Hi,
I have one of those Chinese EL84 tube amplifiers running for about a month now. This morning I heard a crackling sound from my right speaker before it went silent. I looked at the amplifier and noticed the corresponding tube was glowing brighter than the rest. I switched the amp off, changed in another replacement tube and the same thing occurs. The speakers are fine (tested them).
So what would be the cause of something like this?
Thanks.
I have one of those Chinese EL84 tube amplifiers running for about a month now. This morning I heard a crackling sound from my right speaker before it went silent. I looked at the amplifier and noticed the corresponding tube was glowing brighter than the rest. I switched the amp off, changed in another replacement tube and the same thing occurs. The speakers are fine (tested them).
So what would be the cause of something like this?
Thanks.
Sounds like a bad connection at the control grid (pin #2). Pull the tube out and measure pin 2 resistance to chassis ground while power is off.
A failed component (!)
to fix this :
dismantle the amp to get access to it's inner parts. Examine for burned or
damaged components .If none found :
-try to swich tubes between channel to see if the problem follows tubes.
- with schematics in hand, power up ( don't forget to connect a load) and
measure voltages , prefferably with a vtvm. As one channel is working
a golden opportunity is there : compare voltages between the channels.
Any substantial differences pinpoints a possible part of the circuit.
Depending on the circuit, examine the components around the suspected part
for problems.
- when suspected component / connection found ; fix it.
reassemble and verify function
Note that schematics is a essential part of troubleshooting. Of cource you
bought the amp including proper documentation ? If not i suggest you
find a technitian that will repair your amp.
Hi all, thanks for your replies. I was given this schematic
I desoldered the resistors and it seems like they check out on my multimeter. I then soldered them back. When I measured pin 2 for the output tubes, the good side reflected 47kohm to chassis, but on the bad side it was 46. I then tried powering on the amp, switching tubes but the same issue persists (on a dummy load)
The right tube would glow brighter and brighter. At some point there was a blue flash.
I desoldered the resistors and it seems like they check out on my multimeter. I then soldered them back. When I measured pin 2 for the output tubes, the good side reflected 47kohm to chassis, but on the bad side it was 46. I then tried powering on the amp, switching tubes but the same issue persists (on a dummy load)
The right tube would glow brighter and brighter. At some point there was a blue flash.
You'll want to measure the DC voltage from ground to the tube side of the 240 Ohm resistor on each side. They will almost certainly be different.
After that, check the DC voltages at each connection on the EL84 socket. I would be curious about the DC voltage at the junction of the 470K resistor and 0.33uF coupling cap.
After that, check the DC voltages at each connection on the EL84 socket. I would be curious about the DC voltage at the junction of the 470K resistor and 0.33uF coupling cap.
I would just go ahead and change out the two 0.33V cap to something 300+V, like 400V or so. The way that they are connected now, during the power up period, these two cap will see their voltage limit. May be one of them has worn out over time and start leaking.
That is easy to find out with a simple voltage measurement.
To me changing components randomly without any analysis is not smart at all.
This is best that can be done now.
You will want to measure cathode (pin#3) to ground resistance before you measure DC volts that way you can make sense of the DC reading. Measure the resistance with the power off.
In my experience, if a tube is glowing the cathode is grounded (on a cathode biased amp). Check the cathode bypass cap. Also as others have states check the 0.33uF cap, too. If it's shorted your output tube will not be happy at all.
Typical operating voltages can be found in the philips application note :Hi all, thanks for your replies. I was given this schematic
I desoldered the resistors and it seems like they check out on my multimeter. I then soldered them back. When I measured pin 2 for the output tubes, the good side reflected 47kohm to chassis, but on the bad side it was 46. I then tried powering on the amp, switching tubes but the same issue persists (on a dummy load)
The right tube would glow brighter and brighter. At some point there was a blue flash.
https://frank.pocnet.net/sheets/010/e/EL84.pdf
See page 3 top. Here one has 210ohm as cathode resistor, you have 240
What could be confusing is that g1 has a voltage of -8.4V ,
this is related to the cathode. When measuring in you amp this translates
to app 8.4V on the cathode ( pin 3 and the 240ohm resistor)
See what you have on cathode ( pin3) anode ( pin 7 ) and compare between
the channels.
Don't try to measure g1 ( pin 2 ) unless you have a vyvm or very high impedance voltmeter, the values are meaningless. But do check that
the gridresistor is 470kohm .
If these seems similar between the channels and there is a "pop" or hum
in the speakers when you touch pin3 , and it has similar pop or hum
between the channels the output is ok. Then concentrate on the triode.
Thanks everyone for your kind suggestions and help. I'll get some replacement caps and post back once I manage to do some readings.
Hi I took some readings as suggested:
- VDC reading for tube pin #3/240R side to ground: 8.11VDC for R, rises to 10.8VDC for L (good side)
- VDC reading for 0.33uF cap to 470kR gnd: quickly shoots pass 2000mV VDC for R, rises steadily to 14-63mVDC for L
- Pin #3 to ground R: 240ohm for R, 240ohm for L
I also noticed the 100uF on the amp is rated for 35V instead of the 50V as on the schematic.
Should I go ahead and order a new pair of 0.33uF caps to replace the current ones?
- VDC reading for tube pin #3/240R side to ground: 8.11VDC for R, rises to 10.8VDC for L (good side)
- VDC reading for 0.33uF cap to 470kR gnd: quickly shoots pass 2000mV VDC for R, rises steadily to 14-63mVDC for L
- Pin #3 to ground R: 240ohm for R, 240ohm for L
I also noticed the 100uF on the amp is rated for 35V instead of the 50V as on the schematic.
Should I go ahead and order a new pair of 0.33uF caps to replace the current ones?
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Hi I took some readings as suggested:
- VDC reading for tube pin #3/240R side to ground: 8.11VDC for R, rises to 10.8VDC for L (good side)
- VDC reading for 0.33uF cap to 470kR gnd: quickly shoots pass 2000mV VDC for R, rises steadily to 14-63mVDC for L
- Pin #3 to ground R: 240ohm for R, 240ohm for L
I also noticed the 100uF on the amp is rated for 35V instead of the 50V as on the schematic.
Should I go ahead and order a new pair of 0.33uF caps to replace the current ones?
2000mV on the grid ( pin 2) is way to high. Change 0.33 uF and el84 tube
( a sane tube should have more then 8V on cathode (pin 3 ) when the grid is
positive)
The 0.33uF cap should be more then 250V rating, use 400V.
The 35V electrolyte is fine, the cathode won't be more that about 10V.
Thanks, I just ordered the replacement caps. I have another EL84 tube that I will swap out once the caps arrive. Will post back again then.
While you are waiting for a new set of 0.33uf. Take the right channel old one off. The grid voltage (pin 2) should fall back to near zero if that is your only problem.
What was the cathode voltage with the coupling cap not connected?
Something still seems off. Looking at the average plate characteristics the left side looks good; 10/240=41mA which looks normal @ -10 grid volts. But the right channel @ -6v at the grid (8 on the cathode and 2 at the grid) should be 65mA, but the tube only has 33mA passing through it.
Something still seems off. Looking at the average plate characteristics the left side looks good; 10/240=41mA which looks normal @ -10 grid volts. But the right channel @ -6v at the grid (8 on the cathode and 2 at the grid) should be 65mA, but the tube only has 33mA passing through it.
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