1/f noise is quite important at audio frequencies, but not at RF. Noise is not that critical up to 30 MHz anyway, like jhstewart9 wrote, unless you have a very poor antenna/aerial.
When you get an unexpectedly large received signal or a parasitic oscillation, the power amp is more likely to blow up your receiver than a normal RF preamplifier. That might be a practical disadvantage.
That is my only concern and that's why if i decide to make the change permanent i will have to protect the receiver input.
The circuit is a grounded grid so it is not prone to self oscillation and i have checked it thoroughly, the actual rf amplifier is working fine already producing 200-250 watts of power in HF.
The circuit is a grounded grid so it is not prone to self oscillation and i have checked it thoroughly, the actual rf amplifier is working fine already producing 200-250 watts of power in HF.
There are not too many RF experts here on the diyAUDIO forums, but there is a thread devoted to RF stuff. It is here:
https://www.diyaudio.com/community/threads/no-rf-gear-here.309531/
I worked at Motorola starting on the assembly line in 1973 tuning, testing, and assembling two way radios. I left 41 years later as a Principal Staff Research Engineer. I have built ham radio equipment from tubes, transistors, and IC chips, often all three in the same radio.
There is no advantage in using high powered pentode tubes in the front end of any receiver, and several disadvantages in doing so. The large tube is going to generate more noise that a smaller tube that is optimized for RF amplifier use. A grounded grid amp is generally stable but can oscillate under certain mismatch conditions. It expects a near 50 ohm input as it is intended to be connected to a HF transceiver. You want to connect its input to an antenna which could present an unknown impedance leading to possible instability. The antenna could pick up a relatively strong signal from a nearby transmitter which will be amplified and stuffed into your receiver. An out of band signal could also cause instability in the high powered amp. The circuit shown runs on 900 volts. If provoked into oscillation it could produce a very high powered signal that could damage your receiver. The amp doesn't even have to oscillate to be a problem. An amplifier on the edge of instability can produce a large amount of noise in a frequency range outside its normal operating range. This could swamp your receiver making its sensitivity worse than without the amp.
As stated, in the HF bands the received (from the antenna) noise is usually stronger than the noise generated in a well designed modern HF receiver. Adding gain generally will not help the receiver's ability to resolve a signal. The selectivity in your transmit amp is too broad to be of use in a receiver.
The circuit you show has an untuned input and a PI network low pass circuit on its output intended to match the high impedance vacuum tube to 50 ohms and reduce the level of the harmonics generated in the amp. It will not do much for selectivity on HF. You could make a bank of filters, one for each band to improve selectivity but it would be large if it had to handle the power of this amplifier. A simpler solution would be to build a filter bank for the receive path only.
There are (were) some advantages to using a vacuum tube for the first stage of an HF receiver in some operating conditions. I used to live in a large city with many ham radio operators. There was a guy about 1 km from me that ran 1500 watts into a wire antenna on 80 through 10 meters. If he was on the air my DIY solid state radio, or my Yeasu FT-817 was essentially unusable on most bands. The difference between his strong signal and anything that I was trying to receive exceeded the dynamic range of the first stage in either of my receivers. This led to extremely high levels of IMD (the same stuff we hate in HiFi amps) which overcomes the linearity of the amplifying stages turning them into mixers. His single frequency signal appeared in dozens of places on any of the HF and VHF bands in my radios. The only cure for this is to improve the dynamic range of the receiver or filter out the offending signal before it gets into the receiver.
It is possible to build a preamp / filter also called a preselector, to help in these situations. 20+ years ago I built such a device with a 5670 / 2C51 dual triode vacuum tube. It became the first stage in my DIY ham transceiver replacing the dual gate mosfet amp.
Today there are specialized RF amplifier chips with good enough linearity to do the job. Look in a Mini-Circuits catalog. You will need a band specific bandpass filter on the input and possibly the output of the amp to reject unwanted signals if that's a problem for you. I find that the chips are cheap enough to just build a bank of filter-amp-filter circuits and switch them.
If you choose to use vacuum tubes, pick a tube that was designed for RF or IF amp use in a radio or TV receiver. The IF amp in a TV receiver runs in the 30 to 50 MHz range and will work well for this kind of thing. The 6EJ7/EF184 is a good choice. Look for old issues of the ARRL radio handbook from the vacuum tube era. they are full of circuits for this purpose. Some of the old books can be found online.
https://www.diyaudio.com/community/threads/no-rf-gear-here.309531/
I worked at Motorola starting on the assembly line in 1973 tuning, testing, and assembling two way radios. I left 41 years later as a Principal Staff Research Engineer. I have built ham radio equipment from tubes, transistors, and IC chips, often all three in the same radio.
There is no advantage in using high powered pentode tubes in the front end of any receiver, and several disadvantages in doing so. The large tube is going to generate more noise that a smaller tube that is optimized for RF amplifier use. A grounded grid amp is generally stable but can oscillate under certain mismatch conditions. It expects a near 50 ohm input as it is intended to be connected to a HF transceiver. You want to connect its input to an antenna which could present an unknown impedance leading to possible instability. The antenna could pick up a relatively strong signal from a nearby transmitter which will be amplified and stuffed into your receiver. An out of band signal could also cause instability in the high powered amp. The circuit shown runs on 900 volts. If provoked into oscillation it could produce a very high powered signal that could damage your receiver. The amp doesn't even have to oscillate to be a problem. An amplifier on the edge of instability can produce a large amount of noise in a frequency range outside its normal operating range. This could swamp your receiver making its sensitivity worse than without the amp.
As stated, in the HF bands the received (from the antenna) noise is usually stronger than the noise generated in a well designed modern HF receiver. Adding gain generally will not help the receiver's ability to resolve a signal. The selectivity in your transmit amp is too broad to be of use in a receiver.
The circuit you show has an untuned input and a PI network low pass circuit on its output intended to match the high impedance vacuum tube to 50 ohms and reduce the level of the harmonics generated in the amp. It will not do much for selectivity on HF. You could make a bank of filters, one for each band to improve selectivity but it would be large if it had to handle the power of this amplifier. A simpler solution would be to build a filter bank for the receive path only.
There are (were) some advantages to using a vacuum tube for the first stage of an HF receiver in some operating conditions. I used to live in a large city with many ham radio operators. There was a guy about 1 km from me that ran 1500 watts into a wire antenna on 80 through 10 meters. If he was on the air my DIY solid state radio, or my Yeasu FT-817 was essentially unusable on most bands. The difference between his strong signal and anything that I was trying to receive exceeded the dynamic range of the first stage in either of my receivers. This led to extremely high levels of IMD (the same stuff we hate in HiFi amps) which overcomes the linearity of the amplifying stages turning them into mixers. His single frequency signal appeared in dozens of places on any of the HF and VHF bands in my radios. The only cure for this is to improve the dynamic range of the receiver or filter out the offending signal before it gets into the receiver.
It is possible to build a preamp / filter also called a preselector, to help in these situations. 20+ years ago I built such a device with a 5670 / 2C51 dual triode vacuum tube. It became the first stage in my DIY ham transceiver replacing the dual gate mosfet amp.
Today there are specialized RF amplifier chips with good enough linearity to do the job. Look in a Mini-Circuits catalog. You will need a band specific bandpass filter on the input and possibly the output of the amp to reject unwanted signals if that's a problem for you. I find that the chips are cheap enough to just build a bank of filter-amp-filter circuits and switch them.
If you choose to use vacuum tubes, pick a tube that was designed for RF or IF amp use in a radio or TV receiver. The IF amp in a TV receiver runs in the 30 to 50 MHz range and will work well for this kind of thing. The 6EJ7/EF184 is a good choice. Look for old issues of the ARRL radio handbook from the vacuum tube era. they are full of circuits for this purpose. Some of the old books can be found online.
VERY WELL SAID!
I am familiar with all the concepts you mentioned and i have to admit that i have changed my mind and decided to built a preselector with either ECC88 or the ECC84 (or 5670 as i have a lot of them) in cascode with a small difference.
I am reading your post over and over because i can see that there is a lot of packed experience and clues in it that need further study.
I am familiar with all the concepts you mentioned and i have to admit that i have changed my mind and decided to built a preselector with either ECC88 or the ECC84 (or 5670 as i have a lot of them) in cascode with a small difference.
I am reading your post over and over because i can see that there is a lot of packed experience and clues in it that need further study.
The ECC88 grounded grid is an excellent choice for front ends. It can run down to quite low volts and give useful gain. It can also be used as an LO. EF95 is a good mixer as it has low grid cap. The 6EJ7 is great for IF amps and limiting - all good advice. You won't get better than 6dB NF with tubes.
For HF i think 6db are enough i think, my main no no is the use of solid state in front ends and power stages.
Solid state has advantages but i like tubes despite all their shortcomings, having said that i do have solid state rigs for everyday use.
Chris
Solid state has advantages but i like tubes despite all their shortcomings, having said that i do have solid state rigs for everyday use.
Chris
Yep for 30MHz and a large antenna interference will probably dominate over the NF. For VHF and UHF NF is more problematic. You can protect SS from damage but it won't survive a nuclear attack but neither will you.
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I really don't have any use for vhf or uhf other than moon bounce which i plan to try in the future.
Well there is a chance that i might survive but the main reason i don;t like SS is that they are too fragile and unreliable especially those with cpu and a firmware in them.
I really don't have any use for vhf or uhf other than moon bounce which i plan to try in the future.
Well there is a chance that i might survive but the main reason i don;t like SS is that they are too fragile and unreliable especially those with cpu and a firmware in them.