[TenTec] Ig drift on amps
pfizenmayer at qwest.net
Sun Feb 8 14:07:02 EST 2009
Looking at Gary's schematic attachment , it looks to me like the output
network is a PI-L and the padders are at the "midpoint" rather than at the
50 ohm point. Dunno ham designs what TT designs a PI-L for at midpoint, but
some typical ham designs run more like 300 ohms.
73 de Hank K7HP
- From: "Dr. Gerald N. Johnson" <geraldj at storm.weather.net>
To: <tentec at contesting.com>
Sent: Sunday, February 08, 2009 11:01 AM
Subject: Re: [TenTec] Ig drift on amps
> On Sun, 2009-02-08 at 01:03 -0500, Gary Smith wrote:
>> Thanks for the suggestions.
>> I'm not near the amplifier at the moment and can't open the amp to
>> have a look see but I did bring my manual with me and took a macro of
>> the the band switching section of the schematic and since I can't
>> attach it for here, it's on my website.
>> I couldn't see on the photo of the amp just where the padding caps
>> are. Perhaps the idea of using doorknobs in the amp is not practical
>> space wise. It seems as the tuner mentioned in the thread might have
>> more space than the 425's tank?
>> If there's space in the 425, it seems it might be worthwhile to just
>> replace all the padding caps with superior versions than are in there
>> right now. I'd be more than happy to do that. One kind offlist email
>> suggested amp ratings of at least 2.5 at 1.8MHz. I would prefer to
>> use something well suited and add in some overkill factor if that's
>> reasonable, I do want stability and this amplifier is really
>> excellent other than this one current issue. If I can beef up the
>> components to make it more bulletproof, I'm for that.
> A way to look at the current in the padding capacitors is to realize
> that 2Kw at 50 ohms is 316 volts RMS. The reactance of a 220 pf padding
> capacitor at 2 MHz is 360 ohms, so the current in that padder would be
> nearly 0.9 amps. At 4 MHz, 1.8 amps. Then one needs to allow for the
> output voltage being higher, say for feeding 2:1 SWR load, tuned to 100
> ohms resistive. 447 volts RMS, 1.25 amps at 2 MHz, 2.5 amps at 4 MHz.
> Cornell-Dubilier metal cased mica cap rated at > 20 amps and 1KV should
> handle these conditions with no problems.
>> So based on the above info & the schematic section, which caps might
>> I be most likely to want to replace and what would be best to replace
>> them with? I am out of doorknobs except for a couple of fixed vacuum
>> caps but they're the size of a large doorknob.
>> Thanks again for the help, I've apparently got this issue narrowed
>> down quite a bit with everyone's help.
> 73, Jerry, K0CQ
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