That may be a little too optimistic, Mike.
Purchasing matched pairs does not assure they are going to draw equal
current throughout their life, but it does improve the chances.
I have a hunch if you would pull several matched pairs after a couple
hundred hours and check them again, some may not be matched anymore.
On the other hand, there doesn't seem to be any other choice. Seems being
able to monitor and adjust the bias individually just requires too much
extra hardware.
We don't really have that problem here in DL because we're only allowed
750w.
The single 3-500z handles that just fine.
73 - Rick, DJ0IP
(Nr. Frankfurt am Main)
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Mike
Schatzberg
Sent: Wednesday, August 13, 2014 3:01 PM
To: Discussion of Ten-Tec Equipment
Subject: Re: [TenTec] Hercules 444 amp
The matching of the tubes when purchased assures they will draw
approximately the same grid current in the amp.
Proper biasing will help assure the cleanest signal for the selected class
of service. The 3-500 is a zero bias triode, but needs to be properly biased
for good linearity for the desired class of service. Greater bias voltage
will increase the needed drive to reach full output.
73,
Mike W2AJI
Sent from my iPhone
> On Aug 13, 2014, at 3:08 AM, "Rick - DJ0IP / NJ0IP" <Rick@DJ0IP.de> wrote:
>
> Grid current is indeed of major importance.
> I have a question and possible suggestion, but first let me explain why:
>
> We had some dodgy rules here in DL 30+ years ago.
> We were only allowed to run 150w "plate dissipation power" in our
amplifier.
>
> The solution was to use 5 sweep tubes, each with 30w dissipation, in
> parallel, in grounded grid. These were Pentodes.
> We also tried other versions, but grounded grid seemed to be the most
> stable.
>
> As you might have imagined, these would blow tubes like crazy, and
> soon gained the nick name of "Sparkplug Amp" or "Firecracker Amp."
>
> I solved the problem by increasing the negative bias voltage and then
> running it through a string of 1N4007 diodes.
> There was 0.6 to 0.7v difference between any two diodes.
> Then instead of tying all 5 grids together, I ran a separate wire from
> each tube to a tap on the diode chain.
>
> Each tube was set to its correct operating bias (and idle current),
> one at a time. By doing so, at least all tubes had the same "starting
> bias" (idle current).
> I called this "dynamic biasing" of the tubes.
>
> Results: One tube blew immediately and was replaced. It was obviously
> bad from the start. The amp then ran on its original set of tubes for
> 14 years (maybe longer).
>
> I sold it to another contester after about 3 years, when I moved from
> Bremen to Munich. During those 14 years the amp was used in 4 or 5
> 48-hr contests per year, every year. At the end of the 14 years, power
was down about 10%.
> Found out by talking to the OM who bought it. Never saw or heard him
> on the air again, so I don't know how long the original tubes actually
> lasted before one blew.
>
> Output power was originally about 1200w on 80, 1100w on 40, 1000w on
> 20, 800w on 15 and 600w on 10m.
> AND THIS SILLY INTERMOD-SLINGER WAS PERFECTLY LEGAL IN GERMANY!
> (at least from a tube standpoint).
> As I said, we had some dodgy rules here in DL.
>
> Luckily the German Post (our FCC at the time) eventually realized that
> the rule was ludicrous and change the power limit to 750w output, +/-
> 1dB, using any method you choose.
>
> Of course we contesters all ordered American amplifiers!
> That's when I bought my first AL-80A.
>
> NOW MY QUESTION TO THE AMP EXPERTS:
> In grounded grid using triodes, is there any way to dynamically
> balance the idle current of two 3-500z tubes?
> This would not only improves the reliability, it would also increase
> the output power.
>
> 73 - Rick, DJ0IP
> (Nr. Frankfurt am Main)
>
> -----Original Message-----
> From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Bob
> McGraw
> - K4TAX
> Sent: Wednesday, August 13, 2014 1:56 AM
> To: Discussion of Ten-Tec Equipment
> Subject: Re: [TenTec] Hercules 444 amp
>
> The issue I've observed with amps using 2 tubes in parallel, there is
> typically no way to determine what each tube is doing. One needs to
> be able to determine, at least, the individual grid currents. In
> doing so, one can see that the tubes are operating within their design
> range. The same applies for 3 tube and 4 tube amps.
>
> As I pointed out earlier, if the recommended total grid current is 260 ma
> maximum, thus individually 130 ma per tube. If one tube be drawing 60 ma
> then the other tube is drawing 200 ma., which is well above its
> maximum rating. Premature failure of this tube can be expected due to
> excessive grid dissipation.
>
> I did modify a SB-200 by adding a toggle switch allowing the grid
> current for each tube to be displayed. This took some circuit changes
> but being an amp that was converted for 6 meters, this pushes the
> limit of the 572B's and we prefer to keep them in their sockets.
> Otherwise, the previous owner had experienced a good amount of FLASH,
> BANG, POP, followed by oh crap the smoke comes out of the box.
>
> 73
> Bob, K4TAX
>
>
>
>
>
>
>
>
> ----- Original Message -----
> From: "Carter" <k8vt@ameritech.net>
> To: <tentec@contesting.com>
> Sent: Sunday, August 10, 2014 8:32 PM
> 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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>>
>
>
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