Etronic R640 valve swap conundrum.

[john_morris_uk]

I'm in the middle of repairing an Etronic R640. All fairly straightforward and I like the way that they used some quality components in the build. Some of the resistors have drifted off a bit (as expected) but lots of bits are OK so far.

However the first mixer/oscillator valve should be a 6K8G and someone has substituted a X61M This confuses me as by my reading the pinout of the valve is identical except that the grid connections seem to be swapped over. I'll double check if the wiring has been altered, but am I thinking backwards. Is the X61M a straight swap. I'm minded to source a 6K8 (metal or glass) to make the thing original but I'd appreciate any comments from others...

I'm wondering if it's just a nomenclature confusion? I'm comparing: https://www.radiomuseum.org/tubes/tube_6k8.html

with https://www.radiomuseum.org/tubes/tube_x61m.html and they show pin 5 on the X61M going to G (i.e. GT) of the triode side connected to G3 of the Hexode side whilst the 6K8 diagram shows pin 5 connected to G (i.e. GT) of the triode side and what looks like another G1 of the Hexode side. This leaves me confused...

I haven't applied power yet as the set has been through the dishwasher etc (speaker and glass removed etc) so I'm getting ready to apply power soon.

Many thanks John

[john_morris_uk]

I'm wondering whether it's my lack of familiarity with the way that these particular valves are actually constructed? The drawings are approximations to what is actually happening with the triode and the Hexode? Unlike single and straightforward Triodes, Pentodes etc where the grids are all concentric (ish) I assume that things are not quite so simple in the actual construction of one heater and cathode, yet two anodes and associated grids?

[G6Tanuki]

The 6K8 is a valve with a somewhat-unique internal construction!

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

[Nuvistor]

The Brimar equivalents list show the ECH35, 6K8G and X61M as equivalents.
I don’t know if it is a mistake, they look different to me.

[john_morris_uk]

Originally Posted by Nuvistor

The Brimar equivalents list show the ECH35, 6K8G and X61M as equivalents.
I don’t know if it is a mistake, they look different to me.

That's what I thought too - hence my question.

I guess I'll try it and see.

I've got the Variac ready on the bench to start applying power to the transformer. Then its put the 5Z4 in and see if the HT line is OK... All the usual excitement to come.

[Boater Sam]

If you look at the valve museum for X61M it will partly explain ECH35 and 6K8G
and the OM10 are similar and can be substituted.
I have indeed used these as equivalents.
It seems to make little difference which of the hexode grids is used to couple with the triode.

[sentinel040]

I can also state these work as equivalents
Ian

[G6Tanuki]

From memory the 6K8 has a conversion-conductance significantly less than the ECH35 - which could be an issue if substituting them in a receiver that needs to operate on the upper short-wave bands.