How to add an external preamp input in this tube amp?

[whoops]

Hi,this is not a guitar tube amp, it's actually an hi-fi tube amp but I would like to ask some help from you guys.

I would like to have an RCA input so I could connect an external preamp straight into the Tube power amp.
What do you think is the best place to break the connection between the Preamp and the power amp in this circuit?
Schematic attached
Thank you so much for your help

[terminalgs]

Your power amp input begins at V1.  V1 and V2 together are the phase splitters for the push-pull power tubes and transformer.  everything after the plates of V2 have the signal already phase split, and your input is a signal phase so you really can't 'sneak' a auxiliary input in anywhere between V1 and the bias module AP-1002.


If your pre-amp that you want to use produces too hot of a signal for this power amp and it distorts as you turn the volume up:  put a voltage divider between the preamp and the V1 input to attenuate the preamp signal to some clean level as it hits the grid of V1(a).  luckily, you have a voltage divider already there, that 250K volume pot.  All you need is a resistor  between it (cd/tape/tuner input)  and your preamp.  use a trim pot to make a variable resistor to tune the precise value of that resistor.   If this is stereo, do the same in the other power-amp,  or once you figure the trimmed resistance, pop in static resistors if you like.

[terminalgs]

[shooter]

Ok, whoever designed that amp got an A, at least for creativity and complexity! 

I would definitely NOT try and hack anywhere "inside" the circuit as terminalgs said.

Is V1 acting as a "buffer"? I didn't play with numbers on napkins, but I don't "see" any real gain til V2 

[terminalgs]

it looks like cathode of V1.top sends signal to cathode V2.top and grid of V2.bottom,  phase is only inverted on V2.bottom, so plates of V2 top and bottom are out of phase.  V2.bottom is there to only to balance voltages on network between V1.top and V2 top and bottom so biases are the same for V2 top and bottom.


also note that CF1 and CF2 on OT produce NFB signal which is fed to grids of V2 top and bottom . via a 47K/1.6K divider with that 2K pot for tuning in balance.


V2 and V3 are gain stages, V4 are cathode followers to drive power tubes.

[whoops]

Thank you so much, for all your input.
So I will leave it like this. Thanks so much for the advice and explaining the circuit.

[PRR]

It's a cross-coupled phase splitter, then more stuff, then the funny power tubes. It is, as said, all ONE unit.

[shooter]

It's a cross-coupled phase splitter, then more stuff

guessing most of the circuit is for noise/distortion/cross-talk reduction 
I'm off too reading 

quick search only found this, worth drooling over though
although, it doesn't appear to pass the cork sniffer test.

http://www.enjoythemusic.com/magazine/equipment/0510/raysonic_sp_200.htm

[DummyLoad]

some light reading on the van scoyoc cross coupled PI here, and here.


--pete

[shooter]

I like Pearl, but only get blank pages at some spots, the other show’s a Differential driver, which I get from my SStime, long wire, important stuff……
So you take the “source” make a 180 copy, I think I got V1A, and  V2B,  Guessing V2A sucks up the signal at the cathode, passes it out the plate? 

[PRR]

> some light reading on the van scoyoc

I knew there was some pre-history which set-up the form we commonly see.


https://www.bonavolta.ch/hobby/en/audio/split.htm#Cross%20coupled

You can use two tubes if you have a LOW impedance source.

A Shooter insightfully said, V1 acting as a "buffer" to get a low-Z source from a hi-Z line.

[shooter]

I'm still in the deep weeds here;
your bottom pic;
How/where does Tube B(lower R), get an inverted signal on the grid, so the plate becomes in-phase 

[PRR]

> your bottom pic; How/where does Tube B(lower R), get an inverted signal on the grid....

Back up. One Step At A Time. Do you see how the TOP version works?

[shooter]

sorry, missed your reply

Do you see how the TOP version works?

Yup, cathode of V1"A" to grid of V2"A", inverted out



This amp keeps my brain warm in the cold north 

Is this an example of both fixed and self bias?  (co-processing for better efficiency  )

[PRR]

It's an example of topsy-turvy drawing.

Yeah, it has both -50V and 150 Ohms bias. Also four bias trims. I have no idea how to adjust it since there is no way to meter individual plate-cathode currents. (This appears to be that monster Russian "twin"-triode with a common cathode. You'd either have to break the vacuum to get metering onto each cathode separately, or add small plate resistors and take readings at high voltage.) (In general, when you see big caps fitted with little caps, especially cathode caps inside a NFB loop, it is a very New Age design and not always a Good Example.)

[shooter]

I have no idea how to adjust it

Yup, that was my next ? 

not always a Good Example.)

That's the $ ?  what IS good, n hopefully i'll figure out with much help why

[shooter]

So I start reading about grounded grid, got as far as cathode is outta phase with anode
then I found this and gave up!

http://www.extra.research.philips.com/hera/people/aarts/_Philips%20Bound%20Archive/PRRep/PRRep-01-1946-381.pdf

this is the kinda thing that made me quit on the EE and just become a tech 
I'd keep asking when can we breadboard....... when you get the MATH right 
tech school, here's your breadboard kit, go burn something up, BUT, your better know why when I ask ! yess 

[PRR]

Electronics (magazine) Feb 1950
https://www.americanradiohistory.com/Archive-Electronics/50s/Electronics-1950-02.pdf
p 104
A D-C Amplifier with Cross-Coupled Input
By J. N. VAN SCOYOC and G. F. WARNKE
Armour Research Foundation
Illinois Institute of Technology
Chicago, Illinois

[shooter]

Thanks PRR, bookmarked the page, did a 1st read.  Thankfully one of my old "Students" is home on leave so we got coffee! 

I don't know why I find ancient technology way funner than the modern 

dave