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Viking 500 Audio Ideas?? Old Buzzard Input Required




 
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Author Topic: Viking 500 Audio Ideas?? Old Buzzard Input Required  (Read 6634 times)
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WA1HZK
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« on: January 02, 2005, 09:03:28 PM »

Well I got the 833 rig running and everyone tells me that compared to the Viking 500 the new rig is much easier to listen to. Now I'm at the paper & pencil point of figuring what to do with the audio string inside the Viking 500 transmitter. Over the years I have modified the radio in the usual way and have added a pair of 572B's and a Peter Dahl mod transformer. The complaint is that compared to the 833 rig (Gates Peter Dahl Iron) I have no highs in the audio on the 500. After looking at the drawings again I figure it's the usual driver transformer killing the audio. All my other Johnson projects have a phase splitter added, driver transformer tossed & they sound pretty good. I'm thinking of adding the phase splitter to the 500 and a tube like the 815 to generate the audio for the 572B's. The 815 can make 50 watts at 500 volts but I should only need 10 or so. I figured running it on the low B+ (300 V) of the 500 & adding a screen regulator under the chassis might give me something that will fit in the rig. The 500 is VERY cramped for space around the audio stage and I need an 8 pin octal dual section power tube thus the thinking on the 815 possibility. That & the fact I have a half dozen of these tubes. Has anyone come up with an easier way of getting rid of that driver transformer to better the audio of this rig? I cannot have anyone saying I have yelloee audio! gotta fix that. I figured to just load the plates with about 25K & couple into the 572B's with some .1 mf caps & supply a variable bias for those 572B's with a couple 100 K resistors. Look up the schematic, have a brew & scratch yer head. When you have some ideas, blast away.
Keith
WA1HZK
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Steve - WB3HUZ
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« Reply #1 on: January 02, 2005, 11:53:19 PM »

I don't think you'll be able to RC couple the 815 to the 572B's since the latter is running Class B. When the 572B grids draw current, distortion will result.

Get a schematic from a Clegg Zeus (or Thor). It's the one with the 7034 final and 811A modulator (it was a 2 box design like the 500, one for the desk top and the other with the modulator and PS went under the desk). It has a pretty nice direct-coupled cathode follower driver design that may work for you.
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Bacon, WA3WDR
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« Reply #2 on: January 03, 2005, 12:16:27 AM »

I am not familiar with the Viking 500, but I think someone told me that there was no screen modulation in the rig.  If I understand, the modulated stage gets its screen voltage from a fixed supply with no modulation choke.

[edit] OK, I finally got the schematic and manual for the 500 from BAMA.  Evidently the screen of the 4-400 is self-modulated in the 500.  It comes from unmodulated B+ through 20K in series with an adjustable 20K, and there is a 755V string of 0C3 and 0D3 tubes to ground.  The manual says that in AM mode, the screen voltage is 400V and varies with modulation, but modulation does not cause it to hit the 755V regulator limit.

So... the screen of the modulated stage in a Viking 500 is self-modulated.  I learn something new every day!  I have used this self-modulating technique with a pair of 6146s, but never with a 4-400.
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W8ER
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« Reply #3 on: January 03, 2005, 01:42:33 AM »

Keith,

I assume that you checked Timmy's mods over on the AM Window.

http://www.amwindow.org/tech/htm/v500/audio.htm


--Larry W8ER
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WV Hoopie
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« Reply #4 on: January 03, 2005, 12:35:08 PM »

Been searching for 500 mods myself, so far:
If you can find copies of Electric Radio, Issue #37 May 92 has an article on Audio Mods, author Beatty. Off topic; Issue #99 Jul 99 Current Inrush, and Issue #36 Apr 92 Johnson 500, The.....a Classic. Anyone has these issues, I'd like copies when my 500 passes the smoke test.

Craig,
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WA1HZK
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« Reply #5 on: January 03, 2005, 05:22:02 PM »

I have done all of the Timtron Mods and a few others like running 3 KV on the 572B's instead of the same voltage as the 4-400. The last thing to get rid of is that driver transformer. I'll take a look at the Clegg unit. I'm looking for the Level 5 upgrades.
Thanks.
Keith
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Timmy, Sometime in 2007 on a Mountain Far Away..
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WB2CAU
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« Reply #6 on: January 04, 2005, 06:57:58 AM »

Keith,  before condemning the driver transformer, put an audio generator on the input, a 10:1 scope probe on the grids of the 811As, and do a plot of the frequency response going to the grids.  If the response is poor there, work your way backwards until it improves and narrow it down.  If it's good up to the grids, start looking at the mod tranny or perhaps some (too) high value bypass cap across the modulated B+.

Just be very careful while making measurements on a live transmitter.  Use good common sense safety practices.

Eric WB2CAU
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WV Hoopie
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« Reply #7 on: January 04, 2005, 11:24:29 AM »

:-P  I'm looking for the Level 5 upgrade

Is this where creativity kicks in?

Craig,
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W8ER
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« Reply #8 on: January 04, 2005, 03:19:19 PM »

Eric WB2CAU:

What is the guy in your avatar doing!!!  

 :lol:  :lol:  :lol:
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WB2CAU
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« Reply #9 on: January 05, 2005, 07:42:10 AM »

Quote from: W8ER
Eric WB2CAU:

What is the guy in your avatar doing!!!  

 :lol:  :lol:  :lol:



You'll have to ask the beloved K1MAN.  I "stole" the avatar from his website.
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"Life is tough, but it's tougher if you're stupid." -- John Wayne
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