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
> _______________________________________________
> TenTec mailing list
> TenTec@contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec
>
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