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Solid state amp design and protection circuits	Reply
by KD7YQM on September 17, 2009	Mail this to a friend!
I made this amp probably 20+ years ago. It is patterned after several circuits. The moto rf handbook and arrl handbook 1985 edition ;-) It is a 4 stage amp using 8 srf2978 bipolar transistors (mrf454 style package)in push pull config. Actually I have never found a data sheet on these devices. And of course using 4 to 1 power splitter and combiners. A very common circuit I would guess. Over the years I have attempted to drive it with different transceivers usually with poor results ;-( The typical 100w hf transceiver of course provides way too much drive. Blew a few transistors that way trying to reduce the drive. Then using a 3db power attenuator still blew a transistor. This amp will put out 400w with only 10w drive. So I guess my questions are what kind of protection schemes are being used in this type of amp? Very difficult to find circuits like this. Thanks Dennis

RE: Solid state amp design and protection circuits	Reply
by HFRF on September 17, 2009	Mail this to a friend!
It sounds like you are over driving the amp. Ebay has 3 and 6 db high powered attenuators for sale (Henry Radio). These would cut your 100 watts down to anything you want by using multiple units. in series. Another factor is that some HF rigs put out a huge spike from the radio when first keyed. That may be a problem for you.

RE: Solid state amp design and protection circuits	Reply
by HFRF on September 17, 2009	Mail this to a friend!
If you need schematics of ss amps along with their protection circuits, download Ameritrons manuals and see for yourself how commercial ss amps are built. You also can but boards from them.

RE: Solid state amp design and protection circuits	Reply
by AD5X on September 17, 2009	Mail this to a friend!
Check out the in-line attenuators on my website at www.ad5x.com. You can build these very inexpensively. And you can leave your transceiver set to full power so the full power spike doesn't cause problems. I switch in the attenuator with the amp, but the relays I use are fast enough for QSK operation so you can switch the attenuators in and out at keying speed if you have so much attenuation that it impacts your threshold (I only need 2db attenuation with my ALS-600). Phil - AD5X

RE: Solid state amp design and protection circuits	Reply
by KD7YQM on September 17, 2009	Mail this to a friend!
Thanks HFRF ;-) Those ameritron amps sure look pretty. Sure wouldn't want to pop one of those expensive fets. I can probably get some ideas looking at their schems but I wonder how much would be transferable to a bipolar amp. AD5X I have a 3db t-pad in the amp now. Probably need a 6db one. Yes I see those chip attenuators on ebay. I guess I should buy a couple of those. Thank guys Dennis

RE: Solid state amp design and protection circuits	Reply
by KM3F on September 17, 2009	Mail this to a friend!
3db down from 100 watts is still 50 watts. Don't you have a modern rig that can reduce power to at least 5 watts? It's found in the menu. Even 6 db is still allowing to much drive if your sure it only takes 10 watts to drive amp to full output. Normally you don't need all the fancy control circuits if you know what you need, and do it! On FM you can drive full out, but SSB must reduce drive so the peaks only approch about 75% of amp output. On AM, lower drive still more. Remember, these devices suffer from gain compression and generate a lot of IMD when driven to max on SSB or AM. Is there a problem understanding all this? Sorry to come across a bit rough but your post suggest it for being in the hobby 20 + years.

RE: Solid state amp design and protection circuits	Reply
by KD7YQM on September 17, 2009	Mail this to a friend!
km3f the rig I'm driving it with now has a 20w setting. So 3db down would be 10w. No 10w does not drive it to full output. Is this a new rule that I must have a modern rig?? Yes I am aware of the problems of over driving this type of amp. And I am aware of the filtering requirements. Do you know the specs on this device? I have been unable to find a data sheet.

RE: Solid state amp design and protection circuits	Reply
by KM3F on September 18, 2009	Mail this to a friend!
Respectfully, you said 100 watt rig. You didnot say what power control you have or what radio it is. Certianly, I based a response on what you said. If you built the amplifier from a book plan, how much probility would there be that another person would know the specs on a unit you built and in what condition the amplifier is in after having been overdriven and repaired? How do we know what operating angle the bias is set for or what mode you are using to drive it? You need to get a handle on all these things then you can sort out the issues. To come and ask is fine but when we see to many items as unknown, then you have to recognize it and not be to defensive. We can advise but you have to do the work. I don't have any interest in argueing but will point out my position based on what your replies are to a point. No further response from this end because it's not going to be helpfull to you. Good luck.

RE: Solid state amp design and protection circuits	Reply
by K4DPK on September 21, 2009	Mail this to a friend!
Hi Dennis... You said: It is a 4 stage amp using 8 srf2978 bipolar transistors (mrf454 style package)in push pull config. Actually I have never found a data sheet on these devices. And of course using 4 to 1 power splitter and combiners. A very common circuit I would guess. Do you mean four separate stages of amplification, or four different push-pull amps combined with splitters and combiners? If it is the former, how is it configured? Interested in the numbers srf2978. Wonder if they are similar to the 2sc2879? Could you double check that number and I'll see if I have anything on it. Phil C. Sr. k4dpk

RE: Solid state amp design and protection circuits	Reply
by K4DPK on September 21, 2009	Mail this to a friend!
Dennis... Just to continue... If you mean you have four push-pull amps combined with splitters and combiners, it is unlikely just ten watts drive will give you 400 out. What sort of filtering do you have in the output? How many poles? Is it possible you have outputs on frequencies other than the fundamental? That would cause the wattmeter to display much higher than the real fundamental power, owing to the impedance error of the harmonic power. I would expect an amp like this would require 40-50 watts to give 400 watts output. Ten watts is much too low. Phil C. Sr. k4dpk

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