Adding line output to vintage tube R2R tape recorder

[GlassEnvelope]

Hello -

I am going to be rehabilitating a vintage Voice of Music tape recorder and I'd like to add a line output. Most everything I see online on the subject has me just throwing something across the speaker outputs but I don't want to do that especially because there's a two-knob tone stack I want the output to be ahead of.

The most obvious place to me would be to tap into pin 2 (grid) of V2 which appears to be AC only; the output would be after the volume control, which is fine by me. But wouldn't it be best to include some kind of buffer circuit versus just taking pin 2 straight to a line out jack? The 6.3VAC filament supply has a false ground (150-ohm trimpot for hum balance adjustment) so I could conceivably rectify and filter that to power a buffer circuit. Something I might want to be mindful of is that in record mode, there is potential to pick up interference from the bias oscillator and I don't want that coming out over the line output.

[oc disorder]

Hello there, I can't see any reason why you can't just take signal from across the pot, maybe a 0.1 uF from the top of the pot and a sheild from ground would be enough.
An external "buffer" could be a graphic eq or whatever . When transistors came in there were heaps of tube radiograms for sale really cheaply when I was a kid.
I bought a cheap half track 2 speed recorder and took a connection from the "grams" volume control to feed the recorders input. Built up a collection of my favorite songs
that way as didn't have enough pocket money to buy records. If you have earth loop problems you may have to get an audio isolation transformer.
Using half of one of these would probably be ok. https://www.amazon.com.au/Conext-Lin...f_B082GV15Y1/?

[Leo_Gnardo]

If you have earth loop problems you may have to get an audio isolation transformer.

True that, however you have to be very careful where that transformer is placed. Audio transformers sense magnetic fields from nearby power transformers, electric motors and AC power wiring which means poor placement will inject hum into your signal, so some experimentation will be necessary. Also, there are more expensive transformers housed in shielded containers, often a cylinder or cube of mu metal. You may be able to scrounge up a second-hand one for reasonable $$$. Mouser has a likely example, part # 546-140UEX. But it's US$ 72.38, ouch!

The transformer OC suggests shows up "unavailable" but maybe that's because I'm in USA and perhaps that item from that vendor is offered only in Australia. If you're in the USA, or anywhere else, doubtless the same item is available from some vendor or another, you could scan Amazon for it. I'd look at Parts Express or MCM as well for that one and perhaps other offerings. And you can have a look at what Mouser offers here, all a good deal cheaper than the one I mentioned above, but note they are bandwidth limited especially in the bass, have no shield, and have varying primary/secondary ratios. FWIW I have used some of the cheapest (now Xicon brand) in items like DI boxes, and as long as the signal level isn't large compared to the max the transformer's rated for, found no audible deficiency in bass response. So the rolloff below 300 Hz probably applies to signals approaching full whack, and isn't so evident when passing lower level audio.

edit: had another look at OC's suggested transformer. It appears "milli" has been left out of some specs. Very unlikely this can handle 20 amps, with an input power limit of 30 watts. Add the "milli" and now the specs make sense. No telling whether the metal shell actually provides hum shielding. But the price is attractive for an experimenter.

https://www.mouser.com/Passive-Compo...yzmq2lZ1z0w8na

[Helmholtz]

Hello -

The most obvious place to me would be to tap into pin 2 (grid) of V2 which appears to be AC only; the output would be after the volume control, which is fine by me. But wouldn't it be best to include some kind of buffer circuit versus just taking pin 2 straight to a line out jack? .

When tapping into tube circuits it is essential to consider circuit impedance in order to not load down and screw up the existing circuit.
If you want to tap into pin 2, you will need a buffer having an input impedance of at least 20M (!).

A low source impedance output would be the cathode (pin3). You will need a coupling cap to block DC.

Audio (isolation) transformers always need to be chosen for correct impedance. I'm sure those transformer from Amazon are very low impedance and probably not suitable for tube circuits.
A decent high Z transformer must be much more expensive.

If you use the low impedance cathode output, the transformer should be for at least 10k impedance.
But I'm sure there is no suitable transformer for the high impedance grid circuit.

[eschertron]

Mouser sells iso transformers with 10k:10k or similar impedance ratios. If I were to do some non-destructive testing, I'd tap off of each end on R16 (cathode and ground) and run that to an acceptable transformer - could be on my breadboard - and then to the target device (preamp for a HD recorder, perhaps?). Once the impedance and the signal level are established, I'd concern myself then with hard-wiring anything.