The AM Forum

Has anyone had experience with, or is currently operating a controlled carrier with screen modulation set up on a larger Class C tube like a 4-400 or 813 ?  I have been wondering about this since the first time I saw an ARRL handbook with the schematic.  Any reply's will be welcome.

Never had much luck screen modulating the 833-A.

 Yeah, I'll bet. :)  What is the old line, in a triode no one can hear you screen! Sorry about that, trying to multi-task. (I had listed the 833A by mistake).

Hi Bob,

I'm wondering if there is not much difference in performance when running a linear amp in GG, and biased into class C  for AM ---  compared to a screen modulated grid driven amplifier in class C.

They both require careful loading adjustments and both will likely have the same efficiency and power output.   The GG linear amp will be more stable and less work to set up.

But I agree screen modulating a big glass tube sounds kinda sexy... ;D       I'm not a fan of conventional controlled carrier, however.

T

That's because yer doin it wrong!

Patrick! Tell us more, please.  I really want to know.:)

About 20 years ago in ER, Louis Erath W5BM (sk) published a rather interesting article about screen modulating a 4-1000.

The PDF is too large to attach here, but I made a screen shot of page 1, and attach here.

Louis was also famous for his special LWE speaker design. Some of his stuff is still out there. Best bass response ever!


https://community.klipsch.com/index.php?/topic/70763-louis-w-erath-lwe-acoustron-lives-still-building-speakers-networks-amps/

Jim
Wd5JKO

Jim,

Youse gots me interested.

A  4-1000A screen modulated and with NFB... sounds cool.  The need for a negative going,  low impedance  screen modulator would make the GFZ MOSFET driver perfect for this job.  Frank just came out with a new version that will do +- 500V p-p swings.

How do we see the rest of the article?

I have a pair of 4X1s in linear service here now that I don't use enuff  -  that could make a very FB platform...


T

I've often thought that screen modulating a pair of Svetlana GU74B's might be cool. Relatively affordable; plenty of plate-dis.
They don't have that glass-bottle magic, though.

AIWA did something similar in auto sound in the early 90s.  Servo control back to a specially designed amplifier.   This was in the car.

Philips as well, in the 70s.

The Aiwa stuff sounded amazing.  I was a certified installer fresh out of high school, and those little 6 inch woofers sounded like tens when the Aiwa rep came and showed us his car.  Really tight bass and accurate down to lows you never heard a 6 inch SUB (yes, they where subs, not midbass.  Amazing for a 6) play.

Nowadays, I like my Klipsch from the 80s.  Big, woody, but have that amazing Klipsch sound.


--Shane
KD6VXI

I had a two-part article in ER magazine #350 and #351 on the design and build of a single screen modulated 813 modulated with a 572B. It produced 190-175 watts on three bands, 80, 40, and 20m.

It was a direct-screen modulated rig and not a controlled carrier system. In fact, it had better audio than any controlled carrier system.

I was going to add another 813 in parallel but decided to replace the 813 with a single 4-400 and cathode modulate it. A higher voltage, higher current power supply is being built to accommodate the 4-400.

Construction is still in the works but the design has been completed. 

Phil - AC0OB

I remember that article - one of the first issues of my subscription. Seriously planned to build one, but never did. Got me questioning my math, though: I've always understood that in order to get clean modulation, you need to load the final until the efficiency drops to somewhere around 30%. Unless my math is wrong - which is likely - in order to stay under the 813's plate dissipation you'd need an efficiency of almost 60% to get a resting carrier of 175 watts with a single 813. My DX-60 has the QIX mod - all screen modulation without the controlled carrier - but the stock metering is inadequate to tell me the whole story - I'll have to do some tests to find out what sort of efficiency I'm really getting when loaded for best modulation.

Now you’re talking bout stuff I know about, rather than the AM I’m learning about...

Boosting response below a sealed box’s F3 to extend response, via feedback or a filter, is an old idea, one that imo, should be forgotten.  It works but takes up tons of headroom, has tons of speaker (not amp) generated distortion, and cannot get loud.  In the pro audio arena, Bag End has patents and still sells some models.  Like so many things, low end is a triangle...size/low end cutoff/efficiency...choose which side/corner you want.  I’m a horn fan, a Martin Audio and Danley Sound Labs dealer... fun pic attached of a Danley horn (sub is a horn variant) system...Go big or go home!

Ed

Hey Tom......... ya might remember The Slab Bacon's "screem" modulated 4-1000!

Yo Bud -

Yep, "the Slab Bacon with the big meat a-shaking."   Who could forget?

One thing that stands out about his screen modulated 4X1 rig is that it sounded distorted when he first ran it. Slab wanted to peak the output for max RF out. But when he finally loaded it heavily, more plate heat, more color, more dissipation, it cleaned up and sounded great.  But the efficiency dropped down to where a regular linear amplifier would be.  No free lunches.   That extra X4 audio power supplied by a plate modulator is where the power comes from when running at 75% eff.   I think this is the big trade-off with screen mod.

Just like my new series modulated 4D32 X quad 6LF6s rig... almost finished. But I have low expectations of perhaps 50 watts carrier out due to the class A series modulation system.  That same tube will do an easy 150 w out if plate modulated.

The article posted by Jim yesterday about W5BM's screen modulated rig doing 80% eff with low THD makes me wonder.  Could a simple thing like NFB make the rig clean when peaked for maximum output?  Doesn't seem real.  However, with 6KV on the plate, he certainly has a lot of reserve peak power there. The plate must be turning white.  

But I know one thing.... every screen modulated rig I have run and heard others run always needed to be heavily loaded to sound good -  to the point where the efficiency was down around 30-40%.   I think a 2-tone IMD test would show this quite clearly.

The only way might be using a controlled carrier screen mod scheme, but that has its disadvantages too.


T

And here I thought the Klipsch Corner Horns a friend of mine had years ago where the ones.

A horn you can live in is what you got there!!!


--Shane
KD6VXI