3cpx1500a7

[Shockwave]

If you can get the Chinese 3CPX1500A7 for reasonably less than you can get an Eimac 3CX1500A7, I might go for it. The reason is the higher breakdown voltages are exactly what this tube benefits from in terms of reliability.

This tube is noted for grid arcs and arcing between the cathode and filament. The later is eliminated by floating the filiment above RF ground as though it were a directly heated cathode. No bias or drive is applied to the filiminet, it just floats up close to the RF on the cathode.

The improvement in the "P" version of this tube with its larger ceramic insulator, actually does nothing for either of these problems. Other than the fact that the tube has to be tested at a higher breakdown voltage and pass, in order to get the Pulse rating.

That basically means, it has an exceptional vacuum, preventing internal arcs. Many of Eimac's pulse rated tubes (particularly glass envelopes) where manufacturered exactly like the regular version, except they passed a much higher hi-pot, voltage breakdown test.

The 8877 had problems with external tubes arcs in Pulse operation and required the larger insulator. Because improved vacuum, only prevents internal tube arcs.

NOTE: Captain Kilowatt was correct. The Pulse version of this tube in not an electrical equivalent of the 8877 and the difference in filiment voltage, is twice the +/- 0.25 volt tolerance! Treating it as an 8877 will slowly poison the oxide coated cathode. The Pulse version requires 0.5 extra volts and 0.7 extra amps on its filiment, in order to help support the enormous peak emissions in Pulse mode.

 

[linearone]

If you can get the Chinese 3CPX1500A7 for reasonably less than you can get an Eimac 3CX1500A7, I might go for it. The reason is the higher breakdown voltages are exactly what this tube benefits from in terms of reliability.

This tube is noted for grid arcs and arcing between the cathode and filament. The later is eliminated by floating the filiment above RF ground as though it were a directly heated cathode. No bias or drive is applied to the filiminet, it just floats up close to the RF on the cathode.

The improvement in the "P" version of this tube with its larger ceramic insulator, actually does nothing for either of these problems. Other than the fact that the tube has to be tested at a higher breakdown voltage and pass, in order to get the Pulse rating.

That basically means, it has an exceptional vacuum, preventing internal arcs. Many of Eimac's pulse rated tubes (particularly glass envelopes) where manufacturered exactly like the regular version, except they passed a much higher hi-pot, voltage breakdown test.

The 8877 had problems with external tubes arcs in Pulse operation and required the larger insulator. Because improved vacuum, only prevents internal tube arcs.

NOTE: Captain Kilowatt was correct. The Pulse version of this tube in not an electrical equivalent of the 8877 and the difference in filiment voltage, is twice the +/- 0.25 volt tolerance! Treating it as an 8877 will slowly poison the oxide coated cathode. The Pulse version requires 0.5 extra volts and 0.7 extra amps on its filiment, in order to help support the enormous peak emissions in Pulse mode.

I have limited experience with them except for the 2 tube box I built my buddy. With about 4kv plus ive seen 10kw pep for a short test. It ran beautifully at 7500. Its nearly continuous service at 4kw. He could key the mic and play music like a radio station for a very long time, we never felt like we ever reached a time limit. That's how he ran it in skip land, 100w in gave 4kw out. The gain was incredible for a triode. But he was smart. He had ldf5 going to a 20k gamma on an m107. It was an indestructible combo.
he is now a silent key and Im tasked with selling the station.

 

[Shockwave]

I have limited experience with them except for the 2 tube box I built my buddy. With about 4kv plus ive seen 10kw pep for a short test. It ran beautifully at 7500. Its nearly continuous service at 4kw. He could key the mic and play music like a radio station for a very long time, we never felt like we ever reached a time limit. That's how he ran it in skip land, 100w in gave 4kw out. The gain was incredible for a triode. But he was smart. He had ldf5 going to a 20k gamma on an m107. It was an indestructible combo.
he is now a silent key and Im tasked with selling the station.

I have no doubt the amplifier does the numbers you say and more. However, you can achieve longer tube life on an oxide coated cathode, by not undervolting the filament. This is because this type of cathode is dependent on reaching a high enough temperature to produce a protective Cloud over the cathode that prevents it from being stripped of the emissions producing oxides. It is not likely to be noticed in intermittent Communication Service because the reduced output would only be noticed in the pulse mode where maximum emissions are required.

Although, just because you can't see a reduction in performance does not mean it won't have reduced tube life. Just measure the voltage right across the filament terminals of the tube socket. If you're below 5.25 volts, you will get increased tube Life by increasing that voltage. It could be as simple as just squeezing an extra couple of wraps on the secondary of the filament Transformer if there's room. Depending on the Transformer and RF chokes used, you may already be in the ballpark. It looks like a nice amplifier and I'm sure it will make someone very happy.

 

[linearone]

I wish they were still cheap..these were medical pulls. At the time they were $250/ea. Ive seen pulls pop up now and again but its certainly a drier market than it used to be.and they aren't $250 anymore.