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 [mailto:avilaseca@bluewin.ch]
Sent: Tuesday, June 30, 2009 10:10 AM
To: Alex Eban
Cc: dezrat1242@yahoo.com; felipe@dxwatch.com; amps@contesting.com
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
circuits
> a better choice would be a link coupled tank circuit or a multiband
circuit
> similar with the Z-Match innards. This is exactly what it was designed
for!
> At the impedance levels encountered in multitube setups- a few hundred
ohms-
> 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: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of Bill, W6WRT
> Sent: Monday, June 29, 2009 8:41 PM
> To: Angel Vilaseca
> Cc: felipe@dxwatch.com; amps@contesting.com
> Subject: Re: [Amps] Sweep tube amp by DL9AH, made by HB9AWI
>
> ORIGINAL MESSAGE:
>
> On Mon, 29 Jun 2009 11:26:01 +0200, Angel Vilaseca
> <avilaseca@bluewin.ch> 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!
>>
>
> REPLY:
>
> 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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> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
>
>
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