Valve 6K8G

[prajan]

Dear friends
In the valve equivalent sheets it is mentioned that Valve 6k8g is equivalent to ECH35. But in the data sheet base diagram in 6k8g the top cap is g3 whereas in ECH35 top cap is g1. In 6k8g pin 5 is g1, gt whereas in ECH35 pin 5 is gt , g3. Then how both the valves are same? In Mullard sets the top cap g1 of ECH 35 gets directly connected from the top of the first IF transformer then how 6k8g be used in place of ECH35?

Regards
P.rajan

[John M0GLN]

I can't advise you myself as I've never used them, but have you seen this thread?

https://www.vintage-radio.net/forum/...ad.php?t=95027

John

[Radio Tech]

Hello Prajan

Yes indeed you are quite right about the differences in the valve base connections, what I would do in this case is if a ECH35 isn't available and you have no choice but to use the 6K8GT then its just a simple job of changing those two connections over, I know this might not be the ideal thing to do but if it means getting the set to work again then just do it, there is nothing wrong in using the 6K8GT in place of the ECH35 at all its just physical size and the colour of the valve.
Hope this helps.

Ken

[FERNSEH]

ECH35 and 6K8G. These valves can be considered interchangeable. The top cap is the signal grid. No need to swap wires around. It's construction technique where the valve differ, the ECH35 has the conventional arrangement with separate triode and hexode sections whereas the 6K8 was constructed using the very clever method shown in the attachment.

http://www.r-type.org/exhib/aaa0058.htm

Heater currents differ. 6K8 0.3amp. ECH35 0.2amp.

DFWB.

[turretslug]

With older valve types, such as these octal ones and earlier, the "equivalents" may actually be somewhat different in characteristics, though in this case they are at least straight plug-in swaps. The details of what input is connected to which electrode doesn't alter the basic function of the valve.

The ECH35-type arrangement triode-hexode was originally a European design and sought to get the best performance from this type of circuit, particularly above around 15MHz. It was quickly recognised as an excellent design of mixer among European set-makers generally (though with a bit of a squabble from the British valve-making cartel for commercial, not technical reasons). It does, however, have two separate valve-assemblies in one envelope, i.e. the hexode and the triode, built up one on top of the other. This was a no-no for the American market, where the motivation was to churn out radio for the masses with the absolute minimum of expenditure- they were used to the self-oscillating heptode, which came with a few compromises but had just one electrode assembly in the bulb and was quite good enough most of the time. Some cunning folk had the idea of making a triode-hexode with just one electrode assembly- but it meant swapping the order of signal and oscillator grids inside the valve, this became the 6K8. This demanded quite precise assembly of this single structure but once the money had been invested in new fancy jigs, the 6K8s could then be churned out very cheaply in huge numbers- very much the American way.

There was some debate over which type holds the edge in performance but for many applications, they were functionally similar plug-in replacements without worrying about which grid the external connections went to. However, if it's for something like a high-performance receiver with comprehensive short-wave coverage, I'd try to stick to the original type, at least until I was certain that the set was working as well as it could, before trying a swap.

Crossed with Fernseh!

[Boater Sam]

Often used 6K8 in lieu of ECH35 without any problem, in fact I think it works better. And 6K7 or 0M6 is better than EF39.
Just my opinion of course, many will not agree.

[G8HQP Dave]

Maybe the issue here is the difference between an 'equivalent' and a 'substitute'? An equivalent is a valve with essentially the same characteristics as the original which can be simply slotted into place. There may be minor differences but these will not matter in almost all circuits. A substitute is a valve with similar characteristics which can be used in most circuits with no other changes, and in some circuits with minor changes.

Substitutes vary in how close they are to the original. Valve manufacturers used to claim their own designs as substitutes even when they were quite different in some respects.

The next step beyond substitute was exemplified by Brimar with their adverts about 'Brimarization'. This often involved changing the valve socket (not just rewiring it) and several other components.

[kalee20]

The valves are different! The 6K8 has oscillator applied to the inner grid (nearest the cathode) and signal to the outer, whereas the ECH35 has signal applied to the inner grid and oscillator to the outer.

So... bearing in mind that the top cap is connected to g3 on the 6K8 (signal) and g1 on the ECH35 (signal), you'd expect them to work reasonably interchangeably from the outside world's point of view. All's rosy!

However, in the 6K8, you may get some oscillator radiation from the aerial, especially on SW, because by the time the electrons reach the g3 signal grid, their flow has been well and truly modulated by the oscillator. So there will be some oscillator voltage induced on g3 which is connected to the signal circuits. Presence of an RF amplifier would fix any radiation, or some clever neutralisation, so it's best to use whatever valve the circuit was designed for.

[prajan]

Dear friends

Thanks to everyone for a detailed explanation.

Regards
P.Rajan

[Tim]

Do be careful if going from 6K8G to ECH35. the metalising is connected to pin 1 on the ECH35, ( no connection on the 6K8G) and I have had cases where, since the set is designed to use the American all glass valves- the "unused" pin is used as a junction-in at least one case for the HT- so you could potentially get a "tickle" if the metalising is touched when the set is powered up.

[Heatercathodeshort]

Checking the circuits in the immediate post war radio receivers, they often listed either a 6K8G or an ECH35 with no modifications. J.

[Hartley118]

Originally Posted by Tim

Do be careful if going from 6K8G to ECH35. the metalising is connected to pin 1 on the ECH35, ( no connection on the 6K8G) and I have had cases where, since the set is designed to use the American all glass valves- the "unused" pin is used as a junction-in at least one case for the HT- so you could potentially get a "tickle" if the metalising is touched when the set is powered up.

I suffered this very problem in my R1155 receiver. Substituting Mullard metallized valves for the plain glass originals resulted in alarmingly noisy firework displays inside the screening cans because the 'spare' pin anchoring the HT was now connected to the outer (red) metallization!

Martin