I had a tech that was trained at the National Radio Institute in Washington tell me something I wanted some other opinions. 35 to 40 watts PEP on SSB into 2 2879's will be just right. Dead key on am is watt kills amps not the modulated power like on SSB. Does this sound reasonable. I know he is much more knowledgeable on this than me.
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modulated power
- Thread starter tvrc18
- Start date
Depends on who made the amp and what class of biasing is built into the circuit. 40 watts peak power on SSB might be a bit much for a Texas Star 350.
Since the radio you are putting into the amp is probably an Export radio, it probably has an adjustable RF output and a Mic Gain control. This will help adjust the power output down to 25 watts peak power on SSB. That is, if we are talking about a Texas Star 350 or any other amp running 2879's and AB biasing. AB biasing is preferred to have for an amp if you do a lot of SSB. C biasing is used for AM use and doesn't work too well for SSB.
Hope that helps some . . .
Since the radio you are putting into the amp is probably an Export radio, it probably has an adjustable RF output and a Mic Gain control. This will help adjust the power output down to 25 watts peak power on SSB. That is, if we are talking about a Texas Star 350 or any other amp running 2879's and AB biasing. AB biasing is preferred to have for an amp if you do a lot of SSB. C biasing is used for AM use and doesn't work too well for SSB.
Hope that helps some . . .
B
BOOTY MONSTER
Guest
id be more concerned with whats coming out rather than whats going in . i'd keep them under 200 pep output .
A transistor, or tube, can't tell the difference between just a carrier, or carrier plus modulation. It only knows if the applied signal is less than too much or too much. If it's less than too much then that transistor/tube will amplify it. If it's too much then that transistor/tube will quit/die. It's that simple, and the user of that transistor/tube is supposed to know the limits, how much 'too much' is. The user can go to the time/trouble of finding out what that 'too much' is, or he can take the manufacturer's word for it since the manufacturer has already done that. The manufacturer always includes a 'safety factor' in there statements about how much that 'too much' is, and that's a pretty nice thing to do. It saves the user from the expen$e of finding out for themselves.
Then you get into the part about a transistor won't handle as much power using AM mode as compared to SSB mode. The differences there are in what the signal consists of and how you do that measuring. SSB has no carrier, it's all 'modulation'. It's also typically measured in 'Pep' watts rather than 'Avg' watts. SSB is an 'all or nothing' type thingy, no 'constant' input, it's always varying (unless you input a constant tone). If you do use a constant tone for measuring, and use 'Avg' power instead of 'Pep' power to do that measuring, you'll find that the limit for that transistor is the same as for AM mode. That "limit" for the transistor or tube doesn't change. But how you go about measuring that limit certainly can make a difference in the results.
- 'Doc
Then you get into the part about a transistor won't handle as much power using AM mode as compared to SSB mode. The differences there are in what the signal consists of and how you do that measuring. SSB has no carrier, it's all 'modulation'. It's also typically measured in 'Pep' watts rather than 'Avg' watts. SSB is an 'all or nothing' type thingy, no 'constant' input, it's always varying (unless you input a constant tone). If you do use a constant tone for measuring, and use 'Avg' power instead of 'Pep' power to do that measuring, you'll find that the limit for that transistor is the same as for AM mode. That "limit" for the transistor or tube doesn't change. But how you go about measuring that limit certainly can make a difference in the results.
- 'Doc
Those pesky 2600's what where they thinking by not giving it an RF power control???? I have known guys that retrofit them with an external pot to control the power and some guys put a high low switch attached to a resistor to drop the power.
I would not worry about it too much if it was fine before then it should be close to fine now. I doubt the radio doubled it's output power while it was away unless some major work was done to the unit. A few watts on the top end is not going to kill anything. In fact far better for it to be on the top of the swing then more watt's dead key since you seldom hit peak modulated power because of how the English language is made up of vowels and soft sounds!
Now if this unit went from a peak of 28 to a peak of 40 then we have some will see some major issues but a few watt's on top in the swing is not going to be much of a factor especially on SSB.
Ideally I would open the girl up and turn the power back down to the previous safe known value if it was going to be a factor or I would install a switch for high and low power specifically for running it into an amp.
I would never expect to run 40 watts into 2x 2SC2879 even on SSB!!! I would more then likely be expecting to see a 24watts driven into the unit on SSB if I was going to make some broad blanket statement. When it comes to electronics I hate blanket staments because I have seen some huge variation from unit to unit and part to part. It is not uncommon to find that 3 other wise identical units from the same OEM will have power figures from 70%-120% of the theoretical power output based on the votls, amps and the rated power of the transistors. The same can be said about input power you might get 3 widely differing number's for input power that the unit can handle before it starts to distort. This is why I am not a fan of blanket number's. For instance the Amperex 5-500Z tube has a lot more gain then the Eimac 3-500 and it will put out a lot more power then the Eimac version. This is a bad example but my point is that if you had two identical say Ameritron Amps one with Eimac 3-500Z and the other with Amperx 3-500Z even though all the parts in each tube are electronic equivalents to each other the input and output power would be different....Now lets say you have two identical amps with all the same parts well that does not preclude that some of the parts inside of each amp will have widely varying output signatures and thus different drive and output power number's!
So to pull a static fixed number out of his hat like that and make a blanket statement of 40 watt's is ideal drive for 2 2SC2879 transistors is very unwise! I always give a range like 1.5-3.5 watt's is the norm. The above is the reason I do that because you can not just make a blanket statement like he did and expect it to be accurate in the real world!
I would not worry about it too much if it was fine before then it should be close to fine now. I doubt the radio doubled it's output power while it was away unless some major work was done to the unit. A few watts on the top end is not going to kill anything. In fact far better for it to be on the top of the swing then more watt's dead key since you seldom hit peak modulated power because of how the English language is made up of vowels and soft sounds!
Now if this unit went from a peak of 28 to a peak of 40 then we have some will see some major issues but a few watt's on top in the swing is not going to be much of a factor especially on SSB.
Ideally I would open the girl up and turn the power back down to the previous safe known value if it was going to be a factor or I would install a switch for high and low power specifically for running it into an amp.
I would never expect to run 40 watts into 2x 2SC2879 even on SSB!!! I would more then likely be expecting to see a 24watts driven into the unit on SSB if I was going to make some broad blanket statement. When it comes to electronics I hate blanket staments because I have seen some huge variation from unit to unit and part to part. It is not uncommon to find that 3 other wise identical units from the same OEM will have power figures from 70%-120% of the theoretical power output based on the votls, amps and the rated power of the transistors. The same can be said about input power you might get 3 widely differing number's for input power that the unit can handle before it starts to distort. This is why I am not a fan of blanket number's. For instance the Amperex 5-500Z tube has a lot more gain then the Eimac 3-500 and it will put out a lot more power then the Eimac version. This is a bad example but my point is that if you had two identical say Ameritron Amps one with Eimac 3-500Z and the other with Amperx 3-500Z even though all the parts in each tube are electronic equivalents to each other the input and output power would be different....Now lets say you have two identical amps with all the same parts well that does not preclude that some of the parts inside of each amp will have widely varying output signatures and thus different drive and output power number's!
So to pull a static fixed number out of his hat like that and make a blanket statement of 40 watt's is ideal drive for 2 2SC2879 transistors is very unwise! I always give a range like 1.5-3.5 watt's is the norm. The above is the reason I do that because you can not just make a blanket statement like he did and expect it to be accurate in the real world!
To be fair he said 35 to 40 watts would drive the 2 2879's just fine. I see about 35 watts pep and it did do about 25 to 28 before. Sometimes you can see more if you wistle or something but just normal talking it does not do 35 to 40 maybe 28 to 32 or so. I may still turn the alc down a bit to be safe, I do not want to cook my mrf477.
Yes MRF477's are expensive and hard to find and a lot of the stock floating around is counter fit junk! My Lincoln has the MRF455 if ever you pop that MRF477get some perf board and do the MRF455mod much the way the factory did it. Then follow Roger Birds tips on replacing the driver and pre-driver with higher gain lower noise transistors and such. Not to get more power but to run cooler and have more head room and less noise in the system....I also like the front end receive mod's for these units and using Schochtty(sp) diodes.
I would imagine it is on Roger Birds page but any one that has worked on CB's can do the mod. You have two mod's one improves sensitivity and spurious rejection the other protects the front end from over load if you operate in or around a lot of CB'ers running a lot of power or live near an interstate where trucks operate again with more power then is legal. The front end on the HR 2510 and all associated chassis of this kind is flaky when it comes to receiving weak stations and is also easily over driven. Roger Bird and I am prob. not spelling his last name correctly had a page a good 10 years ago online that had all kinds of President Radio mods and his specialty was the HR line of chassis's. Most of the mod's for the other President Radio's though also work ont he HR2510,2600, President Lincoln radios as well!