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Dual 1625 transmitter




 
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Author Topic: Dual 1625 transmitter  (Read 5511 times)
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N4LTA
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« on: January 18, 2010, 09:10:46 AM »

I am just finishing up a dual 1625 AM transmitter for 160-80-40 meters. I have tested the final - two parallel 1625s running about 600 volts. I am using -22 volts as safety bias and that gives me about 45 ma per tube or about 27 watts per tube - which I think is going to be OK.

With no drive applied, the output stage is stable with the pi net switched for any band and the caps rotated throughout their range.

The bias on both tubes will be approximately -90 volts when the grid current is 4 ma per tube.

I am now ready to apply drive and I have a question.

My question is  - do I need to neutralize this output stage?


Pat
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« Reply #1 on: January 18, 2010, 09:17:28 AM »

Are you using partial sheilds on the tubes or did you sink the sockets below the chassis?  These techniques usually are all you need to tame 807s/1625s.

73,  Jack, W9GT
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« Reply #2 on: January 18, 2010, 11:00:29 AM »

The tubes are sunk below chassis with 1" spacers so that the entire tube base is below chassis.

I think I am OK but wanted to make sure. Thanks for the reply.

Pat
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« Reply #3 on: January 18, 2010, 12:03:10 PM »

If you tune the plate cap slowly  through it's range on all bands  and there is not even the slightest dip, your probably ok. Make sure the loading cap is set to the least amount of plate mesh. If there is a problem, neutralize it on the 40 meter band and you should be good to go on the rest. I guess the object it to get the least amount of dip or none at all while neutralizing on the highest band your going to use.
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« Reply #4 on: January 18, 2010, 03:05:41 PM »

For some reason this reminded me of an article in one of the magazines back in the early 60s.

The guy built up a junk box transmitter using an old TV chassis and about a dozen, or more, 1625s (when they were 19 cents each in the B&A catalog) in parallel. He used some sort of voltage doubler or trippler to get his plate voltages and couldn't get it to run above 40 meters due to all the paralleled capacities.

Anybody remember that one?

 
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« Reply #5 on: January 18, 2010, 04:06:28 PM »

May have been Bill Orr - I think he had an article using a TV parts and four 1625s  -two in the modulator and two in the RF final.

Mine has a regulated screen supply and regulated bias supply with a 30H choke to self modulate the screen. I built it as a test bed before I start the 4-400 transmitter RF deck.

I have just come into four NOS Eimac 4-65s and may use them in parallel instead of a single 4 -400.

BTW - I can't see any sign of current change with the load cap at minimum on any setting of the final bandswitch. I think I am OK.

Pat
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« Reply #6 on: January 18, 2010, 05:32:21 PM »

May have been Bill Orr - I think he had an article using a TV parts and four 1625s  -two in the modulator and two in the RF final.

Mine has a regulated screen supply and regulated bias supply with a 30H choke to self modulate the screen. I built it as a test bed before I start the 4-400 transmitter RF deck.

I have just come into four NOS Eimac 4-65s and may use them in parallel instead of a single 4 -400.

BTW - I can't see any sign of current change with the load cap at minimum on any setting of the final bandswitch. I think I am OK.

Pat

Yours is much more sophisticated than the one I was thinking of. It should be a great little transmitter.

The one I was thinking of had tube sockets spread at random over the TV chassis, wherever there were tube socket holes and I think he used the TV transformer. It was just an attempt to do it as cheap as possible with whatever was at hand and using cheap, available tubes (at the time)! It wasn't pretty but it worked.

Sorry for the thread hi-jack! Grin


N4LTA


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« Reply #7 on: January 18, 2010, 06:54:35 PM »

I would build the neutralization circuit anyway, even if its ok in the tests.

My 4D32 rig started out unneutralized, but after adding the 3rd tube, I had spikes of RF when I unkeyed.
Talking with Chuck one day (W3FJJ), he said he had that in one of his rigs and neutralizing it eliminated the problem.

I have added it to the 3x4D32 rig and it also eliminated the big spike, its not totally neutralized, and needs more work.

Brett

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« Reply #8 on: January 18, 2010, 09:58:12 PM »

I think 1625's are a great tube and still hard to beat for dollars per Watt. Actually my first AM rig back in 1988 was a pair of 1625's modulated by a pair and it worked great. I used one of those $4.95 2:1 impedance 100W transformers from Fair Radio that they had an endless supply of ( sigh...) and a TCS mod transformer as the driver iron. I think that transformer came out of the 618-S, but I could be wrong.

As far as neutralizing goes, they did it in the ARC-5 with a pair so it would not be a bad idea. It would not be a bad idea. With Rice (grid) neutralization as shown in this schematic, it will take a 1 - 5 pf cap which is a pretty small trimmer.

Mike WU2D


* 1625Amp copy.jpg (35.2 KB, 376x311 - viewed 567 times.)
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