Greetings all. I want to get some opinions on the best way to interface the impedances of this solid state preamp (left) output to the input stage of this EL-84 power amp. 12AX7 is the input stage. Everything works, but I'd like to make sure there aren't any 'Engineering problems' that might rear it's ugly head. I suppose the impedance should be matched? or not necessarily? Any thoughts?

 

So figure R59 drives C70.

 

The general guideline is a low impedance output drives a high impedance input.

You’re ok.

 

Why does the opamp have a gain of only 1?
Why are there TWO capacitors in series between the output of the opamp and the input of the tube?
Why is C39 only 0.022uF driving R57 that is only 4.7k that cuts frequencies below 1547Hz?
Why is the power amplifier an antique vacuum tubes type that wears out soon?
Why is your schematic a negative image?

 

Why does the opamp have a gain of only 1?

A unity gain amplifier is an electronic amplifier circuit that doesn’t amplify. In other words, it has a gain of 1. I cannot believe I just said that to audio guru!!! But now you're going to smash me.Make it quick I already sent letters to my loved ones. Then again I'm in the US and you're in Canada you're safe until our natural resources run out and then your mine. Your reign is over!!
I travel in the terahertz and you get dizzy at microwaves

 

Why does the opamp have a gain of only 1?
Why are there TWO capacitors in series between the output of the opamp and the input of the tube?
Why is C39 only 0.022uF driving R57 that is only 4.7k that cuts frequencies below 1547Hz?
Why is the power amplifier an antique vacuum tubes type that wears out soon?
Why is your schematic a negative image?

Hmm..... okay...

A) It is a buffer circuit gauntleting other audio paths fed into it.
B) Doesn't need to be --probably lagacy to make prototyping easier. Good catch though. Sometimes a guy looks too deep into a pool, and can't see the water.
C) For tone response- This is a guitar amplifier.
D) All tubes are 'antique' They do not 'wear out soon' if set up correctly, and a quality mfg. is used.
E) It is not a negative image, just a dark back ground. Easier on the eyes. Dark relaxes the eyes. I like high contrast, except when doing mechanical CAD drawings.

And now back to the main subject.... hope this helps.

 

The opamp is probably used as a mixer but the schematic shows only one input.
Of course the vacuum tubes are used in an old guitar amplifier. For the distorted retro sounds.
Many bands replace their tubes before each gig.

 

The all purpose tube amps in the vintage Roberts (and Akai) reel to reel tape recorders (1055 and many other models) are BY FAR the best single 12AX7-EL84 amps ever made. They can do anything! Look into these. I still have some here too. Stunning audio quality and enough adjustable gain to open the pearly gates. They are complete in themselves. Every input and output you could ever want built in. I believe the schematics are findable. The only purpose of an outside preamp would be for signal processing, mixing, effects, etc. before the input. Just plug in your toys adjust the volume and play! I have used these for everything from audio mastering of recordings, recording mic inputs, and yes, mind blowing guitar amps. Audio heaven! Hope this helps!

 

I forgot to mention, nearly all these old Roberts machines are stereo, so these amps usually travel in pairs. There are 2 totally separate amps in there! So you get 2 for the price of one. Even more fun!