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Author Topic: Homebrew amp parasitic frequency  (Read 351 times)

G3RZP

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Re: Homebrew amp parasitic frequency
« Reply #15 on: July 19, 2021, 02:29:32 AM »

Have you ever calculated the RF current in a 40 microhenry plate choke in a stage with say 2500 volts plate voltage operating on 80m? It's surprisingly high, and that current needs to flow the RF bypass at the B+ end. Which it makes it desirable to use an other choke and bypass to keep RF out of electrolytic B+ capacitors...
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VE7RF

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Re: Homebrew amp parasitic frequency
« Reply #16 on: July 19, 2021, 07:45:25 AM »

Have you ever calculated the RF current in a 40 microhenry plate choke in a stage with say 2500 volts plate voltage operating on 80m? It's surprisingly high, and that current needs to flow the RF bypass at the B+ end. Which it makes it desirable to use an other choke and bypass to keep RF out of electrolytic B+ capacitors...

Agreed, BUT... the OP stated it is a MONOband  20m amp.    50uh =  4396 ohms (@ 14 mhz)
 2500  X .62 =  1550 V.  ( RF component).  1550 / 4396 =  .35 amp. 

90 uh won't series resonate on 20m,  so lets try that.  90 uh =  7913 ohms of  XL @  14.000 mhz.
1550 / 7913 ohms =  .196 amp.

I stocked up on 4700 pf @  10 kv.....and also  4700 pf  @ 15 kv  disc ceramic  caps a while back.  Ceramite brand.... which appear to be no longer available.   Typ  2-3 of em are used in parallel for a good bypass on 160m...( with a  225 to  400 uh plate choke).   I see our German friends have found a source for  .01 uf  @  20 kv disc ceramic caps.... and I have no clue where they are sourced from.   They are huge things.

On  80m, with the same 50 uh,  resulting current has now increased to 1.4 amps..which is a bunch.   When real small bypass caps used at the cold end of the 50 uh plate choke, like 1000 pf,  the  1000 pf cap  won't have 1.4 amps flowing through it, more like  1/2 that....with the remaining  1/2  flowing into the series string of HV lytics.

TX  doorknobs, like the  HEC  HT-50 /  HT-57 variety would also work as a bypass cap on 20m, provided they handle the current....which is easy enough to see from their graphs.   Typ  TX  doorknobs work  better for upper HF bypassing, like  20-6m.  The disc ceramics  appear to work better on the lower HF bands, like  160-30m.   Both can be used at the same time, and that is done on some commercial  160-10m amps like the 8K ultra.

The ameritron  225 uh plate choke works good.... up to  1.5 amps dc plate current ( CCS).  Above that, it goes into thermal runaway..and fast.

W8JI has cooked up a bigger version.   1.3"  OD  x  10.5" long  (including the 22 ga wire).   It's  400 uh, and does not resonate on any of the 10  HF bands.  It does resonate on 8.5 mhz...and one higher freq, think it was 16.3 mhz.  Rated for  10 kv  @  5-6 amps.
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W1RKW

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Re: Homebrew amp parasitic frequency
« Reply #17 on: July 19, 2021, 01:24:31 PM »

I think you need to revisit your plate choke configuration.

He never mentioned the value of his plate choke.  50 uh is ample ( what's used in the SB-220)  and will series resonate on 40 mhz.   Those  2.5 mh  ....'safety choke'  will typ blow up if the plate block cap  fails.   Measure the DC resistance of your 2.5 mh  safety choke, and u will see why it will fail.  All  u require is a 50 uh choke...wired between   hot side of load cap..and chassis....and wound with 18-22 ga wire.   What else that works is  12 ga magnet wire wound on a small 1.57" OD  toroid.   The safety choke is at a 50 ohm point.

813's  don't require a  parasitic suppressor...even with 3 kv no load on em.   They barely have gain on 10m.

OP stated 3.17uH in post #1093
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K0IZ

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Re: Homebrew amp parasitic frequency
« Reply #18 on: July 19, 2021, 01:29:22 PM »

Plate choke is 95 turns of 22ga on 1/2" Delrin rod.  Measures 43 uh.  Plate coil is 3.17 uh.  Amp is monoband, just 20 meters.  I checked choke with GDO and didn't find any resonances below 80 uh.  Didn't go higher.

Also the resonance I was getting, which initiated this post, is in the plate coil.  Sitting on workbench, self resonance is 140 Mhz.
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W1RKW

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Re: Homebrew amp parasitic frequency
« Reply #19 on: July 19, 2021, 01:53:00 PM »

got it. disregard last post.
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