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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 1st Dec 2018, 6:27 pm   #1
mikemike
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Default 6K7G / VR100 differences

Hi. I'm restoring a 1944 Phillips PCR radio. This was refurbished by REME in the 1950s so some of the original valves have been upgraded.
Specifically:
The 1st RF amp, originally EF39 is now a 6K7G also marked NR81.
The 1st IF amp, originally EF39 is now a 6K7G also marked ZA5699.
The 2nd IF amp, originally EF39 is now a VR100 also marked 10E/278.

These all appear to be versions of a 6K7G (described as a screened high impedance pentode).

The strange thing is that the NR81 has a completely different construction with a screening structure over the anode. The ZA5699 and the VR100 both appear identically constructed without this screen but with a grey coating on the glass.
Does anyone know the difference (if any) between these valves?
Regards - Mike
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Old 1st Dec 2018, 9:53 pm   #2
turretslug
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Default Re: 6K7G / VR100 differences

The screening structure over the anode in an IO valve sounds like a US construction valve, i.e. an 'actual' 6K7G, highly likely supplied to the UK early in WW2 and additionally marked NR81 for UK Navy stocks. The other two could well be of M-O V manufacture, the first being a KTW63, the second a KTW61 (possibly but less likely KTW62) and labelled for miltary use. The KTW61/62 has a somewhat higher gm than the KTW63/6K7G group but were probably regarded as more or less interchangeable in an IF strip.

I suspect that the valves were replaced as a matter of course as part of overhaul, rather than being regarded as a specific "upgrade"- I think Pye would originally have been close to Mullard/Philips, hence the EF39 specification. During WW2, vast numbers of R1155/CR100 would have employed vast numbers of M-O V series valves, and in-depth spares stock would have been around long after the end of the war.

If nothing else, it's an interesting snap-shot of the progressive rationalisation in UK military valve designations over the years!

Last edited by turretslug; 1st Dec 2018 at 9:59 pm.
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Old 2nd Dec 2018, 1:05 am   #3
mikemike
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Default Re: 6K7G / VR100 differences

Thanks for the reply.

The set has been modified to improve the AGC system which is why, I assume REME have replaced the EF39 with a 6K7G to give greater control of the gain. You can see the cathode resistor has been replaced with a larger value. I found a 1965 Practical Wireless article which shown a similar AGC mod.

As you say it's interesting to see the development of these valves but you'd have thought that valves marked 6K7G would both have the screening structure, as I understood that was the difference between the 6K7 and the G version.
Did they think the graphite coating would provide similar screening?

Interesting.
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Old 2nd Dec 2018, 2:46 am   #4
G8UWM-MildMartin
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Default Re: 6K7G / VR100 differences

Your first one looks like a 6K7G, the other two look like 6U7G (an American 6D6 but on an octal base) or KTW61 or KTW62 or KTW63. (see http://www.r-type.org for pictures of all of these and their services equivalents)
The original 6K7 was a smaller black metal thing, copied in the USSR as 6k7 who made them at least into the late 1950s.
The Mullard EF 39 was a Philips EF9 but with an octal base and small top cap to be pin-compatible with the 6K7, but retaining the EF9's 0.2-A heater current versus 0.3A for the others.

Last edited by G8UWM-MildMartin; 2nd Dec 2018 at 2:57 am.
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Old 2nd Dec 2018, 9:52 am   #5
turretslug
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Default Re: 6K7G / VR100 differences

I think that the graphite coating on the inside of the glass may have been for screening- this was an era when valve-makers were a bit watchful and paranoid with each other and I wouldn't be surprised if either internal graphite or external metallic coatings had patent process implications! Some M-O V power valves, famously the KT66, featured the dark grey graphite, too, and I've heard it said that this also assisted radiation but, as with any technology that's been eclipsed for a couple of generations or so, it's difficult to glean truth from supposition.

The EF39 has an unenviable reputation for shedding or otherwise deterioration of its distinctive red outer shielding coating leading to apparently untameable instability, later North American-sourced versions had a far more tenacious dark grey/near black finish. They obviously put some effort in getting it to stick to the glass bulb very well, but I suspect the preparation would have involved fluorine compounds that would make a modern H + S person turn blue and fall over at the mere thought!

Last edited by turretslug; 2nd Dec 2018 at 10:00 am.
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Old 2nd Dec 2018, 10:52 am   #6
Argus25
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Default Re: 6K7G / VR100 differences

Quote:
Originally Posted by G8UWM-MildMartin View Post
Your first one looks like a 6K7G, the other two look like 6U7G .
I could never understand why the 6U7 came into existence. Its specs are practically identical in every respect to the 6k7, except is has a slightly lower plate dissipation of 2.25W vs the 2.75W for the 6k7. Otherwise the two valves are interchangeable.

In the wartime period huge numbers of 6U7's were manufactured and deployed in military radios (the NZ made ZC1 being one example) the 6U7's were made by all sorts of manufacturers including Philips of North America. Why 6K7's were not stuck to instead, I do not know.
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