820K Grid Leak resistor instead of 470K or 220K which can be the purpose ?

[kagliostro]

A friend received as a present an amp that probably was a Film Projector amp



I've never seen (or I don't remember I have seen) Grid Leak resistors of a so high value

820K instead of the "usual" 220K or 470K

Doing that I think there is a bit less loss of signal and so the signal that arrives to G1 is a bit higher

But if is a way to drive higher the Power Tubes why it isn't seen in other amp ?

The fact that el84 is a High Gain senitivity tube may be the reason ?

On the amp of my friend there is a 12au7 as PI (strange configuration to me) and only an ef86 as Preamp

May this be the reason for those High Grid Leaks ?

Thanks

Franco

p.s.: May be there are errors on schematic, I draw it exacly as my friend did by hand

[thetragichero]

https://www.telefunken-elektroakustik.com/wp-content/uploads/2017/06/EL84-TK-Tube-Data-Sheet.pdf
max grid resistance of 1M for cathode bias
my guess is that the signal level is much lower than a typical guitar amp so it doesn't sound awful. i'm betting it would sound horrible in just about any guitar amp situation

[WiderGates]

Hello,
that's a "normal" direct coupled PI.
Without all voltages of anodes, cathodes, grids and B+ we can only imagine.
But why should it sound horrible?
The PI is free from blocking distortion.

[tubeswell]

A friend received as a present an amp that probably was a Film Projector amp



I've never seen (or I don't remember I have seen) Grid Leak resistors of a so high value

820K instead of the "usual" 220K or 470K

Doing that I think there is a bit less loss of signal and so the signal that arrives to G1 is a bit higher

But if is a way to drive higher the Power Tubes why it isn't seen in other amp ?

The fact that el84 is a High Gain senitivity tube may be the reason ?

On the amp of my friend there is a 12au7 as PI (strange configuration to me) and only an ef86 as Preamp

May this be the reason for those High Grid Leaks ?

Thanks

Franco

p.s.: May be there are errors on schematic, I draw it exacly as my friend did by hand

The PI is a DC-coupled LTP see http://www.valvewizard.co.uk/dcltp.html


Grid leak resistors can (and should) be anything within the recommended limits for output tubes. If you exceed the resistance limit, any charge on the grid won't be able to 'neutralise' as effectively, and there's more possibility that the output tube grid will lose bias due to +ve charge build-up on the grid, especially when driven hard, which we all know is bad.

[kagliostro]

Many Thanks thetragichero - WiderGates - tubeswell

I didn't recognized the DC coupled PI

---

My friend want to guitarize the amp but he don't want to maintain the ef86 as he don't like that tube on guitar amps, he want to sub it for a 12ax7 or, may be, a 12ax7 plus a 6AV6

I submitted him the EF86 as V1 with Morph control as per Merlin's design but he really don't want an EF86 on his amp

Thanks Again

Franco

[thetragichero]

Hello,
that's a "normal" direct coupled PI.
Without all voltages of anodes, cathodes, grids and B+ we can only imagine.
But why should it sound horrible?
The PI is free from blocking distortion.

i'm not sure if this is directed towards me, but in my experience with el84s they can easily go from "nice overdrive sound" to "garbled distorted mess of bees" in a guitar amp. i've had to tweak a few amps to lower the grid resistors (one that comes to mind was a cathode biased el84-powered jcm800 that didn't sound right until the grid resistors were lowered to 150k or 100k)
basically guitar source and preamp likely has a lot more OOMPH than a film projector and film projector amp

[AlNewman]

There's only one preamp stage.  Maybe that's a reason to push it harder at the output?
Plus, there's a large degree of separation between the screen and the plate at the ef86.  You could run it as a triode and place a small value resistor between the 2?  But who wants a guitar amp without a tone stack?