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Old 26th May 2017, 3:51 pm   #1
EdWilliams
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Default Bush VHF62 output valve

I have just finished restoring a Bush VHF 62 radio bought recently. Will post a success story soon.

When I first had it, I measured the voltage at the grid end of the output valve grid coupling capacitor (C65 on Trader sheet) and got a reading of +1.7 V. So C65 was leaky and I have replaced it, along with all the Hunts mouldseals and other waxies.

Now I get a reading of 330 mV at the same point. With the EL34 valve removed I measure only 10 mV. I have run the set for a few hours but the voltage doesn't seem to be changing.

The cathode voltage measures 6.2 V (Trader sheet 6.5 V) and the cathode resistor R36 195 ohms (Trader sheet 180 Ohms), so the current is a bit lower than the 35 mA on the Trader sheet.

The set seems to work well now with plenty of volume. Is there anything to be concerned about re the health of the EL34?
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Old 26th May 2017, 4:32 pm   #2
paulsherwin
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Default Re: Bush VHF62 output valve

The output valve is an EL84. An EL34 is something very different.

I wouldn't worry about a small amount of internal leakage if the valve is getting on a bit. It's likely to soldier on for years like that.
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Old 26th May 2017, 4:58 pm   #3
Boater Sam
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Default Re: Bush VHF62 output valve

EL84 and its forerunner EL41 do suffer from grid current. It is thought to be due to internal leakage.
Bad cases can sometimes be made better by sparking the g1 repeatedly with a piezo spark lighter or a charged capacitor.
At a few millivolts, don't worry, bad ones can produce tens of volts! They still seem to work.
Beware of cooking the output transformer with a really bad one.
Oddly the Russian version EL84 seems to suffer less.

Last edited by Boater Sam; 26th May 2017 at 5:00 pm. Reason: reviewed
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Old 26th May 2017, 6:38 pm   #4
DangerMan
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Default Re: Bush VHF62 output valve

The important thing is that you have seen the cathode current is stable and not excessive, so there's no threat to other components especially the output transformer.

Just enjoy the set, the leakage isn't bad enough to worry about. You might even find that it improves with extended use as the getter works most efficiently when hot and may soak up any gas that is present, especially if the set hasn't been used for many years.
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Old 27th May 2017, 2:49 pm   #5
EdWilliams
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Default Re: Bush VHF62 output valve

Thanks for the prompt responses everyone. I will keep an occasional eye on the EL84 but expect all will be well.

Don't know why I typed EL34 - I even looked up the type before posting.

I will write a proper success story for this radio in the next week or so.

Ed
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