Viking II Interstage Audio Xfmr???

[WBear2GCR]

Well it's time for another weekend update.

The replacement lo B+ tranny has slightly crunchy, hard and stiff wires where they have been heated and come through the end-bell's holes. So, I took the end-bells off and put some nice heatshrink on the leads where they bunch up. Shoulda hit the metal parts with some nice black lacquer, but I didn't.

Looked at the mounting for a while, and decided that it would be good to float the iron above ground, and remove one more possibility of zorch, and possibly improve the cooling, with air able to flow under the bottom (vs. connected badly to the aluminum chassis, and getting minimal thermal conduction). Went with 1/4" lucite/acrylic rectangles under each foot, going just far enough to reach to the actual lamination area for support, and nylon shoulder washers for the top, 6-32 stainless screws with washers top and bottom to hold the sucker in place.

That's just to mount the stupid thing.

Ended up having to take the rear of the enclosure off - if you don't know, and I didn't - the stupid Viking has screws inside the bloomin' case on a flange going top to bottom each side that holds the rear of the case to the front! Geniuses. Well, it wasn't as bad as it looked, taking it orf.

After getting it orf, turned out there was a fair amount of corrosion between the aluminum chassis and the copper plated steel rear case. Not much of a surprise there.

Took that out and hit it with the mildly alkaline Mean Green (those guys should pay me for endorsements!) which cleans copper pretty well, and it evaporated the corrosion. I'll hit it lightly with an abrasive pad, and then mask and spray with a coating or two of lacquer to keep it from happening again. Good for another 25-30 years minimum! 

Ok, so then it was possible to shoehorn the screws in to hold the iron in place. Of course, when they laid out the design, they only thought of it going IN, not doing repairs!! Sounds like a car manufacturer, doesn't it? Works great when the engine/transaxle is hanging in the air, but not so wonderful once it is IN THE CAR!! Duh.

Stuff's in the way. Had to go the other way with one out of three screws to make it work. Completely impossible with that back of the case still on. 

Thinking to myself - "there ought to be a fuse in the ground line coming off this transformer, if there was one, the old tranny wouldn't have been able to fry. That's on the list now too. Gotta make a hole in the top of the chassis for a fuse holder, or else put a chassis mount on the bottom. Either way, gotta have it.

Well, that's as far as I got this weekend... still not on the air with it.

                   _-_-WBear2GCR

[KB2WIG]

        " Ok, so then it was possible to shoehorn the screws in to hold the iron in place. Of course, when they laid out the design, they only thought of it going IN, not doing repairs!! Sounds like a car manufacturer   "


Try 'wing' nuts??  or them tall things they use on the valve cover (not tubes, auto valves)..........   klc

[WBear2GCR]

What are ya sayin there OM??

Who's a wingnut?? 

The guy who designed it was a wingnut foshaw!

The pic above shows the fix, btw.

              _-_-bear

[KB2WIG]

                                                                                .

[WBear2GCR]

Nooooo...

Got the low B+ supply up an running on Sundei.

The idiot who was doing the work got a few wires in the wrong places at first.
So hard to get good help these days... and he remarked to himself when finding the wiring errors on the day that he found them - before it was fired up - HOW THE HECK DID YOU DO THAT YA MORON AND THINK IT WAS RIGHT??? 

Well, it (the low B+) fires up and works.
The stupid 6AL5 "rectifier" they used for the "bias" supply has been replaced with a solid state set of diodes. WTF was the point of using a 6AL5 for a rectifier? It's got a whopping 50ma max rating! Real waste of good vacuum, and other parts if you use it as a rectumfryer... I figure this is a good place for a soylent state rectifier since it will put the negative bi-arse on the grids before the B+ tries to show up...not the other way around.

Replaced some other wax and paper caps along the way.
One was hiding under the 8mfd oil cap that is mounted under the chassis.

I have ideas about the speech amp.
It's going to be changed.
Since everything is out of it now, no reason not to.
Just have to widen out the socket holes for 9pin sockets.

Thinking I will go cascode for the front end, JFET on the bottom of the cascode. Haven't decided on the rest yet. Possibilities include a SRPP for the second stage depending on the gain available from the first stage (gotta go figure it out to know). Or maybe a "parafeed" for the driver transformer... we'll see how it develops.

Gonna try to see if it makes RF next in CW position.

WHHHhhheeeeeeeeeeeeeeeeeeeeeee!

                        _-_-bear

Oh, I painted the area where the copper case contacted the aluminum chassis. The idea is to prevent corrosion. Looks like some moisture got in there and helped, but with the paint there, it won't react if it gets wet again.

               

Nothing like a fast and quick repair to get on the air, eh? 

[flintstone mop]

I'll have to second the suggestion of WA1HZK. The phase splitter way is less complicated than trying to second guess an interstage transformer. The phase splitter circuit is less phase shifting and you can still run your biass for the modulators, if they need a biass voltage.
I made the change on my Elmac AF-67 and nice hi-fi audio.
Fred

[WBear2GCR]

Nah.

Thanks for the encouragement.

I have a strong dislike for single tube phase splitters. I know they are more than good enough for this sort of application. BUT, I have a gen-u-wine old stock Viking II transformer in hand now, so I am not worried about having a transformer. I'll be keeping the B+ off the primary I think using the SRPP cap coupled to the primary. If I pick the value of the cap just right I can probably get a little bump in the response on the low end in just the right place!

Two tube phase splitters, the so-called long tail pair are ok, but inherently not really balanced either. As a result there are a long list of various circuit "enhancements" that make the output balanced fully. In most audio amps that used them, the designer counted on the NFB to force the circuit to work properly - which is a really bad idea. If you want to make the diff amp work right you end up with current sinks and current sources at minimum... more tubes than most folks want to work with (although you can use HV transistors and Mosfets too...).

There is also a nifty crosscoupled phase inverter circuit (in Radiotron fyi) that maintains excellent balance, but 4 tubes - two 12AX7 if you like.

If I was really serious about this rig, I'd do a DC cathode coupled driver to the grids of the modulator tubes. But I'm not terribly serious about it and want to keep it within the bounds of two tube sockets for the whole deal.

So, the plan (assuming there is sufficient gain) is cascode front end, SRPP driver stage, cap coupled to the IT.  Actually I'll have an "extra" tube section if I go with twin triodes like 12AX7s that I could do something else with or use as a cascaded stage for extra gain or even "tone control" !! 

           _-_-bear

[KB2WIG]

        " stage for extra gain or even "tone control" !!    "

Maybe a "BA" boost???     klc

[WBear2GCR]

Poot-tetra,

But I am hoping to get a slight "BA" boost from the resonance of the coupling cap and the primary of the IT. We'll see.

If it sounds like doggie poopie I'll scuttle the thing and do it differently.

Think it might be ok.

             _-_-bear

[kA5WHO]

Bear
    The Hammond 124E is the transformer to use hooked up in reverse.
    Cost more but real universal and heavy duty compared to the 124D.
 Electric Radio 110 and 111 tell all about it in k6AD mods.I have the articles on my computer
  if you would like them I can send them.I also have the stock mod transformer because I don't use
them,if you want them.
  73/dale/ka5who

[The Slab Bacon]

Have we reinvented the wheel yet??   I typically do an audio revamp on one of these (or something similar) in a couple of evenings
while yacking at the mic. Just revamping the audio is a pretty simple job, whether you do the replacement transfoma or the fuzz schplitta, its not that much to do. Just git 'er done so we can hear ya!

                                                 the Slab Bacon

[Steve - WB3HUZ]

Yep. If the tranny doesn't have the response, there's not much you can do. A shunt-fed inductor with DC blocking can sometimes help the low end response, but what it is really doing is eliminating or reducing saturation of the transmformer at lower frequencies. This will reduce distortion not necessarily improve response. Cleaner low end response can sound like improved low end response.

[WD8BIL]

Don't be suprised when you loose all the response you gained in the pre-amps in the Mod tranny.
The Viker mod trannies really go south below 300cps.

[WBear2GCR]

I have the "super kludge" IT transformer standing by...

What it is is a pair of "70v" multitap transformers back to back (8ohm sides connected) that happen to have the about right ratios on both sides after you pick the proper leads. I tested them for freq response and they are quite reasonable on both ends of the spectrum, surprisingly enough - configured as I would be running them, fwiw.

              _-_-bear

[Bacon, WA3WDR]

Just out of curiosity - what is the ratio of the stock Viking II driver transformer?  Also is that turns ratio, impedance of full primary and secondary windings, or primary to 1/2 secondary, etc?  From the DC resistance values in the manual, I get the impression that it is about 1:1 full primary to full secondary, with a center tap in the secondary.

[Steve - WB3HUZ]

I've used a single tranny like that with a center-tap with a tube audio amp to drive the 807s in a Viking II. It worked and sounded pretty good too.

I have the "super kludge" IT transformer standing by...

What it is is a pair of "70v" multitap transformers back to back (8ohm sides connected) that happen to have the about right ratios on both sides after you pick the proper leads. I tested them for freq response and they are quite reasonable on both ends of the spectrum, surprisingly enough - configured as I would be running them, fwiw.

              _-_-bear

[WBear2GCR]

Just out of curiosity - what is the ratio of the stock Viking II driver transformer?  Also is that turns ratio, impedance of full primary and secondary windings, or primary to 1/2 secondary, etc?  From the DC resistance values in the manual, I get the impression that it is about 1:1 full primary to full secondary, with a center tap in the secondary.

Ur probably right - I have one here, but i'll have to run a siganiganal through it to see what the voltage ratios are... will post it here as soon as I get a spare millisecond to actually do something with it on the bench!

                      _-_-bear

[WD8BIL]

Bacon.... it's 1:3 impedence.

[Bacon, WA3WDR]

1:3 - That was a pretty popular ratio.  Thanks, Bud.

[The Slab Bacon]

Bacon & Julie
                      1:3 step up is a very popular ratio for an interstage audio transfoma. this goes back to the days of the early battery receivers of the 1920s. back in those days some of the manufacturers refered to them as "amplifying transformers"

However a 1:3 stepup is usually only good for class A or AB1 grids that dont see any grid current. If you have to push the folowing stage into grid current, You uually need to get closer to 1:1 or even a slight stepdown so you dont load down the preceeding driver stage.

Just my $.02 worth

                                                                    The Slab Bacon