[TenTec] Ig drift on amps

Bob McGraw - K4TAX RMcGraw at Blomand.net
Sun Feb 8 22:36:37 EST 2009


The output of the amp is indeed a Pi-L network.  These work great and have 
been in BC transmitters very successfully for years.  I wish more companies 
would use Pi-L networks in their amps. Better attenuation of harmonics, 
lower currents in the coils and components and a better match.  All at the 
cost of increased costs.

I can't imagine any amp today being able to handle a 300 ohm load on its 
output at rated power.  That's an SWR of 6:1.


73
Bob, K4TAX


----- Original Message ----- 
From: "pfizenmayer" <pfizenmayer at qwest.net>
To: <geraldj at storm.weather.net>; "Discussion of Ten-Tec Equipment" 
<tentec at contesting.com>
Sent: Sunday, February 08, 2009 1:07 PM
Subject: Re: [TenTec] Ig drift on amps


> 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.
>>> 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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