viking 2 audio mods

[kc2etm]

hi im looking for some basic audio mods just to increase freq response i dont want to change tubes etc.. ive installed a higher quality nickel interstage and would just like to open up the amplifier to more freq range for my eq but keep it more or less stock
does anyone know which caps i should change and to what values
thanks
fred
kc2etm

[Don, W2DL]

Just go to www.amwindow.org, there's a bunch of stuff there. No reason to re-invent the wheel!

Don, W2DL

[kc2etm]

hi don ive looked at the two mods they have there but they both use different tubes new circuits etc.. unless i implement those cap changes on the stock circuit but
i dont know if those will help the stock situation or make it worse 

[WD5JKO]

Freddie,

  The mods linked try to open up the freq. response, lower distortion, and allow the 807's to run AB2 such that 100% mod is possible. Your criteria of wanting to open up the response without changing tubes can be done.

   If you just increase the coupling cap size, and cathode bypass cap size, you will enhance the bass. But in doing so DO NOT push the audio much beyond 50% modulation. Pushing the audio will result in a lot of distortion as the level is increased.

   I remember how W5FLV (sk) did his Viking 2, and it was simple, and loud. Ken never had hi-fi audio, but it did pretty well. All Ken did was replace the 6au6 audio driver with a 6ah6 (no socket rewiring), and added a 33v zener diode to the interstage xfmr centertap to stabilize the bias to the 807's. Then I believe he beefed up the coupling caps, and cathode bypass caps. That was it.

Jim
WD5JKO

[kc2etm]

Hi i was looking at the 6ah6 and 6c4 mod
i think that is probably within my skill
level of tube circuits im fine with solid
state but not that familiar with tube
equipment will it really make a dramatic
improvent if its not much i dont know
if its worth the hassel
thanks
fred

[N2DTS]

Something comes to mind here:
Tube circuits are easy and fun to work on, you have to work hard to blow something up.
Its fun to get out handbooks, and study designs, and what others have done, to learn what works and what does not.
Also, owning really old tube ham gear might not work out real well without being real familiar with how to fix and trouble shoot it.

There is nothing really wrong with running a modern radio into an amplifier if you don't like or enjoy keeping old tube stuff working, lots of guys use all sorts of more modern radios.

Most radios don't respond well to just increasing the coupling caps, then you get hum and distortion, and run into power supply limitations.

Brett

[WA1GFZ]

I changed my 807s to AB2 and zener regulated screens., bigger driver transformer, 6aq5 driver.

[N3DRB The Derb]

6ah6 mic amp,  set and forget gain stage 6c4, and a 12b4 driver into the transformer is how I did mine many years ago. you do have to punch a hole right in between the existing holes to do this one though. but lawdy does that 12B4 deliver the goods. The stock SNC driver transformer has got to go, it's only good for 10ma current and will fry quickly if you try to get more through it.

you need at least a 30 ma unit to exploit the increased driver power to get the 807's into AB 2.

at some point I'd like to re visit a V2 and throw a 55 watt S-20 transformer I have in the plus the driver mods.

Hytron HY40's could deliver 135 watts with 750 B+. hard to find but cheap because nobody wants em including the audiophools.
they take about -35 bias, the HY40Z version 0 of course. I'll be looking for more of these this year. lots of power at low low plate voltage.....great 807 sub....but you do have to give em 7.5 volts.

[kc2etm]

hi guys thanks for you replys brett i do like to work on this gear and learn
when i got the radio two weeks ago it was laying in a basement rusting didnt do anything but blow fuses smoke hum and other bad things you could think of
i have the radio working pretty well now and ive learned alot along the way with the help from guys like yourself trouble shooting it is fun and a learning experince but
i was afraid of damaging something by doing the mods i have a ts-440 with the am mods wide tx mod and no one would believe it was a modern rig but if i cooked something doing that i could go down the street and get whatever parts i needed and fix it right up unlike the viking i have to track them down if it's something that is even around but if they are that forgiving as you say i will give it a shot but the thing is i dont know if more drive is what i need i use a preamp 32 band eq noise gate compressor limiter into a eurorack mixer and as it is now i have a 4db atenuator on my board output and i can still only crack the mic gain on the radio to 2 it just gets sickly lound like a good buddy radio i havent really changed anything but the interstage transformer to a 124b nickel core and retubed the whole rig
I was thinking about the k6ad mods are they anygood
thanks
fred
kc2etm

[kc2etm]

Hi i decided to bypass the speech amp and run my mixer board directly into the 124b tranformer and it sounds very nice clean and im getting 100hz to 14k out of it now but i dont think the mixer is putting out enough power to drive the 807s i have a small 5 watt mono amplifier do you think this would work or can i lower the value of the grid resistors to acomplish this same thing
thanks
fred

[WD5JKO]

Fred,

   My first commercial ham transmitter was a Viking I. I was a teenager in high school with a Novice license, wn8pep. The Viking I was a basket case, and I had no idea what to do, and I knew nobody that worked on the old old boatanchors. I had a few books, an old scope, I built an audio generator, and within a year I had that rig going just fine. The book learning on RF theory, and AM got me to pass the advanced class Ham license, and a few months later I passed the 1st class phone FCC exam, stone cold without study. The Viking I experience, and the passion of me, a young kid infatuated with radio stays with me to this day.

   You say that with your ts-440 you can get parts easily, and the old Viking II is full of hard to find parts. I challenge that idea saying that as time goes by the reverse will be more true. Today the state of the art changes so quickly that manufacturers discontinue devices all the time resulting in major issues finding replacement parts. Look at the Tektronix scopes like the 465/475 from the 1970's. These are great to have, but if you need parts, then oh boy! You better get a parts unit, or pay top dollar from someone who knows he has gold. That Viking II on the other hand doesn't have anything in it really that cannot be repaired, rebuilt, or replaced. The biggest fear with these old rigs is a blown transformer. These transformers can be rewound. Try to find some mid 70's or 80's application specific IC in an old solid state transceiver. The word "unobtanium" comes to mind.

  So I suggest you take a audio generator, a scope, a dummy load, and see with that Viking II does. No need for a complicated chain of external audio boxes at this stage. A working Astatic D-104 Microphone is fine. I always prefer to fix before modify. Doing a restoration at the same time as modification can get really confusing. Also many of the posted mods are in no way complete, and the schematics often have errors in them. Some are not well engineered either. So look at those circuits as a reference only. Instead of copying, apply a little theory, and devise your own solutions. You will be glad you did since the transmitter will be a product of your ideas and hard work.

 
Jim
WD5JKO

[N2DTS]

Yes, there are loads of ways to go about it, it depends on what you want to do.
You can run the audio rack into a small 8 ohm amp into an 8 ohm to 5000 or 10,000 ohm ct trans to the modulator grids, or replace the mod tubes with something like KT90 hifi tubes and run AB1 driving the grids with a phase splitter, improve the circuit in the rig, or make a line level input tap, etc.

I think I would not try to make the rig hifi, just nice sounding and clean.


I would also say ts440 parts are harder to buy then stuff for old tube gear.

Brett
 

[WD5JKO]

Fred,

   Having an outboard audio driver as you describe is an idea worthy of considering. You can make it all solid state too if you prefer. So driving those 807's leaves two problems with the 807 modulators, which are, grid one bias regulation is terrible if you run the tubes into class Ab2, and the screen voltage regulation is also terrible since that voltage is derived from a tap on a high resistance bleeder resistor. 

    You can choose to fix those problems, or maybe eliminate the need for those two supplies. Go to www.amforever.com, scroll down to RCA Transmitting Tube Manuals. Then open up the TT4.pdf manual, and look at page 239. Here you see two 807's in class B without any bias or screen supplies. Configured this way, the 807's act like a high mu triode, somewhat similar to a 809. This approach will need an audio driver capable of 5-10 watts rms with a low source impedance.

So your idea has merit. The decision remains whether to repair to stock first, or to take a "squeegee" to remove the stock inboard audio driver, and replace with something external that is all your creation.

Jim
WD5JKO

[kc2etm]

hi everyone thanks for all your help ive read that i can adjust c32 and c46 at the bototm of L21 to pass higher frequencies for instance the one recomended was .0047
to pass up to 5.8k now my question is if i just remove these splatter filters and limit my external audio chain at 5.8 or less will it give me the same results also what voltage caps do you recomend
thanks
fred
kc2etm

[kc2etm]

ok i removed the two caps and l21 now where do i attached the single cap i have the band on the large choke that one end of l21 was connected then the other end was connected to another band which looks to be connected to a wire comeing from the bottom of the chassis  i dont have the choke out of the radio so i cant see it that well will it hurt if dont attach a cap at all for now as the .001 cap i have looks a little wimpy thanks
fred

[kc2etm]

ok i did already remove everything thinking i had the proper cap
just to be sure because i know these caps can ruin the mod tranny from what i read
can i run with nothing while i order one or do i have to reinstall one of the old ones
thanks for your time
fred

[kc2etm]

So for the audio i went the route of an utc
ls-10 transformer right to the 807 grids
completley bypassing the driver section
and driving the transformer external solid
state equipment
is there any other frequency limiting points
in the radio like th l21 caps being removed
thanks
fred

[kc2etm]

so c55 would that be the disc just under t3 on the terminal strip i see it is gone from the schematic  in alot of the audio mods
thanks
fred

[Bacon, WA3WDR]

This may seem contradictory.  Regulated screen voltage is best for lower distortion, because you don't want one 807 affecting the screen voltage of the other 807.  At the same time, I am always in favor of an unbypassed resistor in series with each 807 screen grid, somewhere around 2.2K to 5.6K in value, 2 watts or more, not wire wound.  To retain the audio power, give it about 50V higher screen voltage, and a little more negative control grid bias to control the resting current.

I did this with 8417s in my Viking II (820 ohms 2W on each screen grid).  I later tried it with 6L6GCs in an audio amp (5.6K 1W in series with each screen grid), although I did not regulate or change the screen voltage in that unit (300V plate, 300V screen).  It improved linearity very noticeably in both cases.  I never tried it with 807s, but it should work.

The unbypassed series resistance makes the screen voltage drop at higher drive levels, tracking the audio waveform.  With the right amount of series resistance, this compensates for the upward curvature in the plate characteristic.

I tried both screen grids tied together and through one resistor to a common screen supply, but that made distortion worse.  One resistor in series with each screen worked.

  The Old Bacon, WA3WDR

[kc2etm]

hi so i installed the ls-10 transformer driving the grids but my question is its a 60k
secondary split with centertap so do you recomend i add biger resistors to the feeds
to the 807's in place of the factory 100ohm resistors to raise the impedance of the transformer if so what value do you think would be appropriate
thanks
fred

[Bacon, WA3WDR]

Don't change the control grid resistors.  Significant series resistance will cause unnecessary flattening of peaks. 100 ohms is small, and it serves to protect against parasitic oscillations.

The control grid impedance is very high until conduction, where it becomes lower because the grid conducts.  The tubes are driven into the control grid conduction region to get more plate current on peaks, for more output.

The transformer grid-side winding impedance is different from the effective source resistance presented to the control grids.  The impedance is important because we need voltage swing, and we have limited drive power.  So we don't want too low of a grid-side impedance, because that would mean too little voltage swing, and we also don't want too high of an impedance, because that would give us too little output current capabililty, and the grids need current on positive grid peaks where plate current is maximum.

The effective resistance in series with the source is a function of winding resistance, the plate resistance of the driver stange, and negative feedback.  Generally the lower the better, and negative feedback reduces the effective resistance.

[kc2etm]

thanks i will leave the resistors then
fred
kc2etm