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Re: [Amps] Sweep tube amp by DL9AH, made by HB9AWI

To: "'Angel Vilaseca'" <>
Subject: Re: [Amps] Sweep tube amp by DL9AH, made by HB9AWI
From: "Alex Eban" <>
Date: Tue, 30 Jun 2009 14:42:14 +0300
List-post: <">>
That was done in order to avoid VHF ring oscillation, a common problem with
paralleled tubes.
They ha d a tendency to oscillate at a frequency set by the tray inductances
of the tank circuits and the output capacitance of the tubes. This was the
builder's way to reduce the stray capacitances to a minimum. Bear in mind:
this is not the same as parasitic suppressing! This is intended to avoid
problems stemming from another mechanism.
We did something similar with an 8 tube amplifier producing 150kW for a
short wave radio station.
Alex    4Z5KS

-----Original Message-----
From: Angel Vilaseca [] 
Sent: Tuesday, June 30, 2009 10:10 AM
To: Alex Eban
Subject: Re: [Amps] Sweep tube amp by DL9AH, made by HB9AWI

Hi Alex,

An interesting idea, but since there is plenty of room in the tank 
circuit compartment of the amp, I think I will first try replacing the 
Fuchskreis with a pi-circuit, if the anode impedance of the 5 pentodes 
in parallel allows it.

There is another enigma with this amp: Instead of having the 5 anode 
nipples connected together with pieces of wire, there is an angled plate 
about 2 x 3 inches above the tubes and 3x3 inches on the side made of 
sheet metal and PCB that connects the anode connectors together and to 
the plate cap and HV coil. Never saw such a thing!
I wonder if this is for matching purposes, to add some capacitance from 
anodes to ground? Quite awkward I think, especially considering that the 
whole plate is at anode voltage!

Thanks for your comments

Vy 73 de Angel Vilaseca HB9SLV

Alex Eban a écrit :
> ...if anyone wants to get away from the tested and true PI or PI-l
> a better choice would be a link coupled tank circuit or a multiband
> similar with the Z-Match innards. This is exactly what it was designed
> At the impedance levels encountered in multitube setups- a few hundred
> you get very reasonable "Q"'s and you have to switch only the output side
> coupling coils (relatively  low voltages). 
> Alex     4Z5KS 
> -----Original Message-----
> From: [] On
> Behalf Of Bill, W6WRT
> Sent: Monday, June 29, 2009 8:41 PM
> To: Angel Vilaseca
> Cc:;
> Subject: Re: [Amps] Sweep tube amp by DL9AH, made by HB9AWI
> On Mon, 29 Jun 2009 11:26:01 +0200, Angel Vilaseca
> <> wrote:
>> Advantages of this tank circuit are: simplicity, no bandswitch contacts 
>> to burn nor associated wiring to cause stray resonances.
>> Potential inconvenients I can think of : Safety issue when 
>> bandswitching, Stability?
>> Only trial and experimenting will tell!
> This is an ancient design and was pretty much abandoned years ago for
> good reason. Take a look at handbooks from the '30s and '40s and you
> will find it there. There are two major drawbacks to it:
> 1. Harmonic suppression is poor. The antenna is connected across a
> coil (or part of one). At higher frequencies (think TVI), the coil
> acts as a simple high-pass filter, just what you don't want. Compare
> this to a pi-network where the antenna is connected across a capacitor
> and just the opposite happens.
> 2That alone should be a show-stopper, but there is one more.
> 2. Band switching requires switching two taps for each band. Again,
> compare to a pi-network which only switches one tap. 
> As soon as the pi-network became well understood it almost totally
> replaced the tapped-coil output. TVI reduction was a major factor in
> adopting the pi-net, as you will note in discussions of the circuit in
> handbooks of the early TV era. Another mechanical factor is that a
> variable capacitor is much simpler and more reliable than a variable
> (or tapped) coil.
> 73, Bill W6WRT
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