The AM Forum

At a hamfest over the weekend I scored a nice Viking II one owner, appears to be factory wired. It is not a CD version however.

My question is, what are the best upgrades to the audio section?
I have read here and other places about the weak link in the audio, replacing the driver transformer with a Hammond 124B, etc, and I am wondering if it would be better to build a 75-100 watt capable modulator and outboard the audio.  If I keep the factory audio section, I plan to run line level audio into the second stage of the unit.
All opinions and comments are welcome. Thanks.

Electric Radio Magazine #324 and #327 have the latest upgrades so i would consult those issues.

Whether it's a VII or a VII-CDC version, the audio circuits are pretty much the same.


Phil

Check with Janis-AB2RA.  She has been reworking some design issues on the Viking II and her work is very good.   She is in the process of writing a very comprehensive technical paper on the transmitter.  Her email is on her QRZ.com page.

I do have a Johnson Viking II and they are great transmitters.

73,
Joe-W3GMS
 

This works

https://www.amwindow.org/tech/htm/viking2.htm

A few years ago I was helping a neighbor get a Viking-II going and he had me put one of those Hammond 124B transformers in it. It was little to no better than the stock transformer. Notice the specs for it give a low frequency cutofff of 150 HZ. Not exactly HI-FI. The primary saturated too making things grungy. This was while using a triode connected 6AQ5 audio driver. When I did up my own Viking-II I used a 6AQ5, and for the driver transformer a UTC S-19 mod transformer back connected (it will fit under the chassis). I kept the 6AU6 mic amp, and ran inverse feedback from the secondary of the mod transformer to the cathode of the 6AU6. This reduced the gain considerably and it came out just right for a a high level input. It's one of the best sounding transmitters I have and has been trouble free for over 20 years. Good luck with your project.

I triode connected the 807 modulator tubes and fed them with outboard audio.....the stock audio section is clipped and left intact ..
The outboard audio is transformer coupled to the grids of the 807s..Sounds like Arthur Godfrey....Good Luck...Steve

Thanks to everyone. I am familiar with Janis page Wireless Girl.  In fact I read it about three times before I bought the Viking 2.  I will go re-read it. and I will digest all that you have contributed, thanks again to you all.

Two ends of the spectrum - inboard mods or an outboard modulator. The later is easier and will yield better audio. I used a solid state amp and a tube audio amp output transformer with a heising modulation reactor to modulate the pants of a Viking II. Arrangement at the link.

http://amwindow.org/tech/htm/ssmod.htm

Two inboard mods at the links.

http://amwindow.org/tech/htm/viking2.htm

http://amwindow.org/tech/htm/vikinga.htm

Hi Bob,

First thing to do its to make sure everything works the way it should.

Has it been sitting unused for 30 years? Are the tubes all good? Any damaged components? Dry/bulging electrolytics that should be replaced? Sweaty resistors? Dusty/dirty air variable capacitors? Arced band-switch contacts?

Give it a good going-over and fix what should be fixed. Clean the cabinet and knobs. Make it like new. Then park it your favorite band and operate it for a while.

You may find that you kinda like it... just the way it is.


Don

Pretty much every Johnson and even my 833 rig started with this preamp and phase splitter. No transformers required.
Drives the Mod Tubes of 100 watt transmitters just fine.
Keith


(https://criticalradio.com/833%20Rig%20Project/Pre%20Amp%20-%20Phase%20Splitter.jpg)

I’ve used Janis’s design in my Viking 2 but had to make some changes. For her design you have to use the E version of the Hammond transformer. Initially I buil it as designed and had response issues, significant roll-off from 500 Hz on up. After talking to Tim (HLR) he suggested I turn the interstage transformer around, putting the secondary on the PP 12AX7 and the primary on the grids of the 807s. This made a huge difference. The input to the 807s is flat from 50 Hz to well over 15KHz. Janis’s dual 12AX7 design has tons of gain so drive is not an issue. I still have a little roll-off at the output of the 807s but I’ve not yet reduced the value of the plate bypass caps or the cap across the 807 output. That is next (awaiting some .001 1KV caps), then solid stating the supplies and regulating the screens.

One consideration with Janis’s design, you’ll meet to resize the chassis holes to fit 9 pin tube sockets. It wasn’t an issue for me as I have a set of punches but it may be if you don’t. The second 12AX7 will be pretty snug against the back of the meter shield but it will fit as long as you keep the new hole centered.

Janis was very helpful while I was building and troubleshooting, as was Tim.

Good luck!

One other consideration: W2BTK posted his upgrade to the AM forum on QRZ.com. His design uses toroidal transformers in place of the modulation transformer and his audio chain is a solid state stereo amp. I’ve heard it on the air and it sounds really good. I was going to go that route when I ran into the response issues but decided to first try Tim’s suggestion. I couldn’t bring myself to scrap all the work I’d done to date. Had I not already done Janis’s upgrade I probably would have tried this solution, in part because it’s a little outside the typical approach box.

Here’s the link if you’re interested: https://forums.qrz.com/index.php?threads/using-toroidal-power-transformers-as-a-modulation-transformer.641974/

For enlarging holes in transmitters for small tubes, a unibit works wonders.

I have unibits that cover from 1/8 hole and can step in 1/16 steps to an inch.  4 different bits.

Mine where not cheap.  But, I did buy a cheap 4 pack of them  that where titanium covered in a nice presentation box for like 20 bucks from China.  Use in the drill press.

For holes from 3/4 to 6 inches (large TX tubes), I use a hydraulic punch and dies.

Unibits are a tube guys best friend.

My next purchases in the chassis building department  is going to be some square punches.

--Shane
KD6VXI

The phase splitter is the diagram that Keith did post can be improved quite a bit. In order to have perfect phase splitting, the two cathodes should be driven by a current source or a high resistance. BUT, the grids should be referred to GROUND, not to the cathode potential like in the diagram to have perfect counter-phase signals. This is an error. This circuit does NOT work well as phase splitter if the grids are referred to the cathode. Than the anode resistors should EQUAL for an ideal phase splitter. The fact that tey are not is an indication that there is an error in the diagram. The better the two signals are matched the less is the even harmonics distortion.
I propose to give the left tube a positive voltage of approx 60 Volts using a voltage divider from the +300VDC and connect both 1 Mohm grid resistors to this divider. You can omit the 1 K cathode resistor and only use the 10 kOhm. The 10 KOhm should be high compared to the cathode impedance of a tube which is 1/S. 10 kOhm is not that high but acceptable
I use in general a J-FET or transistor as a current source to drive the cathodes instead of a high resistor. Than you do not need such a high positive voltage at the grids, have more signal headroom and you have a more constant current.

This topic is discussed at length here, see link below. The guitar amp folks DO NOT want perfect balance. The tonal quality is sometimes better with some 2nd harmonic distortion. Maybe that is why Arthur Godfree sounds so good.  :P

http://www.valvewizard.co.uk/dcltp.html

Jim
Wd5JKO

I thought you want to make a low distortion modulator, not a guitar amp
I have a completely different experience Jim I did repair almost all guitar tube amps in the south of Spain and produced as well tube amps ( https://www.radiomuseum.org/r/highfre_lola.html ) . The guitar players that I knew were all in the opinion that, when you overdrive a tube amp, the sound is worse when even harmonics start to develop. Some Fender amps did that due to distortion in the splitter. We did do a lot of testing with Fenders and other amps and with my design, and they were all in agreement to avoid  even harmonics.

QUOTE:   The guitar players that I knew were all in the opinion that, when you overdrive a tube amp, the sound is worse when even harmonics start to develop. Some Fender amps did that due to distortion in the splitter. We did do a lot of testing with Fenders and other amps and with my design, and they were all in agreement to avoid  even harmonics.
[/quote]

Very interesting....What kind of music were the guitar players in Spain playing...?