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Building an Amp

LeapFrog said:
By purchasing and using trimmer capacitors of the appropriate range. Just where you stick the trimmer caps is something that a builder will have to tell you because I honestly don't know. But I have seen them on top of the Transformers.

After the appropriate capacitance is discovered the builder will measure and replace the trimmer with a regular and permanent capacitor, but not always. If you hear someone say: "I mic'ed that in" there are talking about removing the trimmer capacitor and installing a "silver dipped mica".
Sometimes the trimmers are left inside, I imagine that can become costly.

LDMOS is in reference to the new mosfet technology made popular by nxp. (blf188xr)
There are boards available for purchase on eBay and the device requires a 50 volt power supply.
Click to expand...
Now I'm starting to understand more. Crushers post made yours make more sense to me. Thanks!
 
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LeapFrog said:
No problem, sorry I couldn't help you more, but I just don't have the experience. Crusher did a great job explaining what you are getting into. (y)
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Yeah, just the kind of stuff I'm looking for. I appreciate ya'll taking time to write out responses..
 
LeapFrog said:
By purchasing and using trimmer capacitors of the appropriate range. Just where you stick the trimmer caps is something that a builder will have to tell you because I honestly don't know. But I have seen them on top of the Transformers.

After the appropriate capacitance is discovered the builder will measure and replace the trimmer with a regular and permanent capacitor, but not always. If you hear someone say: "I mic'ed that in" there are talking about removing the trimmer capacitor and installing a "silver dipped mica".
Sometimes the trimmers are left inside, I imagine that can become costly.

LDMOS is in reference to the new mosfet technology made popular by nxp. (blf188xr)
There are boards available for purchase on eBay and the device requires a 50 volt power supply.
Click to expand...
So, trimmers are put in just long enough to figure out what value needs to be there then replaced with the right capacitors? That way the trimmers can be used again. And getting the right caps is how the amp is tuned?
 
Correct

I have to tell you up front. If your doing this to save money, then forget it. You'll end up spending twice as much the first few times to build the amp as you have to buy the stuff then after you get working, you have to buy more. Remember certain places have a minimum order. Plus a decent starter capacitance meter is going to cost about $100.

If your doing it to get a better understanding of how it works go for it. Have fun.
 
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Crusher said:
Correct

I have to tell you up front. If your doing this to save money, then forget it. You'll end up spending twice as much the first few times to build the amp as you have to buy the stuff then after you get working, you have to buy more. Remember certain places have a minimum order. Plus a decent starter capacitance meter is going to cost about $100.

If your doing it to get a better understanding of how it works go for it. Have fun.
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Even if I never get around to building one myself one thing is for sure; I know a lot more than I did before and I feel pretty confident I could probably do most repairs on my own in the future.
 
Crusher said:
Also on the trimmers, the 464/465/469&4615 caps, they work good. But they have been known to arc while in use. Especially if compressed nearly all the way.
Click to expand...


Well don't use ARCO capacitors. :LOL:

ctm-425.jpg
 
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Yea, I did. With tools he mentioned he can build splatterbox, not an working amplifier.
Luckily not my neighbour will use that "amplifier"
Mike
 
sp5it said:
Yea, I did. With tools he mentioned he can build splatterbox, not an working amplifier.
Luckily not my neighbour will use that "amplifier"
Mike
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Now see there. You just gave me some useful info, which is better than making it look like you never read my post. I understand that it won't be tuned, but if you had read through the other post you would have seen that I understood that and may have to send it to someone who can tune it. Plus you asked if I had any of the tools that I specifically said I did not.
 
Crusher said:
A couple of meters, so you can watch input vswr and output power at same time. Few extra watts on output isn't worth it if input starts to go up. Also need a power supply with a current meter and voltage meter. This will help when tuning if doing the math, you can figure out efficiency. ( this will let you know if doing crazy watts but not pulling current that it should be or if it's pulling to hard). You will need an assortment of trimmers. 1 for input, 1 across input transformer, one across output transformer and 1 for output tune. A scope is nice especially dual channel so you can compare input signal to output signal. When first keying, start with a low carrier/ dead key. Turn the mic Gain way down. Don't want a bunch of swing till you have it half way turned up. On the input transformer, you may or may not need a trimmer capacitor. Depends on device. Remember, most plans involve Toshiba transistors. The HG 2879 is a different animal and will not tune the same as the plans. I could go on and on. You will need a decent capacitance meter with some degree of accuracy. Otherwise you'll be putting fixed parts in that aren't correct. You will need a dummy load. Tuning on the antenna will cause false reading. When building 2 device homebrew amplifier, you will need to use right ferrite material on transformers. If not, things heat up quicker and may not match up as well. 43, 61&67 are common ferrite materials. I've said enough.

As to LDMOS, if he doesn't understand tuning. How do you expect him to understand bias or bias set point or how to do active biasing/ ( bias only only when keyed and grounded when in standby). If anyone has done one and it doesn't have thermal tracking. Watch what it pulls current wise until device cools down while in standby. Not to mention want bias applied before actual Drive is supplied. So some kind of sequencer is needed. Or footpedal, no keying circuit.

Ok, this 11meter junkie on standby.
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I do understand what you are saying about the biasing, and understanding/not understanding. I am found of the site I linked before as I found it to be a point, for myself that I learnt from, as well as the youtube vids done by KF8OD.
The "fourth" thing on my list to do after our move to Mt. is to really start to put the parts together to build a LDMOS amp., and I am prepared to ( I hope not) blow it up one time if I get it wrong. I am getting parts to put 3 amps up. I have MORE to learn first before I touch an Iron to solder on this project though... I really thing the LDMOS is going to be the way to go from now on, if your not doing tubes.
 

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