Ampex mx-10 clone

[smilan]

Hi, I built an Ampex mx-10 preamp clone using a Bogen T200 input transformer with the following circuit:
https://i.ibb.co/4K8JvSz/125972809-132619804974443-1633951602894638334-n.jpg
The preamp works, but the signal on the output is distorted.
When I've connected a signal tracer probe to the grid of V5a (12au7) the sound was clean but on the cathode of V5a the sound was distorted.
I replaced the tube and the problem remains the same.
I've wrote the voltages on the schematic and it seems to be OK for V1 and a little bit low on V2 and V5a. I'll change some resistors to getting to the right voltage, but do you think this is what make the output distorted?
How can I troubleshooting this issue?

 

[gridcurrent]

EF86 pins 2 and 7 refer to internal shields and are not strapped together internally.
Thus, no connection of 12AU7 to circuit common via R10.
Pins 2 and 7 are indeed wired together in a real MX-10.

Level trim circuitry including components R13, R14, and R15 is not your friend.
Questionable in principle, the signal is induced is positive feedback.
Due to an abundance of negative feedback, level difference between channels with healthy tubes is very small

 

[smilan]

EF86 pins 2 and 7 refer to internal shields and are not strapped together internally.
Thus, no connection of 12AU7 to circuit common via R10.
Pins 2 and 7 are indeed wired together in a real MX-10.

I've took the EF86 out of it's socket and measured dc resistance  between pins 2 and 7 and indeed there's an internal connection between pins 2 and 7.
So when one of those pins are grounded (as pin 2 is grounded in the original Ampex schematic) there's a connection of 12AU7 to circuit common via R10.

Am I missing here something?

 

[mjrippe]

I just checked a half dozen EF86 types including vintage Amperex and Mullard as well as modern JJ and Sovtek EF806 and the old Soviet 6Ж32П.  ALL of them have pins 7 and 2 tied.  Perhaps the early Phillips tubes did not, but everybody else seems to.

 

[smilan]

I found the problem, the distortion caused due to a cold solder joint.
With R10 (22k between the cathode of the 12au7 to ground wia pins 2 and 7 of the the EF86 all works great!

 

[templemark]

Just curious. What did you use for .33uf 400V cap?

 

[smilan]

Just curious. What did you use for .33uf 400V cap?

I used what I had in that time, a .47/600v.