Viking II modifications

[N6YW]

Greetings everyone.
I am now getting into the restoration of the power supply of my Viking II and am considering some of the various mods that have been talked about in recent times. My questions are brief and to the point.

Speech Amplifier:
Instead of using the interstage 3:1 transformer, couldn't I achieve the same thing by using a long tailed phase inverter (12AT7 etc) capacitive coupled to the 807's? It seems the nickle core replacement transformers were discontinued by Hammond. Instead of the 6C4 as a gain amp, would the the option I suggested work?

6AL5:
The bias supply tube could be replaced with silicon rectifiers. Has anyone done this? If so, what is the mod?

Screen regulator mod:
I read of a Mod by N4JK that used 2 OA2 gas regulators for the screen supply which makes sense. Is there a better way of supplying the required HV without using the big 20K dropping resistor?

Modulation transformer:
It has been told to me that the modulation transformer kinda sucks in these things. If so, what are the replacement options? It's a little cramped in there.

Pointing me to earlier threads would be helpful as to reduce clutter in this forum.
Thanks in advance.
Billy N6YW

[KB2WIG]

I'd recomend you check out the AM Window web site.
B,

http://www.amwindow.org/tech/htm/mods.htm

Thars a carload 'o mods for you. Check out the Ranger and Valiant mods along with the VK2. 

Its not a direct answer to you specific questions, but if you look through them, you'll find stuff of interest.  (Like the tossing of the big 20K 'space heater' and using the LV power supply fer the screenz)


gud luck

klc

[KM1H]

There was a 2 part aricle on the V II audio mods in ER in 1998, perhaps someone has the info handy otherwise I can look tomorrow.

The 807's run AB2 so you need a transformer to replace the POS that is in there. It is the primary reason for marginal audio.

My oil filled HV cap was bad so I replaced it with a pair of 100uF/450V in series with 50K equalizers.

The 6AL5 lasts almost forever, no need to mess around with it.

The ER articles go into the screen regulator and scrapping the 20K and also say why other articles are bad to piss poor.

The mod iron is fine for 100W output as in the V I, its not up to 120W at 100% or higher for 6146's. There isnt much spare room. Dont know if the DX-100/Apache iron is any better or would fit.

The same article would cover the V-1 as the audio circuits are identical.

Carl

[Pete, WA2CWA]

Actually it was a 3 issue series on Viking II modifications (June 1998 through August 1998). Lots of great info there.

[KM1H]

I thought the 3rd one was for the CW folks

[N6YW]

There was a 2 part aricle on the V II audio mods in ER in 1998, perhaps someone has the info handy otherwise I can look tomorrow.

The 807's run AB2 so you need a transformer to replace the POS that is in there. It is the primary reason for marginal audio.

My oil filled HV cap was bad so I replaced it with a pair of 100uF/450V in series with 50K equalizers.

The 6AL5 lasts almost forever, no need to mess around with it.

The ER articles go into the screen regulator and scrapping the 20K and also say why other articles are bad to piss poor.

The mod iron is fine for 100W output as in the V I, its not up to 120W at 100% or higher for 6146's. There isnt much spare room. Dont know if the DX-100/Apache iron is any better or would fit.

The same article would cover the V-1 as the audio circuits are identical.

Carl

Thank you Carl.
The reason why I thought of the phase inverter idea was from my work with guitar amplifiers running Class AB1. With 2 stages of amplification, a PI would work well with a Push Pull modulator setup like the V II, at least I would think. I could easily compensate for frequency response by NOT using a crummy transformer.
If I had room, then back to back output transformers would work but what a waste of time... I mean it's a V II not a Collins 20V! Hi.
I am looking at not using a transformer at all because the one that everyone used is no longer in production. I will press on and see if my idea works and it is easily reversed if not satisfactory.
If I was going to use outboard audio processing, then I would only need one stage of amplification prior to a PI or interstage transformer. I had even thought of using a Jensen or Cinemag microphone transformer and the Jensen mic preamp circuit just to see if it would work.
I guess my mental handicap is showing. I can repair and build anything, but engineering this stuff is very time consuming for me because of Math deficiencies. Ughhhh!

[KX5JT]

You might want to PM Darrell, WA5VGO.  I think he built a phase inverter for his Viking I (maybe it was II, but same audio section anyway).  He and I were talking about that a year or two ago.

John

[N6YW]

Thanks!

[KB3DKS]

 If you want to get radical, my VKII was a not well built kit and needed a driver tranny as well as a basically a rebuild. I do have a factory built that is basically modded for a D104 so decided to just forget about the original audio in the rough one.
  First off solid stated the supply, mounted two 50uf in parallel (Marshall Caps) in the holes left from the 5R4s and mounted the ceramic octals in place of the 807 sockets. Installed a pair of Sylvania 6CA7s, no Russian garbage, used the low voltage supply for the screens and stiffened it up. The Sylvanias can run 400v G2 easily. Added a bias trimmer.
 Feeding the grids of the 6CA7s with a backwards line output transformer, maybe 600 to 8K. Would have to look. Tried out several I had to find the best ratio and effective drive. Also split off the modulator B+ from the plate supply after the new diode string, added another 10H 200ma choke followed by another pair of caps series up to give 100uf.
  There are some other small things that I forget but it has been working this way for a couple years with an outboard processing chain and no problems. Stock mod tranny too.

Bill

[kb3rdt]

Mine has a 12ax7 for speach amp and as audio driver I have a 12by7 mod driver tranny still in there I get bout 150% audio  but my power suply is ss up's the voltage a bit I can run D-104 no amp if i want but I have a MPF102 amp to run a eq little for bass! I am on 40 Meters nothing up on 75 yet need some antenna work!

                                                                                                                       Carl  KB3RDT

[WD5JKO]

If you want to eliminate the driver transformer and just use R-C coupling from a phase inverter stage, then you are limited to class Ab1 for the 807's. This will be fine for local skip contacts, but your audio will get lost trying to work further out. On 10m I have a lot of receiving noise, and many AM signals on 29 Mhz are severely undermodulated. Then I here the Timtron, WA1HLR with his Vikink I using a pair of 811's as modulators and the three diode ultra-Mod circuit. Not only does he sound great, but the audio jumps out of my speaker heads and shoulders above the noise. Most other stations at the same or higher S-Meter reading are IN the noise.

An alternative to the transformer is R-C into a push pull cathode follower stage. Go a step further and try the power FET instead. The guy at Tubelab calls this Powerdrive:

http://www.tubelab.com/PDcookbook.htm
http://www.tubelab.com/powerdrive.htm
The use of a CCS:
http://www.tubelab.com/CCS%20circuits.htm

Running the 807's in class Ab2 dictates the need to regulate the screen voltage (300v), and the grid 1 bias (about -33v). Omitting these steps means you'll just distort as you draw grid current on audio peaks.

Jim
WD5JKO

[WA1GFZ]

I'm running under 300 volts zener regulated AB2. You have to drive G1 pretty hard.
Steve QIX has a good SS driver for the V2. My driver would also work with a few value changes.

[N6YW]

If you want to eliminate the driver transformer and just use R-C coupling from a phase inverter stage, then you are limited to class Ab1 for the 807's. This will be fine for local skip contacts, but your audio will get lost trying to work further out. On 10m I have a lot of receiving noise, and many AM signals on 29 Mhz are severely undermodulated. Then I here the Timtron, WA1HLR with his Vikink I using a pair of 811's as modulators and the three diode ultra-Mod circuit. Not only does he sound great, but the audio jumps out of my speaker heads and shoulders above the noise. Most other stations at the same or higher S-Meter reading are IN the noise.

An alternative to the transformer is R-C into a push pull cathode follower stage. Go a step further and try the power FET instead. The guy at Tubelab calls this Powerdrive:

http://www.tubelab.com/PDcookbook.htm
http://www.tubelab.com/powerdrive.htm
The use of a CCS:
http://www.tubelab.com/CCS%20circuits.htm

Running the 807's in class Ab2 dictates the need to regulate the screen voltage (300v), and the grid 1 bias (about -33v). Omitting these steps means you'll just distort as you draw grid current on audio peaks.

Jim
WD5JKO

Your description of Tim's VI sounds interesting. It's all about the audio punching through with authority without distortion. The deeper I look into this subject, it becomes a matter of which hot blonde to choose.

[kb3rdt]

I got lots of audio in viking 2 with D-104 stright in I run audio gain at 2 1/2 never can reach 3 the audio clips off I don't have the 3 diode limiter in mine I'm thinking about it!

[N6YW]

I finally got around to popping the covers. Pretty clean throughout and mostly original except for a problem centered on the 6AL5 rectifier circuit which had a flameout and migrated throughout the filament line.

[N6YW]

I stripped the old Lytics out. The bias supply units were toast.
I am building up a daughter board that will contain the upgraded power supply components. What I would really like is if someone could post o photo of theirs to help me arrange the proper wiring scheme as this one seems modified or not totally stock. It helps a lot visually.

[N6YW]

And...
This is what I had in mind for my Mic pre (speech amp) after looking at the various options presented on this forum and elsewhere.

[kb3rdt]

need get on 40 meter ask derk about his viking 2 he has an outboard pair 811A's using radio shack audio amp to go into the outboard WN3B built one also but he use an 6l6 audio amp lots of punch there!

[The Slab Bacon]

Real simple:

Look over on the AM Window site for the 12AX7 - 6C4 audio mod. It is easy and simple and sounds very good driving anything AB1 that doesn't require being pushed into grid current. I have installed this circuit (or some slight variation of it) into bugoodles of transmitters and they all sounded good.
That is more than enough to drive a pair of AB1 807s. Which is plenty for a vik 2. (Keep in mind that an 807 is nothing more than a 6L6 with a 5-pin base and a plate cap)

Use the K.I.S.S. rule........................

[N6YW]

I replaced the crappy 7 pin socket in the photo I posted. Punching the new holes today for the speech amp sockets.

[N6YW]

The low voltage/bias capacitor daughter board is complete and installed, resuming the repair of the filament leads, and other little wiring issues. onward...
BTW, this is before the leads were attached and the board cleaned of flux. The board material is 1/8"
Garolite, which is virtually indestructable.

[WA1GFZ]

Move the 500V caps apart a bit,  1/2 plate voltage between cases with thin plastic between them. This way the plastic won't burn through. 1/16 inch will be fine

[W1RKW]

I never had any luck with the 12AX7/6c4 mods with my V2.  I chalked it up to being a crappy mod transformer. I bypassed the interstage transformer and tickled the 807s with an audio transformer in reverse and an outboard amp.  Also did some power supply mods and repairs.  But I still wasn't satisfied with the end result with those changes as well. I figured the mod transformer was either wimpy or toast.  So now the V2 is an exciter for an 813 after burner.

[N6YW]

Why? These caps are not wired in series.
The filament test passed! Wiring in the new speech amp commences. I decide to go with the simplified 12AX7A/6C4 arrangement to see how it works out. The static voltages that I read are as follows:

6146 PT=754, SC=298, GD= -80vdc neg.
807   PT=750, SC=432, GD= -37.8

I plan on loading up the amp this afternoon to see how it puts out into a dummy load.
 

[N6YW]

On 40 meters, I managed 110 watts into a 50 ohm load. No smoke yet.