Metalwork - Lathe and Mill topics: 2026

Saturday, 3 January 2026

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

Acom amps also use this valve and its bigger brother 1500. The circuits are downloadable.

There is a lot if protection built in.

If you find a willing volunteer, with said amp, you could try it in that.

I have one here, and would happily swap it over if you lived closer.

I have not seen valve testers, that take this series of valves.

If you have an Acom dealer near you, get in touch.

Andrew

Get BlueMail for Mobile

On January 3, 2026, at 16:46, Jim Shorney via groups.io <jimNU0C=gmail.com@groups.io> wrote:

Interesting. Full disclosure, I have no experience with this amp but I did peruse the schematics. I do have some experience with tetrodes (NCL-2000). There is a lot going in there. Are the proper voltages actually making it to the tube sockets in TX? Does the step-start function normally with no tubes installed? It is possible you have a high voltage fault in one of the tubes. It might be worthwhile to test each tube by itself in the amp with airflow from the open socket blocked off.

There is an active Alpha group on Facebook that may be able to provide some guidance.

On Sat, 03 Jan 2026 08:19:30 -0800
"David Mitchell via groups.io" <wb2pjh=gmail.com@groups.io> wrote:

Hi Jim,
I've got a fair amount of technical experience and checked all the voltages at all the test points and all were FB. The Penta 4CX1000A subs that Alpha used were notorious for screen grids shorting out, but finding nothing when testing each tube with my VOM, need to find someone reputable who can test these tubes in a proper test fixture to see if they're failing under load.

I neglected to add that the two 20 amp main fuses did NOT blow, only the 2A fuse that runs the control circuitry and CPU.

Thanks,

Dave WB2PJH

--

73

-Jim
NU0C

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

Interesting. Full disclosure, I have no experience with this amp but I did peruse the schematics. I do have some experience with tetrodes (NCL-2000). There is a lot going in there. Are the proper voltages actually making it to the tube sockets in TX? Does the step-start function normally with no tubes installed? It is possible you have a high voltage fault in one of the tubes. It might be worthwhile to test each tube by itself in the amp with airflow from the open socket blocked off.

There is an active Alpha group on Facebook that may be able to provide some guidance.

On Sat, 03 Jan 2026 08:19:30 -0800
"David Mitchell via groups.io" <wb2pjh=gmail.com@groups.io> wrote:

> Hi Jim,
> I've got a fair amount of technical experience and checked all the voltages at all the test points and all were FB. The Penta 4CX1000A subs that Alpha used were notorious for screen grids shorting out, but finding nothing when testing each tube with my VOM, need to find someone reputable who can test these tubes in a proper test fixture to see if they're failing under load.
>
> I neglected to add that the two 20 amp main fuses did NOT blow, only the 2A fuse that runs the control circuitry and CPU.
>
> Thanks,
>
> Dave WB2PJH

--

73

-Jim
NU0C

-=-=-=-=-=-=-=-=-=-=-=-
Groups.io Links: You receive all messages sent to this group.
View/Reply Online (#96196): https://groups.io/g/Amateur-repairs/message/96196
Mute This Topic: https://groups.io/mt/117041575/693790
Group Owner: Amateur-repairs+owner@groups.io
Unsubscribe: https://groups.io/g/Amateur-repairs/leave/4530700/693790/458576413/xyzzy [jhjoshan@gmail.com]
-=-=-=-=-=-=-=-=-=-=-=-

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

Hi Jim,

I've got a fair amount of technical experience and checked all the voltages at all the test points and all were FB. The Penta 4CX1000A subs that Alpha used were notorious for screen grids shorting out, but finding nothing when testing each tube with my VOM, need to find someone reputable who can test these tubes in a proper test fixture to see if they're failing under load.

I neglected to add that the two 20 amp main fuses did NOT blow, only the 2A fuse that runs the control circuitry and CPU.

Thanks,

Dave WB2PJH

_._,_._,_

Groups.io Links:

You receive all messages sent to this group.

_._,_._,_

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

Can you apply power with the valves removed, see if you still have high current?

Abdrew

Get BlueMail for Mobile

On January 3, 2026, at 03:18, 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

Friday, 2 January 2026

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

-=-=-=-=-=-=-=-=-=-=-=-
Groups.io Links: You receive all messages sent to this group.
View/Reply Online (#96192): https://groups.io/g/Amateur-repairs/message/96192
Mute This Topic: https://groups.io/mt/117041575/693790
Group Owner: Amateur-repairs+owner@groups.io
Unsubscribe: https://groups.io/g/Amateur-repairs/leave/4530700/693790/458576413/xyzzy [jhjoshan@gmail.com]
-=-=-=-=-=-=-=-=-=-=-=-

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

_._,_._,_

Groups.io Links:

You receive all messages sent to this group.

_._,_._,_

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 -

-=-=-=-=-=-=-=-=-=-=-=-
Groups.io Links: You receive all messages sent to this group.
View/Reply Online (#96183): https://groups.io/g/Amateur-repairs/message/96183
Mute This Topic: https://groups.io/mt/117041575/693790
Group Owner: Amateur-repairs+owner@groups.io
Unsubscribe: https://groups.io/g/Amateur-repairs/leave/4530700/693790/458576413/xyzzy [jhjoshan@gmail.com]
-=-=-=-=-=-=-=-=-=-=-=-

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

Get BlueMail for Mobile

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

Get BlueMail for Mobile

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?

_._,_._,_

Groups.io Links:

You receive all messages sent to this group.

_._,_._,_

[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

_._,_._,_

Groups.io Links:

You receive all messages sent to this group.

_._,_._,_