Calling all Amplifier Design Experts

[AB1QP]

Ok All This is not about me actually doing the modification. This is about me learning what when where and why. I was asking those questions to get more education on Tube amplifiers and why the extra circuitry was added to the 811H besides the fact it has a fourth tube what is the purpose? I hope I am making myself clear enough to get those answers.

Thanks;
WA6DJM
Always Learning, Always Improving, Always Striving for a ounce of Perfection
K3DJM1@gmail.com

You do understand that marketing, and marketing considerations drive the product requirements, design, and manufacturing, not pure engineering excellence, don't you?

AB1QP

[K1ZJH]

Ok All This is not about me actually doing the modification. This is about me learning what when where and why. I was asking those questions to get more education on Tube amplifiers and why the extra circuitry was added to the 811H besides the fact it has a fourth tube what is the purpose? I hope I am making myself clear enough to get those answers.

Thanks;
WA6DJM
Always Learning, Always Improving, Always Striving for a ounce of Perfection
K3DJM1@gmail.com

What extra circuitry are you referring to? Gas arrestors were added to the cathode
circuit on some Ameritrons to protect the exciter in case the tubes arced over.

Pete

[WB2WIK]

What extra circuitry are you referring to? Gas arrestors were added to the cathode
circuit on some Ameritrons to protect the exciter in case the tubes arced over.

Pete

They also added a neutralization circuit which adds complexity and cost.

Tom W8JI who designed these amps has posted several times he never wanted to do either one of them, he thought an 811A amp was silly in the 90s, let alone the 2000's, but they pressured him into it.

[WA6DJM]

Well thanks to all for the bit of education and I will continue to Study more about it so maybe one day I will build my own homebrew, but till then Study and learn.

WA6DJM

[WB2WIK]

I don't know the whole story there but would agree the recent 811As seem pretty bad, destroying the usability of any amps using them today.

Of course it's also a 75 year-old design, eh?

My best friend's (at the time) dad worked for RCA in the 60s and 70s and when they stopped making these tubes -- which was a really long time ago -- I remember him saying, "That ends the era of properly built and tested tubes like this. You'll never see them again."

He was pretty right. I bought a pair of brand new RCA 811As in 1969 for the modulators in my homebrew 4-400 AM rig. They were $9.90/each, brand new. They're still working perfectly after 43 years.

[AF6LJ]

Ok All This is not about me actually doing the modification. This is about me learning what when where and why. I was asking those questions to get more education on Tube amplifiers and why the extra circuitry was added to the 811H besides the fact it has a fourth tube what is the purpose? I hope I am making myself clear enough to get those answers.

Thanks;
WA6DJM
Always Learning, Always Improving, Always Striving for a ounce of Perfection
K3DJM1@gmail.com

These are all good things to know...

I'm of the belief that from the very beginning if you cannot gain 6 DB, it's not going to be worth your trouble. 10-13 DB is more to the ideal.

[WB2WIK]

These are all good things to know...

I'm of the belief that from the very beginning if you cannot gain 6 DB, it's not going to be worth your trouble. 10-13 DB is more to the ideal.

In terms of getting stronger, that seems reasonable.

But it can also pay to have an amplifier capable of much more power than we'll really run; usually keeps it more linear and lower IMD at the power we'll actually be using, if well below the max available.

Henry for years used to push the 3CX3000A7 on hams. Good reason: The 3CX3000A7 can run twice the power of an 8877, and at the time didn't cost any more! So, although it took more heater power and so forth it was a really economical tube for a 1500W output application and just "loafs" at that power level, since it can easily produce triple that. Some hams "got it" but many never did. I think they really didn't understand: You get 6 dB of "headroom" by maybe using a few dollars more a year in electricity, and the actual cost of the tube was the same as for a popular tube rated for only half as much power.

And they didn't charge much more for the 3CX3000A7 models, for many years.

It's a tube that has such low IMD at 1500W output you could run PSK31 at legal limit and nobody would complain about distortion...they might notice how strong you are, though.

[N4BBQ]

If I wasn't happy with its power output, and before I butchered it to the point that only a CB'r would buy it, I'd just sell it and get an AL-80A or AL-80B.

Or if I was just bored, I might take up an additional hobby...bike riding comes to mind.

[WA7PRC]

I bought a pair of brand new RCA 811As in 1969 for the modulators in my homebrew 4-400 AM rig. They were $9.90/each, brand new. They're still working perfectly after 43 years.

The long grid lead makes 811s not so good at higher RF frequencies but they work well at audio. I've used four in parallel-pushpull to modulate four 807s in parallel. We wanted LOTS of overhead in the modulator, and got it.

[AF6LJ]

In terms of getting stronger, that seems reasonable.

But it can also pay to have an amplifier capable of much more power than we'll really run; usually keeps it more linear and lower IMD at the power we'll actually be using, if well below the max available.

Henry for years used to push the 3CX3000A7 on hams. Good reason: The 3CX3000A7 can run twice the power of an 8877, and at the time didn't cost any more! So, although it took more heater power and so forth it was a really economical tube for a 1500W output application and just "loafs" at that power level, since it can easily produce triple that. Some hams "got it" but many never did. I think they really didn't understand: You get 6 dB of "headroom" by maybe using a few dollars more a year in electricity, and the actual cost of the tube was the same as for a popular tube rated for only half as much power.

And they didn't charge much more for the 3CX3000A7 models, for many years.

It's a tube that has such low IMD at 1500W output you could run PSK31 at legal limit and nobody would complain about distortion...they might notice how strong you are, though.

That makes sense and I believe one should have as much headroom in solid state amplifiers as one can afford due to gain nonlinearity.