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Old 3rd April 2013, 05:37 AM   #1
london is offline london  Canada
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Default Amplifier circuit from tube radio

I am newby and say hello to everybody here.

I currently salvage parts from a viking51-102 tube radio.
http://pacifictv.ca/schematics/viking51-102manual.pdf

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.
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Old 3rd April 2013, 10:27 AM   #2
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Not easy to read that schematic....
Anyway, after a quick look, looks like the 6SN7's and the 12AU7 are in the audio amp section. To make it short, the first 6SN7 makes the preamp section, the 12AU7 drives the tone controls and the second 6SN7 drives the 6v6 output stage.
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Old 3rd April 2013, 11:26 AM   #3
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What kind of amp are you trying to build ?

Audio or instrument amp ?

If you´re building a small audio amp i suggest you get a couple of SE output transformers and turn it into a small stereo amp.

Keep the 6SN7,6V6 and power transformer/choke add new OPT´s
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Old 3rd April 2013, 06:13 PM   #4
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Hi London - you've got a double-post here, two threads running on the same topic. Maybe the moderators can combine them?

Anyway, I posted a lengthy reply on the other thread, reposting here:

Quote:
Originally Posted by rfengineer2013 View Post

Calling London... (I've always wanted to say that on the radio, have to settle for this I guess )

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 correct 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 connection from the tone switch or remove the capacitor completely and install 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 replace the 2-wire line cord 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 failing 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 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!

~ Sam
Attached Files
File Type: pdf Viking Audio Amp Conversion.pdf (107.0 KB, 25 views)
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Old 3rd April 2013, 06:30 PM   #5
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There's also a supplied phono-jack if you don't want to do anything at all to the amp.

Still best to replace the old capacitors and convert it to 3-wire line cord.

~ Sam
Attached Files
File Type: pdf Phonojack.pdf (56.9 KB, 16 views)
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Last edited by rfengineer2013; 3rd April 2013 at 06:46 PM.
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Old 4th April 2013, 01:39 AM   #6
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Tank you very much sam
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Old 4th April 2013, 01:42 AM   #7
london is offline london  Canada
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Quote:
Originally Posted by flatheadmurre View Post
What kind of amp are you trying to build ?

Audio or instrument amp ?

If you´re building a small audio amp i suggest you get a couple of SE output transformers and turn it into a small stereo amp.

Keep the 6SN7,6V6 and power transformer/choke add new OPT´s
Audio amp for my rc80m and some others like mp3 or tuner
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Old 4th April 2013, 02:10 AM   #8
london is offline london  Canada
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Quote:
Originally Posted by rfengineer2013 View Post
There's also a supplied phono-jack if you don't want to do anything at all to the amp.

Still best to replace the old capacitors and convert it to 3-wire line cord.

~ Sam
Thanks a lot.
I do need the phono jack. I recently just restored Garrard RC80M. It need preamp.
According the print on this viking. Phono output signal goes out through peramp tube 6sc7 pin 5. it also receives B+ voltage from pin 5 and pin 2. phono output signal through T.02/600 cap--switch 1f---volumne control, then 2 T.o2 caps--12au7 pin7.

Do I need to keep 6sc7 and 2au7 if I need phono preamp circuit?


BTW, If I want to make a stereo(extra tubes), is it possible to let two channels share one power transformer?
thanks.
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Old 4th April 2013, 05:02 AM   #9
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Quote:
Originally Posted by london View Post
Thanks a lot.
I do need the phono jack. I recently just restored Garrard RC80M. It need preamp.
According the print on this viking. Phono output signal goes out through peramp tube 6sc7 pin 5. it also receives B+ voltage from pin 5 and pin 2. phono output signal through T.02/600 cap--switch 1f---volumne control, then 2 T.o2 caps--12au7 pin7.

Do I need to keep 6sc7 and 2au7 if I need phono preamp circuit?


BTW, If I want to make a stereo(extra tubes), is it possible to let two channels share one power transformer?
thanks.
The comment on using the existing phono connector was to not really change anything at all. Just replace the electrolytic and paper caps, add the 3-wire line cord and use the radio as-is. The selector switch on the radio probably has an "Aux" or "Phono" input that switches the external audio input through the existing audio amp circuit.

If you decide to do a rebuild (which doesn't seem necessary), then my suggestion would be to tap into the 6SN7 gain/phase-splitter stage and not use the rest of the audio path.

As far as the power transformer goes, I couldn't say whether it is sufficient to power a second audio channel. If you convert the output to single-ended, possibly. Unlikely if the transformer has enough margin to add a second PP output stage.

Adding a second channel to this is really into the area of Major Changes, not a trivial task at all. Lots of good designs here and on the web if you want to search around - but at that point you're possibly better off selling the Viking as an "antique" and using the money to buy something new and appropriate for your needs.
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Old 10th April 2013, 05:17 AM   #10
london is offline london  Canada
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Thanks for all the replies. I am just wondering where I should put my volume control pot.
According the original print, 1Meg pot is ahead of 12au7.
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