• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Amplifier circuit from tube radio


2013-04-03 5:23 am
I am newby and say hello to everybody here.

I currently salvage parts from a viking51-102 tube radio.

Try to use the 2 6v6, power xformer, o/p xformer. preamp, circuit. not sure what 12au7 and 6sn7 do. Will they be part of amplifier circuit if I am building the amp myself from those parts or they belong to the radio side?

any suggestion is appreciated.
If, as I suspect, a guitar amp project is what you are working on, this thread belongs on the Instruments and Amps "board". Thread movement by the moderators is quite likely. Also, you may get objections about gutting what appears to be a pretty nice unit.

The tone control stuff complicates things, but what you have is a 6SN7 section voltage amplifier feeding a 6SN7 "concertina" phase splitter that, in turn drives the push/pull 6V6 O/P tube pair. The 12AU7 is providing voltage gain too.
Calling London... (I've always wanted to say that on the radio, have to settle for this I guess :D )

The Viking engineers provided a good way to follow the signal path - just follow the heavy lines on the schematic. Very thoughtful of them.

Working back from the output transformer (OPT) on the right-hand-side of the schematic. Both of the 6V6 tubes and the OPT are the push-pull output stage. You need to keep that section and all associated parts.

The 6SN7 uses one triode (pins 4-5-6) as a phase splitter. This circuit provides the proper voltage waveform and phases to properly drive the 6V6 stages - you need to keep that too.

The other triode (pins 1-2-3) is an audio amplifier stage. You need to keep that.

Everything else (except, of course, the power supply) is essentially in the "radio" world. The 12AU7 is wired as a detector (pins 6-7-8) and first audio amp (pins 1-2-3) - you probably don't need the extra audio gain (it was necessary to amplify the small detected radio signal) and you certainly don't need the detector. Thus, the 12AU7 is unnecessary.

I suggest tapping in at pin 1 of the 6SN7 that drives the 6V6 tubes - that is your first, best "clean" audio input. You probably don't need the Tone control stuff - the radio might have, but a plain audio amp doesn't.

Leave the 1 Meg resistor to ground (pin 1 of the 6SN7)) - that is your grid-return resistor. Either lift the 0.02uF (T.02) cap from the tone switch or remove the capacitor completely and use a new audio cap - either way, pin 1 of the 6SN7 is your "audio input" for this purpose.

The rest of the signal circuit (from the antenna to the tone switch) you can consider "spare parts". Remove the B+ connections to the rest of the circuit as shown in the attachment - you can leave the heaters connected or remove the extra tubes, they won't affect the audio portion.

Replacing the old capacitors in this radio is essential in the power supply section - the electrolytic capacitors dry out with age and will fail spectacularly if you apply power to one that is past it's use-by date! You will be best off replacing all of the other capacitors as well - the paper caps degrade with time.

I also recommend that you get rid of the 2-wire line cord and replace it with a modern 3-wire cord. Connect the black wire (Line) on the new line cord to a fuse (add a proper fuse holder - there does not seem to be a fuse in the original radio) and wire the other end of the fuse to the power switch. The white wire (Neutral) connects to the other transformer lead

Most importantly securely bolt the green wire (Earth) to the chassis using a star-lug! Very important safety issue - your metal chassis needs this to prevent accidental electrical shock in the event that something inside this old radio fails (like the power transformer insulation due to overheating, for instance).

Safety first!! :)

Also, don't bother to wire-up the 2-prong AC "phono-motor" plug on the chassis - it's a serious safety hazard to use it!

If you want to go to the trouble of installing an IEC connector (a "PC"-like power connector for the AC line), it's possible - but will require you to put a large square hole in the chassis to mount it. If you don't use an IEC connector, make sure to properly strain-relieve the AC cord to the chassis using a Heyco to securely attach the line-cord to the unit.

You will also need to drill a hole for your input connector - so no matter how you look at it, you're going to have to do some metalwork.

This will give you a single-channel audio amplifier - not stereo.

As a side note, you might be able to wire up the 6U5 "Magic Eye" tuning indicator on this radio as an audio level indicator. It's cool looking, it's green - but that is way past my word limit for tonight :).

Finally, remember that you are tinkering with a high-voltage piece of equipment that can be quite dangerous unless you really know what you are doing. As with all postings on this site, you are responsible for your own safety and the safety of others who will use this device. No liability for any of these suggestions is taken by me or any other poster here - be safe, be sane.

Good luck with your project! :cool:

~ Sam


  • Viking Audio Amp Conversion.pdf
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