Amp Guru's - Which valve to swap?

[Stevieb76]

May not even need one of our resident gurus to answer this one, maybe just someone not as completely inept as me..

I have an old telefunken ECC83 that I want to try in an EC Vibro Champ. Pretty sure the pic below (googled) is the same amp, so if I was just changing the one preamp valve, would it be the one far right? And is the second from right solely for the tremelo?

Also, is there any reason not to change just the one valve? Any advice welcome, as I aint got a clue..

[ICBM]

Yes, that's all correct as far as I can tell.

[Stevieb76]

Thanks John!