[Amps] 200-ohm tank and "un-un" in HF amp design?

[Jim Thomson]

Date: Tue, 22 Oct 2013 15:00:28 -0700 (PDT)
From: Dennis OConnor <ad4hk2004 at yahoo.com>
To: "amps at contesting.com" <amps at contesting.com>
Subject: Re: [Amps] 200-ohm tank and "un-un" in HF amp design?


### Some of these folks oughta go back to whatever university they spent huge $$$  to attend,
and ask for their $$ back !   Ditto  with  RF consultants.  Strap the bandswitch correctly as per
my previous post.  DON’T use braided wire for the various taps, use silver plated  cu strap. 
Install a 100 pf NPO cap between chassis and cathode.   Install correct sized coil between plate
block caps and  C1 tune cap. Presto, eff on 10m skyrockets.  That  HF-2500E ..with the 3 tubes
is supposed to do 3500w out. 1200w out on 10m sucks. 
Install the above tricks....and you will almost triple the PO on 10m.   Heck, even the 11m folks have
this all figured out..its all old news.   You also need bigger ga wire for the 10m coil on the tuned input,
and don’t get the tuned input Q  too high on 10m either.   If you actually measure the PO  of the 
PI tuned input with too high a Q.... you will see that the PO is way down  vs the  power applied to
the input of the tuned input.   I had that problem on my hb 3CX-3000A7 amp on 15m.  With
200 w into the tuned input... I measured just 160W  on the output side..into a dummy load...
using calibrated wattmeters  on each side of the tuned input.   The fix was to increase the 
uh a tiny bit.... by tapping the coil by an extra one quarter of a turn on 15M.   This was
a manually tuned..tuned input..with a tapped 4 uh coil, used on all bands.   Coil was made
from 6 ga wire. PO rose to 195 watts  on 15M.    Ok, now way more drive to the cathode,
more grid current, more plate current..and more PO.    A 17 turn coil with a 1.5 inch ID was used
to make the 4uh coil.    Two broadcast caps used for the C1 + C2 caps. 


Jim   VE7RF




Roger - Palstar is no longer in the amp business.
I have the first HF2500E he built. I flew down there a few times to watch the build. 

I flew there to pick it up on a Friday afternoon when they told me they had a 'small problem' with uneven power output.? He did demonstrate that on 20 meters the amp would make his dummy load beg for mercy.

Said he had a call in to a big name amplifier designer (commercial stuff) to discuss it Monday morning and I should call him on Monday afternoon.
Called on Monday and they had given up, didn't know what to do.The consultant said it should produce power and he could not see a problem on the schematic.

What's the problem - I asked - because you are sitting on thousands of dollars of my money and I have burned a bunch of avgas and I still don't have an amp and I'm getting annoyed? 

So, then he told me that it would not tune much above the bottom of the ten meter band without losing power - would only make about 1200 watts at the top of the band.
Oh that - I said - why the hell didn't you say so in the first place, sheeesh.

So, I told him how to fix it and I would be down on Friday afternoon to pick up my amplifier for CQWW the following weekend.
I'll let the list figure out what it was just from my sketchy description and how I had them fix it. - most of you should be able to do so.
Anyway, Bill lost money on the redesign (super good design job, too) and wasn't selling enough amps to cover the payments to Commander for the rights so he returned the rights to Commander.
Anyway, it is a great desk top amp. Pumps out power quietly.

?
denny / k8do