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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 15th Oct 2022, 11:37 am   #1
OldTechFan96
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Default Hacker Serenade Crackling Amplifier

Morning all,

Recently I have been working on a Hacker Serenade which I found locally. The gentleman I bought it from said it was in use until recently.

The amplifier consists of a tone control unit (TC101) using an ECC83 and a power amplifier (A201) using two ECL86s.

The rectifier valve and the ECC83 look to be original Mullard types. The ECL86s have both been replaced. One is a Siemens and the other is a Philips. Visually, they both looked to be in good condition.

When in fine fettle, it should produce 10W of output.

Both the tone control and the power amplifier have had various out of tolerance components replaced.

The initial power up was done via a lamp limiter. The amplifier sat for half an hour playing music to itself with out any problems.

Once full mains was applied to the amplifier, an occasional crackling noise could be heard coming from the speaker after a minute or two.

I decided to monitor the voltage on the grids of both V1b and V2b. HT was also being measured.

Over a period of just under four minutes after switch on, the control grid of each pentode section became more and more positive while the HT was slowly pulled down.

One grid measured 9v and the other 3v before I switched things off.

I could not believe that two new coupling capacitors could be leaky! Or so I assumed! I tested each 0.047uf/400v capacitor for leakage at 500v and they were fine. I temporally tacked them back in with solder. I will replace them.

At this point I remembered that ECL86s were prone to internal leakage.

Is this what I'm seeing here? Some kind of triode-pentode leakage?

Thanks for reading.
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Old 15th Oct 2022, 2:13 pm   #2
Robert Gribnau
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Default Re: Hacker Serenade Crackling Amplifier

Probably parts of the barium oxide coating of the cathode came loose and got deposited on the control grid. When the control grid gets hot enough (something that can easily happen in a cramped valve like the ECL86), it starts to act like a cathode, so it starts to emit electrons. This stream of electrons causes a voltage drop over the resistance in the control grid circuit, making the control grid positive with respect to ground.
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Old 15th Oct 2022, 2:14 pm   #3
yestertech
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Default Re: Hacker Serenade Crackling Amplifier

Given that the coupling caps have already been replaced, it’s almost certainly down to ‘leaky’ ECL 86s. These valves do not age well.
I recently went through my stock of them, using a test rig with an output transformer , PSU and copious metering. Over 60% of them showed a rising grid voltage and accompanying rise in anode current within 10 to 15 minutes of being powered up.
They all went in the bin.
Not one of Mullard’s better efforts, although way past their design life of course !
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Old 16th Oct 2022, 12:50 pm   #4
OldTechFan96
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Default Re: Hacker Serenade Crackling Amplifier

I had a funny feeling that the ECL86s would be the problem.

Should I be concerned about leakage with NOS ECL86s?
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Old 16th Oct 2022, 1:06 pm   #5
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Default Re: Hacker Serenade Crackling Amplifier

Check the ECL86 pentode G1 resistors, if they go very high they can make the problem with the valves much worse. Worth checking the cathode components as well, those being faulty could cause the valves to pass too much current and again not help the life of those valves.
The valves will likely be faulty but easy to test those resistors/ capacitors just in case.
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