It sounds like maybe you lost grid bias. With a tetrode amp you should always check that all operating voltages are present and correct before you put the tubes in.
On Fri, 02 Jan 2026 19:18:36 -0800
"David Mitchell via groups.io" <wb2pjh=gmail.com@groups.io> wrote:
> I tried to contact Penta and got no response, but if you think that they test their own tubes, I'll give it another shot. The amp had been working fine but quit in the middle of a QSO. When I opened it up, the wirewound resistor in the startup circuit was fried and one of the three plate blocking caps was shorted. I replaced everything but as soon as I applied the HV, the plate current skyrocketed and the resistor started heating up fast. I've checked the sockets, the resistors in the RF deck, and everything checks out.
>
> Replacement tubes cost more than the amp is worth, which is why IMHO the next step is to get the tubes tested by someone who knows what they're doing.
>
> Sorry for the typo - the tube is obviously a 4CX1000A.
>
> 73,
> Dave
--
73
-Jim
NU0C
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