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Author Topic: Amp Build Project - is the GS-35B a good or bad choice?  (Read 26774 times)
KO7I
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Posts: 134




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« on: September 10, 2011, 10:03:15 AM »

I have been wanting to build a HF amplifier covering 160 – 10 meters for many years. I am looking at using a GS-35B as the PA tube. I have read a lot of things about this tube – some good – some bad.

While I am a CW op 1st, I do desire is to have a clean SSB signal. I have read those messages claiming the Russian Tubes are designed for pulsed radar application and they are non-linear. What is not clear about the GS-35B is what type of tube it is.

No matter what I build, it will have the following I will be using a pi-network tuned input, Class AB bias for SSB, and a Pi-L output tank. QSK keying will be a must.

So the first concern I have is, is the GS-35B suitable for SSB service or not? If I am dealing with a suitable for CW service tube only I need to know that up front.

Thanks and 73,
Don KO7i
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W6WRT
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« Reply #1 on: September 10, 2011, 12:54:10 PM »

I have built two GS-35b amps and was disappointed with both of them. I am almost exclusively a RTTY op so I can't speak about linearity, but the problem I had was poor efficiency. With both amps, I had to run about 3000 watts DC input to get 1500 watts RF out. In both amps, I replaced the tube with an 8877, making no other changes except to reduce the drive and change heater voltage, and the efficiency went way up. Now 2500 watts DC gave 1500 watts out. Both amps were run with 2900 VDC under load anode voltage.

At least one other European ham reported similar results on his website.

The GS-35b is tempting because it's cheap, but I wouldn't use it. If space is important, use an 8877 or if it isn't, a pair of 3-500Zs would be a good choice. The 3-500Zs have the advantage of quick warm up, my only complaint about the 8877. They are also cheaper than even a Chinese 8877, the one I use.

73, Bill W6WRT
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N3JBH
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« Reply #2 on: September 10, 2011, 03:25:40 PM »

Not sure  as i never built a amp from scratch. But if i had to by a tube and build a amp around it i think i be very tempted to use a 3CX3000A7 tube  yes it is some what taller then the other tubes. But i think it give you amazing head room as the say. I often thought of having one(amp) made for me just because of that reason.
and the tube is less expensive then the 3CX1500A7 as well... Jeff
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W6WRT
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Posts: 26




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« Reply #3 on: September 11, 2011, 04:06:16 AM »

The 3CX3000A7 is a popular tube but be aware of a few drawbacks. For one, a new one is breath-takingly expensive. You will almost certainly end up using a used one. Not a problem if you can get a guarantee from the seller. Second, the filament transformer will be hard to find and expensive when you do find one. The heater power alone is almost 400 watts! Third, the warm up time is long, six minutes as I recall. The only Eimac data sheet I have does not list the warm up time, but I am pretty sure the figure of six minutes has been quoted by others.

Aside from those drawbacks, the tube is a good one. It has good gain, can be operated zero bias, thereby eliminating the cathode zerer, and should last a lifetime. It also can be mounted directly to the chassis, eliminating the need for a socket but requiring a certain amount of mechanical ingenuity. The -F7 version has wire leads, simplifying mounting a bit.

73, Bill W6WRT
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N3JBH
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« Reply #4 on: September 11, 2011, 06:16:44 AM »

I don't know about all that Bill the Henry 8K Ultra manual stated there was minimal warm up time needed  and it could be used with in seconds of turning it on.
Also RF parts sells the Taylor brand new for just $659.00 So price was not bad either. I do give you credit though on the heater power. That is a big one. I also think Henry stated they preferred what was then known as Green stone Brand tubes over the more Expensive Eimac Tube. So there seems to be some good news on that tube from what i been reading... Jeff
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KO7I
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« Reply #5 on: September 11, 2011, 08:38:24 AM »

It is a tough choice, still wondring about the linearity issue of the GS-35B.
Bill, W6WRT, do you remember how much power you had to drive those GS-35B in order to acheive 1,500W out, a full 100W or more?
I checked the W8JI web site and see that at one time the 8877's were having a lot of problems.
Regarding the 3CX3000, too much power for me. the filament power consumption is a bit excessive. One would have to look into venting exhaust air outside. Great for bragging rights at a hamfest - short on simplicity and practicality.
I have a pile of parts sitting here, vacuum varibles, turns counters, plate chokes & caps, 7.5V/21A & 5V/30A filament transformers, fillament chokes, HV filter caps, K2AW diodes, and a bunch of other crap. I have sat on the pot long enough and it is time to build. I guess I could go with a 4-1000 (I have a pair of tubes). Unfortunately a 4-1000 with both grid and screen grounded is less efficent than the GS-35B.
Decisions - Decisions.
thanks and 73,
Don KO7i
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W6WRT
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Posts: 26




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« Reply #6 on: September 11, 2011, 10:15:17 AM »

ORIGINAL MESSAGE:
Bill, W6WRT, do you remember how much power you had to drive those GS-35B in order to acheive 1,500W out, a full 100W or more?

REPLY:
To reach 1500 watts out required less than 100 watts. As I recall it was about 80 or so.

73, Bill W6WRT
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KO7I
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Posts: 134




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« Reply #7 on: September 11, 2011, 12:35:55 PM »

80 to 100W ain't so bad for my style of operating. With the placement of a small fan on the heatsink of the exciter that would be OK.
For RTTY your concern would be valid and I agree with your choice.
Thanks & 73's,
Don
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KC2RLY
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Posts: 40




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« Reply #8 on: September 12, 2011, 07:45:51 AM »

The 3CX3000A7 is a popular tube but be aware of a few drawbacks. For one, a new one is breath-takingly expensive. You will almost certainly end up using a used one. Not a problem if you can get a guarantee from the seller. Second, the filament transformer will be hard to find and expensive when you do find one. The heater power alone is almost 400 watts! Third, the warm up time is long, six minutes as I recall. The only Eimac data sheet I have does not list the warm up time, but I am pretty sure the figure of six minutes has been quoted by others.

Aside from those drawbacks, the tube is a good one. It has good gain, can be operated zero bias, thereby eliminating the cathode zerer, and should last a lifetime. It also can be mounted directly to the chassis, eliminating the need for a socket but requiring a certain amount of mechanical ingenuity. The -F7 version has wire leads, simplifying mounting a bit.

73, Bill W6WRT



The 3cx3000a7 has no such long warm up time. It is an instant on, once you see full heater v, you can apply hv and drive.  The big plate dissipation is quite nice to have, imagine firing into a 20:1 swr and not caring!
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N3JBH
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Posts: 2358




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« Reply #9 on: September 12, 2011, 10:12:32 AM »

I would love to hear are resident amp expert chime in on the topic of the 3Cx3000A7 tube and his thoughts of it for a very heavy duty  amp. From what i have read if you use a pretty much run of the mill 100 drive from a exciter it will do the 1500 watt  output so unless your going to go gun ho and drive it harder there shouldn't be any issues with it being abused and over driven to wild power level's...   My thoughts is it just make a all around all mode full legal amp with out a lot of concern about duty cycle. Jeff
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KO7I
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Posts: 134




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« Reply #10 on: September 12, 2011, 09:16:28 PM »

Jeff, There are none of here who question the rugidness of the 3CX3000 as a PA tube for the reason's you have accurately stated. Like you I do not believe a ham can damage a tube of the magnatude while operating it legally.  Unfortunately it does not fit my operating preferences. It kinda comes down to something like this, I am happy pulling my 5th wheel trailer with my Chev Duramax pickup. While yes I could re-fit the bobtail of a MAC truck to pull my 5th wheel I will stick to my little chevy. The more I read about the GS-35B the more I keep reading about it being developed for UHF Radar applications and AM or SSB service. I want to be loud - but I want to be clean too. Based upon what I am reading there are better choices for SSB service PA tubes. Thank you for your comments and the time you expended responding. Good DX & 73, Don KO7i
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N3JBH
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Posts: 2358




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« Reply #11 on: September 12, 2011, 09:40:21 PM »

Oh i am sorry Don i did not mean to suggest that for your amp.... I guess looked that way i guess i was caught up by some the comments of others  was all. i agree with you it not what you really needed. Just was i guess hijacking your posting for some my info. I again am certainly sorry for bad behavior here and wish you the best luck. 73 Jeff
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ZENKI
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« Reply #12 on: September 12, 2011, 11:30:39 PM »

The 3cx3000 is one of the one most linear tubes you can buy. Its also the smallest tube that you can buy that can be rebuilt by people like Econco. So as an investment its money in the amplifier bank. Besides, the only people who wear out a 3cx3000 are the brain dead CBérs, it will last you your ham radio career. Spending all your time building for tubes that  is no longer being made is a senseless practice really. It would have made a  good choice if you could have walked down to the surplus end of town and bought a bucket full for $1 These days you have to deal with a lot of crooks before you get some decent tubes.

I run a a 3000 and I always get comments on how strong and clean my signal is  up in the 80 meter phone DX window. I wish other operators with these Russian Tetrodes and triodes would also buy a 3000. Their signal quality is shocking. Its so obvious now that you can pick the 3500's, 8877, 8874, 3cx800's from the typical russian amplifier tube crowd. Their IMD performance is so bad and noticeable because signals are also so strong in the 75 meter DX window.  Russian is a dirty word!
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KO7I
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Posts: 134




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« Reply #13 on: September 13, 2011, 06:34:31 AM »

So how many CFM of air does a 3CX3000A7 require? From what I am reading about it, one would have to vent the exhaust air outside via some thing like a dryer vent. How big of a motor do you need to run the fan? 1/2 hp? Surpressing the ambient noise from the fan would require the amp and the fan be mounted within a acquistic enclosure.
Don KO7i
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N3JBH
Member

Posts: 2358




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« Reply #14 on: September 13, 2011, 11:55:38 AM »

Don if you ran that tube at honest legal limit 40 cfm would do... that be fairly easy to do. If your going to drive it for all it's worth well now your talking some air. i suspect you have to probably triple that number as far as a blower Hp rating dont know but i am sure you find it if you looked.
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