The AM Forum

I'm working on a Viking 2 restoration for AM use. 

My goal for this radio is to produce high quality, low distortion audio within reasonable bandwidth constraints like 150hz to 3.5kc.    Not looking for high bandwidth, just high quality.   My parameters for this are to generally stick with the original architecture and iron, all controls "work as original", re-built/re-designed with modern components as required, but no active semi-conductors.  So far, all the power supply filter caps have been "up-sized" by a factor of 8 to 10, all the rectifiers replaced with SS, and a Harbach soft-start module added to the HV.  The modulator screen will be regulated, derived from the low voltage supply, and switched on for AM TX by the PTT relay.  The original 5R4 HV rectifier sockets are being recycled to hold a pair of VR-xxx tubes for the modulator screens. 

I expect to be running this TX barefoot so want to produce as much PO as reasonably possible.

After reading and re-reading everything on the Internet and in back issues of ER, the K6AD audio modifications in ER #110 and 111 seem the most compelling.  Those mods preserve the 6AU6 audio tubes and will work with the Hammond T124B inter-stage transformer that I have on-hand.

However getting normal 2X 6146 power levels with near full modulation requires adding an additional audio stage and is still marginal, power wise.

Considering that a fundamental shortcoming of the original architecture is the inability of the 807's running in AB1 to fully modulate the 6146's, I wonder if it wouldn't be easier the convert the 807 modulator tubes to 6146's in AB1 than to add the additional audio driver circuitry to support running the 807's in AB2 with low distortion?

This seems like it would be an easy conversion, particularly since I'm rebuilding the screen and bias supplies anyway.  But I haven't heard of this being done, and I wonder if anyone on the board has any experience, thoughts, or ideas about this.

73 Jack KZ5A

 

   

I built a zener regulator for my 807s and run AB2. I put a 6aQ5 in the driver.

Viking 1 & 2s, Rangers, Valiants, Apaches, and others, there are more mods out there than Grandpa's secret recipes for barbecue sauce! ! ! ! ! !

You got to look over as many as you can and pick the one that works for your needs and skill level to do. I have done bunches of them over the years and have kinda settled on the 12AX7 speech amp ones. Whether or not to use a fuzz inverter or driver is strictly dependent on the mod tubes used and the class of operation that you want to run them in. (Or how much audio you need to have on tap)

If you dont need to push the modders into the grid current range, a fuzz inverter is preferable. But a driver and driver transfoma helps keep it simple. (kiss rule)

Jack,

   As Bacon pointed out, there are so many options to follow here. It depends so much on your ability, and time available devoted to these modifications. I cannot see the ER issues you mentioned, but it is typical for posted mods for these rigs to not be 100% complete. There is usually a gotcha somewhere that you have to figure out.

   One suggestion might be to look at the original design, and then weigh the pros and cons of it. What did Johnson do right, and where did they fall down? Then based upon your already stated goals, try to take a stab at it based upon your experiences, and motivations. This way the changes will all be yours, and you will learn a great deal. When I do this myself, I have a blast getting there.

Jim
WD5JKO


Oh, look at the VR tube data, and pay attention to the "minimum DC anode supply" specification:
http://www.mif.pg.gda.pl/homepages/frank/sheets/141/g/GL874.pdf

The problem is NOT the 807s, in this case. It's the 6AU6's inability to drive the 807s. I use a variac on the plate supply of my hotrod Viking 1. I typically run 135 watt carrier with the 4CX250B final at 100%+ modulation using the 807s. The trick is to drive the 807s sufficiently which the 6AU6 cannot do.

Slab's 6AQ5 is a great choice or, like mine, I use a 12AX7A input amp followed by a 6C4 into the interstage transformer. The 6C4 has twice the drive of the 6AU6. Checkout the mod here:

 Viking 1 and 2 Audio  (http://www.amwindow.org/tech/htm/viking2.htm)

Good luck, have fun and be safe!!

FWIW, The RCA tub manuel has a ckt fer runnin the 807's in special triode connection.  10 watts of drive gives 120W output. This is enough to make the Vikings sing.

Hear is a link.

http://www.youtube.com/watch?v=g8huXkSaL7o


klc

Slab, Jim, and .....

I'm a little vague on what a "fuzz inverter" is ???

The original design certainly has it's interesting aspects, and for the most part seems to accomplish the designer's goals.   One of those was the "high intelligibility" audio passband which I'm sure seemed like a good idea at the time but resulted in the Viking 2's reputation for thin audio.  This limitation was compounded when the finals were upgraded to 6146's in the Viking 2 without a corresponding upgrade to the modulator section. 

I have looked at several "audio upgrade" articles for the V 2, most of which focus on the low level audio stages and don't deal with the basic problem of the original 807's not being able to fully and cleanly modulate a pair of 6146's.

Originally I worked up a design for new low level section based on the 12AX7/6C4 approach but with a 6EU7/6C4 circuit partially plagiarized from a 60's vintage Heath Hi-Fi amp and a better inter-stage transformer.  This would be a big improvement but still requires running the 807s in AB2 which gives rise to a bunch of complications I'd just as live avoid.   AB1 is much simpler to implement well due to the more consistent impedance presented to the driver.     

The June/July/August 1998 ER articles are by far the most comprehensive and well explained look at the Viking 2's shortcomings that I have run across.   The author's solutions were too "transistor-ish" for my purposes but the technical analysis of the root problems was excellent.  In addition to a solid discussion of the AB1/AB2 807 issues there is also a good look at the 6AU6 audio stages with what seems to me like a good low distortion re-design that preserves the 6AU6s.

The only "gotcha" I detected in that article was doing away with the 20K HV bleeder resistor which is probably OK for AM only, but looks like it would not maintain minimum current to keep the HV from soaring on CW during key up.   I could be wrong on that, but that's my understanding of choke input filters.

So baring hearing a "been there, tried that, didn't work" my direction on this project is to replace the 807 modulator tubes with a pair of 6146W's (B's) running in AB1, preserve the 6AU6's with new circuitry, use the Hammond 124B inter-stage transformer, and also replace the finals with 6146W's.

The lower screen voltage on the 6146 modulators (~200V) relative to the 807s (~300V) makes deriving it from the LV supply practical assuming a provision is made to switch it on and off along with the HV.

The power supply mods are focused on substantially improving the dynamic regulation, eliminating the voltage drop of the tube HV rectifiers and regulating the bias and mod screen voltages.

The RF section will probably end up unchanged except for replacing the usual old out of tolerance R's and C's.

If all this works as hoped, I'll end up with a really good sounding, well modulated, 125W AM TX with no new holes, no transistors, no empty sockets (the rectifier sockets are being recycled to hold VR tubes), and no changes to the normal operating procedure.   The only externally visable changes will be the  VR tubes in the ex-rectifier sockets.

This is a long term project, no big rush.   It has already caused me to review/relearn a bunch of stuff that I had forgotten decades ago and has been a very enjoyable endeavor.   I'm taking pictures as I go, mostly to help me remember which wire went where originally.  Maybe if it works I'll post the details.

I'm always looking for input and appreciate everyone's comments and suggestions.

73 Jack KZ5A






 

Jack,

   You definitely have a plan, and I'd say run with it. If your like me, 90% of the fun is in getting there.

   Heck, some would argue that 6146 are poor audio tubes, but I still recall that Acrosound application note where a pair of 6146's made "enough sound to fill a stadium", and did so with high fidelity. They used tertiary windings for the screens, and cathode follower drive though. Also the resting class AB1 idle current might need to be set at the tubes Pd rating for low IMD.

   So taking this approach you are running away from the pack, and doing something pretty much original. Good luck with that. I'm no stranger in that arena, pushing a Retro75 to 30w PEP output, or running a single EL-34 on 20m SSB grid driver, AB1, running a pair of 7591's on 160-80-40m at 65w PEP RF output SSB, or 15w AM. It's fun breaking away from the pack.  ;)

Jim
WD5JKO

Well, ok, Jack.

I still totally disagree with your evaluation of the capabilities of the 807s considering I am modulating a 135W carrier with a pair in AB1 now. The Viking Bud is well known to many on this board and it's operation can be easily verified.

CAN I GET AN AMEN ON THAT???

But, as Jim points out, getting a plan and working it to conclusion is a lot of fun. I hope to hear ya strappin' into Northern Ohio soon!!!

I will second, third, and AMEN that Buddly. The Viking Bud has gobs of sparkling clean full-range audio. I'm also puzzled by the comments regarding the capabilities of a pair of 807's. I have never found them to come up short in the power department for handling the task of modulating a 100 watt final. I also agree that the issue with the Viking I and II series is in the low level audio stages. Before my grand expansion into a rack to allow for a pair of 811A's, I followed the 12AX7 -> 6AQ5 -> heavier driver transformer route and the 807's handled the task quite readily. As the RCA data sheets indicate, with +600 volts on the plate, a pair will produce +60 watts in AB1, which is more than enough to cleanly modulate a pair of 6146's to 100% all day long.

Then again Jack, if you come up with something better, go for it! That's more than half the fun of this whole adventure. It'll be fun for you and we'll all enjoy following your progress!

73,

Rob W1AEX

Jack,
       A "Fuzz Inverter" is amers slang for a phase inverter. If you are running tubes that make enough audio running AB1, you only need to tickle the grids with some voltage. 807s and 6L6s are fine with this. 6550s really get it done just barely tickling the grids with some voltage, and a phase inverter is OK-FINE to drive these tubes in AB1.

However if you want to beat the snot out of a pair of 807s in AB2 (much more outpoot) or run anything in class B where you must push them into the range of drawing grid current, a transformer coupled beefy driver is the way to go.

Typically in class A or AB1 a step up driver transformer is used, where in the classes of operation that want to see grid current, a 1:1 or even a step down driver transformer is often used.   (dont beat me too hard, Don  ;D  ;D ) 

Slab,


Ahhh.   Yes, I would consider a phase inverter to be a better arrangement than most any inter-stage transformer for driving an AB1 stage, however I already have what I think will be a good xfrmr on-hand and preferred not to add a tube socket.   Might end up doing that anyway later if the transformer gives me any grief.

WD8BIL

I don't have anything against 807s, I'd rather keep them, but..... my understanding of plate modulation is that the modulator should deliver roughly the same PO as the RF stage and I don't see how one would get there with 807s in AB1, but then I don't know what a "Viking Bud" is either???

Hopefully I will be back up on AM this weekend with my HT-37/SB-200 setup ;D ;D.  The Halli needed a new LV filter cap (can) and I thought I was going to have to use dynamite to get the old one loose from the chassis. 


73 Jack KZ5A

Well Jack, the Viking Bud is my (my name is Bud) Viking 1. I've modified it with a 4CX250B in the final. I've solid stated the power supplies and put the plate transformer primary on a 140% variac. I get just under 1050VDC on the plate with 250ma current that's @ 260W input. The full carrier in CW hits 170 watts.

Now, in phone the modulator is turned on and the plate sags a bit. There's a big power resistor in to drop the plate voltage to the 807s to 750VDC. I drop the carrier to 135 watts and the 12AX7A - 6C4 - 807s FULLY modulate the carrier beyond 100%.

It's been working that way since 1973!!! Only changed the 807s once. I thiink it was 1998????
25 years on a set of 807s singing for their supper ain't bad in anyones book!!!!

 
That's one school of thought and it works well. But I've found that a 1:2 power ratio works just fine. Ifn you want "overhead" then the skys the limit. 200 watts will mod 100 watts too but why??

Whatever you choose I hope to hear it soon. Remember... we're all in this together!!!

 Viking Bud  (http://www.wd8bil.com)

I don't think the 807s are running AB1.

I agree with Buddly. Given enough drive, the 807s in the Viking I and II will easily modulate 100 percent. At about 100 watts out, I was able to get 120-125 percent modulation with mine.

AMEN 807s are a good tube for modulators and look the part
AB2 gives you more clean plate voltage swing. but you need to motivate the grid 1 a lot harder.

Frank said:
Thus the 6AQ5 or 6C4 or (insert any tube stronger than a 6AU6 here) upgrade in the low level audio section.


Yes, Steve, they are if you set the idle current according to the manual ala R13's tap. You can, however, set the screen voltage to idle them in AB2.

The VK Bud has a variable BIAS supply so I can keep it in either class when changing the plate voltage with the scrote knob. I typically run it in AB2.

As others have stated already. The greatest defeciency with the Johnson design is the 6au6 lower stages, not the 807's. The 6au6 driver stage is at best barely up to the task. To keep the original socket, use a 6aq5 or 6c4. For real umph, drill and blast a new 9 pin socket and use a 12ax7. My father has used a 12au7 push pull with amazing results. The 807's only need to make 50% of the final power to modulate the class C amp to 100%. With the 6au6 left in there, that will assuredly never happen.

I agree completely that we can make the 807's play ball in the Viking I or II transmitter. That said, if somebody wants to redesign to get 100 watts of audio in class AB1, a single pair of 807's won't do it within ratings, so another tube choice that will do it needs to be looked at. Tubes such as the 6550, KT-88, KT-90, KT-100, KT-120, and yes even the 6146 can make that kind of power without drawing grid current. In fact the 6146 ICAS at 750V plate can make 120 watts of audio, and do so class AB1.

So Jack, if you want to go that route, by all means do so. As far as all the QRM your getting, that is what passband tuning is all about.

Jim
WD5JKO

It's not QRM, Jim. If you consider his original claim;

KZ5A said:
it simply isn't true. The 807s have no problem modulating 2 X 6146s to 100% in AB1. Many Vikings 1 and 2s have proven this time and time again. I'll meet ya tonite on 3885 with my Viking at stock power with the bias on the 807s set for AB1 and prove it.

Jack's notion of a 1:1 power ratio is where his original statement above shows an error. You simply do not need 100 watts of audio to 100% modulate a class C RF stage that is setup correctly.

It may be a different story when trying to modulate a class C triode but we ain't talking there. The Viking 2 plate modulates the final stage at 100 watts output. In this configuration the 807s have more than enough scrote for the job. That is undeniable.

The answer, for Jack, will be found in what Jack ultimately decides he WANTS to do.

In theory, you need audio power equal to 50% of the DC inpoot to the final to modulate 100%. So for 180w inpoot (I think that is what a V-2 runs) you should, in theory need 90w of audio power to moduate it. However...................

In practice, it is nice to have the ability to produce audio power equal (or somewhere close to) the carrier outpoot power to have some extra "headroom" and be to make in excess of 100%. This really makes for good "Munky Swing". Also, this will also reduce distortion as you are not beating the snot out of the modulator.

Where's the fun in NOT making a tube sing for it's supper?  ;D

The Viking 2 manual states phone output is 100 watts. 807s can handle that no sweat!

You be the MAN Bud!!!

   Well said. Not discussed thus far is that modulation transformer. Between its turns ratio, and power rating my bet is it has to go to get that audio headroom. Consider the Heath Apache with its ~ 2:1 step down ratio (imp is 11K/3K). That thing by design just cannot get it done. Anybody know the Viking II mod transformer turns ratio?

   Johnson by design had several built in limitations to the audio headroom:

1.) Hi-Z audio driver (6Au6) that cannot supply any grid current to the 807's, so class A, and AB1 are it.
2.) A 807 grid bias supply that is high impedance. Anything done to fix the 6AU6 problem is then offset by the bias supply that shoots more negative as the 807's draw grid current, AB2. That same bias also cascades to the RF stages as pulsating grid modulation.
3.) As soon as the 807's start to draw some serious plate current, the Hi-Z screen supply (tap on HV bleeder) caves in on audio peaks, further limiting the power.
4.) The modulation transformer is sized such that it can only handle so much audio with the 250ma DC current flowing through the secondary. The turns ratio might be deliberately limiting as well.

My last Viking experiment used the Stancor 125W multi-match transformer with outboard audio.

Jim
WD5JKO
 

Buddy,

The "Viking Bud" sounds really interesting. The 4CX series of tubes are hard to beat in terms of "bang for the buck".   There is a set 4CX300A's with sockets and chimneys in my parts collection that may find themselves in my SB-200 after I finish the Viking 2.   Last year's RTTY contests took a toll on the T-160L's and they are a little weak now.

Did you change the HV filter in the Bud to capacitor input or just use the variac to raise the output voltage?  Also wondering if the screen is modulated.

In any case it's good to know that the power iron in the Viking can deal with fully modulating 135Ws long term.


I think this a function of the finals being upgraded in the V2 to 6146s from the 4D32 of the V1 without a corresponding increase in the modulator.


Steve,

"My understanding" is that the Viking 2 modulator in stock form operates in AB1 up to around 75-80% modulation.  Above that it goes into AB2 and generates tons of distortion due to the inability of the 6AU6 driver to provide the necessary power cleanly.  That is the issue dealt with by the various 6C4/6AQ5 mods.  The alternate approach I've run across is to just run abt 80W of carrier.

All,

One of my regrets in this project was that my Viking 2 was "toast" when I acquired it so I didn't have an opportunity to experience it in stock form before setting out to modify it.   That eliminates a handy frame of reference for evaluating potential mods.  There is a bunch of mod info on the internet and in old ER magazines but none of that is as useful as the comments and experiences related on this forum.

The difference between "plate modulation" and "plate and screen modulation" is no doubt the source of the confusion on my part about the requirements for modulator power.  The RCA TT-5 manual has a note recommending plate plus screen modulation for the 6146.         The only 4x 6146 rig that I am familiar with is the B&W 5100 which also does plate plus screen modulation.  Eimac also recommends it for tetrodes in general in some 1952 vintage application notes.  So I guess its just a tetrode thing and I'm stuck with it.  There is still the basic "conservation of energy" principal involved, meaning the output can not exceed the total of the RF power and the modulator power, i.e. "there are no free watts".

In the Eimac notes I did run across a discussion regarding the "degree" of screen modulation.  Thinking abt it, totally screen modulated finals (DX60) tend to have PEP~=CXR, fully plate modulated triodes generally have PEP=4x CXR, and apparently tetrodes fall somewhere on the line between depending on the degree of screen modulation.

Seems to boil down to either upgrade the driver to support AB2 or upgrade the mod tubes to supply the needed power in AB1.   Being the contrary old codger that I am, doing something differently than it has been done before has a lot of appeal.   It's all about having some hollow state fun anyway.  If it was just about getting on AM I could just stick with my HT-37 or, heaven forbid, put my K3 on AM.   

So I'm going to proceed with my 4 - 6146B's configuration and see how it comes out.   If it works there will be yet another Viking 2 mod floating around the internet.  If not I'll have to come up with a "plan B". 


73 Jack KZ5A



 

Good deal, Jack.

And it's a gooder thing you're doing bringing the 'ol girl back to life!!!

Yes, I did switch to cap input on the power supply. It is now a pi filter with new caps at both ends. I have considered, from time to time, shoe horning in a seperate screen supply but the gain would be negligable so I usually settle on 'if it works don't fix it"!

One thing I did do to the plate and mod iron is pull the bell ends off and drill holes in them for increased ventilation. After 38 years I guess I can conclude it helped!!

 

Jack,
        You wont be the first to do 6146s as modders. I did 2 apaches that way 5 or 6 years ago. ;D  ;D They work just fine. On one I even used a DX-100 mod tranny
(Made for a pair of 807s)

A valiant also uses 6146 for modders. 3 in the final modded by a pair.

If you want to keep it running in AB1 class, why not use a pair of 6550s, they're designed to run in either Class A or AB1 service. (Or a pair of the KT series bottles) With 600+V on the plates, they'll really swing the munky!!

Edit, I went to the shack, and pulled my old Viking I mod transformer out of the spider pit of iron. The secondary is a dual winding, so I suppose that offered flexibility should we want to use the modulator as a driver to some real tubes.

Anyway using a 1khz audio generator, and a scope, I measured the turns ratio. I put 8v P-P across the 807 side primary, and I get 5v P-P across the two secondaries in series. So 8/5 = 1.6 turns ratio, or 2.56:1 impedance ratio.

So lets say you tune up the Viking II to 250ma (plate plus screen currents) at 700V. The modulator load impedance will be 700/.25= 2800 ohms. Multiply this by 2.56 and the 807's will see 7168 ohms plate to plate. If the modulator tubes could swing the plate down to 100v (from a B+ of 700v), then each plate will swing from 100v to 1300v or 1200v peak. Across the whole primary, one tube will dip to 100v when the other peaks at 1300v. Convert that to RMS, take 1200 X .707 and we get 848 v AC rms. Since the load impedance is 7168 ohms, and we can put 846 volts RMS into that impedance, the power would be E^2/R or 846^2/7168 = 100 watts RMS.

So, the Viking II mod transformer with a 1.6:1 turns ratio, 700v B+, and the final loaded to 700v@250ma will allow 100 watts of audio at the clip point. This assumes the transformer does not saturate at that power level. If it does saturate, you can Taylor the low frequency response, or get the DC current off the secondary (Modified Heising).

Jim
WD5JKO

Thanks Jim.

That explains why the one in the Viking Bud has survived my abuse all these years!!!

The only thing the manual gives is the inductance.... 1.5 to 1.75 H.

And yes, there are hookup diagrams in the manual for using it to drive a higher power modulator.

Good work!

Remember that core saturation and good insulation were not factored in.

So with the variables I came up  where we make 1200v peak across the mod transformer primary with a 700V B+ on BOTH the modulator and the RF final, when we factor in the turns ratio of 1.6 to get the modulated B+ peak voltage: 1200/1.6 = 750v. So 750/700 = 1.06, or 106% modulation capability. If the turns ratio were more the ideal ratio of 1.4, then we could modulate to 122%. Take the Apache mod transformer at 1.9:1, and we then have only 84% capable modulation...Hence the need of the Turbo Mod.

Those calculations I gave were a first pass with variables picked out of the air. So try varying the parameters such as the plate voltage minimum where I selected 100v (EB min). A typical multi element tube will start to go non linear as the plate swing goes lower than the screen voltage. Here the beam power tubes have an edge over the Pentodes, and both have an edge over the Tetrodes. Remember the KT series of tubes where KT stands for "Kinkless Tetrode"?

Tubes like the 807 with a screen voltage of 300v or so, will show a big difference with EB min in class Ab1 versus Ab2. Other tubes like sweep tubes, and even the 6146 where the G2 voltage might be 175V or so, will swing further in class Ab1. This is all generalization though, and there are good tubes in each category. Lots of variables to juggle.. :-)

Jim
WD5JKO 

This thread is getting to be highly educational. ;D ;D

Jim, thanks for the detective work and the example.  I put all that into a spreadsheet and played with it a bit.  Assuming a efficiency of 59% for the class C  stage, the RF PO and the mod PO match closely and track with various final plate voltages and loadings, maintaining a 1 to 1 ratio.  Do mod transformer losses figure into that ratio significantly?

Buddy, I'd be interested to know how much HV the Bud has with the variac set to a nominal 115 VAC, idle and loaded.  I'm curious about how much difference there is in real life as opposed to theoretically ::) ::) between choke input and cap input.   Also how much cap are you using?

 

73 Jack KZ5A

   Jack, You could factor in the transformer winding resistances. On my Viking I transformer, the primary is 101 ohms each side of centertap. Each secondary is 37 ohms, or 74 ohms total. While at it, I put my Beckman DM27XT on the inductance range, and I get 3H on each side of the primary, and each secondary is 1.2H. This meter runs at 1 Khz or thereabouts and does not take into account reduced inductance from DC bias of the core when the secondary has 1/4 amp flowing through it.

  I would think a good transformer would be about 95% efficient, a SWAG.

Jim
WD5JKO

6146's are not the best choice for modders BUT if you would add some negative feedback to the circuit you could lower the plate resistance from several thousands of Ohms to several hundred Ohms ... this would lower peak power distortion ....there are several ways to do this ....consult old handbooks or iffin you are Very lucky Tim just might speak

   Well I am not Tim, but maybe we can smoke him out.  :)

Looking around the net, the 6146 has not been embraced by audio folks. Here are a few reasons:

* They don't like the plate cap for safety reasons
* Susceptible to VHF parasitics, need grid stopper resistor in AB1
* Need higher peak grid swing compared to high Gm audio tubes like 6550. This requires a low distortion driver at high peak levels. This eliminates simply driving with a hot cathode phase inverter like the triode side of a 7199, so therefore another stage is required, or transformer drive is needed.
* These tubes like a low Z path to fixed bias, 100K max. Other more popular tubes might get away with 270K to 470K.
* Found many references to a stiff G2 voltage requirement, and at ~ 200v. This means Ultra linear operation with screen taps on output transformer is out. The Acrosound folks got around this with the TO-350 using tertiary screen windings. Still using conventional P-P with a regulated G2 supply should work fine.
* Matched sets of 6146 are not available. You need to match them yourself, or have independent tube bias adjustment.

Here is a 6146 audio tube amp that seems to overcome the specific 6146 needs:

http://www.bonavolta.ch/hobby/en/audio/6146.htm

Also the legendary Acrosound ciruit:
(page 11)
http://www.clarisonus.com/Archives/Trans/Acro55.pdf

Jim
WD5JKO

Jim,

Welllll...... I'm not an "audio guy" ::)

Great info, so looking at these points one at a time.


I like them, actually prefer them to keep less HV routed around under the chassis.


I noticed that several mod schematiics have something on the order of a 100 ohm R at the grid connection, which I included in my plan.  Is that what you are referring to???


The K6AD 6AU6/6AU6 circuit I'm using is alleged to work well with the 807's which have only slightly lower G-G drive voltage requirements than 6146's per the RCA TT-5 manual.  So I think I'm OK there.


I'm a little vague on this one, my circuit only has a 100 ohm grid R and a 10K R between the grid and the bias supply filter cap.  I don't understand how one would get to a 100k between the grid and the bias supply so I''m probably missing something here.


Regulating the mod G2 voltage has always been part of the plan regardless of tube selection.


I picked up a dozen 6146Ws ("B" type) so time back and found 3 ~closely matched pairs among them.  So I'm good there.  I had already provided for independent cathode current measurement by putting a 10 ohm R in each cathode.   Hadn't thought about providing independent bias adjustment but that sounds like a good idea and shouldn't be too hard.


WD4BFS

Negative feedback is also part of the plan.  I've seen several implementations and haven't really settled on one.   This might one of those things that has to wait to be evaluated in the working TX.  In any case I plan on making this adjustable.


All,

I think it is time to commit all I've learned here to paper and produce a schematic that I can scan and post for discussion.   Got the old clearprint paper and schematic templates in front of me now.

Thanks to all.

73 Jack KZ5A

Ok,

Here is the Schematic as it stands today.  I haven't calculated some of the PS values and some caps and zeners are "composite" objects.  For instance the HV filter cap is actually 3 x 250uf @ 450VDC.

Comments, suggestions, and criticisms all welcomed.

73 Jack KZ5A

Forgot to attach the drawing, obviously need more coffee this morning.....

73 Jack

Jack,

At nominal line (117 here) Static (no drive) voltage is @775VDC. At 100 watts out it drops to 754VDC.
It's been a looooonngg time since this thing was stock but ifn I remember right the unloaded plate was around 700- 710.

Buddy,

Looks like about 10%, some of the literature I've read suggested it would be more like 30%.

Did you keep the 20K HV bleeder in the circuit?

73 Jack

the bias adjust will not work as drawn .... the secondary of the transformer would need to be ac coupled with a blocking cap on at least one of the secondary legs ....this is ok unless you are planning on driving into AB2 which would be dicey with 6146 anyway .... I would not use a balancing bias shift as drawn but a voltage divider then to 100k grid resistors ....this is to cut down on coupling cap size into grids ....hope this helps andmakes sense

Good Stuff!

   I was thinking the same thing, then I realized that pot wiper sets the AC center of the drive instead of change the bias voltage. This is because the driver transformer has no CT. So this acts as an AC balance.

   To vary the bias, id'd just put a pot across the 75V zener, and bypass the wiper. This provides 0-> -75v mod bias instead of -50v fixed.

  Adjusting the bias, and adjusting the AC balance are good things. Then adjusting each tube bias independently throws a wrench into the circuit. Would be easier to just match the 6146's.

More to come!

Jim
WD5JKO

Jack,

   I suggest you zero in on the 6AU6 driver stage. Here we will have resistive and core losses in the interstage transformer, and your two 10K's and 5K pot place a 25K load on the transformer. So just from the 25K load, with a bias on the 6146's of -50V, and full drive to the limits of Ab1 operation, each grid will swing from -100 to 0 volts; grid to grid you got 100v peak, or 70.7 v AC rms. Take P=E^2/R, 71^2/25000, and we have 200mw. The transformer losses might match that as well, so lets figure your going to need 1/2 watt of drive.

  To get 1/2 watt from a single ended class A amplifier with transformer coupling, the efficiency if everything is perfect is around 50% at the clip point. More than likely you will be at ~ 25%. This means your 6AU6 DC operating point needs to be somewhere between 1 to 2 watts. So this means you might need a lot more plate current than your circuit currently has.

  I suggest you look at the single ended pentode load line tutorial by Patrick Turner:

http://www.turneraudio.com.au/loadmatching1sebeamtetrodes.htm

and here is a 6AU6 data sheet:

http://www.r-type.org/pdfs/6au6-1.pdf

Note, the Heathkit Apache using EL-34's as modulators in Ab1, use a triode connected 12BY7 as an audio driver.

I think the 6AU6 might work, but to do so you got to crank up the juice to it. Knowing what the G2 voltage is will be helpful. Maybe make that determined by a voltage divider instead of just a series dropping resistor.

Jim
WD5JKO

I was a bit surprised too, Jack, till I measured voltages. At the first cap there's just over 800 loaded so that Johnson choke has quite a bit of resistance. Yes the 20K bleeder is still in there.

Revision 1.1 attached.  Fixed a couple of boo-boo's, added a few things, and tweaked a few values.


Why?  Neither the original design or any of the mods I've looked at do so.  Or maybe I'm misunderstanding what you are saying?

The two 10K R's and the 5K pot across the secondary are intended to do two things.  One is to apply bias in the absence of a center tap on this particular transformer (Hammond T-124B), the other is to act as the load for the 6AU6 driver.  This is strictly an AB1 design, so the grids not present a load to the driver.  My thinking was that the driver would play better if it was working into a nice matched resistive load.  Or maybe that's just an RF thing?

The pot in the center was intended to be used to balance the tube idle currents, I hadn't thought about the effect on the AC drive balance.  But after thinking about it some more, it probably will not effect the relative bias due to the lack of grid current.   I can definitely see how it would effect the AC balance however, since it is tied directly to the 150 uf bias supply filter cap and the wiper will have a low Z to ground. 

So I guess that pot is a good thing, by accident....even a blind hog finds an acorn occasionally. 8) 8)

The final value of these R's are to be determined in testing later, these values are just SWAG starting points.  Same with the mic transformer load resistor.

According to the RCA TT-5 manual and my other AB1 6146 rigs, the bias for a 6146 in AB1 is -50V.  I intend to "adjust" this in testing by changing the Zener as required.   A pot across the 75V bias would work fine for the AB1 mod bias, and save some soldering iron time, probably put that in the next revision.

Jim,

I was listening to 3890 this AM and heard you mention that you are using PC software to do your audio testing.  Wonder what you are using and your impressions of it.   The station PC has tons of power (4 cores@3200mhz) and a vacant slot so that's probably going to be the best approach to making the distortion measurements needed to tweak this design.  All the hardware audio distortion meters I've looked at have been a bit too pricey.  I could put a SB Audigy Z2 in it or just "borrow" the EMU 0202 from the LP-PAN setup for testing.

Audio test equipment is at the top of my Ham-Com need list.  I'll be looking for a scope, and an audio generator.

Your last post came in while dotting the I's and crossing the T's on this one.   The tutorial looks good, it will take me a bit to digest it.

Other than the change to 6146s, this audio mod is largely the K6AD mod from ER #110/111, which allegedly works well.  One potential difficulty arises from the greater voltage swing required by the 6146's relative to the 807's in the K6AD circuit.   The 6AU6 driver circuit is supposed to provide a 300V swing on the primary side,  with a 3/1 turns ratio,  100V on the mod side.   Not much room for losses.



73 Jack KZ5A

 

Jack,

So how did my Retro75 + 813 sound when hitting 150% positive peaks? :-)

  I purchased the Dr. Jordan audio test software, the "educational" version. This version is somewhat neutered, but still does a whole bunch of stuff. For $50 or so ($/Euro conversion) it comes with a CD, and USB Dongle. It is registered to whoever buys it by name. Those Germans are efficient.

Look here about that software:
http://www.tubelab.com/testing.htm

There are some free evaluation downloads. I like the audio generator. You can run two at once for two tone output.

http://www.dr-jordan-design.de/Downloads.htm

Also see attached schematic for a higher powered 6AU6 driver where we get rid of the interstage transformer coupling issue. Food for thought. Looks like they run about 12ma plate current on the 6AU6..

Jim
WD5JKO

This has probably already been covered and ruled out for some reason, but couldn't you just replace the 6AU6 with a 12B4?

You would then have a way better driver tube.

You would need to replace the 7 pin miniature socket with a 9 pin miniature, but the tube would fit in the same space. Seems like you could keep the driver transformer.

Perhaps there wouldn't be enough gain????

Dave

Jim,

Sounded really good, looked good on the pan-adapter as well.   

I've got to the point where I can recognize several folks without listening, just from looking at the pan-adapter.  You can see everyone's signal width and modulation patterns in great detail.  Same display as the Flex.

I'm not stuck on the 6AU6 driver,

0934 Z just heard you CQ on 7290 but my barefoot K3 PW isn't making it to Round Rock!!

73 Jack

Oops, 0934 local

73 Jack

Anyway, back to the 6AU6 driver...

The 1960 ARRL Handbook has a 6146 AB1 modulator alleged to make 120W with 750V.  One of my other sources of inspiration.  It uses a 6C4 as the driver running it off of the 200V supply shared with the 6146 screens.   Same 3 to 1 driver transformer ratio, same 3.5W Max Pd in class A.

The K6AD first 6AU6 stage has less gain and much better fidelity than the Johnson design.  The K6AD second 6AU6 was changed to pentode connection to "make up" for the lower first stage gain.  So I'm thinking putting a 6C4 in place of the 6AU6 driver, without doing the 12AX7, is probably a non-starter.

A 6AQ5 would have higher max Pd, but I don't know about more gain.

BTW The T-124B measures about 400 ohms on the primary and 1500 across the secondary.

Time to QRT for the day.

73 Jack KZ5A

  Good Deal. Just poking around and looking at that. The 6C4 is a good choice for sure, but low gain. For kicks I dug out the 6AU6 triode connected characteristics (grid 2, grid 3 tied to plate). Looks like the 6AU6 in triode mode has higher Gm (4800 versus 2200 uMho), and higher Mu (36 versus 17) than the 6C4. The plate resistances are about the same. Looks like the 6C4 would need ~ 2X the AC drive for the same plate swing..

http://www.tubebooks.org/tubedata/HB-3/Receiving_Tubes_Part_1/6AU6-A.PDF
The last chart for 6AU6 shows triode connection

http://www.tubebooks.org/tubedata/HB-3/Receiving_Tubes_Part_1/6C4.PDF


Could make the Viking II audio input a line level input...Don't have to add that 12AX7 :-)

Good Night OM

Jim
WD5JKO

hello i just picked up a viking 2 my v1 tube is replaced with a 12ax7 and v2 tube 12by7 have not looked under the hood yet see what mods been done yet so far that's what i seen my clamp tube is gone too any idea i dont have no documents yet!

Compare it to a Heathkit DX-100. If it is close there are plenty of mods out there.

RDT,

When I started looking at the original circuits, the clamp tube looked like something to be disposed of....

However, after digging a bit deeper, it looks to be necessary to keep the screen voltage from going to full HV in the absence of drive.   

Maybe yours has some other arrangement (mod) to accomplish this task?

73 Jack KZ5A

Hi 


Finding a PDF file with a schematic of class AB2 807 modulator,when browsing the internet.


Gito.N

well they took it from the top and put it on the bottom i'm not sure what all 3 tubes are but 2 of them are oa2 other not sure of but it's not a 6BQ5 but i like how it is wc3e mod it i am on 40 meters 7.290 k5iia has his other rig was a home brew pair of 813's x 813's

Turned the corner today from removing old parts to adding new parts.  The first picture shows the modulator with all the "retained" parts.... the two 7 pin sockets, the level control, and the mod transformer.   The second photo shows the new 6146 octal sockets mounted.

Thanks to W7TFO for the sockets.

73 Jack KZ5A

I love how clean the aluminum chassis stays with the old Johnson rigs. Makes it a pleasure to roll your sleeves up and get to work. That looks brand new in there Jack!

After getting side tracked on several other projects I'm finally getting back to working on my Viking 2 project.

The design has evolved a lot in the last months and is more HB than Viking now.  It now is a pair of 4X150A's modulated by a pair of 6146B's in AB1 .

An external (modified) HW-23B now provides HV for the modulator, LV, -Bias, and +48VDC for the negative peak limiter, T/R relay and fans.  The oil HV cap has been replaced with three 225 uf 450V computer grade caps,  all four rectifier sockets have been reused to hold VR tubes.   There is a Variac and a soft start module in the PA HV supply.  The LV transformer has been replaced with a 6.3 @ 10A filament transformer.

The modulator circuit is a amalgam of ER and AMphone circuits featuring an adjustable balance phase inverter, adjustable bias, regulated screen, adjustable negative feedback, and a powered negative peak limiter circuit.

A second cabinet, about the same size as the V2, holds the VFO 122, the HP-23, the Variac, and three nice old RCA meters that will monitor mod and PA HV, mod and PA plate current, and NPL current (which will be used to monitor and set the audio drive level).

It still has the original mod transformer which will be a limiting factor but hopefully I will run across a nice 125W multi-Z mod transformer to replace it.   Being able to adjust the PA voltage independent of the Mod HV will give me a lot of flexibility in accommodating the stock transformer or taking best advantage of a better one later.

The picture shows it all dummied up with the tubes installed  and some of the chassis mods finished.  It still needs new holes in the rear apron, a home for a 9 pin tube socket for the grid-block keyer mod, and three new pots installed on the right side (mod bias, mod balance, and mod feedback).  Then I'll be ready to start wiring....... ;D ;D ;D

73 Jack KZ5A

That is quite a layout and the 4X150's are cheap and reliable too. I have box of them lying around.
All of the comments of this thread have my head swimming as to which way I will go with my V-2.
Being an audio guy, I can vouch for the 6550's being a wonderful tube and has been used very effectively
for a lot of different designs. Good 6550's are getting expensive too.
A good friend of mine, Larry Janus KB4YXT who owns Tube Equipment Corp (http://tubeequipment.com/)
Told me about his setup where he uses a Mcintosh MC-40 amplifier to modulate his audio, along with an Optimod and other cool stuff. Anyway, my point here is he suggested using two PP output transformers back to back with the secondary of the #1 or driver transformer tied to the secondary of the second transformer, thus the primary of #2 transformer drives a pair of 6550's. Interesting approach.
I am a big fan of the 6BQ5/EL84 tube also. My best selling guitar amp uses a pair of these to produce 18 watts of Whoopass. So, the comment before with the schematic of the EL84 PP UL stage pushing the modualtors looked promising as well.
I am just stuck as to where to go with this. To be honest, I am bummed because there are so many different directions to go and my engineering chops suck. I need encouragement and a prodding into a general direction with noted high level results. A radical redesign of my V-2 isn't the scope of my project and I really want to stay away from that stupid tone sucking interstage transformer.
Anyway, you are doing brilliant work and I look forward to your completion of this fine project.

I use 6550s to modulate a single 6146 with 600v on the plates.  Plenty of audio, 6550s running in AB1.

Fred

N6YW,

I looked at several possibilities for mod tubes.   Based on the Z ratio of the stock mod transformer, the PA needs to operate at a lower HV than the Mod to "match up" with the stock transformer better.  Assuming I didn't get my logic reversed....   That puts a kink in most of the popular audiophile tubes in that they are only spec'd up to ~600V or less.   

The 6146B's have a relatively low screen voltage, which is supposed to be a good thing, will operate at 750+ V, and I have a dozen on hand.  The 4X150A's are something I have wanted to build a TX with since my teens, are designed for this sort of service, and will work at the V2's design plate impedance over a fairly wide range of power levels with the adjustable HV.   

Overall, this project has definitely gotten out of hand in terms of complexity but should yield a good AM TX that I can enjoy for a long time.  With the stock mod transformer I am expecting to be limited to running 80 to 100W of CXR, however with a better mod transformer the rig should be good for 150W of CXR.  Or at least that's the theory.... 8)

73 Jack KZ5A

Jack
Thank you again. Your postings are very interesting and it looks as if you may have yourself quite an audio powerhouse brewing there.
I have a bundle of these too: http://www.ebay.com/itm/Pair-Triad-A65J-Output-Transformer-f-Old-Ham-Radio-Receiver-Tube-Audio-Amp-/270863021672?pt=LH_DefaultDomain_0&hash=item3f10b1b668

This transformer is the same as  used in a Spectra Sonics recording console that I parted out.
In keeping with a class A audio output stage driving the 807's, does this appear to be a decent candidate?
Better than the stock iron?

I spent quite a lot of time modifying a Viking 1 'bout 30 years ago for good audio.  The final configuration is a pair of 6146 modulators.  It has a 12AX7 as the first and second audio stage, a 12AT7 phase splitter/ driver RC coupled to the 6146 modulators.  Eliminating the driver transformer was very important in enabling the application of negative feedback around the modulator.  The 6146 are a little dirty, but 6-8 dB negative feedback cleaned them up just fine.  I ended up running the modulator screens at a bit over rated voltage in order to get more power from the modulator and it is capable of 120% positive peak modulation.  I have a switch that allows me to run a D104 directly or a transformer coupled 600 ohm line from my station processed audio chain.

One thing I found necessary was replacing the modulation transformer with one with a better impedance ratio.  In my case I used an old Douglas 100 watt multimatch, but I'm sure there are better and easier to find alternatives.  In your case, the higher voltage on the modulator tubes will overcome the poor impedance match of the stock mod iron, and the negative feedback will help the bass response.  However, a better mod tranny would still be a significant improvement.

My souped up Viking 1 is one of my favorite transmitters and still on the air.  Have fun with yours!

Charlie

Funny how things work...
A buddy of mine just popped up online and is giving me an Eico 730 modulator chassis, complete.
I'm sure all he wanted were the tubes, which were likely Mullard's.
This thing has some promise to drive a bigger RF deck. Check out the Schematic here:
http://www.amwindow.org/misc/gif/eico730.gif
Maybe this would drive the 807's using 6V6's instead of EL34's? Or, would the EL34's be okay?

N6YW,

If you play these back to back you would effectively have a 1 to 1 transformer.  I think the inter-stage is a 3 to 1. 

If you want to keep the transformer, Radiodaze has a Hammond HX124B which is a high nickel content 3 to1 and is the best readily available "hi-fi replacement" Viking inter-stage transformer I am aware of.  The center tap is on the wrong side so you have to feed bias to the modulators via a pair of R's. 

I was originally going to use this transformer (bought one), but somewhere along the way got persuaded that a phase inverter was a better idea for my purposes.

The "best" Viking info I ran across was the June, July, and August 1998 ER magazine series of articles by K6AD.  These are a great read for anyone involved with a Viking.   Most of my mods are based on those articles and the rest came from the folks on this board.

The quick and easy approach to better Viking audio seems to be bump up the audio coupling caps, replace the inter-stage transformer, and don't drive the 807's past AB1.  Beyond that the mods that regulate the 807 screens and get rid of the R13 HV bleeder are highly recommended.   Also replacing C46, L21, and C32 with a proper splatter filter is an easy and in-expensive way to set your high frequency roll-off where you want it.   I "designed" mine with the ARRL "Elsie" software and found the parts at Mouser.  Another quick and easy enhancement is to replace the ni-chrome meter shunts with precision resistors.

These Vikings are kind of like a Harley, you can modify them till Hades freezes over or you can just enjoy them as they are.

73 Jack KZ5A

I have neg feedback around the mod transformer power suply is solid stated also ptt but still looking at audio stages has nice audio what I been told also run what you bring!

Take a look at 4CX150 heater voltage. It may be high if the source is the LV transformer.
Both 5 volt windings in series with the primary will get it a lot closer and really drop the operating temperature of the LV iron. It must be running very close to saturation at 120 VAC line. Really cool modification!

WA1GFZ,

The PA's are 4X150A's, 6V heaters.  I replaced the LV transformer with a Stancor 6.3V @ 10A and will use a small dropping R for the 4X150s 6V filaments.  LV comes from the external HP-23B as does HV for the modulator.

73 Jack KZ5A