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Opinions/Advice on Triode Strap method

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I just completed restoration of an old PA Amplifier.

Triode strapped 1603 (premium quility 6C6) as teh microphone input tube to a triode strapped 6C6 (which is also the Pickup "PU" Input) to a triode strapped 6J7 which drives a phase splitting transformer and finally to a paair of 807 for 40 Watts or so out.

The 6J7 triode strap is conventional with G2 and G3 strapped to the anode.

The preamp tubes however (1603 and 6C6) have G2 strapped to the anode but G3 strapped to the cathode.

Any reason for this? Was it a screwup or did they know something I don't?

Cheers,
Ian
 
Many pentodes have an external shield connected to G3 - so connecting it to the plate eliminates this shielding. Type 77, which is a plug-in sub for 6C6, for example. 6J7 has an external shield, but if it's metal, there's still a shield if G3 is connected to plate.

This is most likely the reason for strapping G3 to cathode/ground.
There are other reasons. Beam power tubes should not have 'G3' strapped to anode s it's not really a grid but a set of two beam formers on the grid sides. Normally if G3 is on a pin on such a tube it's intended tu supress snivet oscillations and will be specified for up to several tens of volts maximum voltage WRT cathode. The pinout of the tube often reflects this and strapping G3 to G2/Plate may cause internal arc-over. There are also various pentodes that need the same consideration.

Strapping G3 to anode (when possible given previously stated constraints) somewhat increases tube gm. How much depends on the tube - sometimes it's a trivial amount, barely measurable, sometimes it's a usable percentage, especially in tubes that were designed for 'dual control' i.e. where G3 can be used as an additional control grid.
In some cases there are considerations which turn out to be almost the oposite of the internal arc-over problem. One such example is the GU50 power pentode. The pinout from the internal structures is such that it will work reliably with higher plate voltages (less chance of an arc-over between G2 and G3) triode strapped with G3 connected to plate, than with G3 connected to cathode or ground.
 
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