[AMPS] Re: 6m amp blocking caps

[2]

>
>Hi,
>
>Speaking of blocking capacitors (and at the risk of starting a
>range war)--
>
>My 6m amp, which uses an 8877 and a pi network, seemed to suffer
>from the same problem as Lance's -- I could get 1500 W out, but
>just barely.  It seemed strange when I can easily get 1500+ W
>out of my 2m W6PO-style 8877 amp.
>
>Since both Lance and I were using two 500 pF caps in parallel 
>for the blocking capacitors on our 6m amps, and since he reported
>improved output by changing to two 100 pF caps in parallel, I
>thought I'd give it a try.  Only I thought I'd take it a small 
>step further and use two 75 pF caps in parallel instead of the 
>2 X 100 pF caps that Lance used.
>
>So yesterday I swapped out the caps, let the amp warm up, and
>applied some drive.  When I released the PTT, the grid current
>meter came to rest on the left sided peg.  After some investigating,
>I found that one of the filament feedthrough caps was developing
>a short to ground (only about 800 ohms resistance to ground).
>I wrote off the failure of the feedthrough cap as a coincidence,
>replaced the faulty component, and put the amp back together.
>
>Once back together, the amp warmed up and -- good, no more reverse
>grid current reading.  But as soon as I applied a small amount of
>drive and then released the PTT, the reverse grid current was back.
>Checking, I found that the same feedthrough capacitor had failed.
>
>This morning I replaced the feedthrough capacitor once again and
>replaced the 2 X 75 pF blocking caps with 4 X 120 pF (4 instead of
>2 to better handle the current and to significantly increase the
>value of the blocking caps).  Now the amp tunes up just fine -- no
>more blown feedthroughs -- and I'm seeing 200 more watts out with
>similar plate current readings and drive requirements (when compared
>with the amp with 2 X 500 pF caps).
>
>My question:  why did the feedthrough caps blow when I had the 
>2 X 75 pF caps in as blocking caps?  

When DC blocking caps are paralleled, a parallel-resonance is created 
between the capacitors in the RF output path.  The additional resonance 
may be problematic.    

>I assume there was RF voltage
>getting to the feedthroughs, but I don't understand why.  The amp
>uses a bifilar wound filament choke and the feedthroughs that failed
>were on the transformer side of that choke.  And I don't really
>understand why I should be seeing increased power out of the amp now.
>
>By the way, I now get 1500 W out with .90 A of plate current, 2600 V
>key down, and 40 mA of grid current.
>
>73,  Arliss  W7XU
>
>Lance Collister wrote:
>> 
>> Just in case you have been wondering about YOUR 6m 8877 amp...
>> 
>> I have been using a homebrew 8877 amp (Pi-L output) for the last six years 
>(since I
>> started on 6m). The efficiency always seemed significantly lower than my 
W6PO 
>8877
>> amp on 2m.  At one point, there were a number of other people with similar 
>problems.
>> I was able to make some improvements by making changes to the input 
circuit, 
>but was
>> not satisfied with the results.
>> 
>> In talking last night with Barrie, W7ALW, he mentioned he was thinking about
>> changing the blocking capacitor in his 6m 8877 amp before he hooked it up 
>again for
>> 6m EME.  I asked what that was all about.  He explained that the ONLY thing 
>he had
>> ever seen the Elmers on the AMPS email reflector agree on, was that the 
>blocking cap
>> should be about 4 times the value of the tuning capacitor.  I don't know if 
>that
>> includes the stray capacitance or not, but I know my 1000 pF blocking cap 
>(two 500
>> pF doorknobs in parallel) sure did not fit that guideline!
>> 
>> So, since 6m was dead up here today as usual, I decided it was a good time 
to 
>open
>> things up and operate.  I replaced the two 500 pF blocking caps with two 
100 
>pF caps
>> (just because I happened to have them handy). I might have even gone down 
to 
>trying
>> a pair of 50 pF caps, but I only had one of those in the bin here.  Anyway, 
>the
>> result was about 10-15% more output, and much smoother tuning!  So, even 
>though
>> there was no DX in my log today, the amp (with the 20 year old tube) is 
>working
>> better than ever!  ;-)
>> 
>> TNX, Barrie!  GL and Happy Holidays to all!  Vy 73, Lance
>>
>
>
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>


-  R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.  
end


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