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Old 13th Feb 2012, 10:29 pm   #1
BobGreen
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Default Valve Linear Amplifier project

I am indulging my youth by planning to cobble together a 200Watt ++ Linear for my FT101B. I have the power supply and chassis lined up as well as 4 x 807s and a 5763 driver. Now searching for 100pF HV and a 1000pF variable, RF chokes and T/R relay etc. Any offers for the bits I might need?
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Old 14th Feb 2012, 10:22 pm   #2
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Default Re: Valve Linear Amplifier project

Quote:
I will have to lay on the additional 10V 5A for the 'space' heater and voltage doublers for more anode volts. I will build the tank for all the Watts
This will be ok for heaters, and a doubler on the PSU might work - first area of concern will be "is the HV supply stiff enough to allow for good IMD performance?"

Building the "Tank" or anode circuitry for "all the watts" is fine in theory, I am guessing you will be going for a pi network as the output?

If this is the case you will almost certainly come a cropper

The anode impedance of the 807 amp will differ greatly from an 813 amp - your pi network will need to transform Rp to 50r or there abouts

Assuming 4x 807, in full abuse mode

80ma@700V per valve

320ma total anode current @700V

Rp will equal somewhere near 2200r

Single 813 1500v on anode - 180ma anode current

Rp will be somewhere near 8000r

Thats a massive difference, and will doubtlessly require a redesign for the anode circuitry.....

Sorry to be the bearer of bad tidings.....

Sean
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Old 14th Feb 2012, 10:38 pm   #3
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Default Re: Valve Linear Amplifier project

Sean, You are absolutely right so will get the 807s steaming first and go on from there with advice from you or your avatar bottle... not 813? Oh! one small point is getting my licence as tutor(ess) has gone silent for a while.

Bob
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Old 15th Feb 2012, 9:25 am   #4
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Default Re: Valve Linear Amplifier project

If you want a really impressive 200W linear that has the extra advantage of not requiring any tuning at all, then why not try a distributed amplifier?

I have some notes of one I saw consisting of eleven 4CX250B's which gives a minimum of 200W continuous from 10KHz to 220Mhz, so covers a few bands!
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Old 15th Feb 2012, 10:10 am   #5
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Default Re: Valve Linear Amplifier project

The delay line design would do my head in. 11 x 4CX250Bs could push out frightening amounts in straight parallel. I may end up with 6 or so 807s as they don't cost a lot.
I once used and studied a 90MHz Cossor Scope with a distributed Y amplifier. About 10 valves driving each CRT plate.

Bob
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Old 15th Feb 2012, 2:16 pm   #6
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Default Re: Valve Linear Amplifier project

I just came across this.....

The mathematical theory of the operation of the plate line of a distributed amplifier is developed using matrix algebra as a tool. The effects of a finite number of current generators placed at regular intervals on what would otherwise be an ordinary lumped-constant transmission line are determined. The theory predicts the manner in which the output of a distributed amplifier is changed with propagation constant of the grid and plate lines, termination of the grid and plate lines, number of tubes, and grid driving voltage. Some of the deductions made were tested using a six tube distributed amplifier; and, the experimental results were found to be in general agreement with predicted results.
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Old 15th Feb 2012, 2:45 pm   #7
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Default Re: Valve Linear Amplifier project

My homebrew Tx has 2 x 6146s , running 430v @ 180mA. I make that 2.38K. They match to my 50ohm aerial via the pi-tank no problem. Is the concern about using 4 x 807s that they won't match properly via a pi network?

Cheers

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Old 15th Feb 2012, 3:09 pm   #8
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Default Re: Valve Linear Amplifier project

Aub,

No, that isnt the concern.

The OP stated that he would build the tank circuit to cope with 813s, but use 807s in the meantime - in this case, the 813 circuit design will not match the Anode impedance of 4 or 6x 807s......

that's the problem.

Sean
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Old 15th Feb 2012, 9:20 pm   #9
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Default Re: Valve Linear Amplifier project

Ah, ok understood.

Aub
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Old 17th Feb 2012, 10:28 am   #10
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Default Re: Valve Linear Amplifier project

Quote:
Originally Posted by GMB View Post
If you want a really impressive 200W linear that has the extra advantage of not requiring any tuning at all, then why not try a distributed amplifier?

I have some notes of one I saw consisting of eleven 4CX250B's which gives a minimum of 200W continuous from 10KHz to 220Mhz, so covers a few bands!
We build these type of amplifiers for use in broadband high power systems used for EMC testing, they work well but harmonics in band are a problem, -15dBc!.
It might be worth looking at some of the Russian GU series valves, cheap to buy and very robust

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Old 26th Feb 2012, 7:06 pm   #11
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Default Re: Valve Linear Amplifier project

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Originally Posted by Sean Williams View Post
Aub,

No, that isnt the concern.

The OP stated that he would build the tank circuit to cope with 813s, but use 807s in the meantime - in this case, the 813 circuit design will not match the Anode impedance of 4 or 6x 807s......

that's the problem.

Sean
Sean, I have now planned for 5 x 807s but have wound another 4V secondary on the xfmr with a 813 heater in mind. Do you have an 813 tank circuit design to share? If so I will try to make allowance for it.

Bob
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Old 17th Mar 2012, 7:22 am   #12
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Default Re: Valve Linear Amplifier project

Nice photo of the 4CX250 distributed amplifier!

In case anybody is interested, these things were a British invention that sort of languished until a paper was published at Stanford. Look up Ginzton, Noe and Hewlett. Yup *that* William R Hewlett.

The distributed amplifier plays two nice tricks: The Gm of all the bottles add, making for mor gain than one device would give, and the ability to drive a conveniently low output impedance directly without transformation. secondly, the inductors forming the transmission line prevent all the anode capacitances ganging up in parallel to wreck your bandwidth.

You pay a few prices for all this though. There's a lot of heater power which is a seasonal benefit/penalty. And everyone will tell you that the termination at the 'inside' end of your anode line soaks up half your output power. They're right, sort of.

First note that a distributed amplifier needs TWO transmission lines. One for the anodes and one for the grids. The two lines can have different characteristic impedances, but they MUST have the same time delays between valves.

The amps use not real transmission lines, but approximations built out of lumped components. This gives them an upper cutoff frequency. Think of them as tapped filters with equal value inductances and capacitances per section. The lowpass response is handy in an HF linear amplifier, but a nuisance if you want max bandwidth. Modelling the line with more sections of smaller L and C values increases the BW. Ultimately you can build a real transmission line inside a vacuum tube and the Travelling Wave Tube (TWT) or 'twit' to its friends is born.

One thing rarely mentioned is that distributed amplifiers have a band where the power wasted in the anode line load is nulled. If you think of all the signal paths from input to output, they are all the same length... go so many stages along the grid line, go through a bottle and go the rest of the stages along the anode line to the output and the total number of filter sections is the same. This is why both lines/filters must have the same time delay per stage. Everything arrives at the output in phase.

Not so for the input to the anode line load resistor. Each different route has a different delay, and things arrive with staggered phases. They partially cancel and the wasted power is reduced. Great!

I played around with one of these beasties and found that I could design something with a cutoff above 30MHz and with the waste power in the line termination kept down to a few percent of output provided the frequency stayed above 1.8MHz. It needed 30 stages to do it (a problem for 4CX250Bs, but no problem for cheap SMPS power MOSFETS)

To get lower distortion, I redid the thing with a different time increment, still 30 devices, but distributed on a pair of output lines configured in push-pull.

You don't have to do the maths. Visit the Linear Technology website and download their free copy of SPICE. It's very powerful, it's not crippled with any artificial limits and it's free. A bit of hacking around and you can create models for your favourite valves, or you can find them done by other enthusiasts on the support websites. It's a great tool for playing 'What if' and unlike building hardware for silly ideas, no-one else need know and ask embarrassing questions.

CHeers
David
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Old 24th Mar 2012, 7:42 am   #13
Roger ZS1J
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Default Re: Valve Linear Amplifier project

I am curious why you want to increase the output power of a Yaesu FT101 by a factor of two, or to be precise 3dB. This will only give you half and "S" point increase at the received end. To make a real difference you really need to increase a transmitters power by a factor of 10 to make 2 "S" point difference.

Rather build an amp with a pair of 813s in grounded grid. Good 1 kW out on all bands except 10.
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Old 24th Mar 2012, 8:14 am   #14
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Default Re: Valve Linear Amplifier project

I have just constructed an HF linear using a pair of 811A's in GG. It runs 1275V on the plates, PI coupled output - 300W with 50W drive @ 400mA.

Much easier than mucking around with a heap of 807's

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Old 11th Apr 2012, 9:48 am   #15
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Default Re: Valve Linear Amplifier project

Way back in the early 60s I made an HF Linear amplifier using a 7094 Tube in passive grid. Thinking about it today with more experience I think that going passive grid was messy. You needed a dummy load to dump much of the driving power and also the 7094 required a stabilised HT voltage which was done with a dropping resistor from the EHT and some OA2s and and OB2. The efficiency was quite good as the 7094 was rated at full power up to 60 mHz. 30 watt drive would give a good 300 watts out.

Just out of interest I built a G2DAF design linear much later but was very disappointed with its performance and also very messy using the 6X4 (I think) as the rf rectifiers to power the screen grids. Eventually I modified it to GG and what a difference. Worked an all bands.

Still think you are going the wrong route with 807s. They are much better when used in class C.

73
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