Capturing audio from 1951 Revere T-100 reel to reel

turner3d

New member
I have a box full of old reel-to-reel tapes and the original 1951 Revere T-100 they were recorded on and would like to transfer their contents to digital. I'm not worried about the actual process - I have and know how to use Sound Forge, but I am having a hard time figuring out what I need to do to connect this unit to the PC for capturing audio.

The issue is that there is no line out or headphone jack on this unit - the only output is a speaker jack. It is a tube unit and I have no idea what the output impedance or voltage might be, so I'm guessing that connecting this directly into my sound card is probably a really bad idea. I have attached a schematic of the unit if it helps.

Does anybody have any suggestions on what kind of interface I need to keep this thing from frying my sound card? Maybe an isolation transformer of some sort?

Thanks in advance!
 

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  • revere t-100 schematic.pdf
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What a brilliant chap! Posting a schematic! SO helpful.

But, OOO, that's an oldey! Ok, yes you are correct, plugging into the speaker jack will open circuit the transformer secondary and not be good. I doubt it would actually cause damage since you would not be driving the OP valve (6V6) that hard (but you never know with a traff THAT old!) but the sound quality would likely be very poor and it might even go instable. You are also correct in that it is the only way to get a signal out unless you are prepared to "go inside"? We can discuss that later if you like.

So, speaker jack it is but you need more than just a jack plug to computer input (3.5mm jack?). First you need a load resistor. You say you have no idea of the speaker impedance (aka Z) but it is to your credit that you are so aware! Actually it doesn't matter a lot. It is probably 3 Ohms but could be 15R (how big is the speaker? ) but 8 Ohms would be fine in either case, in fact anything 3 to 20R will serve.

The resistor is soldered across the jack plug (TS mono) and now you need a 3.5mm stereo jack plug cable to 'f' all to solder to it. Stereo because you want the mono tapes to record on both channels I guess? The 3.5mm lead will have a shield and two core. Strip the core insulation back 10mm and twist together then trim and solder to the centre tag of the 1/4" speaker jack. Shield of course goes to the sleeve. If you like I can draw that up and post a scribble.

A better solution would be to take the signal off inside the recorder but this would involve the use of an external Audio Interface with a "high Z" input.

Dave.
 
I would use one of my passive direct boxes, pad engaged, between the output and an external speaker. But I have those things just sitting around.
 
I would use one of my passive direct boxes, pad engaged, between the output and an external speaker. But I have those things just sitting around.

Yes, good idea if you have the kit but I would still prefer a resistive load, a speaker can colour the sound.

Dave.
 
Yes, good idea if you have the kit but I would still prefer a resistive load, a speaker can colour the sound.

Dave.

Would the speaker make any difference to a parallel resistive load that was substantially higher impedance than the speaker itself?
 
Would the speaker make any difference to a parallel resistive load that was substantially higher impedance than the speaker itself?

Yes, because the source impedance driving the speaker is going to be a lot higher than zero (pentode). Next time you have that rig setup chap, just gently tap the speaker cone!

In the OP's case I dare say the original quality is not high and I am being picky! But, "there's a right way to do it......"

Turner3d, you mentioned an isolation transformer? Might be an idea if you run into hum, aka ground loop troubles and IF you do there is a very cheap, £5.00 two channel one on Amazon that is easily good enough for this task. N.B. You will still need a load primary side of any iso-traff. (ooo! Tis early) And in that event you are now looking at "a bit of a project" and best gathered together in a tin. Load, traffs, jacks, di-da)
Dave.
 
Thank you so much for taking the time to consider this - it is very much appreciated! Just to be clear, you are recommending that I go straight from the speaker jack to the sound card input with a resistor (between 3 and 20Ω) wired in parallel to the connection? Do you have any idea how many watts this resistor should be rated for? I'm guessing that in order to keep the signal/noise ratio nice I'll probably have to turn up the deck's volume a bit. BTW, I realize that my "sound card in" of the schematic is mono - I'm planning on just wiring to one side of the stereo 3.5mm plug.

Sorry for so many questions, and thanks again for the assistance!
 

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  • t100-interface.png
    t100-interface.png
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Thank you so much for taking the time to consider this - it is very much appreciated! Just to be clear, you are recommending that I go straight from the speaker jack to the sound card input with a resistor (between 3 and 20Ω) wired in parallel to the connection? Do you have any idea how many watts this resistor should be rated for? I'm guessing that in order to keep the signal/noise ratio nice I'll probably have to turn up the deck's volume a bit. BTW, I realize that my "sound card in" of the schematic is mono - I'm planning on just wiring to one side of the stereo 3.5mm plug.

Sorry for so many questions, and thanks again for the assistance!

Do not apologies! All good stuff. Yes you need to have a decent amount of drive from the valve and if you blew the 'its off that 6V6 you would only get 5-6 watts (at 10% distortion!) So, a 5W R should be fine. If you have any kind of AC voltmeter (and you strike me as the kind of chap that would) look for around 0.5 to 1volt across the load, that should keep you well above 'system noise' but well below distortion. However, possible snag.

You are going into (ugh!) a PC soundcard and they tend to be neg 10 devices (300mV ave) and so 1 volt is about their limit and so, get it right for the valve, overload the soundcard!
Easiest solution is a pot across the load and the SCd input goes from wiper and earth. Any old pot will do and almost any track value. 10k log is pretty ideal but it really won't matter.

So, recap: Set for around a volt then pot down to get -12dBFS in the software average, max peak -6dBFS, if running 16bits. Should you be able to run at 24bits, neg 18, peak -8ish.

Dave.
 
Having had to do this more times than I can remember about 50+years ago, the best solution in my opinion --- assuming that you can use a soldering iron !!! --- would be to go inside the unit and find the volume control (a 500K pot) and solder (say) a 0.1uf capacitor to the center terminal of the pot and then on the other side of this connect an unbalanced microphone wire (ie active and earth/shield wires -- eg a guitar type lead) with the active wire going to the capacitor and the earth/shield wire going to the earth pin on the pot --- you should be able to see this quite easily and probably the pot terminal on the RHS when looking from the rear of the pot.

This connected audio cable can then be (somehow depending upon any space --- possibly need to drill a small hole on the case) taken out of the recorder's case. Then solder what ever type of plug you require on the end of this cable. For connection to a PC, I would probably recommend a stereo 3.5mm plug with both of the active connectors soldered together (to give you dual mono signal) and connected to the active wire in the cable and the earth wire going to the earth on the stereo plug.

This way, although a bit complicated, in my opinion will give you the best result and the output will most probably (although I have not looked up the valve specs) be at a maximum (ie volume control turned fully up) of about -10db, which your sound card should be designed to handle when set to (say) Line In.

Hope that you can understand all of the above and that it helps a bit in your quest.

David
 
Having had to do this more times than I can remember about 50+years ago, the best solution in my opinion --- assuming that you can use a soldering iron !!! --- would be to go inside the unit and find the volume control (a 500K pot) and solder (say) a 0.1uf capacitor to the center terminal of the pot and then on the other side of this connect an unbalanced microphone wire (ie active and earth/shield wires -- eg a guitar type lead) with the active wire going to the capacitor and the earth/shield wire going to the earth pin on the pot --- you should be able to see this quite easily and probably the pot terminal on the RHS when looking from the rear of the pot.

This connected audio cable can then be (somehow depending upon any space --- possibly need to drill a small hole on the case) taken out of the recorder's case. Then solder what ever type of plug you require on the end of this cable. For connection to a PC, I would probably recommend a stereo 3.5mm plug with both of the active connectors soldered together (to give you dual mono signal) and connected to the active wire in the cable and the earth wire going to the earth on the stereo plug.

This way, although a bit complicated, in my opinion will give you the best result and the output will most probably (although I have not looked up the valve specs) be at a maximum (ie volume control turned fully up) of about -10db, which your sound card should be designed to handle when set to (say) Line In.

Hope that you can understand all of the above and that it helps a bit in your quest.

David

I agree, best way to do it but there is a snag. The signal inside a valve pre amp is at a fairly high impedance, the input to a PC will only be 10k at best and could cause distortion but certainly if the OP is up for the task, worth a go, cannot damage anything.

Had he a proper AI it would have a high Z input and Bob's yer mother's brother.

Dave.
 
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