[Amps] Feedback ( No Pun) from Eimac 3cx1500d7

[Gene Bigham]

I wrote and asked about this tube, here is what I got back:

"Thanks for your Email.

I have been receiving inquiries for the past couple of days regarding this new product. 

It was an idea that languished for a period of time, the individual that suggested this tube was a ham I have known for many years. We promised to sell him a tube for his moneymaking idea, namely conversion of existing amplifiers such as the Drake L4B, to replace the 3-500Z that Eimac no longer makes.  I guess I finally convinced engineering to make a tube sometime last January. It was just completed last month but the effort to check it was put off for one reason or another. Finally it was sent to the lab engineers to check  out.  RF test data indicates it is a good tube where 80 to 100 Watts of drive power is available. It likes to be run at 4000 Volts (more so than 3000 V as grid current tends to drop with increasing plate voltage).  The 3CX1500D7 is "bulletproof" compared to the smaller oxide cathode tubes presently being used in amplifiers made for the amateur radio market. I brought in an old Drake L4B and we see that it will work in that equipment as long as the air flow is increased by adding a blower to the air intake opening on the back of the linear. Also a gasket must be added between the OD of the plate cooler on the tube and the ID of the glass chimney on the existing amplifier, and the unused socket covered over. The gasket can be made of silicone rubber and stuck on with RTV or silicone rubber.  Teflon might be useable if one can keep it from slipping down (it can't be glued to the chimney!) but no one has figured out how to use that material yet. Something needs to reduce the loss of air from the top of the chimney around the anode cooler on the tube. A special pyrex chimney would be nice but too costly (besides we don't have glass making equipment any longer).   Last week my boss decided to send out a press release, announcing this tube. Unfortunately we have not built up a stocking level yet, therefore delivery is going to be rather lengthy, I presume it will be between 60 and 90 days. (That should make one available for a late summer project, right?)  You may be delighted to know that we are introducing a new "buy-direct" plan whereby Hams can pay us directly (we plan to accept Visa or Mastercard sales only) and we ship this tube directly from the factory, thereby keeping the price manageable. If distributors were selling this tube their normal 35% markup would make it much less desirable.  This plan will not apply to other products already established as distributor items. Selling for $599 per tube, it is obvious that a single 3CX1500D7 will cost somewhat more than two 3-500ZG.  But we believe the reliability will be superior to some of the Chinese 3-500Z that are on the market (did you ever wonder why they sell matched pair of 3-500Z?)  Also if you take into account the extra headroom this tube has it starts to show virtues that are worth the investment. I just spoke with one individual who feels this way to the extent that he plans to call our customer service tomorrow and order one. 

Here is data from the (tentative) 3CX1500D7 data sheet:



TYPICAL OPERATION*, under 30 MHz:

Anode Voltage......   3.0      4.5           kVdc

Cathode Bias Voltage...     0        12           Vdc

Zero-Signal Anode Current1   168        98          mAdc 

Anode Current (max signal)   0.72      0.64         Adc

Grid Current1......     247      180          mAdc

Driving Power1.....        80        70          W

Anode Dissipation1...       660      930         W           

Anode Output Power1..      1.45     1.95         kW

Input Impedance ....        73         72         Ohms

Resonant Anode Load Z.   2650     2900        Ohms

Intermodulation Distortion

         Products2 , 3rd order.. -29      -34    dB

                           5th order...  -43      -37    dB



* measured data (single tube)

1. Approximate Values     

2. Referenced against one tone of a two-equal tone signal







I have observed a 3CX1500D7 running at a plate voltage of 5500 Volts, with 800 mA plate current, and with a drive power of 105 Watts it put out 3000 Watts exactly.  The power gain was 14.5 dB and the plate dissipation was 1400 Watts, still within ratings. The tube is completely stable at that power level, but I don't recommend you try that in a linear designed to run at 1500 Watts or less, as component damage will certainly result!.  The Henry amplifiers always seemed to be an odd in some respects.  I have seen one up close and I rather like the plate supply.  Why they used series connected filaments is unknown to me! My hunch is that they wanted to keep the wire size down on the filament chokes!  Maybe there was a desire to prevent individuals from being able to operate with a single tube or something, I just do not know.  Ted Henry Sr. himself probably knows but I will leave posing him with that question to someone else.   

I hope this helps you decide if this tube is one you want to use in your "upgrade" or retrofit into an existing linear. Let me know what you think!

I'll mail you a copy of the data sheet as soon as they are available.

Best 73,

Reid Brandon  W6MTF

Applications engineer

CPI Eimac division

301 Industrial Rd.

San Carlos  CA  94070

Tel. 650-594-4175 or 1-800-414-TUBE(8823)   press 1 at the prompt and ask for me."