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Author Topic: Amp Build Project - is the GS-35B a good or bad choice?  (Read 26791 times)
K9FV
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« Reply #30 on: September 28, 2011, 06:42:02 PM »

Thanks Ken,
One last question (I hope HI HI). At full power out, what was/is the grid current? I am picking out meters and want to make sure I scale them correctly. I will have 3 meters, 5kV Plate V, 2A Plate I, and thinking 300mA for Grid I.
I guess a better question is what should the grid current for a GS-35B be limited to?
Thanks, Don

Don, you asked a question I've been searching for an answer to.

With my GS31b 6 meter amp, 2600 vdc loaded, CW "key down" for a 1,000 watts output I'm about 300mA (maybe a tad over? - my grid meter pegs at 300mA).  With 800 watts output "key down" in CW mode I have about 225mA, then move to SSB and a peak reading watt meter will peak in the 900+ watt range.

You are correct, the cathode current max is 1.4 amps - REMEMBER, this is the total of anode plate current plus grid current.  i.e. if plate current is 800mA and grid current is 200mA your total cathode current is 1.0 amps.

I have talked to a couple of folks who have built several GS35b amps who "claim" to "normally" run 400mA to 450mA grid at 1500 watt output.  This person "claim" he knew a guy who ran 750mA for a test without blowing the tube.

I am "told" by "some" folks the GS31b and GS35b tubes are basically the same tube, except the GS35b has the bigger copper heatsink and is pushed harder than the GS31b tube.  Note on spec sheets the GS31b has a good bit longer life than does the GS35b tube.

I think I would scale my grid meter for 500mA at least for a legal limit amp - and am thinking of re-scaling my grid meter to 400mA?  Naw, with it scaled to 300mA it helps keep me from pushing it too hard.  I do need to get a spare tube on hand...  just in case:)

73 de Ken H>
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KO7I
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« Reply #31 on: September 28, 2011, 08:08:41 PM »

Ken,
I think I will instrument the amp with a 5kV plate V meter, 1.5A anode I meter, and a 500mA grid I meter.
73,
Don KO7i
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W8JI
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« Reply #32 on: September 28, 2011, 08:56:46 PM »

Ken,
I think I will instrument the amp with a 5kV plate V meter, 1.5A anode I meter, and a 500mA grid I meter.
73,
Don KO7i

Normally oxide cathode tubes have gold plated grids. If the grids are gold plated, you can get into trouble fast with extra grid current.

What do the data sheets say about grid dissipation? That is where the correct answer would be.
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ZENKI
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« Reply #33 on: September 29, 2011, 06:09:14 AM »

And what do the data sheets say about the IMD performance? Everyone seems to want to thrash the living daylights of these Russian tubes, its no wonder we hear all these raspy IMD spewing signals on the ham bands from these Russian surplus tubes. Hams are getting as bad as CBérs just pursuing the goal of power output  purely on the basis of cost and not the resulting signal quality. Please gentlemen ask not only what the power output is, also ask the question how clean the tube is at its nominal rated output.
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W8JI
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« Reply #34 on: September 29, 2011, 07:01:26 AM »

And what do the data sheets say about the IMD performance? Everyone seems to want to thrash the living daylights of these Russian tubes, its no wonder we hear all these raspy IMD spewing signals on the ham bands from these Russian surplus tubes. Hams are getting as bad as CBérs just pursuing the goal of power output  purely on the basis of cost and not the resulting signal quality. Please gentlemen ask not only what the power output is, also ask the question how clean the tube is at its nominal rated output.

That's why I'm asking what the data sheet says about grid dissipation. If you look on line at 3/4 or more of the himebrew articles, they have no consideration for anything. Many of them won't even work correctly as documented.
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K9FV
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« Reply #35 on: September 29, 2011, 07:29:52 AM »

And what do the data sheets say about the IMD performance? Everyone seems to want to thrash the living daylights of these Russian tubes, its no wonder we hear all these raspy IMD spewing signals on the ham bands from these Russian surplus tubes. Hams are getting as bad as CBérs just pursuing the goal of power output  purely on the basis of cost and not the resulting signal quality. Please gentlemen ask not only what the power output is, also ask the question how clean the tube is at its nominal rated output.
Which is why I've asked several times how to measure grid dissipation - so far I've not been able to find an answer I can understand, or have the ability to measure.  

The only way I have to tell about the signal from my 6 meter GS31b amp is to ask "what does it sound like"- I've asked several folks from close (<50 miles) to DX (>1,000 miles) contacts.  EVERY person has said the audio sounds really clear, not muddled, no splatter (as they could tell) - the ones who could hear me with no amp, say there is no difference in sound of my voice/signal from no amp to amp at full output.

Grid dissipation for the GS31b tube is listed as 22 watts.
{edit to correct to GS35b} Grid dissipation for the GS35b tube is listed as 26 watts.

If someone can guide me with a method to test grid dissipation I would be very happy to test.

As I understand it - the main issue with exceeding grid dissipation is to shorten tube life?

The harmonic suppression is more a function of the output network design than what tube?  The Pi-L (Thank you Owen and Tom for encouraging use of Pi-L vs Pi) design has about -45db for 2nd and almost -60 db for 3rd harmonic per Elsie.  There is also a shorted stub for 2nd harmonic that will short HV to chassis in case of doorknob blocking capacitor.
 
Thank you all for any help and guidance in this never ending quest for knowledge.

73 de Ken H>
« Last Edit: September 29, 2011, 08:10:59 AM by K9FV » Logged
W0BTU
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« Reply #36 on: September 29, 2011, 08:06:27 AM »

Full specs of the GS-35B are listed at http://gs35b.com/gs35b/index.html. The max grid dissipation is 26 watts.
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K9FV
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« Reply #37 on: September 29, 2011, 08:12:02 AM »

Awww shucks Mike - you saw my most common error in writing....  a typo!  That should have posted specs for both GS31b and GS35b tubes, but typo has both listed as GS31b.

Thanks for catching.
edit: Corrected original post.
73 de Ken H>
« Last Edit: October 02, 2011, 08:56:16 AM by K9FV » Logged
KO7I
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« Reply #38 on: October 02, 2011, 08:44:15 AM »

Ken,
I think I will instrument the amp with a 5kV plate V meter, 1.5A anode I meter, and a 500mA grid I meter.
73,
Don KO7i

Normally oxide cathode tubes have gold plated grids. If the grids are gold plated, you can get into trouble fast with extra grid current.

What do the data sheets say about grid dissipation? That is where the correct answer would be.
I am chosing 500mA not as a maximum drive level, but more for appearance and meter deflection. I do not want a meter "hitting the peg". My hope is a nice mid range deflection that looks pleasing to the eye while operating.
I agree that the grids would get smoked at 500mA.  Wink
73, Don KO7i
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K9FV
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« Reply #39 on: October 02, 2011, 09:02:04 AM »

Normally oxide cathode tubes have gold plated grids. If the grids are gold plated, you can get into trouble fast with extra grid current.
What do the data sheets say about grid dissipation? That is where the correct answer would be.

Sure wish I could measure grid dissipation, but so far have not found an answer to how it's done with simple equip I have.

The Russian tubes must not be gold plated grids - I've talked to one person who says he normally runs his GS35b at 450 mA, and others don't even have grid current meters so who knows what they are running.  Common statement is "GS35b tubes are tough" and no-one has reported a failure. 

One person even reported running his GS35b tube at 700mA with no failure.  I have no idea if true, or if that one time, or what.

I'm running my GS31b tube around 200mA - we'll see how long it lasts.  I've got a spare ordered....  just in case.

73 de Ken H>
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W8JI
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« Reply #40 on: October 02, 2011, 12:47:25 PM »

Ken,

I've seen many Internet schematics of amplifiers that do not read grid current accurately.

Keep that in mind where you see 500 mA currents and good life. :-)

73 Tom
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K9FV
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« Reply #41 on: October 02, 2011, 02:19:37 PM »

You are correct on the internet Tom - LOTS of good info, and not so good info out there.  The trick is how to determine which is which.  You remember the hassles I had designing the 6 meter amp - you so kindly reviewed the schematic for me, and Owen spent a good bit of time with me on getting the input and output circuits correct.

Now, I think it's pretty good - the actual input and output inductor and capacitor values agree pretty close to calculated values.

If I blow the tube - it's only a $90 tube so it's not the most expensive mistake I will ever make.  BUT I wish not to blow the tube, that's why I'm limiting grid current to 200 mA and cathode current well below an amp (cathode is spec'd at 1.4 amps).

Thank you again for all your help Tom - and a GREAT BIG thank you to Carl, Owen and all the others who helped.

73 de Ken H>
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