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 The crackling 6M5
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 Return to top of page · Post #: 1 · Written at 8:07:03 PM on 11 April 2020.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1910

Thread on the Kriesler 11/77 reminded me of this silver migration issue.

The 6M5s in the 11/77 are too new to suffer from this so worry not.

You may have noticed that the pinout of the 6M5 has Pin 1 - G2 right next to Pin 2 - G1. An unfortunate choice as it turns out.

The very early 6M5s made by Mullard had a different shape of glass base to later B9A envelopes. They have a distinctive outward flare on the base. Their pins are also silver plated.

Can you see what's coming?
Yes, silver migration across the glass from Pin 1 to Pin 2.

This can cause crackles and the occasional loud bang as the silver molecules are blasted out of the way.

You can actually fix them temporarily by scraping the glass between pins 1 and 2.

This propensity of silver was not known at the time.

As a side note, if you listen to Winston Churchill's wartime speeches that were made from the cabinet war rooms (using a small console that is still there) you will occasionally hear the unmistakable sound of silver migration crackle. This console uses some of the early baseless valves by Mullard - silver plated pins of course.

If you are ever in London I'd highly recommend a visit to the Cabinet War Rooms. Around the back, just off Downing St.

 Return to top of page · Post #: 2 · Written at 10:10:29 PM on 11 April 2020.
Robbbert's avatar
 Location: Sydney, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1380

6N8 and 6BH5 are two more valves with G2 next to G1 and the same silver-plated pins.

 Return to top of page · Post #: 3 · Written at 6:25:43 PM on 13 April 2020.
Padge's Gravatar
 Location: Umina Beach, NSW
 Member since 9 April 2020
 Member #: 2410
 Postcount: 33

Well this all intrigues me Ian and Robbbert with a triple b. Only because Ian mentioned the 11/77. So back to this particular issue Ian. The noise I have from my unit is a background noise and doesnt seem to have any effect on the sound quality apart from its just there. I am starting to think that now you have said this that maybe I have a silver migration issue. I was starting to think it was those two paper caps 100pf 600volt as I think they maybe noise filters? All of the valves in the chassis seem to have a nice colour( orange) and seem to glow nicely but admittedly I think the 12ax7 although its colour is nice doesnt seem to have the brightness the others have. But this maybe due to its role in the chassis. I did notice a couple of valves had black on the pins rather than nice and clean looking. So I sprayed some contact cleaner into all of the valve sockets and it did not seem to help the situation.Im gonna have a listen to Winston Churchills speech to see if its a similar noise. I should say that I cant seem to fault any resistors or caps apart from the 2 you mentioned which I am unable to test.

 Return to top of page · Post #: 4 · Written at 6:56:04 PM on 13 April 2020.
Vintage Pete's avatar
 Location: Albury, NSW
 Member since 1 May 2016
 Member #: 1919
 Postcount: 1993

So record player has the crackle yes?
So does the radio yes?.
The crackle is in both channels Yes ?

 Return to top of page · Post #: 5 · Written at 1:40:51 PM on 14 April 2020.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1910

The colour the valves glow is irrelevant. The colour of the pins is irrelevant, they use a nickel alloy.
You 11/77 is too new to have these 6M5s and so is not affected.
The 100pF caps are mica, not paper. They are there to ensure high frequency stability in the amplifier, google Barkhausen's stability criteria for an explanation. In that p[osition and that age, silver migration is a definite possibility inside the cap.
As I suggested before, pull the 12AX7 out while the unit is running (perfectly safe to do this, although it will make a loud bang in the speakers) and see if the noise is still there or gets worse.
Another way you can test the 100pF caps is to measure the voltage across the 270k resistors with the 12AX7 unplugged and the chassis powered. There should be zero volts across those resistors. Any voltage you measure is caused by leakage in the 100pF caps.

 Return to top of page · Post #: 6 · Written at 10:53:05 AM on 11 June 2020.
Keith Walters's avatar
 Location: Sydney, NSW
 Member since 16 January 2008
 Member #: 219
 Postcount: 55

You also get other types of contamination, such as cigarette smoke, that can make the radio moisture sensitive. A good scrub with soap and hot water will work wonders.

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