5E3 clone volume drop

monkeybanana

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Hi all,

I built a 5E3 years ago and it has been working perfectly until today. I am only getting about half the volume now. I was playing this morning and poof volume dropped. I did hear a slight pop last night and thought it was from a tube. Both channels behave this way. Volume, tone, and low input work as they should. Everything checks out so I don't know where else to look.

Here is what I have done so far:
-tried a different speaker
-swapped tubes,
-checked voltages (see chart below)
-made sure coupling caps were not leaky
-checked continuity for each wire and ground connections I also jumped any under board connections in case they were broken
-chopsticked around and looked for bad solder joints and pulled on wires to see check for bad connections
-checked socket pins
-unsoldered primary OT leads made sure they were not open, secondary shows continuity
-electrolytics tested fine with my ESR meter but I swapped them anyways

What else can I check before I rebuild this amp?
FA2D4C9D-DBC6-4868-BB89-D3DFB7C0D189.jpeg
 

corliss1

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Plug in a nice sine wave test signal and measure AC voltage along the signal path. See if that drops anywhere. This is basically what using a scope or signal tracer would show.

(assuming your meter is smart enough to read AC where DC is present...some do, some don't)
 

2L man

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Power tube cathodes should have same voltage!

If OT input has 385V and Anodes are 382V and 374V there is something strange. Some difference comes when inside half of the coil has shorter length of wire and bias current cause lower voltage drop but 3V against 11V is way too big difference. Possibly coil has a short?. Measure primary half resistances!!!

Cathode bypass capacitor might have failed? It drop output but not to half though and it is new.

Measure filament AC voltages between sockets as well.
 
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schmee

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Check your heater wires at the tube sockets. maybe you have a cold solder joint there. Especially the power tubes. Usually, if you do, wiggling the heater wires will produce a loud 'pop'
 

monkeybanana

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Power tube cathodes should have same voltage!

If OT input has 385V and Anodes are 382V and 374V there is something strange. Some difference comes when inside half of the coil has shorter length of wire and bias current cause lower voltage drop but 3V against 11V is way too big difference. Possibly coil has a short?. Measure primary half resistances!!!

Cathode bypass capacitor might have failed? It drop output but not to half though and it is new.

Measure filament AC voltages between sockets as well.
Thanks that helpful. The 250 resistor measured okay so maybe it is the transformer I was really hoping that's not the case!. I don't remember the resistance but I was a able to get a reading at the primary so I thought I was good. I will sub in another transformer when I get a chance.

The filament AC is okay and reaching all the tubes.
 

monkeybanana

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Check your heater wires at the tube sockets. maybe you have a cold solder joint there. Especially the power tubes. Usually, if you do, wiggling the heater wires will produce a loud 'pop'
Unfortunately the heater wires are not the culprit. I measured at al the nodes and pulled on them when I chopsticked around.
 

2L man

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Thanks that helpful. The 250 resistor measured okay so maybe it is the transformer I was really hoping that's not the case!. I don't remember the resistance but I was a able to get a reading at the primary so I thought I was good. I will sub in another transformer when I get a chance.

The filament AC is okay and reaching all the tubes.
Measure OT primary half resistances from B+1 to power tube anodes socket 3. There is 150...250 ohms / half on typical 5E3 OT. If you don't get solid readings it might come when multimeter resistance measuring send current pulses and OT inductance ruins the resistance measures. Then put loudspeaker to output socket and it should change inductance so that primary resistance readind comes solid.

Primary half resistances should be only max 25% apart. First coil half diameter comes smaller when it is wound first and second half is wound over the first needs longer length of wire and its resiatance is higher, although wound count is the same. Usually HiFi OT coils are wound to few layered sections and primary halfs can have same resistance. Some instrument OT as well but this increase their price when winding work takes bit longer time.

If resistance difference is higher than 25% there might be shoted coils if wire insulation has broken. This is possible to test quite easy without changing new OT. If amp has a HV switch secure tape it to Off position. If there is also HV fuse remove it. Sometimes HV fuse is on PT CT circuit and removing does not prevent HV to come to amp circuits but HV switch should prevent this. My 5E3 Deluxe has two switches.

Remove power tubes and bend two about 1mm thick wire pieces to U-shape and connect filament socket 2 to anode 3 on other power tube socket and 7 to 3 on another. Now when you switch only the Mains switch On the 6,3VAC filament voltage connects between OT primary ends. You should hear quiet mains hum from loudspeaker. Now measure what the filament AC voltage is between socket pins 2 and 7. Then measure what is the voltage from other filament to B+1 which is OT supply and verify other filament to OT CT. They should be exactly half the filament voltage. If they are not half there is shorted wound on OT primary which has lower than half the filament voltage.
 
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monkeybanana

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Measure OT primary half resistances from B+1 to power tube anodes socket 3. There is 150...250 ohms / half on typical 5E3 OT. If you don't get solid readings it might come when multimeter resistance measuring send current pulses and OT inductance ruins the resistance measures. Then put loudspeaker to output socket and it should change inductance so that primary resistance readind comes solid.

Primary half resistances should be only max 25% apart. First coil half diameter comes smaller when it is wound first and second half is wound over the first needs longer length of wire and its resiatance is higher, although wound count is the same. Usually HiFi OT coils are wound to few layered sections and primary halfs can have same resistance. Some instrument OT as well but this increase their price when winding work takes bit longer time.

If resistance difference is higher than 25% there might be shoted coils if wire insulation has broken. This is possible to test quite easy without changing new OT. If amp has a HV switch secure tape it to Off position. If there is also HV fuse remove it. Sometimes HV fuse is on PT CT circuit and removing does not prevent HV to come to amp circuits but HV switch should prevent this. My 5E3 Deluxe has two switches.

Remove power tubes and bend two about 1mm thick wire pieces to U-shape and connect filament socket 2 to anode 3 on other power tube socket and 7 to 3 on another. Now when you switch only the Mains switch On the 6,3VAC filament voltage connects between OT primary ends. You should hear quiet mains hum from loudspeaker. Now measure what the filament AC voltage is between socket pins 2 and 7. Then measure what is the voltage from other filament to B+1 which is OT supply and verify other filament to OT CT. They should be exactly half the filament voltage. If they are not half there is shorted wound on OT primary which has lower than half the filament voltage.

I measured the primaries again and they were off more than 25%. Bummer. Swapped out the OT for another and volume is back. I did in the past turn the amp on without a speaker. It was only for a few secs but maybe it was enough to fry it.

Thanks for your help and detailed response. Back to Neil Young!
 




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