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 Wanted: 6G8G Valve
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 Return to top of page · Post #: 1 · Written at 9:21:34 PM on 22 June 2018.
Skymaster's Gravatar
 Location: Ashfield, NSW
 Member since 7 April 2018
 Member #: 2237
 Postcount: 53

All,

The 6G8G in my Radiola 517M is on the blink. Had a silent radio for the last few days, with no heater running. Pulled all the valves today, let it run with just heater current for half an hour before it even remotely warmed up. After putting the others back in, it seemed to work for a whie, and then audio all disappeared again.
Thermal camera is showing it's barely getting warm.

I'm wondering if anyone has a spare they'd wish to sell on; please feel free to get in contact; my email address is unhidden.

Looking through my collection, I seem to have the following Double Diode/Pentode valves available.
Not sure of any of them might be suitable drop-in replacements.
2B7, 6AR7GT, 6BV7, 6N8, 8LT8


Cheers
Mike


 
 Return to top of page · Post #: 2 · Written at 11:17:53 PM on 22 June 2018.
Robbbert's avatar
 Location: Sydney, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1113

The first thing to try is to unplug the valve then resolder the pins. Any valve with a glued-on base is susceptible to dry joints in the pins.

I doubt any of those "replacements" will do the job, in fact very few of them will even fit in.

The best bet would be a 6B8G, if you have one. Just plug it in and it should work.


 
 Return to top of page · Post #: 3 · Written at 10:58:14 AM on 23 June 2018.
Skymaster's Gravatar
 Location: Ashfield, NSW
 Member since 7 April 2018
 Member #: 2237
 Postcount: 53

Thanks Robbbert.
I just consulted the spreadsheet of the spares I have, which list the model number, type of valve and quantity, I might have to add the base to it too Smile

I'll give the base resolder a go; certainly can't hurt.


 
 Return to top of page · Post #: 4 · Written at 11:04:51 AM on 23 June 2018.
Tallar Carl's avatar
 Location: Latham, ACT
 Member since 21 February 2015
 Member #: 1705
 Postcount: 1445

I have nos 6G8G.


 
 Return to top of page · Post #: 5 · Written at 10:40:54 PM on 23 June 2018.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3865

6B8 & 6B6 have identical pinouts 6B8 is normally run with less volts and 6B8 uses different grid & screen volts, so without mods we do not go there. I would however, make a point of checking the 6B6 plate resistor and the ones on the grid one bias of the OP valve. Those have a very high attrition rate.


 
 Return to top of page · Post #: 6 · Written at 8:52:43 AM on 24 June 2018.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 607

Hi Skymaster, if you did not mind experimenting the 6AR7 is a similar double diode detector amplifier.
You would have to shuffle the socket connections around to connect up the electrodes properly.
The 6AR7 is a later valve with more gain and slightly different cut off characteristics so may react differently in the particular circuit.

On the other hand it may fall within the circuit parameters and you would never know the difference!
I would not hesitate, being me, to wire it up and try.

If it has to be original, and you don't like circuit mods, then a new 6G8 is the only way to go if the original is faulty.

Marc and the other guys are right, check the parts around the valve and test the heater of your valve.
Obvious way to test the valve is to hook the pins of the base up to the 6 volt supply with clip leads, with the valve lying on the bench and MEASURE the current! Put you multimeter in series on the amp range with one clip lead. Check against the standard, my data book says 0.3 amp at 6.3 volt.
This is pretty fundamental electricity testing 101!

And you can squint inside the valve and see if there is a cheery glow or not. That's called observation 101.

I don't know that much about electronics so do the simple 101 things first.
Cheers, Fred.


 
 Return to top of page · Post #: 7 · Written at 2:08:31 PM on 25 June 2018.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1349

Fred is right, as usual.

The 6AR7GT is an improved replacement for the 6G8 and was designed by AWA for reflexed operation. It has an integral shield.

It has VERY different pinouts though.


Pin..6AR7...6G8
============
1.....H..........n/c
2...Shield.....H
3...Anode..Anode
4....G2......Diode 2
5...Diode 2..Diode 1
6...Diode 1...G2
7......K...........H
8......H...........K

Cap is G1. "Shield" must be grounded.

I had the same problem 50 years ago with a 6G8 in Mum's old STC radio. Resoldering the heater pins fixed it.


 
 Return to top of page · Post #: 8 · Written at 5:16:02 PM on 25 June 2018.
Skymaster's Gravatar
 Location: Ashfield, NSW
 Member since 7 April 2018
 Member #: 2237
 Postcount: 53

Thanks all.
I haven't yet had a chance to pull the radio back apart again. It looks heater related as I can't see the glow from the top when it's not playing well.

I'll disasseble it and give the re-solder a retry, and then if still no avail, either look at fitting the 6AR7 (sounds like a big job, with the tight wiring in place) - or reaching out to one of couple of people who offered some new valves.

Cheers
Mike.


 
 Return to top of page · Post #: 9 · Written at 9:59:00 PM on 25 June 2018.
Skymaster's Gravatar
 Location: Ashfield, NSW
 Member since 7 April 2018
 Member #: 2237
 Postcount: 53

Tonight I pulled the valve out, fired it up in a standalone socket off 6.3V. Needed a jiggle before the heater started glowing.

Unfortunately the meter I have doesn't 'do' AC current, so I rigged up a little jig to for a test.
28R resistor in series with the valve, across a 12.8V supply.

Measuring the drop across the resistor (7.4V) and valve (5.4V), I was able to calculate current, and therefore resistance of the heater. At 6.3V, that resistance would mean 296mA flowing, right on the mark.

The pins were all very dull, so I decided to resolder all of them, popped te valve back int the radio; and had instant life. I'll run it for a few hours tonight and tomorrow, but it appears that a dry joint is the culprit here.

Thanks all for assistance again! Greatly appreciated.

Mike


 
 Return to top of page · Post #: 10 · Written at 7:11:23 AM on 26 June 2018.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 607

That's a well done and electrical fixing 101 at its finest.
These radios don't need an Einstein with 6 degrees in outer space theory to get going again, just basic common sense.
That's all I use, note I do all this tinkering but have NO formal training in electronics or radio or anything really.
If I had the training I might start to get doubts about what I am doing!
Well done again!
Fred.


 
 Return to top of page · Post #: 11 · Written at 12:34:56 PM on 26 June 2018.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1349

UN-common sense it is, Fred!


 
 Return to top of page · Post #: 12 · Written at 5:38:22 PM on 26 June 2018.
Robbbert's avatar
 Location: Sydney, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1113

Glad to hear the resoldering did the trick.

For 10 minutes of your time and at no expense, you now have a radio that works.


 
 Return to top of page · Post #: 13 · Written at 8:30:59 PM on 26 June 2018.
Skymaster's Gravatar
 Location: Ashfield, NSW
 Member since 7 April 2018
 Member #: 2237
 Postcount: 53

That's it.

Thanks all. I wasn't aware that the bases could become 'bad' like that. So that'll be the first trick up my sleeve next time.
Once again your help is very much appreciated - thank you for taking the time!


 
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