Johnson Viking II Audio Mod

[w5hro]

There is an audio modification that I came up with years ago for the Viking II transmitter. The main problem with the Viking II is the audio interstage driver transformer. I came up with a really simple mod for this problem which requires removing the 2nd 6AU6, which is not needed, and replacing it with a 6C4 connected in a hot cathode phase inverter configuration and the driver transformer can then just be eliminated. There is more than enough gain in the first 6AU6 speech amp tube and the 6C4 (6C4, 1.5 x to 1.75 x) to drive the 807's.

The mod is much easier than finding another suitable driver transformer because it simply eliminates the transformer which eliminates the problem. The other part of the mod is to change all of the resistor and capacitor values in the audio chain to the exact values show in the RCA "Resistance Coupled Amplifier Charts". The equipment maker’s back then and even still today design audio chains with more restricted frequency response in mind and shoot for narrow communication type audio. They never used the correct component values shown in the RC amp charts for that reason.

I should probably put together a schematic of the mod and post it, but I currently don’t have one. Everything is from memory. I have attached a pic which shows the "Hot Cathode" phase inverter configuration and the 6J5 can be substituted with the 6C4 with some minor component value changes. The same type of mod could probably be performed on the other Johnson rigs as well because the audio driver transformer is always the main problem with the audio chain in those transmitters.

[N3DRB The Derb]

many years ago I used the driver transformer in the AES catalog, added a 6c4 with a internal gain control, and used a 12B4 to drive the mod tubes. 12B4 is a outstanding mini audio driver tube. Beats a triode connected 6AQ5 by a mile. There's so many ways to do stuff.

[Steve - WB3HUZ]

But since the 807s are running AB2, it's more than just voltage gain that is important. Some power is required.

[N3DRB The Derb]

a triode connected 6aq5 driver tube is a popular modification for that tx. even when the 6au6's are changed out for 6ah6's, you tend not to have enough gain. The added 6c4 voltage gain stage fixes that and the 12B4 drives the crap out of the mod tubes with a super low plate impedance. The stock driver transformer in the JV1 and 2 is junk. it can only take 10 ma current and is undersized and saturates just like you say. the cure is a better transformer and a driver tube that can deliver power as well as voltage.

I just wanted to relay what I did to my JV 2's,  not beat you down.  

I went P-P phase inverter with my HB rig with the first half of the second tube and the second half driving a pair of 2A3's. 6SJ7 or 6SG7 I think. There's no end to how things can be done.

 

[N3DRB The Derb]

I'm trying to remember he exact tubes. The first mike amp was a pretty hot gain metal job, not a dual triode, which went into the first triode section of a dual triode which phase split and dumped everything out into nothing but push pull from there on all the way to the 2A3's. this of course canceled any even order weirdness all the way to the mod transformer, which was a 300 watt Thordarson CHT plug and jack job.

it took me 2 years plus to get all the parts to build that rig but only about 4 or 5 weeks to actually build it and have it on the air.

A ham out in Ohio has it now, he says he's have arcing problems somewhere. I'm real glad it never got torn down for parts or other stuff, and if he decides to not persue it, I hope he'll let me buy it back from him. He's had it 4 or 5 years and I dont think ever had it on the air. Go to my gallery if you want to see 2 pics of it.

[WA1GFZ]

Stock V2 runs AB1 with plenty of screen voltage on the 807s.

[Steve - WB3HUZ]

The point is that if you use a capacitor coupled phase-splitter as a driver and the 807s pull screen current, distortion will result. If not, all is good.

Stock V2 runs AB1 with plenty of screen voltage on the 807s.

Not sure what the point is, but even with as much as 300V on the 807 screens it only takes 200mW at 96V p-p on the grids in the Viking II AB2 configuration, and that’s if there was 750V on the plates. Below 750V it requires less. The one 6AU6 and the 6C4 (6C4, 1.5 x minimum in hot cathode) is plenty of drive.

[WA1GFZ]

Also Steve if you need to drive grid 1 above zero you need power. My set up I run about 15 volts positive. G2 is around 200 volts. I would think it would take a cathode follower on each phase to drive grids that hard.

[Bacon, WA3WDR]

I used a high transconductance dual triode as cathode followers driving the grids of my 8417 modulator tubes in my Viking II.  It worked pretty well.

[Steve - WB3HUZ]

That's what I was getting at. I was pretty sure a cap-coupled phase-splitter wasn't going to work.

Also Steve if you need to drive grid 1 above zero you need power. My set up I run about 15 volts positive. G2 is around 200 volts. I would think it would take a cathode follower on each phase to drive grids that hard.

[KB2WIG]

it looks lonely sitting there.............

[w4bfs]

Interesting thread ... I think I may understand what is causing the confusion ...

Steve said  "The point is that if you use a capacitor coupled phase-splitter as a driver and the 807s pull screen current, distortion will result. If not, all is good."

I think you meant to say control grid current which as you move from ab1 to ab2 as the control grid goes positive with resp to cathode ... the grid impedance drops and at some point cannot faithfully follow the the phase inverter input signal with resulting didtortion ... the important distinction to make is that SOME grid current may be drawn before distortion sets in ... its all in the driving and load impedances ....gotta go for now ...73...John

[Steve - WB3HUZ]

Nice catch, Beefus. I did mean grid current.

It's more than just a change in the driving impedance that would be a problem. When the grid draws current, the bias is changed (just like grid leak in a Class C RF amp). Generally, shifting grid bias results in distortion and wouldn't be a recommended mode of operation. Just how much grid current one can "get away with" or when the distortion becomes noticeable or objectionable is the gray area here.

HRO wrote

From what I remember the whole thing is set and operating somewhere between AB1 and AB2 and not at full AB2.

What do you mean by this? In class AB2 there is grid current and in AB1 there is not. In many cases, the only thing determining whether an amp is running AB1 or AB2 is the amount of drive.

[Steve - WB3HUZ]

Why would I make this up?

I never said you did. I'm sure it worked. Most stock AM rigs of the 50's had 5-10% audio distortion, so I would imagine your mod was at least that good. My point was that, in general, it's not good practice to drive an AB2 amp with an RC coupled phase-splitter. You clearly took some steps to ensure it would work in this particular application rather than just hooking it up and letting it rip.