[TenTec] Ig drift on amps

Dr. Gerald N. Johnson geraldj at storm.weather.net
Sun Feb 8 13:01:27 EST 2009

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.
> http://doctorgary.net/Titan425TankSchematic1.jpg
> 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.
> Gary
> KA1J

73, Jerry, K0CQ

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