[Amps] Overdrive protection

[Mike Tubby]

At the risk of being somewhat controversial ... why are we using ALC 
these days?

I have many radios but currently use an FTDX5000 on 6m and transvert to 
2m.  None of my tube or solid-state amplifiers have ALC. Even on HF with 
my trusty TL922 I do not use ALC.

My approach has always been to tune up the amp at full power on CW/FM, 
switch to SSB, reduce drive to around 2/3rds.  My 8877 on 6m produces a 
very nice 1000-1100W with about 30-35W of drive.  I can use plenty of 
speech processing and a Heil headset with a "DX" element to create a 
"wall of sound" type signal for contesting and yet the signal is narrow, 
has almost no splatter and doesn't suffer overshoot or need ALC to hold 
it back.

Basically I'm using the 2/3rds rule - most so-called "linear" amplifiers 
are not... at best they're approaching linear for the first 2/3rds of 
their output (to around 65-70% of their rated output) after this they 
become increasingly RF compressors. The 1dB compression point on some 
amplifiers is shocking.

I cannot see the point of (over) driving an amplifier and effectively 
using ALC to "hold it back" ... it just seems to me that if you're doing 
this then your amplifier isn't big enough in the first place - to this 
end I have to say I'm with Mr. Measures... a 3CX3000 or 3CX5000 
under-run at 1500W PEP is just going to be SO SO much nicer than a small 
amp thrashed to within inches of its life ;-)

Leif, SM5BSZ, has done some work looking at an FT1000D's output and the 
affects of ALC:

http://www.sm5bsz.com/dynrange/alc.htm


With solid state amplifiers things get more interesting again, since you 
don't tune them up in the traditional sense.  My experience with 
building single and double BLF578 amps for 144MHz has shown that there's 
a significant trade-off between gain, efficiency and linearity depending 
which you pick to match on the load-line. Its the difference between 
920W out at an apparent 73% efficiency (with poor IMD) or 800W out at 
63% with good IMD... if you wan the magic kilowatt then use two of them 
to get 1200W with each "loafing" at 600W.

The difference dropping from 1500W -> 1200W output is less than 1dB - a 
fraction of an S-point but can make your signal 6-10 times cleaner and 
reduce your electric bill.

I wonder how many people on here run their amplifiers at below the 1dB 
compression point? and without ALC?


Mike G8TIC/M0VHF




On 26/07/2012 21:50, David Jordan wrote:
> Isn't this a really old concern?  I remember having an FT-1000D where I
> noticed that instantaneous spike on key-down.
>
> I noticed in the menu of the Kenwood TS-480SAT you can select the response
> time of the transmit relay.  I always thought that was Kenwoods approach to
> solving this concern?
>
> 73,
> Dave
> Wa3gin
>
> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
> Behalf Of Richard Solomon
> Sent: Thursday, July 26, 2012 4:39 PM
> To: amps at contesting.com
> Subject: Re: [Amps] Overdrive protection
>
> Not "all" of us know which they are ... is there a list somewhere of those
> rigs that exhibit this nasty problem ? Maybe if we boycott those rigs then
> the manufacturer will modify them.
>
> 73, Dick, W1KSZ
>
> _______________________________________________
> Amps mailing list
> Amps at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps