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Converting old hi-fi amp/tuner to a line-level signal 'warmer'
Old 23rd May 2006
  #1
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Converting old hi-fi amp/tuner to a line-level signal 'warmer'

A few years ago, I remember reading about a guy who converted old valve tuners into signal processors.

I have an old valve hi-fi amp and tuner - does anyone know if it's possible to adapt either of these into accepting a line level signal input output device? An electrical engineer told me that you could take the power amp valves out so that they don't need to drive a speaker, which would just leave getting the sound out at line level...

I have no idea how to adapt the tuner though...

It would be interesting to do as the amp makes my cd player sound great! Is this possible?

Cheers

Zedric
Old 24th May 2006
  #2
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brianroth's Avatar
 

Many (most?) hifi units were totally passive from a given line input to the line out jack (typically "tape" out), so there is nothing in the signal path besides some switches.

Active stages included the phono preamp, EQ amp, and power amp.

I would consider using a dummy load across the speaker output, with perhaps a pad across it. Also keep in mind the inputs are unbalanced high Z.

Bri
Old 25th May 2006
  #3
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paully's Avatar
 

Don't run it without the power tubes. Simply removing them will change (raise) the operating voltages globally. Not good if there are other active sections, such as a tuner, in the unit.

Either do what Brian suggests, or find a tech that can pinpoint the summing spot where all inputs (including the tuner section) feed the output tube section. Then decouple, ground the output tubes' input point, and reroute the L/R signals to a seperate pair of output jacks.. which you'll have to install. Line level, if not available/acceptable at one of the stereo input jacks currently in the unit, can be achieved by reconfiguring the input (load) resistance of any available stereo input pair.

This whole procedure is pointless if you like the sound that the output tubes are generating, in which case Brian's suggestion is the only option. That'll leave you with not really just a pre-amp, but a fullblown amp that's been stepped WAY down. A resistive network can then be added to the dummy load to give you a line level signal. It's a trick used by guys to feed a seperate Leslie speaker from the internal speaker of a Hammond organ that has a built-in speaker/amp.

Unless you have a schematic or a knowledgable tech, don't fool around in there, as there are high voltages associated with tubes, and you'll be shooting in the dark.

Best, Paul
Old 25th May 2006
  #4
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I've heard of your idea before and known people who tinker in this way extensively. My only experience was with an old Scott Hi-Fi from the 60's I believe. Yeah this can be a cool way to get some interesting sounds. Staying pre 60's hifi you can find all sorts of cool stuff. Preamps with xformer coupled preamp stages, odd filter sets, etc. The biggest deal with using these is interfacing. Level and impedance matching on I/O will have to be figured out. On the input side you're most likely to find phono inputs. Outputs are likely to be xformer coupled speaker outs. You can find some good options for input interfacing from Jensen xformers. They've got all sorts of models for different applications. On the output side you can probably get away with a simple resistor voltage divider to bring speaker down to line levelish. A pure resistive load may not be the ideal thing the output xformer wants to see but it should do the trick just fine. Othewise you could look into some sort of reactive load device like a THD hotplate. Good luck tinkering.
Old 25th May 2006
  #5
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Great!

That's just the sort of lead I needed - I'll get in touch with Jensen on the amp side of things...

I was thinking about the valve tuner thoough - although the line out would be line level the only thing going in must be the receiver.... What would you suggest there? Looking at the four valves in the tuner alone, I'm thinking mmmmmmm!!!

Thanks
Old 25th May 2006
  #6
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Paully,

Thanks for advice - yes I'm not experienced enough (or at all!) to do this, but I just wanted to see what would be involved.

I suppose a valve compressor would be the way to go, but I'd just been curious over the low-budget possibility of hot-rodding an old tuner/radio...
Old 25th May 2006
  #7
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paully's Avatar
 

Zedric,

I'm still not sure exactly what you've got there. Is it an integrated amp (pre-amp + amp), or an integrated amp with a built-in tuner? If you have the wherewithal, shoot a small JPEG pic of the front and back and attach it to a 'reply', along with any info on the unit. I don't know why you would want to include the tuner section into a pre-amp only circuit (or how) . The unit's input tubes, output tubes and related circuitry are the only things of importance.

Paul
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