The AM Forum

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

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

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

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

I thought the 3rd one was for the CW folks :o

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!

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

Thanks!

 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

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

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

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.

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.

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!

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.

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.

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.

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!

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........................

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

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.

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

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.

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.
 :)

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

If y'all are going to use the low voltage supply for the screens be sure to relay switch that voltage. You don't want screens on with no plate voltage.

Got it!
I have several able bodied relays in my arsenal. I should just relay switch both Plate and Screen supplies simultaneously then.
Nearly complete with the speech amp mod. 2 more hours max.

Speech amp is finished, for now. I installed the tube underneath because the top side is a Pain to replace tubes from. Telefunken ECC83 and a Mullard 6C4 will do the work.
Onward.

It works.
The audio is sufficient to drive the transmitter into 100% modulation. However, the audio response is restricted as evidenced by the sucky interstage transformer. I swept the audio bandwidth with my audio oscillator from 50 to 4000 cps and the result was not gratifying. The audio peak occured at 400 cps and dramatically dropped off at a high of 2500 cps. The low end dropped off  around 150 cps. I then monitored my transmissions with my 756 Pro and using my trusty D-104 non-amplified mic, it sounded okay but not great.
I definitely need to address the coupling to the modulators. This is an easy fix with a good transformer, but is likely best addressed with RC coupling rather than a lossy transformer.
At least I was able to re-energize the transmitter after decades of non-use and repair the numerous issues that had plagued it. I found the reasons why the calamity happened in the first place... poor cricket.
I am pleasantly surprised to find that the single ended style filament arrangement does not induce any hum into the carrier or the audio. Very quiet. The carrier looks great on the 475 scope and the 756 panoramic display.
If the 807 is much like a 6L6, then I would assume biasing them properly would be a very key component in getting the audio to really swing and sound natural. Just thinking out loud here.

I Like run mine 210 mils plate bout 90 watts out but i seen mine do 110 watts no smoke 45 min key down into dummy load played a mp3 player into the input mic jack i said passed my test!

  I ran the screen supply thru the other half of the plate switch and use a bnc relay to attenuate the rf drive. That way I can switch that relay for spotting/zero beat. My intention is to put in a solid state relay for the plate trans and a regular one for the screen supply. Make push to talk easier and less beating on the plate tranny with zero switching.
Wonder what triode strapping the 6CA7s would do?
Bill

Billy,  Good deal getting this beast up and running.

   As to that transformer (interstage), there a some things you can do to make a crappy transformer perform better than its designer ever imagined. I will name a few:

1.) Resistance load the secondary. One circuit on the AM Window used 39K grid to grid on the 807's. Might try that , or 47K. Within class Ab1 swing, the driver tube will only see a varying reactive load and then adding the swamping resistor will dominate this causing the driver to behave better so long as it can provide power to the resistor.

2.) Choke couple the primary and capacity couple the audio to the transformer to the driver plate. This gets rid of the magnetizing DC current and its detrimental effects that restrict a large portion of the transformer BH curve. To do this you need something like a > 10 Henry choke. These can be pretty small at current ratings of < .03A.

3.) If you have a surplus of gain, add a negative feedback circuit from one 807 grid to the 6C4 input. Maybe Taylor for 6db gain reduction. The circuit on AM window by 'Ed' applies NFB across two stages (6C4 + 807's). I'd be hesitant since phase shift across two transformers might lead to instability. Also the 6C4 cathode is bypassed eliminating any NFB. Many of those circuits on AM window are best used for ideas only. So take audio from one 807 grid (whichever provides NFB), capacitor isolate the DC, and load with something like a 100K pot to ground. Then take the wiper (NFB adjust) to the grid of the 6C4 with a series resistor like 470K to sum the NFB with the audio from the driver stage.

Jim
WD5JKO

Thanks for the tech tips. When I get to my computer, I'll take a look at those ideas in detail. Also relating to the 807 modulators, I plan on measuring both tubes to see what condition they are in and how much resting current they're drawing. Its possible that the bias isn't correctly optimised for best audio. I will scope the output of the Mod Trans to see if any crossover notch distortion is present.

Jim
Thank you. As to the swamping resistors, the 47K's would be directly to the grids of the 807's just like many AB1 designs, right? There is also a post stating that disconnecting the secondary center tap and using two 10k bias divider resistors feeding the secondary leads to supply grid bias.
I am curious as to the actual layout for the inductor mod you suggested for the driver tube.
Really interesting information here, learning a lot from this project.

I am contemplating substituting the transformer with this PI arrangement for feeding the 807 modulators.
With the other 12AX7A in the circuit, I wouldn't need the 6C4
What do you guys think?

Where did you come up with that circuit??   Are you sure it will work??  The .1uf cap coming off the one grid to the 100ohm resistor shorts the signal to ground.

I also see other issues with the circuit.

Fred

Fred
This is a standard "Long Tail Pair" phase inverter used in many audio amplifier circuits. Most notably, Fender guitar amplifiers in most of their "AB763" designs. It is basically a phase inverter stage that has gain over the usual distributed load phase inverter. The .1 cap coming off the one grid is there to isolate the DC present on the grid so the feedback signal from the output stage can couple a sampling back into the phase inverter.
Hence, we have "negative feedback".
Because the speech stage of the transmitter is just an audio amplifier, I see no reason why it wouldn't work. How well it works is the question here.

OK, I think I see how it works,  I overlooked the common cathodes.  I'll have to try that set-up to see how well it works.

Thanks for posting it.

Fred

Some questions regarding the use of an external modulator.
I have an Eico 730 driver modulator that I have finished restoring. By itself, it sounds fantastic especially after performing some mods to improve the audio. My question is, how would I go about properly interfacing the 730 to drive the RF stage of my Viking II?
Here is the schematic of the 730:

Billy,

   I see three major shifts in your approach in the same number of days. Realize that all have merit, and I believe each could be crafted into a very satisfactory modulator. You asked me a question a few days ago, and I am now just getting back to this. Since then you have moved on twice so I will just save the keystrokes.

  The Eico modulator might do a fine job. You will want to key it with the Viking, and wire it in as if it was the internal modulator. This is manageable, but could get tricky. Make sure it wires into the Phone-CW switch so that the modulation transformer secondary is shorted when in CW position.

Jim
WD5JKO

Jim
You are correct!
I'm not convinced that what I have done so far is optimal.

I agree mostly with the others, but I want to emphasize just how hard it is to drive 6L6/807 grids in AB2. Keeping in mind that the plate resistance of the 12AX7s is pretty high and the grid drive gets really squashed on the peaks due to grid current of the 807s, you need something herkier than a 12AX7. A cathode follower driver is in order here, an afterburner so to speak! The tube you use should be a 6SN7 (or maybe even a 5687) running on as much as 450-500V on the plates (especially if yoy ss, i hintend to use a good amount of NFB). Guitar amps almost never use AB2, even the powerful Ampeg SVT series. True Hifi amps built in the fifties never ran AB2 that I can recall. This why everybody keeps harping on the interstage XFMR thing. Yeah, I know a single ended class A driver has alot of 2nd distortion and it's a PITA to find a decent XMFR, but they ARE still available new from Hammond and other mfgs. The MOSFET circuits the others have mentioned are an elegant solution to the dynamic load problem of the 807 grids drive discussed here. In my dusty files of tube amp schematics, I have some suitable driver circuits I can send if you desire.
Good luck,  Lenny

Lenny
Seasons greetings and thank you for the offer. I am always open to anything that helps my thirst for knowledge and radio fun. I am getting ready to fire up the Gold Dust Twins for the first time on Christmas Day.
After that, the Vike gets all of my attention again. I wish I could find the original nickle core Hammond transformer just to see how it sounds. I guess at this point anything will be better than the one that resides in the rig now.
Cheers.

Now I like to know how you can tell if it's AB1 or AB2!

                                                         Thanks Carl

Good question! How can you tell whether or not the  amp is running AB1 or Ab2? AB2 IS AB1 for probably 90% of a sine wave. It's just a little before the crest, the actual crest and a little after the crest that grid current flows. The major difference between the classes is just how hard the amp is driven. How one measures this isn't so simple though. I doubt seriously that you could see any grid current flowing with a DVM because the duration of the pulses is short. When I had access to one, I used to use a current probe on an oscilloscope to see the pulses. Current probes just aren't everywhere and good ones can cost more than the scope!
However, in a driver that doesn't have enough scroat to drive the grids hard enough under grid current is easy to see on an ordinary scope with conventional voltage probes (where the ground clip has to get grounded). First, remove the output tubes. Connect the scope probe tip to the grid of the output tube socket. Connect a variable level tone generator to the amp input. Turn on the amp and raise the tone level while watching the scope. At some level, the tops of sine waves will begin to square off and that is the unloaded clipping point. Make a note of this voltage. Now repeat this with the 807s in their sockets. If your driver can't deliver almost the same undistorted voltage as before, you are into the grid current region (AB2) complete with bad distortion. If the clip voltage is the same, it indicates that you aren't quite getting out of AB1 and into AB2. This could caused by not enough driver voltage swing. If the peak clip voltage stays almost the same and you are getting 120 watts out with 700 volts on 807s, you have one potent driver! In practical tube drivers, one never sees this. Without NFB, it is almost impossible to not see some driver droopage during the grid current flow. I haven't actually scope tested any of the MOSFET drivers, but their very low on-resistance promises them to be practically constant-voltage source drivers.
How can you tell if you are using AB1, AB2 or B? Consult the scriptures (tube manuals), my son! There are guys like WA1HLR (the Timtron) that can quote the operating voltages of the various tubes and their various classes like chapter and verse. A guy that has some pretty good tutorials on tube amps is: www.funwithtubes.com.
Merry Christmas and Happy New Year!   Lenny

Lenny...
It's official, you rule.
Thanks in abundance and have a cool yule.

Thanks!

Marry Christmas and Happy New Year!

                                       Carl

Put a regular old 0-10ma meter in series with the bias going to the driver xfmr CT. If there is absolutely no needle movement you are in AB1, if it barely wiggles on voice peaks you are in AB2 and if it moves more than that you are splattering everywhere.

Any analog VOM or VTVM can also be used as an indicator.

Carl

Dear Carl,
Gotta love the old analog meters! Even though I first learned to use them in the sixties, I sometimes have to be reminded of what they can do that can't be done with a DVM! I'm also somewhat embarrased to see that you provided an answer in a couple short sentences which took me a treatise to cover! I sometimes think that my two semesters of "Technical Writing" in college has gotten the better of me.
Thanks, Lenny

They also sub for an oscilloscope to see AC ripple on a B+ line....if it moves its bad. Ive used a Simpson 260 or Triplett 630 as a multipurpose tool more than once usually because Im basically lazy and it was the simplest way ::)

I am now on the air with the Vike.
It plays pretty well considering I haven't done all that has been laid before me.
I am now noticing a popping noise "On Air" that sounds like it may be the big space heater in the screen supply.
So, it begs to ask the question where can I find the mod for doing a real smart method of doing this?
I suppose a separate transformer and filter could be used but...
The keep alive negative peak diode mod beckons as well.
So, just point me to the proper URL and I can go from there.
Thanks in advance.