Metalwork - Lathe and Mill topics

Re: [Amateur-repairs] Company Needed to test 4CX1000a Tubes

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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Re: [Amateur-repairs] Company Needed to test 4CX1000a Tubes

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

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Re: [Amateur-repairs] Company Needed to test 4CX1000a Tubes

That is a typo. A 4CX10000 won't fit inside an Alpha 8410

On 1/2/2026 10:49 AM, Andrew Lenton wrote:
> Just noticed the extra zero, Ok 10KW output, will need 1Kw drive in
> common grid mode, or 500 Watts if grid driven!
>
--
73 de Mike, W1NR

THAT was the equation. EXISTENCE!... SURVIVAL... must cancel out...
programming!

- Ruk -

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Re: [Amateur-repairs] Company Needed to test 4CX1000a Tubes

Just noticed the extra zero, Ok 10KW output, will need 1Kw drive in common grid mode, or 500 Watts if grid driven!

Not to mention you will need a 20kW transformer assuming 50% efficiency!

That is 28A at 240V per phase, double. The current at 110Vs!

Interesting project, your ham licence allows you 1.5KW, well you will have headroom for digital modes!

What am I missing, is this for SW broadcast? Maybe FM?

SSB you could get 12KW PEP, still less than Italian stations 😎😎!!!

Andrew

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On January 2, 2026, at 14:25, David Mitchell via groups.io <wb2pjh=gmail.com@groups.io> wrote:

I'm trying to finish the repairs on my Alpha 8410 and would like to know if there is a company that has the proper fixture to test the two Penta 4CX10000a tubes? I checked for obvious shorts between the grids, cathode and plates and they all look OK. There aren't many repair facilities around that service amps that use these tubes.

73,

Dave WB2PJH

Re: [Amateur-repairs] Company Needed to test 4CX1000a Tubes

You amp is probably the best test bed for your valves?

Andrew

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On January 2, 2026, at 14:25, David Mitchell via groups.io <wb2pjh=gmail.com@groups.io> wrote:

I'm trying to finish the repairs on my Alpha 8410 and would like to know if there is a company that has the proper fixture to test the two Penta 4CX10000a tubes? I checked for obvious shorts between the grids, cathode and plates and they all look OK. There aren't many repair facilities around that service amps that use these tubes.

73,

Dave WB2PJH

Re: [Amateur-repairs] Company Needed to test 4CX1000a Tubes

Penta claims to test them, did you already reach out to them?

Check with Econco or Kennatron?
What are the symptoms?

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[Amateur-repairs] Company Needed to test 4CX1000a Tubes

I'm trying to finish the repairs on my Alpha 8410 and would like to know if there is a company that has the proper fixture to test the two Penta 4CX10000a tubes? I checked for obvious shorts between the grids, cathode and plates and they all look OK. There aren't many repair facilities around that service amps that use these tubes.

73,

Dave WB2PJH

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Friday, 2 January 2026