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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