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Author Topic: Homebrewing a gg 811A amp... 2,3, or 4 811's?  (Read 4922 times)
KF6PQT
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Posts: 39




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« on: July 24, 2006, 11:01:46 AM »

For my first homebrew amp, I want to use 811a's because they are cheap and plentiful... so are the sockets!

Been  looking at a lot of old ARRL handbook schematics, and the AL-811 schematic.

Here's the issues I'm working with...

I need to sort thru the pile of plate transformers in the garage to find out what kind of plate voltage I can pull off... I figure I wont need much more than 1.5kv.

I need to see what I have in the way of a filament xformer... I know I can buy one from my surplus place that'll do 6.3v @ 20 amps for $35, but that's likely overkill.

I'll be happy if I can build something that gives me 500w on 40m... anything over that is gravy.

What are the pro's and cons of going with 2, 3, or 4 811a's?

Thanks,
J
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KA5N
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Posts: 4380




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« Reply #1 on: July 24, 2006, 11:39:05 AM »

I suggest using two 572B's which will give you 500 watts with ease.  The 572B and 811A use the same tube socket and have the same basing, but the 572B has a plate dissapation of 160 watts compared with the 65 watts of the 811A.  With 811's you would have to use three or four tubes (and neutralize if you use four).  The cost for tubes is not that different and the 572B's will last for years and years if not abused.  The 572B's can pull half an amp at 2000+ plus volts (about 1200 watts max) and put out 600 watts.  This is the description of a Heath SB200 and there are hundreds of those around in use after 20+ years.  The tuned input of the SB200 can stand a bit of improvement and it doesn't have 160 meters so there is room to improve.  If you stick with 811A's look at Ameritron AL811 and AL811H (you can download the manuals at the Ameritron site.
GL
Allen
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KF6PQT
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Posts: 39




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« Reply #2 on: July 24, 2006, 12:26:05 PM »

Thanks...

Question, how does the plate dissipation rating of a tube relate to its potential power output?

Or is plate dissipation more of an indicator of how much "abuse" a tube can withstand?

Thanks,
J
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WB2WIK
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Posts: 21837




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« Reply #3 on: July 24, 2006, 12:59:03 PM »

Not just abuse, it's a very important rating.

Typically in Class AB2 (like a GG amplifier), an amplifier's output is expected to be about double (2x) the plate dissipation rating.  So, a pair of 572Bs should be good for (160*2*2)= 640W.  A pair of 811As should be good for (60*2*2)= 240W.  It's a huge difference.

But the 572Bs, to run that much power, require a higher voltage plate supply (2.4kV vs. 1.5kV for the 811As).  If you run 572Bs at 1.5kV (like you would 811As), they won't run more power, but will definitely have more "headroom."

WB2WIK/6
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KF6PQT
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« Reply #4 on: July 24, 2006, 03:06:38 PM »

Excellent info, thanks!  Some gaps in my understanding are definately filling in.

So, lets go back to 811a's... the more of them you have in the amp, the greater the effective plate dissipation across the entire amp?

How come 3 or fewer 811a's do not require neutralization, but a 4th does?  I was under the impression that running such a triode GG was self-neutralizing?

Thanks again,
J
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WB2WIK
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« Reply #5 on: July 24, 2006, 03:43:06 PM »

Not necessarily.  Whether an amp requires neutralization or not depends not only on the tubes, but the entire circuit, including how well shielded the input and output stages are from each other.

There's no magical quantity that requires neutralization.

GG amps can absolutely require neutralization, although many don't.

"More" 811As will definitely run more power, of course.  The extra output power comes at the cost of more filament power and more plate current.

Because the source impedance (plate load impedance) is effectively Ep/Ip, if Ep stays the same and Ip increases, as you add more tubes in parallel you also reduce the plate load impedance; so an output network that's optimized for 2 tubes won't be optimized for 3 tubes or 4 tubes.  You really need to determine how many tubes you're going to use in advance, and stick with that.

The most important part of such an amplifier design is right there in the output network, its bandswitch and other components.  These are also the parts most likely to fail if not selected correctly.

WB2WIK/6
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