The V69 is a odd circuit is yours the version with the solid state output?
As Ruud posted it could be the capsule
I have one and started to trace it and then gave up because of epoxy covered parts and it seems to have a tube just to have a tube so I did not feel it was worth my time to trace. Also the tube is upside down and IMO the capsule wires are to long that go to the tube.
Something that might help is remove the tube and place the body back on for shielding and power it back on on does it still hiss? I have not done this and an possible issue with this is it unloads the B+ and heaters power supplies and I did not trace how the heaters and solid state circuit are powered, don't know if they share the same supply. So hold off doing this test
This splits the circuit almost in half. Sometime splitting a circuit in half is a good first step in troubleshooting
If it still has hiss it is probably the solid state section or less likely the power supply.
If the hiss goes away check the capsule. Do a search at this forum or the web for how to do this.
The reason for the above test is this circuit has a first stage with a 12a?x both triodes in parallel, cathode biased, plate out with a charge amp feedback cap from plate to input.
It then looks like it goes to a circuit that looks like a Schoeps like circuit(This is a GUESS as best I can tell with out removing the epoxy) JFET in two NPNs? out. if you look at a picture in the link below
https://www.onsemi.com/pub/Collateral/2N6426-D.PDF
A link to a site with pictures and adjustments, this one does not have epoxy on the circuit board
http://www.mark-o-matic.com/v69-mod.html
