Selmer Stadium TV19/T - Wìch is the purpose of 2 x .001 cap connecting G2 to OT

[kagliostro]

I was observing this circuit as an interesting odd schematic because it uses ef86 tubes on the vibrato circuit



what slipped out from my eyes is the presence of two .001 capacitors that connects G2 to the OT  (and plate resistors on Power Tubes)

as far as I can understand the value of the cap has influence on the frequency that cross the capacitor and for those frequencies the capacitor

acts as a short

if I short G2 to the OT I obtain triode functioning but as I can understand it is limited on the bandwidth by the cap value

more there are 100R plate resistors

Is this thing more a triode functioning pentode or an Ultra Linear arrangement ??

EDIT: I've reconsidered the thing and probably those cap are there as to cut some fequency

I would like to understand how this thing works

Thanks

K

[PRR]

Your image link is not working at this moment (server may be down); I assume you mean this one:

http://www.vintagehofner.co.uk/selmer/schematic/sta.html

They have nothing to do with G2. They are connected to B+, which is heavily bypassed to ground with 32uFd. The other ends connect to the *plates*, or *across* the OT windings.

EL84 we assume 8K across the primary, pencil 4K on each side. 0.001uFd against 4K is 40,000Hz. FAR above the audio band. Has "no effect" on musical tone.

In an AM radio we would use a much larger capacitor across the primary to roll-off above 5KHz and reduce static noise and adjacent stations.

There is NFB around this amplifier, and such caps "can" help tame the supersonic response so that NFB does not turn into PFB at some supersonic frequency. However the NFB in this amplifier is very small. First triode has gain of 50, EL84 to 15 Ohm tap is gain like 1.2, overall gain 60. OTOH the NFB is 5K to 100 or 500:1.... even when the load impedance runs high (bass resonance or extreme treble) there is practically no NFB really happening.

Perhaps the prototype was tested under a high-power radio transmitter and these caps were needed to prevent radio programs from modulating the guitar. Or maybe they had too many 0.001 caps around the factory.

[kagliostro]

Many Thanks PRR

I changed the host of the image I posted, I hope now you can see it

The schematic you posted is the original and those I posted is a redrawn version, the way is draw this schematic confuses me, in the schematic you posted is evident that all is as you say

As drow in the original schematic the connection remember the Zobel Network, also if the two caps aren't directly connected to ground (they are, but in the way you explained)