeg V serie. When one tube goes down it takes with him the hum balance pot and the amp works but quite noisy. It happened mostly when the tube died, not because of running the amp without a load.
Could be a tube shorting out inside due to playing time and unloading the amp to swap speakers. Those new production tubes have short lifetime and can blow anytime under heavy loads, not all but you can have a bad one even after a few months without issues.

[norburybrook]

I agree most likely open secondary and the resulting flyback voltage. Once one side or the other arcs, then there is a path for the remaining energy to dissipate. I'm not a fan of flyback protection diodes. They can become damaged without your knowledge, and then you're left wondering why the amp sounds funny. Just tell the owner to put it on standby before unplugging the speaker.

The crazy heater voltage has to be due to capacitive coupling between the windings. That will be drained away once the filament winding has a ground reference as Steve says.

ah hence replacing the resistors made that drain and it was back to normal. Once I'd seen that high reading I was then reluctant to carry on with anything as I thought the transformer had shorted internally.

@Steve L, yes I was indeed measuring with reference to ground so again you were spot on.

would a 500 ma HT fuse have been any help in this situation? i.e. would it have done before the resistors and LED pilot lamp and save all this headache?

and last question to the gurus, whats the safest way to do speaker swapping/testing? we've been using standby on off but I'm wandering if in this case someone might have missed it or perhaps the amp was still charged but open when someone hits a loud note.


again, thanks chaps, very informative.

M