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1 3cx1500a7 or 2 3cx800a7's????
- Thread starter dxhound
- Start date
There's nothing wrong with 8877s on the whole. Yes there were some bad production years. Maybe mid to late 80s. I lost one for no good reason here. Several others have given me great service. They have grid structures especially built to deliver low IMD in grounded grid service. This translates into low grid dissipation numbers. One must be careful of tuning. Always monitor grid current or the tube can go bye bye very quickly. This is the price paid for an unusually linear tube in a simple circuit. The same is true for any tubetype capable of low IMD. Look at the grid dissipation of a 4CX1500B.
As mentioned but worth repeating. The tube type does not set efficiency. You get about 60% peak efficiency in AM or SSB linear service. This is (P.E.P. output / peak power input) times 100 to make it a percentage.
When you see RMS thrown around get out the boots. All RMS means is equivalent to DC power to create X number of calories of heat. RMS values change drastically depending on the waveform. Radiophone is a complex waveform and cannot be treated as a sinewave in practical applications.
BTW, I spent 9 years figuring out what to do for 160 before I ever transmitted there.
As mentioned but worth repeating. The tube type does not set efficiency. You get about 60% peak efficiency in AM or SSB linear service. This is (P.E.P. output / peak power input) times 100 to make it a percentage.
When you see RMS thrown around get out the boots. All RMS means is equivalent to DC power to create X number of calories of heat. RMS values change drastically depending on the waveform. Radiophone is a complex waveform and cannot be treated as a sinewave in practical applications.
BTW, I spent 9 years figuring out what to do for 160 before I ever transmitted there.
I looked at the Ulvin site for a minute. They claim 10kw from 3000 watts dissipation. Sure. No problem. To do that without breaking stuff requires bastardizing the waveform. It won't even be phone anymore. It will be more like radar. You couldn't do that for 20 seconds on the ham bands. That's also why some guys on the chicken band get stomped by other guys with far less bird watts.
On sheer dissipation, two 3cx800's have 100 watts more. New tubes, a 3cx800 is about $600 each new, while the 3cx1500 is around $900 new. They are probably about an even match.
As others have mentioned though, it's not just about tube dissipation. It is about efficiency and also about the capabilities of other components in the system. A transformer rated at 2000 KVA ICAS probably isn't going to cut it for legal limit no keydown time. If you want an amp that can run day and night at us legal limit in any mode, you might look at converting an older military amp such as the AM-3924, part of the URT-23 transmitter. It has two 4cx1500's and will run full power out 24/7.
If I had to choose, I would probably choose the twin 3cx800's. A couple of reasons.
1. it's easier to cool two tubes that it is one. There is more surface area to cool and typically you will need a smaller cfm blower. The two tubes will also present less backpressure to the blower.
2. in the case of the 3cx800's, you can also switch to 3cpx800's and increase the plate voltage, see example here: http://www.hamradiomarket.com/articles/Titan P Hybrid.htm. It simply adds options to your available tube list
3. Did i mention the blower? Also, less blower noise since it is smaller and requires less airflow.
Paul
As others have mentioned though, it's not just about tube dissipation. It is about efficiency and also about the capabilities of other components in the system. A transformer rated at 2000 KVA ICAS probably isn't going to cut it for legal limit no keydown time. If you want an amp that can run day and night at us legal limit in any mode, you might look at converting an older military amp such as the AM-3924, part of the URT-23 transmitter. It has two 4cx1500's and will run full power out 24/7.
If I had to choose, I would probably choose the twin 3cx800's. A couple of reasons.
1. it's easier to cool two tubes that it is one. There is more surface area to cool and typically you will need a smaller cfm blower. The two tubes will also present less backpressure to the blower.
2. in the case of the 3cx800's, you can also switch to 3cpx800's and increase the plate voltage, see example here: http://www.hamradiomarket.com/articles/Titan P Hybrid.htm. It simply adds options to your available tube list
3. Did i mention the blower? Also, less blower noise since it is smaller and requires less airflow.
Paul
You have great taste in amplifiers.
Also the Harris HFL-1000, HFL-3000, AM-7223 or AM-7224. Sunairs too.....
Just a dream for an inner city kid, I want a full sized 160 meter loop but, I am settling for 80 thru 10 operation only on a OCF Dipole.
.
Sorry, slight typo, 2000 KVA would be more than enough for amateur legal limit. Obviuolsy I meant 2KVA.
As someone already mentioned, Harris makes some great amps too. Another thing to look, especially if this is for 160 meter AM is a mono-band homebrew amp. There are a variety of solid state mosfets which would operate in enhanced mode at the 160 Meter frequency range which are "cheap - cheap - cheap" You could put together an AM or CW only monoband amp very effectively this way.
Paul
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