1/2 6SN7 (cathode) feeding G2 of Power tubes ???

[kagliostro]

In Pinterest, without looking for something similar, I've seen this amp where 1/2 of a 6SN7 tube has the cathode connected as to feed the G2 of the Power Tubes
If I read corlrectly G2 are at 293V and B+ is only a bit higher, so ... why there Is the necessity of that connection ?
Please, can someone explain to me which Is the purpose for that architetture

Many Thanks

Franco

[sluckey]

It's used to lower the screen voltage. Notice the 56K and .45M voltage divider that puts 290V on the grid? Well, since this is a cathode follower the cathode will be a little more positive than the grid. In this case, 293V, which is also applied to the screens. So, you lowered the screen by 33V while allowing the PI to operate at higher voltage.

I don't see any advantage to doing this, but the designer did manage to find a use for the otherwise unused second triode in that 6SN7.

[HotBluePlates]

... 1/2 of a 6SN7 tube has the cathode connected as to feed the G2 of the Power Tubes ... can someone explain to me which Is the purpose for that architetture

... I don't see any advantage to doing this ...

One could say it regulates the screen voltage, and does a solid job of filtering out ripple (due to the 56kΩ, 450kΩ divider feeding the grid).

The designer may have been prompted because they had an extra 6SN7 triode handy, and also because they wanted choke-filtering of the phase inverter but "extra choke & cap" for the screen was just too much money.

[kagliostro]

Many Thanks Friends

Franco

[PRR]

Note that the 6SN7 heater-cathode is working FAR beyond usual rating.

History says it can work that way for days or decades but not forever; and may depend on factory quality and the Made On Monday problem.

I would not say "regulate". It is dropped (6V6 @10KCT does not need 330V on G2) and filtered (expensively; a 2K resistor and a 20uFd cap must be cheaper). There's lots of single triodes; or 6SJ7 triode-strapped makes a fine triode. 6J7 as triode was common in radio broadcasters to reduce the number of types kept as spares.

[kagliostro]

Ciao PRR

Many Thanks also to you

Franco

[HotBluePlates]

Note that the 6SN7 heater-cathode is working FAR beyond usual rating.
... There's lots of single triodes; or 6SJ7 triode-strapped makes a fine triode. 6J7 as triode was common in radio broadcasters to reduce the number of types kept as spares.

Perhaps it was a Gibson amp that had a 6V6 as a screen regulator?

I should have known the screen current would be more than the 6SN7 could handle, but I didn't want to spend time looking it up.   

[PRR]

> the screen current would be more than the 6SN7

Is it? I think that part may be OK. The heater-cathode insulation is what I was questioning. Rating is 200V (I did look that up) and cathode is 293V. Heater circuit potential is set in the preamp, but unless it is very strange it is likely low, lower than 90V.