The Viking Saga Begins...

[N8UH]

Tim, is that 600V coming from the main plate supply?

Yes, the voltage would be coming from the plate supply, where R13 used to be, at L1 and C9. I had toyed with the idea of taking the voltage from the LV supply, but it seemed better to go with the 600V and regulate it. I could be mistaken though, wouldn't be the first time!

-Tim

[WQ9E]

Tim,

Keep in mind the 600 volt supply may be quite a bit higher than 600 depending upon line voltage and if you ever SS the rectifiers it will go quite a bit higher.  It would be a good idea to measure the current flow through the VR tubes and adjust the dropping resistor if it is excessive; maximum current will occur with minimum loading of the plate supply since these tubes are operating as shunt regulators.  You would want to make sure that voltage is not applied to the VR tubes except in AM mode and of course never if there is no plate voltage applied to the 807 tubes if you choose to use the LV supply instead.

Keep in mind that the octal 150 volt regulator (0D3) will handle 40 mils to the 0A2 30 mil rating so that would give you a bit more leeway to handle voltage/current variation.

[WB4AIO]

Okay! Let's see how this works. I combined everything in what I hope is the right way. The big question marks are the coupling and bypass caps. Also, I would like to configure the input for a low-Z dynamic mic, so some massaging is in order there I'm sure.

What do you guys think?

You'll want to make sure that the 807s have the right voltages (and load impedance) for class AB1 operation, as the resistance-coupled driver cannot drive them into AB2. The high plate load resistors on the first two stages might limit your highs somewhat; I am not sure how much.

I personally don't like the purposely-unbalanced driver plate resistors to force higher positive peaks -- that adds unnecessary distortion. In my view, it's better to just design a modulator to faithfully reproduce whatever waveform is fed into it (then you can do all your audio shaping, if any, in your external audio preamp chain, and the transmitter won't alter the sound you've achieved). The male voice usually has plenty of natural asymmetry anyway; sometimes even too much, hence the good sales of the Kahn Symmetra-Peak among broadcasters.

There are many schematics for low-Z microphone preamps out there, the output of which could be fed into the hot end of the gain pot.

I would also add that an audio signal generator (many computer programs can do this) and scope are essential for seeing and understanding what your new modulator is actually doing.

There's a whole world of audio ideas out there, as I am sure you're seeing, so build and explore and learn and enjoy.

With my best,


Kevin, WB4AIO.

[WD5JKO]

Tim,

  I just read through this thread, and you got plenty of good input to ponder.

You did ask about eliminating the driver transformer, and someone else mentioned cathode follower drive to preserve class AB2 operation. I am linking such a circuit from the late Fred Nachbaur with his marvelous 'Dogzilla' amplifier project using six 807's in Push Pull Parallel, class AB2:

http://www.dogstar.dantimax.dk/tubestuf/dzart-4.htm

Look around at the various sections of Dogzilla. Might stimulate some ideas for use in your modulator.

Fred would have made a great AM ham...

I believe cancer got him...tragic.

Jim
WD5JKO

[w4bfs]

Tim,

  I just read through this thread, and you got plenty of good input to ponder.

You did ask about eliminating the driver transformer, and someone else mentioned cathode follower drive to preserve class AB2 operation. I am linking such a circuit from the late Fred Nachbaur with his marvelous 'Dogzilla' amplifier project using six 807's in Push Pull Parallel, class AB2:

http://www.dogstar.dantimax.dk/tubestuf/dzart-4.htm

Look around at the various sections of Dogzilla. Might stimulate some ideas for use in your modulator.

Fred would have made a great AM ham...

I believe cancer got him...tragic.

Jim
WD5JKO

thanks for including this, Jim .... I am working on a design for a pp 4-125 modulator to be driven by a cathode follower ...will post in a new thread ....73 ...John

[N8UH]

I'll have to say, I'm staggered with the combined knowledge on this site! I have a lot to ponder here! I think I'll have to keep working on this in the background. I keep swaying back and fourth between going transformerless, going with a line-level input, or keeping things traditional. I can see this getting messy, I may need more Vikings!

I think for the time being, as I'm eager to get this rig on the air, I'll take a more traditional approach and just replace the driver transformer with the PT-124E (that's the current choice, right?) , replace the second 9-pin socket with a 7-pin for a 6C4 and go for the AM Window mod. I think I'll keep the regulator idea in, as I don't like the idea of the orininal monster-bleeder-resistor-divider theme.

Then, when I get this new idea hammered out, gut it out and try something new!

Thanks again for all the help and support. This site is great!

-Tim

[WD8BIL]

Hi Tim,

I've got the AM window 12AX7/6C4 in my Viking and it works great. A few things:

1) Changing the first socket for the 12AX7 socket was easy. I just had to hand ream the hole a bit to get the new 9 pin socket in.
2) If you want a low impedence input look for a 600/15K ohm audio input transformer.
3) The Vikings use fixed bias on the 807s and set the idling current with the screen voltage.(ala R13) If you're gonna regulate the screen as shown you'll wanna consider an adjustable bias supply.
4) Even with the stock rectifiers my plate voltage hangs around 675VDC. With the solid state diodes it runs at 710VDC. So you'll wanna measure the actual plate voltage and figure the regulator's load resistors accordingly.
5) the coupling caps could all be .1uf. But with the stock mod transformer you're not gonna get booming bass response so anything larger wud be a waste.

[kg8lb]

Also attached, for your viewing pleasure is a pic of the finals. I'm putting these up on a shelf...

  Saw the same thing happen to the finals in a friend's TX. He had been using the same tubes for quite a few years. At the time it happened he was trying to "tune" his antenna coupler. It seems he ran for an extended period of time with very light loading on the finals and caused the envelopes to "suck in". Art (W8TMS , SK) was a broadcast engineer and cited the light plate loading as the cause. He put a fresh pair of 6146Bs from the same batch in the transmitter and continued to operate.