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Message |
    
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 1694
Registered: 9-2004
| | Posted on Wednesday, May 24, 2006 - 11:14 pm: | 
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hi, i posted this here because i want to discuss, specifically, power supplies for linear amplifiers.
i have a few questions that will come out as the thread moves along.
hopefully, this will turn out to be very helpful for people looking to get a base amp.
ok, first; does anyone have a competition style base amp?
doesnt matter which guy made it, as they are pretty similar, but i am looking for custom, not boomer and palomar.
if you have one, would you mind looking at the inside, at the power supply section? (be careful!)
i would like to know what value the cap(s) are, and if the bridge rectifiers are wired in series, if there are two.
any and all details about comp. amp power supplies is appreciated.
i am going to build a 2x2879 base amp, and from what ive seen, the power supplies in these are nothing more than a transformer, recifier(s), and a big filter cap or two.
they dont really regulate them, im guessing because they dont care about the voltage drop.
any and all comments are needed.
question 2:
how many amps PEP will a 2x2879 amp running class AB1 draw.
lets say at 15 volts.
i want to know how big i need to make my supply.
i hope i can get away with 30 amps, but i dont know.
thanks for all help,
matt |
Audiophile73
Intermediate Member
Username: Audiophile73
Post Number: 119
Registered: 5-2004
| | Posted on Thursday, May 25, 2006 - 4:24 pm: |
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Depending on how you bias your transistors and how many volts you will run will depend on the draw. I am assuming you will be running this amplifier on DC current. Usually a 2sc2879 will do about 150 watts. Like I said depending on drive and voltage you could see anywhere from 300-400 watts out of an amplifier that has 2 of those transistors. So 15 volts doing 400 watts is 26.667 amps. As long as you have a power supply that will do that you should be in good shape. If you need any more help let me know. Good choice on transistor though tough, ruggid and they offer good gain. |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 1704
Registered: 9-2004
| | Posted on Thursday, May 25, 2006 - 11:10 pm: |
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im building the CBcity intl. tri power kit.
i want to run the amp class AB1 as it will be used mostly on SSB.
i know this will reduce efficiency somewhat, so im hoping that the draw will be 26 or less.
i dont want to have to go with 2SC2290's just because i cant find a transformer with enough current.
the one i found will give 18 volts at 30 amps.
im looking for details and the pics ive seen dont offer enough.
if you have an amp you could look inside and describe the power supply to me id appreciate it.
thanks for the help,
matt |
Chad
Advanced Member
Username: Chad
Post Number: 857
Registered: 11-2004
| | Posted on Friday, May 26, 2006 - 10:20 am: |
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I have and X-Force XT-40012 on the bench now. The Cap is rated at 25V with a 30V surge, I cannot remember the value off-hand. There are 4 rectifiers in there running in paralell attached to an aluminum heatsink. With a 120VAC supply attached the output voltage of the power supply is 20.8VDC open circuit, unregulated. I have not loaded it as the amp section is awaiting a couple finals and resistors. I'll look at the cap rating tonight, it's a single can style cap used in big computer supplies, etc.
One thing about this amp I don't like is the fan noise. WOW! It's like my air compressor when it's running!
Chad |
Patzerozero
Senior Member
Username: Patzerozero
Post Number: 2902
Registered: 7-2004
| | Posted on Saturday, May 27, 2006 - 7:55 pm: |
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chad....you dirty dog, you....x force 
they don't worry about voltage regulation on the custom amps because they are built to take 22 volts or more, though lifespan is not neccessarily 'max'. most power supplies built by these guys supply 18 or more volts when 'ON', but drop to 14 or less when under load.
i have a 2 x 2879 davemade that will key 150 & can swing over 400 on 15 volts & trip the breaker on a 40 amp continuous PS, at 14 volts in the mobile, it'll pull almost 40 amp peaks & swing around 400. |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 1707
Registered: 9-2004
| | Posted on Saturday, May 27, 2006 - 11:15 pm: |
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chad, parallel, yeah, that makes more sense.
i read in the ARRL handbook something about running them in series, but in looking at the pics ive found, it seems that they are parallel.
thanks, yeah i would like that cap value.
where do you think they buy the transformers for these things?
thanks pat also, both of you have the experience that i dont.
i appreciate the insight.
the problem im running into is that the only transformer i can find is the MFJ that will do 18 volts at 30 amps.
its around 35.00, so using two seems to be too expensive for me.
i know where the big boys get their stuff from, but that place doesnt sell power transformers.
what gives?
i know they're out there!
matt |
Chad
Advanced Member
Username: Chad
Post Number: 861
Registered: 11-2004
| | Posted on Sunday, May 28, 2006 - 2:08 am: |
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Pat, the X-Force is meerly in for repair, I have no intention on using it, maybe a good testing though  I like my neighbors so most testing will be into a load.
KV, I'll get the value tomorrow, don't forget tio look into industrial types rated for twice the voltage at 220V. Also remember the full-wave rectified voltage will be considerably higher than the AC output voltage. You also have to be VERY careful paralelling transformers, If the tolerances are off then they will push each other around, especially idf there is a phase shift of some sort. Best to do the combining in the DC domain and at that point I only like doing that with bi-polar supplies. Best to find one big mama tranny. I'll look for markings ont he transformer but if I remember right I already did thinking of you
Chad |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 1713
Registered: 9-2004
| | Posted on Sunday, May 28, 2006 - 10:40 pm: |
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chad you rock.
didnt even consider looking at the 220 trannys.
good point.
i have to admit that i didnt know that the voltage available at the DC end of the bridge rectifier(s) would be higher than the transformers rating.
i will not be trying to parallel two transformers for the exact reasons you stated. no thanks.
so, if i get the 18 volt 30 amp transformer, and use two bridge recifiers in parallel, what would be the approx. DC voltage coming out of them?
no load of course.
and im beginning to realize that i need to have 30 amps continuous, maybe 35.
so i seem to be moving backwards at the moment.
one of these days i will find a transformer.
i hope its not too heavy, as im sure the YL is going to throw it at me when i get home.
she is getting tired of signing for packages for me. TEE HEE!
thanks guys,
PS chad, got my kl 300p finished and i will be testing it tomorrow if all goes well.
matt |
Chad
Advanced Member
Username: Chad
Post Number: 863
Registered: 11-2004
| | Posted on Monday, May 29, 2006 - 9:30 pm: |
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The cap is 47,000 uF, can type wiht screws on top. Make sure you get a high temp version (105 Deg C) I prefer Panasonic brand, they seem to last the best in stressful situations. I'll figure out the final voltage formula for you.
Congrats on the KL300, let me know how it worked out! You doing a little work at a time or shotgunning it?
Chad |
Chad
Advanced Member
Username: Chad
Post Number: 866
Registered: 11-2004
| | Posted on Monday, May 29, 2006 - 10:27 pm: |
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I went out and measured the voltages in the X-Force.
122VAC in (line voltage at casa de Chad)
15.72VAC out of the transformer
20.82VDC at the cap.
The cap tank will charge to the peaks of the rectifier output give or take a couple volts. AC voltage is measured in RMS sooooo take the transformer rating and divide by .707 and that will be pretty darn close to your unloaded voltage. This is using full wave rectifiers.
Your 18VAC transformer will produce around 23-25VDC filtered. The larger the cap the better it will hold this voltage before it sags to the RMS output voltage of the transformer.
BTW there are MANY companies that make large torridal transformers, have you looked into these?
Chad |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 1721
Registered: 9-2004
| | Posted on Monday, May 29, 2006 - 10:58 pm: |
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ummm, im very embarrassed.
i wish i could just lie to you guys and say that everything is great.
you would never know otherwise, but i just cant do it.
i have screwed up the amp. im only bummed because it worked before and now it doesnt, but the amp was cheap, and i am learning alot.
here's what happened:
i have been doing little bits at a time, but i have not been testing the amp in between mods.(i know, i know)
i have been doing this at work, as home life is way to busy for radio stuff right now.
i changed R9 to a 10 watt 27 ohm resistor (same value, higher rating)
changed C17 to a 40-280pF trimmer.
stock was 150pF.
changed C23 to a 15-120pF trimmer (500v) stock was 47pF.
changed C8 to 47uF for longer SSB delay.
this seemed to work fine.
finally took the amp home and decided to test it in the house before mounting it in the car.
dont ask how, but i HOOKED THE COAX UP BACKWARDS!!!
OMG, i cant believe i did it, but i did. so embarrassed!
anyway, because i had done the mods, when i first keyed the amp and heard the relay fluttering; i didnt assume it was hooked up wrong; i thought it needed some adjustment to the trimmers.DOH!
obviously no ammount of adjusting was going to work, and then i found my mistake and switched the cables.
now, the amp will key, (relay clicks once) but on low, with the new 10 watt resistor, i could not calibrate the meter on the radio.
switched to HI and could.
dont know why, its the same value as stock (27 ohms).
went to calibrate SWR meter coming out of amp, and no power output, just enough to jar the needle but not even a watt.
did i blow the RF power transistors? the resistors near them?
no smoke, but they were getting hot quick.
any help or advice will be heeded. (i promise)
one more thing:
on these trimmers, it seems as if im going to turn the screw right out of the thing if i go too far counterclockwise (left).
is the screw going to fall out, or is there something to stop it?
it is definitely moving in an outward direction when turning.
thanks for any and all advice and help,
and laughter is OK, i deserve it.
once again for good measure, DOH!!!
matt |
Chad
Advanced Member
Username: Chad
Post Number: 870
Registered: 11-2004
| | Posted on Tuesday, May 30, 2006 - 1:53 pm: |
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You shouldn't have to increase the power handling of the input circuit, R9 only works in "low." In high it's a simple pass thru to the drive transformer. 2 things to check, put the original R9 back in, but before doing so lift one leg of the replacement R9 and make sure the tolerances are the same, 10W resistors can vary a bit. Also keep in mind that many 10W resistors are wire-wound and can have problems in RF circuits acting like a choke. Use film resistors whenever possible in circuitry that is CARRYING RF. If you want to use wire wound for the AB bias mod then go ahead, it's just carrying DC. 5 watt resistors would be fine here, are smaller, and probably easier to find in the film variety, remember that it won't see the brunt of the full radio output because R8 is dumping part of that power to ground, a pad has two legs to the "L" I'd drop the old one in and see if it goes. The amplifier still has fixed gain, to increase it¡¦s needed drive R8 and R9 have to be manipulated to a different resistance value.
Did you do the bias mod? If so, that could be generating some heat in the output circuit but shouldn't generate that much, it WILL run hotter because efficiency is decreased a bit.
Why the trimmer caps? I could see one on the output but the input SHOULD be pre tuned via the tank network on the input of the amp. Replace C17 with the stock one and see what that gets you. You can leave C23 as a variable but I doubt you would see much. You may want to pre-tune it with an LCR meter so you are in the ballpark when you first fire it up, those caps can be touchy. Remember the advantage of a solid state amp is the ability to use WITHOUT tuning
Now, hooking it up backwards, what can happen?
First, do you have receive with it hooked up correctly? If you do not then you may have taken out the preamp circuit. Shoving power bakkids thru the preamp circuit may have made it mad. The preamp relay (RL1) is activated whenever the unit is NOT keyed, the preamp is TR4 and support components. To disable the preamp remove TR3, this controls the preamp relay. Mine is removed, I like it that way but to each his own, it's a quick fix later to get it going but this will tell you something, lets get it on the air, preamp or not.
WITHOUT the preamp circuit what will happen is that you will key the amp FROM the output. The keying circuit starts at the south end of L4, it will key but not see any further input, unkey then key again because essentially it's a straight line thru that is broken by the action of keying the amp. Any stray RF COULD have been amplified and shoved back to the radio causing oscillation, this CAN damage output transistors, radios etc. Is the radio OK?
I do not know how the preamp will affect the keying while working backwards, it seems it may have worked once ļ
The trick now is baby steps. Ideally I would probably put it back stock and do things one step at a time starting with the output tuning cap (if you really want it), then some L-Pad work, and possibly a preamp repair When doing a shotgun approach it's tough to find out where things went wrong. This, compounded with a known connection procedure flub can be frustrating, I've been there, too many times
Even when I know a repair and have done it a thousand times I repair in blocks and test, rinse, lather, and repeat. This is the only way you can be assured that you will not have to work backwards later. It also saves money in parts
Also remember that when working with RF electrical principles can be thrown out the window, 27 ohms may not be 27 ohms at 27 meg, IMPEDANCE now takes precedence over resistance. The design of the components and even physical board spacing play a VERY important role in a properly designed RF amplifier. I came from an AF world and am still VERY much involved in it. I'm still learning RF too and the learning curve is steep, I love all forms of amplification and my Forte' is very high power AF amps, I fix the ones nobody else can (if it's worth it.) RF was a BIG eye opener for me and caused me to proceed with caution, I am in NO WAY ready to be a designer and when doing repairs will put it back to stock as often as possible, There's a level of Voodoo involved that must be understood to do it properly, I very much do not believe in the pull a turn here, spread this, increase this, decrease that. A trained engineer built the thing to do what it does, I in no way feel that I have the training to go in blindly, without a proper test rig, and start changing values around. I firmly believe in proper engineering practices in doing modifications, this is the ONLY way to keep harmonics down and ensure the longevity of the device.
If you have a transistor tester it would not be a bad idea to pop out the finals and test them.
Best
Chad |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 1727
Registered: 9-2004
| | Posted on Tuesday, May 30, 2006 - 7:31 pm: |
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thank you chad!
i know how long it takes to type a post that long, and i really appreciate your time.
especially since you've got an antenna in the air now, and can actually talk!
ok, good point about the 10w resistor!!!
its wirewound.
i saved the old part and will put it back in.
im going to put it back to stock, leaving the SSB delay cap, as its not a problem so far.
dont know if the radio is totally ok or not, havent looked, but i do still have receive through the amp.
i put the preamp on a switch, and it still works. (well, it raises the noise level from s-1 to s-7 if you can call that working)
i did not do the AB1 mod, and im glad i didnt now!
more confusion!
the only transistor tester i have is the cheapo plug the legs into the slot kind.
not sure if i can use this on these.
after putting the amp back to stock, i will test it and let you know how it went.
PS, i know absolutely nothing about torroidal trnasformers used in this application.
how do they work?
any special considerations?
why dont we see them in the comp. base amps?
if you know of a source, a link would be great.
thanks again,
matt |
Chad
Advanced Member
Username: Chad
Post Number: 879
Registered: 11-2004
| | Posted on Wednesday, May 31, 2006 - 9:08 am: |
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Dunno why we don't see them on more RF stuff, they certainly are in broadcast stuff! One advantage to a torrid is that they can be made pretty low profile, this allows a powerful transformer in as little as 2 rack spaces. I'm pretty darn sure you know what a rack-space is
I'll look for some links, one guy I know on the net is a torrid whiz, he's also bipolar and can be one mean aussie
I was getting ready to buy the parts for the AB Bias mod. Although it's no biggie to me because mine is not mobile..... The power switch is all ground based. Power is ALWAYS applied to the finals. When doing the AB mod there will ALWAYS be voltage dropped across the resistor, even when off. It will be like having a 100 ohm resistor across your car battery at all times. I did not think about that till last night. I was thinking about modding the power switch for total power, but I dunno if the switch can handle the current of the amp (the switch activates the LED and keying circuit). Being able to shut off the entire power would allow you to put in a switch that would allow class C or class AB operation by tying the CT to ground OR .7VDC... Thoughts?
Chad |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 1740
Registered: 9-2004
| | Posted on Thursday, June 01, 2006 - 12:40 am: |
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umm, my head's swimming as it is.
RF theory is no joke!
i am going to build lou franklin's tri power linear amp. i just bought the PC board and plans.
looks pretty cool and not too tough.
having the board helps tremendously.
his amp design switches bias when you switch from AM to SSB. C for AM and AB1 for SSB.
pretty cool!
thanks for all the help, and if you come across anything, be sure to post a link or email me.
matt |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 1755
Registered: 9-2004
| | Posted on Monday, June 05, 2006 - 5:08 pm: |
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well, the amp is back to stock.
and it works!!!
yay.
it mustve been just too much stray RF floating around in there with the extra lead length etc...
the amp works better than ever right now.
i added the preamp switch. (cut the trace to the middle leg of TR3, and added SPST switch)
and also a dual fan which kicks butt!
the power switch kinda threw me for a bit, as i wanted to wire the fan to the on/off switch on the amp, and assumed that i wanted to splice the positive wire. WRONG!
as chad already pointed out; the switch is all ground based, and the negative wire was the one to splice.
works great, and i learned alot.
glad to have the amp back,
matt |
Road_warrior
Senior Member
Username: Road_warrior
Post Number: 1576
Registered: 7-2004
| | Posted on Monday, June 05, 2006 - 11:34 pm: |
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Glad to hear that kid! |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 1768
Registered: 9-2004
| | Posted on Tuesday, June 06, 2006 - 10:34 pm: |
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yeah, it was starting to get dicey there for a few days, with me wondering whether or not i had frankenstein'd it too much to bring it back to life.
i have to say, the SD1446 pills were tough sonofaguns!
they withstood being way mistuned, and being hooked up backwards and keyed more than a few times.
good to hear from you on the thread road warrior, i didnt know anyone was watching.
got a solid state base amp that you can open up and tell us whats in the power supply?
take it easy,
matt |
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