Viking 2 Modulator

[kb3rdt]

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!

[kg8lb]

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

[KZ5A]

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

[Gito]

Hi 


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


Gito.N

[kb3rdt]

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

[KZ5A]

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

[W1AEX]

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!

[KZ5A]

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.......

73 Jack KZ5A

[N6YW]

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.

[KA2DZT]

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

Fred

[KZ5A]

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....

73 Jack KZ5A

[N6YW]

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?

[K3YA]

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

[N6YW]

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?

[KZ5A]

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

[kb3rdt]

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!

[WA1GFZ]

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!

[KZ5A]

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