The AM Forum

Hi Guys.... I've been working on a small amp project to get a bit more power on 160 meters so I decided to build the amp with 160/80/40 meter capability. Check out the photos. This is just the RF deck. I'm already drillin' n blastin' on the power supply.

This "lockdown" has created time to do stuff!!

Very nice looking rig, nice workmanship, looks like a very fun project, keep us posted..
AG5UM

Looking good I want to look under her skirt.

 :) N8QPC

Very nice, Buddly!

I can see the Dentron influence with the orange coils... :-)


You made the right decision about multi-band capability. Nothing worse than a one-band rig that gets little use. Now you can row around the bands as the seasons change.

Maybe a Variac or resistor for the  muffin fan will make it real quiet.

Yes, let's see the big bottom!


Good chatting with you on the air yesterday, OM.

T

I shuda known there were under-skirt lookers here! :)

OK, here's the underside. Left to right;

Relays - switch T/R and bias from cutoff to operate.
Bias diode - 8V Zener
Tuned input PCB for 160/80/40. spots available ifn I ever wanna go to other bands but probably not. I got the
         Henry 2K Classic for 20M on up.
Filament Choke - self explanatory!
Of course, the Tube sockets.

This bias diode is floating. This will allow me to put the grid current shunt in the power supply and use the PS for another triode GG amp. I won't hafta come up with meters for each deck. All metering will be on the power supply. I got a 3-500Z on 6 meters planned.

As for the fan Tom, it's a 24VDC fan. I'll be running it off the 12VDC supply I'm using for the relays and lights. I already tried it out. Quite acceptable.

Very nice Buddly. Should be a nice glow factor on that amp!

Buddly,

Underside view: Unless I am missing something....   I would hang some .01 filament choke bypass caps right at the choke input, where the choke wires first touch the ferrite. Shorter path to ground.  (or simply add additional caps there)

BTW, be sure to add a class "C" biasing  position (class B/C)  so you can run the amp very efficiently using the AM carrier as the bias.  I have a 4X1 linear that uses this technique and the AM efficiently is much improved.  Less plate color and the same IMD numbers.

T

nice looking amp .... looking forward to hearing it on 40M

reminds me of a Little Feat song about spot-check Billy ... he glued mirrors on the top of his shoes  :D ;D

Tom, you can't see'em but there are .01s on the choke input right at the terminal strip. The biasing idea is a great one.... thanks.

Hi Buddly,

Yeah, it's hard to see in the picture… is the "terminal strip"  at the rear panel or hidden under the choke?  If at the rear panel, then the leads are about 6" long to the terminal strip. Add .01's right at the choke to ground and decrease this length for RF. It just gives a closer bypass for the circuit to work against.

I assume you are using zero bias, grounded fil CT now?  Add some forward-biased series diodes in the fil xfmr center tap lead until the idle current is slightly below cutoff and you will have a good class C/B amplifer AM mode.

You probably already know this but just for the archives:  In practice I have been able to bias a linear amplifier far into class C for AM... it just took more and more RF carrier drive to bring it back out into class B for a clean signal. But there is a point where you see a big difference in efficiency while still having enough RF drive to do the job.  Experiment using a spectrum display on your SDR and you will be impressed what can be done for AM.  But, on ssb it will be all class C distortion since there is no AM carrrier to bias it more positively. Thus, there needs to be one (switched) bias position for AM and one for ssb if you use this method.


T

It wouldn't be hard to add a few .01s at the choke. Probably a gooder idea.

The present bias is coming from an 8V Zener in the cathode/B- path.(see pic) The 47K resistor holds it in cutoff during receive. I'll play around with some other zeners I have and see what will work best for a Class B/C setup. switching between the 2 zeners is an easy addition!

That should work FB.

What will you be using as an RF driver, the hi-fi Drake T4-X?


I've been toying with the idea of building up a single 813 in class C, plate modulated by a powerful but tiny class D solid state audio amplifier (Chinese cheap)  and winding up a large toroid core as a modulation transformer. (8 ohms to  4K)    A couple of the locals have beautiful sounding transmitters using this audio modulation technique.  I still need to axe a lot of questions to get up to speed on it.

Just thinking about it as a cool summertime rig. 

T

Well, on 40 meters it'll be the Drake most of the time. On 75 and 160 I plan on using the Derb Memorial FT101E!

Nice looking amp Buddly.

Looks like the filament former is off the chassis? So taking 16 A, how long a run and what gauge wire?

On the filament RF choke, what is the perm? I used a toroid on a pair of 811A's GG and it worked fine. However, I ran a Z measure on some from the junk box and at the load current of 10 A, they did not play well. I think I wound up with a perm of 900 or so and 2 inch diameter toroid from Fair-Rite.

Alan

Hi Alan...… yes, the perm on this one is in the area of 800-900 also. I've used it before on other 572 amps and everything was fine. This one is a 2 3/8" dia. Fair-Rite #31 mix and has a few more turns to be a bit more effective on 160 meters.

And yes, the filament transformer is on the power supply chassis. The connecting wires are #8 and only 18" long.
My line voltage runs a bit high most of the time so the small voltage drop will probably be more beneficial than not!

It's on the air! So far no smoke!
Pictures are a bit fuzzy from my cellphone but 2400VDC plate voltage and 1KW out on 40 meters.

(don't know why the pics got turned?)

In free air, you could get by with #14 wire.

I doubt you'll see much, if any voltage drop with #8.  #12 would be fine in an enclosed sheath.  You've upsides your wire twice!

Will be nice and stiff, though.  No imd increase due to yoyoing VFil.

--Shane
KD6VXI

Looks nice and sounded great on 40m today. Nice to work you after so many years, Buddly.

Congratulations Buddly. That's a nice looking amp!

A Dentron HV meter and orange coil.   Denny would be proud!

You have enough extra room in the PS to rent it out.  A rare thing around here.

I like the cool trip and reset option there Buddly.

Now run some 2-tone IMD tests and see where it starts to fall apart so you can run it within that limit.  If you can hold better than -35DB 3rd IMD at 1KW out, that would be very impressive. (Probably 1KW at -30DB....   or -35 DB at 500 watts is more realistic)    I'd be very curious.

If you don't have an SDR receiver to see the spectrum, just tune in your own signal on a receiver with a good S meter to measure the 3rd order peak.

T

Saw Denny at the Richmond FrostFest in February.

Here's a few more looks including the under-skirt of the power supply.

Buddly,

I heard you on 3885 tonight using the new amlifier at about 8:30 PM tonight. You had a nice high end, hotter than usual.  I like your new sound.

Your major power band audio bandwidth looked well contained at +- 5 KHz on my sdr. 

The amplifier stayed clean as you increased power and modulation level, though I wud need a louder signal to evaluate any further.  The 572B is a FB linear tube.

Good job, OM!

T

Thanks for the report, Tom! Check is in the mail OM!

Thanks Steve, yes it does add a nice glow during night ops!

Congrats on the hi hi FB build Budly. Looking gewd. 8)

like the sloped front rack!