Alpha 374A As An IPA

[Mike/W8BAC]

I am putting into service a converted Gates Vanguard 1 transmitter. It uses a 4CX3000 in the final running class AB and requires about 100 watts carrier plus modulation to run properly similar to a Henry 8K. My exciter only delivers 25 watts plus modulation so I need an IPA to make up the difference.

I have been looking for a broad band solid state amp to fit the bill but haven't found it yet. I have an Alpha 374A sitting on the shelf. It runs a pair of 3CX400A7's. I was wondering if I tune this amp up properly in CW (low plate voltage) and just lower the drive in AM to, say, 10 or so watts carrier (a 10 Db gain amp) do I risk anything other than efficiency? The tubes are so expensive I don't want to risk a problem. What are your thoughts? If you need voltages or other spec's to help, just ask. Thanks in advance.

Mike

[w4bfs]

Hi Mike ... first thing that comes to mind is to slip out those 400's and slip in some 4cx250b's ... as I recall on that amp that B+ was low and I think it would work ok, particularly at this reduced power level ... as I recall the tube bases were the same or just similar?

p.s. the Bill Orr connection ... tie control grid to cathode ...ground screen...apparent gain = 5

[KM1H]

The Alpha is a grounded grid triode amp, the 250's are tetrodes. It aint a swap and the sockets are completely different.

To answer the question, yes it will work as long as you are very careful to not overdrive the big amp. In fact, you can pull one of the 8874's and just use one tube. With the price of those tubes there is no sense of wasting a national resource   Secure or remove the plate strap and tape some cardboard over the socket. At 100W out the single tube is well within its ratings and the extra airflow will keep it very happy.

The only possible problem would be if your driver cant handle the resultant input match or doesnt have a built-in tuner.

Carl
KM1H

[w4bfs]

Ok Carl ... got my alphas mixed up as to tube sockets ... however if you were planning to run the 374A as an ipa for an extended period, the 4cx250/4x150/7034 series will work with a socket change (maybe fil xfmr as well) ... yes they are tetrodes but the Orr connection uses them as triodes ... very low distortion due to heavy inherent feedback ... input circuit redo is probable ...would only need one tube...73...John

[KM1H]

I guess I dont understand your reasoning John.

Why ruin a perfectly functioning amp with a good market value with an inferior tube?  Have you ever been into a 374A? Do you understand its circuitry? How would you configure the new circuit much less find room for the air system socket without a major hackorama?

Carl
KM1H

[w4bfs]

Went and did some googleing ... the 8874 uses a 11 pin socket (Johnson 124-311-100) and the 4cx250B uses a special 8 pin loctal socket ...the fil voltages/currents are similar and use similar deratings with freq ... the anodes and base structures dimensions are nearly exact ...so it comes down to replacing the 11 pin socket with an 8 pin socket ... the screen ring is grounded .... pull the spec on both tubes and satisfy yourself ... the 8874 is nearly unobtanium (unless Svetlana still makes them - anyone else ?) lotsa 4cx250b still out there on Epay .... should be able to do as no-holes mod on this amp so can reverse it back to stock ... BTW if you can find some 8122/8072 family tubes, might not even need a socket change ... hope this helps Mike and Carl.... 73...John

[Mike/W8BAC]

Thanks Carl and John for your replies. Funny, I thought my post might bring a few more responses but your input did satisfy my question.

John, Thanks. I bought this amp new in 1980 and at the time I couldn't afford it but I just couldn't resist. Even thou it was expensive I never regretted the buy. The amp was used with my S line station for many years and never missed a lick. As a matter of fact the original tubes are still in the amp with countless hours. I babied this thing for 28 years now and it is still as dear as it was the day I bought it. I know, silly, right. If at some point in time I get rid of it I'll sell it in the same condition I got it. I don't have the heart to change the tubes but that's just me.

Carl, I looked at the manual for the amp and Alpha put a toroidal matching transformer in the input to provide a 50 ohm load to the exciter with linear characteristics. Hopefully when I pull a tube I'll still see the same load. I'll take some measurements of the tube base and find a suitable plug to block the cooling air. Pulling a tube is the right thing to do and for now the Alpha will live a new life. What I'll do next is look at some of the solid state designs Steve/QIX and the rest have done. I might build a solid state IPA next.

Thanks again for the input.

Mike

[W2JBL]

 if you really want to use 100 watts drive on that you might want to consider using a bigger radio drive that amp- Viking-II, DX100 etc. if you try to drive it with an IPA as big as that Alpha your residual noise and distortion numbers will be really bad. and (Bill Orr) will back me up on this: NEVER drive an amp with a class B or AB2 driver. it just multiplies the distortion. my take on it would be to just use 25 watts of drive from a really clean exciter. that should be all you need. you have 3000 watts of plate dissapation and a TON of gain available. 25 watts should easily give you a 375 watt carrier (legal limit on AM) out with tons of headroom for audio. that is a good thing. leave the "Alpha for a driver" stuff to the knuckledraggers like Wally and Richard and the CB kiddkies on 3878 SSB. that's why they are SO wide... the secret to a great signal using linear AM is a low distortion amp with lots of headroom.

[KM1H]

JBL, while I somewhat agree in principle the OP is trying to use what is on hand.

The Alpha is an extremely linear amp, 8874 3rd order IMD is in the high 30's and he has plenty of overhead. In any case its a lot cleaner than any 1940's designed tetrode. And I suspect is cleaner than most Class C ham rigs.

Back when I was using the NCL-2000 with 8122's the best IP3 IMD was -25dB.

The average ricebox exciter isnt as clean, typical IP3 on all but the best, is in the mid 20's so its still a matter of GIGO and thats also where the residual noise comes from.

As far as multiplying distortion thru several amps I disagree. The final output will never be any better than the worst stage but the distortion products dont add or multiply. Its a rather complicated result dependent on phase relationships from stage to stage and even some cancellation is possible.

Carl
KM1H

[Mike/W8BAC]

Hi Chris,

if you really want to use 100 watts drive on that you might want to consider using a bigger radio drive that amp- Viking-II, DX100 etc.

I have a Viking II that has Timtron's audio mods sitting on a shelf and a suitable receiver as well. It wouldn't be hard to press that system into service. I am running an SDR-1000 right now as an RF exciter and receiver to drive a 20V-3 and my intention is to use the  same exciter to drive the Gates.

  if you try to drive it with an IPA as big as that Alpha your residual noise and distortion numbers will be really bad. and (Bill Orr) will back me up on this: NEVER drive an amp with a class B or AB2 driver. it just multiplies the distortion.

Trying to drive an amp with an amp can cause unintended problems, I agree. I wouldn't want to push a distorted signal that high and the IPA setup would have to be monitored closely. On the other hand Henry did this when they offered the 3CX3000 amp. In reality a 100 watt Viking II is a series of amps as is the SDR rig.

my take on it would be to just use 25 watts of drive from a really clean exciter. that should be all you need. you have 3000 watts of plate dissapation and a TON of gain available. 25 watts should easily give you a 375 watt carrier (legal limit on AM) out with tons of headroom for audio.

On 160 and 80 the 25 watt exciter works fine. Moving to 40, 20 and 10 meters is a different story. The 4CX3000 wants to see an increasing amount of drive in the higher bands. The Viking II output drops in the higher bands as well.

leave the "Alpha for a driver" stuff to the knuckledraggers like Wally and Richard and the CB kiddkies on 3878 SSB. that's why they are SO wide... the secret to a great signal using linear AM is a low distortion amp with lots of headroom.

The 20V-3 is the first AM rig I have owned that has the ability to Exceed the limits of my license and do it all day long. During the three years restoration and conversion process I thought many times about how much fun it would be to dump a kilowatt on 3.880 and have some fun. Reality is I never did that. As a matter of fact I get more pleasure out of holding the 375 watt line. I could also raise hell with an intruding side bander very easily by messing with the input audio gain. Instead I pay close attention to tilt and overshoot and brick wall my signal to 6KHz. I can't sit in judgment of  poor operating practices if I use poor engineering practice myself. If the sideband interference is trouble I'll pull in the filter skirt.

I'll see this through Chris. If it doesn't work right it's back to the drawing board. Life is too short for QRP and 10 meters is the target when conditions require a little more umph. Thanks for the comments.

Mike

[Mike/W8BAC]

It Works!  NO, It works great! I pulled one tube and blocked off the forced air ports. After insulating the unused plate cap I closed it up and lit it off. After warm up I did a low power tuneup and hit the exciter tuner. Perfect so far. On high tap the power output is a bit over 5db gain and rock stable. Grid currant is nominal. I can't measure input impedance but it doesn't seem to have changed. Plenty of head room and it looks textbook on the spectrum analyzer.

Thanks all for the help. I wouldn't have thought of that on my own.

Mike