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Author Topic: Mixing 3CX800A7 & 3CPX800A7  (Read 4373 times)
KO4NX
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Posts: 179




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« on: May 13, 2005, 06:24:20 AM »

Hello All:

I currently have a spare Eimac 3CX800A7 for my Ten Tec Titan. I now have the chance to purchase a Eimac 3CPX800A7 for a good price.

My question is, can these two tubes be run in the same amp together?
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K6AER
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Posts: 5745




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« Reply #1 on: May 13, 2005, 01:42:12 PM »

You can probably run the two tubes together with out much problem but the CPX version is about ¼ inch taller than the standard version. This gives the CPX a higher flashover voltage. Compare the tube capacitances from the Grid to Cathode and from Plate to Cathode. If they are close it should not be a problem. The CPX will not provide higher output with out higher plate voltage.

Now having said that, if you don’t have two tubes of the same type CX or CPX you could run both but pay careful attention to the grid current. Don’t exceed the maximum current for the lowest tube rating.

I would replace both tubes with the same type in order to have better linearity. Putting a 3CPX800A7 in the TenTec is a waste of the tube with out replacing the HV transformer  with a higher output unit and beefing up the amp for the high output CPX tubes.
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N0TONE
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Posts: 173




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« Reply #2 on: May 13, 2005, 03:17:06 PM »

The two tubes are exactly the same physical dimensions, and have exactly the same internal structures.  The ceramic insulator on the "P" version is longer, and the anode mounting structure is slightly shorter, so that the overall length is the same.

Capacitances don't matter - you're going to put them in parallel, so the anode-to-grid, or cathode-to-grid capacitance of the paralleled pair is simply the sum of the two.

"Matched pairs" are matched in terms of the required grid bias voltage, and the mu, nothing else.

Most likely, they'll work just fine in parallel.

AM
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