[TenTec] Hercules 444 amp

[Robert]

Best experience and best advice is to ways tune the amp for max output while keeping drive and loading such that maximum recommended plate current and maximum recommended grid current are not exceeded even for a brief moment.   Then if one wishes to operate at less power, reduce drive but do not touch any TUNE or LOAD control. 

Also make sure you know the antenna or load is good (low SWR). before one activates the amp. 

I find that many "grounded grid amps" are not true grounded grid amps but have the grid above ground by a resistor bypassed with some value of C.  In some instances a RF choke is used with some value of  C. These designs are easier to build with less protection ckt required but have much higher likely hood of instability. A VHF oscillation will quickly destroy a band switch and in doing so will exceed greatly the grid dissipation of the tube(s). 

The correct solution is to take all grid pins to ground, 3 of them, using a 1/4" wide copper strap at each one that is less than 2" in length.  Some adjustment in bias may be required to attain correct idle current.  One will then find the tuning to be much smoother and flash overs will greatly diminish if not completely disappear. 

73
Bob K4TAX





Sent from my iPhone

> On Aug 10, 2014, at 9:51 PM, "Duane Calvin" <ac5aa1 at gmail.com> wrote:
> 
> Sure, Carter.  My Centurion came new with two RF Parts 3-500ZG's.  This was
> fairly early in the life of this tube configuration (graphite anode variety)
> and manufacturer, and TT had just started advertising they were using them.
> After about 2 years of occasional use, generally running between 600-900W
> (and all settings well within the safe zone), one of the original tubes
> developed the infamous grid/cathode short.  I replaced the original tubes
> with a new matched-pair of 3-500ZG's from RF Parts, and shortly after that I
> sold the amp (with a used spare tube).  I know lots of guys who have had
> Heath and other amps with EIMAC 3-500Z's in them for years with very few
> problems.  Yes, mine could have been a late early life failure, but at that
> time I was reading of others having similar problems with these tubes.  That
> was a few years ago - maybe RF Parts has improved their relationship with
> the factory that makes these tubes and all is well today.  So, my "data"
> consists of one in-person experience along with a number of other reported
> failures around the same time, for whatever that's worth.  The implication,
> to me, is that the tubes at that time were not of the same quality level as
> EIMAC's.
> 
>    73, Duane
> 
> 
> Duane Calvin, AC5AA
> Austin, Texas
> www.ac5aa.com  
> 
> 
> 
> 
> -----Original Message-----
> From: TenTec [mailto:tentec-bounces at contesting.com] On Behalf Of Carter
> Sent: Sunday, August 10, 2014 8:33 PM
> To: tentec at contesting.com
> Subject: Re: [TenTec] Hercules 444 amp
> 
>> On 8/10/2014 2:08 PM, Duane Calvin wrote:
>> This was certainly true of the  EIMAC tubes, as long as you cared well
>> for them.  I've not had as good a luck with the current
>> Chinese-sourced tubes (specifically, the 3-500ZG's from RF Parts,
>> which the Centurion uses.)
> 
> My Centurion (which I obtained used 20 years ago) finally had one of its 
> tubes [rather spectacularly] go to the great beyond.
> 
> Unless I get really lucky and find a NOS pair at the e-place, it seems 
> like my only other option is a matched pair from RF Parts.
> 
> Could you please elaborate a bit on your statement above that says "I've 
> not had as good a luck with the current
> Chinese-sourced tubes (specifically, the 3-500ZG's from RF Parts)".
> 
> 73,
> Carter K8VT
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