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Tank curcuit Woes




 
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Author Topic: Tank curcuit Woes  (Read 5980 times)
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N8ECR
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« on: February 26, 2005, 10:47:03 AM »

Gentelman:  I have been converting a GE XT1-A to the amateur bands particular 75m.  I have removed the 8005 driver and installed a 813 instead to get rid of Neutralizing woes. and seems to work well so far, however now it is time to Neuralize the 833A's, and I have found resonance in the pi-network on 75m.  Problem:......

Every time I turn the driver (813) medium voltage supply/off then back on again, my resonancePoint on the 833A's  changes causing me to run the controls again  to find were the grid current is max, and the slight rise in cathode current on the 833A's  dips once again,  this happens every time

Has anyone else seen this happen, what is going on?Huh
I have never had this happen before.  However every thing else up to that point of drive on the 833A's is looking good , and seems stable.  And the output of the pi is not terminated yet , I recall that push-pull finals require  antenna temination to be disconnected, However the final in the XT1-a is a pi-net  Maybe I should teminate the out put of the pi-net first???

Still this tuned curcuit souldn't jump around like this unless something is itermitted, such as the image load impedence!!

IDEA"s any one!!!!! :ctf:
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k4kyv
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Don
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« Reply #1 on: February 26, 2005, 01:09:35 PM »

I would terminate the output network into some kind of a dummy load.  I always neutralise my homebrew pushpull final rigs working into a load.  I have found just checking for rf in the final tank circuit with no load can give a false null when neutralising, and that changing the final tank capacitor adjustment affects the null.  My neutralising instrument is an old Mirage swr/wattmeter, set to maximum sensitivity, working into a noninductive 50 ohm glo-bar resistor, macthed to the link-coupled output with a resonating capacitor.  The idea is to place a nonreactive load on the final tank circuit while neutralising.  I have also found that it is often necessary to physically disconnect  the HV lead from the power supply to final to get a clean neutralising indication.  Just turning off the ac voltage to the plate transformer to kill the HV won't always work.

I have a somewhat similar problem with my Gates BC1-T.  It uses a pair of 807's in parallel to drive a pair of 833A's in parallel, with pi-T-L network to the antenna.  The final is grid-neutralised with the same tuned circuit serving as driver plate and final grid tank.

I have tried mine on 160 only.  The neutralising (surprisingly) holds up perfectly from one end of the band to the other without any readjustment, but I have to always make sure the driver plate/final grid tuning is such that the 807 cathode current is dipped exactly.  If the circuit is tuned even slightly to one side of dip, I may get more grid drive, and the final PA plate current to the 833A's jumps up, but the rf output drops and the 833A plates start glowing orange, indicating a sustantial reduction in plate efficiency.

I just tune mine up carefully, and have had no problems.  As soon as I can acquire a good stable oscilloscope that will display rf waveforms up to several megaHertz, I plan to examine the waveform of the rf at the grid of the 833A's and see if I can determine what is going on.  I suspect some kind of parasitic oscillation in the 807's, 833A's or both.
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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WA1HZK
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« Reply #2 on: February 26, 2005, 01:37:10 PM »

For what it's worth I chose to use a standard balanced input and parallell connected Pi-Net on the 833's in my rig. For RF drive IPA I used a 4-400 connected as a triode, grounded grid with a regular filiment choke & a cap into the fil. This works pissa if you are going to drive the rig with another transmitter with 25 watts or so. To neut. the 833's I connected about a 15 pf 10 KV cap from the plate to the input ckt as shown in the ARRL handbook. about 1/2 of the cap is used to neut. the transmitter. It's not exactly what you are doing but after trying almost everything else over two years to come up with a stable input ckt & neutralizing circuit for the transmitter the "cheap & dirty" 4-400 solved all of the problems. I also chose the 4-400 because it is happy running from the same plate supply as the 833's. Easy 175-200 ma drive now for the 833's.
Keith
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KL7OF
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« Reply #3 on: February 26, 2005, 01:57:12 PM »

What are you using for a plate choke?    Does it have resonance on the freq you want to operate on?
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Steve W8TOW
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« Reply #4 on: February 28, 2005, 08:32:43 AM »

PI Net or PP, I always neutralize finals into an appropriate load.
If it is PP link coupled, I use about 1000 pF in series to ground with a
10-12 turn link, 80m 470 pF w/ 8-10 trun link, 40m 270 pF w/ 6 turn link;
to achieve 50 ohms.
For the PI Net, I also turn thru resonance with the tank output circuit while neutralizing just to see what happens!

I would check the resonant freq of the plate choke,  and also the
RF choke for the screen and plate of the newly installed 813....

gl es 73 steve
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Always buiilding & fixing stuff. Current station is a "Old Buzzard" KW, running a pair of Taylor T-200's modulated by Taylor 203Z's; Johnson 500 / SX-101A; Globe King 400B / BC-1004; and Finally, BC-610 with SX28  CU 160m morn & 75m wkends.
73  W8TOW
N8ECR
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« Reply #5 on: February 28, 2005, 08:58:31 AM »

I put a 50 ohm load on it this week end and retuned, I didn't see the massisve detuning I saw with no load. But there might be a sight bit still.

My plater choke is likly a 2.5 mh job, but of an excessive size.
I didn't install a choke in the screen lead to the 813, Might go back an do this.  I went right to the High voltage bleeder, to tap off needed voltage.

I will see how it goes from here, my time is limited lately.
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Steve W8TOW
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« Reply #6 on: February 28, 2005, 01:33:13 PM »

I think not only a rf choke in the screen, but consider a paracitic choke...
this would be common practice.
if you have  a "Z-1" choke, I think that should work nicely...
gl es try to keep your head above the water this week mike!
73 steve
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Always buiilding & fixing stuff. Current station is a "Old Buzzard" KW, running a pair of Taylor T-200's modulated by Taylor 203Z's; Johnson 500 / SX-101A; Globe King 400B / BC-1004; and Finally, BC-610 with SX28  CU 160m morn & 75m wkends.
73  W8TOW
N8ECR
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« Reply #7 on: February 28, 2005, 05:02:57 PM »

I don't ever recall seeing a choke, or a parasitic choke in a screen circuit.  :Huh:  

But have seen a low value resistor used in multi-poob stuff likly used for equilization.

however that modified circuit (813) took care of a lot of problems I was having.  And boy I found out why engineers like to use regulated voltage there, it really changes with different loads, I guess I should known that too.

Just a little more drive to come up with and I am good for a go...  Likely not going to happen till I get back from down South. :lol:
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Steve W8TOW
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« Reply #8 on: March 01, 2005, 08:48:30 AM »

yup, all the West Coast Radio Handbooks show a paracitic choke in the screen circuit, a very well regulated screen supply and as you mentioned, sometimes even  a bit of modulation on it....all kinda little secrets from Bill Orr and Frank Jones et al.....
I typically saw a big bank of VR tubes in such rigs and now of course some zeners will do...
glad it is working tho....vri 73steve
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Always buiilding & fixing stuff. Current station is a "Old Buzzard" KW, running a pair of Taylor T-200's modulated by Taylor 203Z's; Johnson 500 / SX-101A; Globe King 400B / BC-1004; and Finally, BC-610 with SX28  CU 160m morn & 75m wkends.
73  W8TOW
N8ECR
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« Reply #9 on: March 01, 2005, 11:01:17 AM »

I have found a slight resonance near 4 mc.  this could be the devil , however I was using a MFJ antenna anilizer which I have found to give arronius readings, picking up other stuff from other circuits.  But there might be some thing there that needs futher checking.

I have changed the whole driver and final to be confiqered like that of Don's/K4KYV (Grid Neutralized) Gates circuit discribtion above.  I feel it is a real stable tank circuit for my needs.  I was getting nowhere with the original final arrangement and triode driver.
With all the inductance in the original BC coils, I should have concidered Push-Pull tank

Steve:  If I get time I will get some of them West Cos handbooks out and see what they were up to,  I never seen Parasitic supressor in the screen of a tube.
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