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Old 26th May 2006, 03:57 PM   #1
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Default Help on AMC CVT 3030

Hi,

Im trying to repair an integrated tube amp (AMC CVT3030) wich uses two EL34Gs per channel. One channel was reported "not working".

The first thing I did after cleaning the amp was to have a look at the printed circuits. In one channel R524 was blown (0,25w 100kOhm connected to the wiper of a bias pot). After replacing this resistor I set the bias pots to minimum bias and turned the amp on. Nothing blew but the tubes the tubes glow quite hard.

Im afraid my knowledge of tubes is very basic and since the tubes are soldered in it is a bit difficult to try another set.

Is there a simple way to tell if those EL34 s are still OK?

thanks,

William
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Old 26th May 2006, 04:16 PM   #2
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Hi William,
Do they have any oval discolourations on the plates anywhere? I would check the pot if the resistor burned out. Also the coupling cap for leakage.

Start adjustments with the maximum negative voltage on the grids. That would be minimum cathode current.

-Chris
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Old 26th May 2006, 05:36 PM   #3
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Hi,

no discolourations, pot is ok, cap measures 5uF.

The only thing I see is that on one valve theres a small metal cap between the plates. The other valve shows two wires instead of the metal cap. The cap is about 4mm x 5mm x 2mm and forms a sort of box over those wires.

William
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Old 26th May 2006, 05:41 PM   #4
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Hi William,
Try to remove the entire module. Look at the tube with the missing box. Rotate the assembly and see if any metal bits show up, you may need to shake it a little. Look around the lead wires inside the envelope near the base.

Those tubes are an extreme pain to replace. I hope you don't have to do this. Can you post or PM a picture?

-Chris
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Old 26th May 2006, 06:25 PM   #5
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heres the one without the cap
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Old 26th May 2006, 06:26 PM   #6
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heres the other one
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Old 26th May 2006, 06:32 PM   #7
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Hi William,
Are you sure the other part isn't hiding up in the silvered portion??? I hope!

That's how they flash the tube. The compound is on the metal piece (the getter) and it's heated to form the silver coating (flashed). The getter is no longer useful, however we can't have it running around free inside the tube.

If it's found loose, you must replace both tubes (to get a pair). Install only good tubes due the the problem of removal. JJ or Electroharmonix would be my choices. Try them in another amp before soldering them in.

-Chris
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Old 26th May 2006, 06:53 PM   #8
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Hi,

yes it is. You can see a litle part of it. Question is if this is a problem or not. Are there any simple ways to measure the tubes without turning the amp on?

William
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Old 26th May 2006, 07:09 PM   #9
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Hi William,
The getter is still attached then. Good.

The only way to test a tube is either in circuit and under power (measuring the voltages on the pins) or pulling it and substituting another tube. The latter is very difficult because the tubes are soldered in. I break them into pieces and remove the pins one at a time. Much faster, and the tube is guaranteed bad after that.

-Chris
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