Viking 500 Audio Driver/Modulator

[WA1HZK]

OK
So I was talking with the APE man yesterday about getting some decent frequency response from my Viking 500. I have done all of the usual pre-amp mods up to the 6B4 driver but it and the driver transformer are still in place. My modulator triodes are 572B's running 3000 Volts on them and the mod iron is by Peter Dahl. I'm convinced that the choke hold on the audio is the usual Junkston driver transformer. I want to dump the driver transformer but I still need some power going to the 572B's to get them lit up.

What have others done here? I was thinking of a power phase splitter using maybe an 815 that will go into the 8 pin octal socket of the 6B4. Or maybe going to tetrodes like a pair of 813's as the modulator tubes. This would allow a smaller phase splitter but require a screen supply. If I run them as triodes, I'll need to slam them with some power. I would like to use the existing octal 6B4 socket & drive the 572B's. Maybe my only choice is a new Hi-Fi Driver transformer? I have a brand new UTC S-19 Driver transformer in my hand? Anybody ever use those? Will they pass audio? The 6B4 is a dual triode. It could probably be set up with a phase splitter in front of it and run as a PP driver.

Your Thoughts?
PS, I could give a crap if the rig stays original even if it goes for 3K.
Keith
WA1HZK

[The Slab Bacon]

Keith,,
        The a 6B4 is actually a single triode somilar in characteristics to a 2A3. The 6B4 actually has 2 sets of elements in 1 bottle, but they are wired in parallel as 1 triode. My old buzzard black crackle rig uses a pair of 6B4s in P/P to drive a pair of 811s.
         How much power do you actually need to drive the grids of the 572Bs?? An 815 wuth both sections in P/P can actually make 50w of audio if you run it AB2 with 500v on the plates! An 815 is actually a pair of 2E26s in one bottle. A 6N7 might do it if you run it with both sections in P/P they can make 10w in P/P and are a small metal octal tube. Use a small driver transformer to to drive the grids of the P/P driver tube and R/C couple them to the grids of the 572Bs. Use a circuit similar to the 2E26 driver / 4-400 modulator circuit used in many brodcast transmitters. Just a thought.
             
                                                                                     The Slab Bacon

                               

[WA1HZK]

I'm wondering about R-C coupling into the grids of power tubes? Will that work? I think a good driver transformer is needed there or possibly use the same cky that drives the 845's in my 833 rig:
http://www.criticalradio.com/833%20Rig%20Project/845%20Audio%20Driver.jpg
This uses 6J5's (small triode) to RC to the 845 grid. The phase splitter drives the 6J5's:
http://www.criticalradio.com/833%20Rig%20Project/Pre%20Amp%20-%20Phase%20Splitter.jpg
I know this works for the 845's in class A P-P  operation but I never tried it with 811/572B types.
I could add a power stage in the Power Supply Modulator deck & remove the 6B4 driver & replace it with a phase splitter...
I have a feeling I need like 10 watts of audio to light up these triodes.
Keith

[Don, W2DL]

I also think it would be very hard/impossible to drive the triode mod tubes into class "B" with an R/C coupling. I am just finishing up a new rig using triode connected 813's as modulators, driver is a pair of cathode follower 807's (probably don't need that much) directly coupled to the 813 grids. The bias for each 813 is individually set by the bias on the 807's, which have a separate pot for each tube. Input to the 807's is R/C, and no driver transformer is needed as the cathode follower 807's can directly drive the 813's into whatever grid current is needed. This is the same sort of system used in the Gates BC1-G bcast xmtr, only they used the 807 cathode followers to drive a pair of 833A's. I figured if it was good enuf for them, I couldn't go wrong!

73
Don, W2DL