To any and all S/S amp designers

[Unit 148]

...not copiers!

I have a one-off job up for grabs for a negotiable fee.

Here's the schematic of one of my Metron/Transword Communications amp's:

I would like this circuit modified to incorporate 16 X 2SC2879's.

So that requires new input splitter, output combiner and hopefully a superior bias circuit to the stock design.

We have a good cad guy here at work that can work with RF but he is no component level designer.

I am looking to produce an as clean as possible 1500W pep output for SSB use.

I have a very hi-res copy of this schematic for anyone interested.

PM me if you don't wish to discuss on the board.

I'm happy either way.

 

[Shockwave]

I'd be careful what you ask for in this case. To do the job right not only requires 16 new matching transistors, it should have double the heatsink, double the bias current capabilities, and double the capacity of the output filter. The meter and shunt will not handle 16 transistors either. You might have been better off just buying a pair of amps and trying to find someone to combine them. Asking someone to build you the missing half and make it match the first, sounds like problems waiting to happen. Especially if you want the output filtered and working on all the bands.

 

[Kamikaze]

Hambone rigs using pairs of 2879s typically make 125 watts P.E.P. maximum.

Each 2879 is good for 50 - 65 watts P.E.P. before you hear from your neighbors on the band. Yes, I know this is a whole lot different from CB stuff.

8 2SC2879s and 600 watts out would be right on the edge of reasonable.

Linearity would need to be sacrificed in order to get 1500 watts Po from 16 2879s.

The Metron's bandpass filters and output switching relays would not be happy.

Maybe some MRF 454s will show up for a good price somewhere.

 

[eagle1911]

^The 2sc2879 datasheet states 100W PEP max @ 12.5V 28MHz. Maybe you're thinking of the 2sc2290? Unless the datasheet figures are not taking IMD into account, but I can't remember seeing a power transistor datasheet with a published max output power that was so high that excessive spurious emission is generated. I have always been under the impression that those numbers are sort of a guide to help avoid that issue along with overdriving.

 

[bob85]

eagle1911,
the 2879 datasheet shows imd @-24db for 100w which is poor, imd climbs fast after 60w pep,
2879's would not be my choice of device for a decent solid state amateur amplifier

 

[eagle1911]

Hmm.. I see. -24db isn't too great, that's true. That's weird since IIRC they are pretty common in some HF rig's PAs, but I suppose in those cases pairs of them are being used to produce only 100W total.

 

[Unit 148]

Shockwave & Co,

You make some very valid points.

Airpax produce the circuit breaker in the Metron.

I would propose not just a heatsink to sit under 16 devices but a copper spreader and some forced air cooling with tunnel design chassis with inlet & outlet like our FM TX's & PA's:

TX1000 FM Transmitter from BW broadcast

Now, regarding the 2879. I've seen pep ratings of 100w pep out off of a 12V rail and 120W pep from a 14V rail but I hear what several of you say regarding IMD.
Are there any other off the shelf 12V device that offers better performance at HF that the Tosh? I thought only the MRF421's were up there but are now unobtanium.

Finally, output filters.

I have another guy that seriously into magnetics and who has done consultancy work for SGC so upgrading the output filters does not present a problem.

Any other thoughts?

 

[Shockwave]

Once you reach specifications of 25 amps and 250 watts of collector dissipation, the weak link in the chain in terms of IMD is the actual supply voltage of 12 volts. In order to surpass the 2SC2879's power and IMD specifications, you would need to choose a transistor with a higher operating voltage. Either 28 or 48 volt devices.

As I recall the amp you have is fairly old. It likely has transformers and combiners made of material mix that has slightly different permeability then today's modern devices. This could create an imbalance between the original bank of 8 outputs and the 8 you will add. Be aware of this possible issue since it could dictate replacement of all core material.

 

[Unit 148]

Hi Shock,

The idea is to redesign a new board to hold 16 devices. I'm aware of more modern ferrite cores so may find something superior to the early type 43 which I believe was considered best for 2-30MHz.

Interesting your comments re 28V devices.

I may fire one up with say 14V and see how we go.

Tnx

 

[Shockwave]

A 28 volt device will perform poorly on 14 volts in terms of both power output and IMD. The 28 volt transistor is designed to operate spectrally clean and produce best output into a higher impedance load then 12 volt devices. In this case it has more to do with the actual operating voltage then the parts specifications indicating it can handle more then the applied. We don't need the extra headroom operating the part below rated voltage may offer. We need the reduction in current the higher voltage offers while providing the same relative output level.

 

[Unit 148]

Hi Shock,

OK, so we've come full circle regarding operating characteristics at 12V.

Let's just stick with 12V for now.

Is there any devices out there which at 12V can offer superior performance to that of the 2979, taking not just IMD but RF output and cost into the equation?

I can see no other way of achieving circa 1500W pep output from 100W+ input at an acceptable IMD and cost per watt than with this device - unless you know otherwise?

Thanks for your input.

Phil

 

[eagle1911]

Hi Shock,

OK, so we've come full circle regarding operating characteristics at 12V.

Let's just stick with 12V for now.

Is there any devices out there which at 12V can offer superior performance to that of the 2979, taking not just IMD but RF output and cost into the equation?

I can see no other way of achieving circa 1500W pep output from 100W+ input at an acceptable IMD and cost per watt than with this device - unless you know otherwise?

Thanks for your input.

Phil

You might have better results using MOSFETs instead of BJTs, however I can't say exactly what that would entail in terms of modification. FETs require a different, but simpler, bias scheme.. normally a simple voltage divider will do the trick, but you'd need to bias in accordance with the datasheet specs for whichever transistor you use. It's possible to make 1 kW+ with only 2 MOSFET devices, but at a bit higher than 12-14V.

 

[Shockwave]

The 2SC2879 is the king of the 12 volt RF devices. Anything better is going to require more volts.

 

[Unit 148]

Thanks guys.

Yes, Shock, that is what I thought.

Without me going a searching is there any truth in what I read somewhere of there being some not so good 2879's out there?

Are there some second rate or ripped off copies of this device being produced by the Chinese?

What are your thoughts regarding matching up devices, is the same date or batch code good enough or should one carry out a thorough test of spec on each device?

Thanks for your continued interest.

 

[mchaos]

Sorry to put the paddles on an old thread but its an interesting topic and I wanted to toss in my 1 and a half cents in..

I have built recycled amps, and I have built bigger amps into enclosures you would not expect to find inside the case. Completely custom and from scratch etc...

Aside from the obvious issue of cooling, and internal space... To do this your basically going to be rebuilding the entire inside from scratch. New board, custom cut pill holes, and tracing, Transformers, etc, basically a brand new amp. Because you would be building everything new with the exception of the enclosure, it is going to cost more to build this custom then to go out and buy something made for what you want.

I have spend more time and more money then I should have building custom amps just for the sake of a fun project, but in this case your better off selling that amp for what you can get, and buying one that does what you want.

I'd only do something like that for my self just for something to do, over time. Right now I have plenty of projects to occupy my time.