I need help troubleshooting a EL84 push pull amp. - diyAudio
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Old 24th June 2005, 08:18 AM   #1
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Question I need help troubleshooting a EL84 push pull amp.

I've just bought a second hand power stage. The circuit is a push pull with EL84, ECC83 phase inverters and ECC82 pre-driver. There are 2 problems:

A - The left channel is working, but the volume is very low.
B - The right channel sounds normal, but it makes white noise (even with the volume turned down).

I have tried the following:

1 - Check all connections.
2 - Change all valves for new ones: no difference here.
3 - Check all the voltages: they are very simmilar in both channels.
4 - Adjust the bias according to the manufacturer specifications (the instructions are labeled inside the amp).
5 - Remove the ECC83 valves: the noise is still there!. So problem B is not coming from the driver stage; it is some component of the output stage that is noisy (and it's not the valve) any clue here?

Thank you very much.
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Old 24th June 2005, 09:56 AM   #2
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I've made a new test:

I've swapped the right/left output transformers: no difference here, so the two problems are not in the output transformers.

A curious thing: one of the many times that I've switched on the amp, the noise was not there! and it was still working (with the left channel very low, anyway) I've already "touched" all the parts while the amp is working, re-solder all the points to make sure that there are no weak connections, but I can't reproduce that situation again; the noise is back and constant
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Old 24th June 2005, 10:02 AM   #3
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Replace the coupling capacitors on the control grids of the output tubes. It might be a good idea to replace all coupling capacitors in the amplifier.
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Old 24th June 2005, 10:35 AM   #4
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Thank's Frank. I'll start with it.

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Old 24th June 2005, 11:51 AM   #5
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It could be a good idea to change all electrolytic caps too.
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Old 24th June 2005, 12:16 PM   #6
SY is offline SY  United States
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FWIW, the output transformers are the least likely culprits. But I guess you know that now.

After replacing the coupling and electrolytic caps, if things aren't working, it's time to do some basic voltage checks. Maybe even before starting to replace caps... Check the anode voltages on the EL84s. They should be in the 275-330V range. If there's a cathode resistor/electrolytic cap parallel combo in the EL84 cathode circuit, check the voltage across it- it should be something like 10V. Check voltage drops across the plate resistors of the 12AX7s- the voltage drop in volts should be something like 50-100% of the plate resistor's value in Kohm.

A scope will, of course, be a very powerful tool, especially for tracing the oscillation in the right channel.

If you've got an analog voltmeter and know EXACTLY what you're doing, you can load the amp's output with an 8 ohm power resistor as a dummy load, stick the voltmeter across it, then use your finger as a signal source causing the meter to swing when signal gets through. It's easy then to find where the signal stops. If you don't know EXACTLY what you're doing, this test method could cause you great harm, maybe death, but it's an old repairman's trick.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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Old 24th June 2005, 02:40 PM   #7
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use your finger as a signal source

I prefer to use a sine wave from the PC soundcard

I've found the solution to problem A (low volume at left channel):
After long time checking the path of the signal with the scope, it was finally a short circuit at the speaker connector!

Now I'll concentrate fighting problem B. The noise is there about 3 of every 5 times that I switch on the amp (after cooling down)

Thanks for your help
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Old 24th June 2005, 05:28 PM   #8
SHiFTY is offline SHiFTY  New Zealand
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Its a solder joint, or bad tube connection.

Try wiggling the small signal tubes around with the amp on... Also try tapping them with a pencil, could be a noist tube.

If that doesnt work you could try, VERY CARFULLY, running the amp upside down. CAREFULLY, with a plastic pen or the plastic side of a screwdriver, push on various components until you find the source of the noise.
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