Dear Felipe,
What a coincidence! I just got this quarter-century old amp with
obsolete tubes, and almost at the same time, the CQ DL publies an
article about a similar design!
I have mailed the DARC to see if I can obtain a copy of the article.
Thanks to you for the info.
About the Fuchskreis:
The tank circuit of this amp is made of just a variable capacitor and a
coil in parallel.One side to the plate cap. The other side to ground.
Output coupling is taken from a tap on the coil.
Bandswitching is not for the fainthearted!
There is a hinged subpanel on the amp front panel that gives access to
the tank circuit. To switch bands you open the subpanel and (yes!), with
your bare hands, you modify the position on the coil of two banana
plugs. The coil is made of 10 mm thich tubing and each turn has a hole
on it to fit a banana plug . One link to ground, the other one to the
amp output. The unused part of the coil is not terminated.
Sounds a bit scary, but on the other hand (no pun) the coil is kept at
ground potential at all times by one of the links and there is "only "
300 volts between anodes and ground (600 from anodes to cathodes).
Compare that with 3000 V for a GS35B for example. The plate capacitor
looks sturdy enough. As a safety measure, you could also temporarily
clip a jumper between the coil and the outside of the chassis while you
shift bands. The wire would keep you from closing the subpanel when
you're done, so you would not forget to remove it.
Electrically, I tend to think that this circuit should have basically a
high-Q, mostly depending on where the output tap is located. The
diameter of the tubing used for the coil is quite large, so there should
not be excessive heating of the coil. I will see if stability is an
issue when I try it. VHF parasitics should not be a problem with these
sweep tubes. Their efficiency at 30 MHz is already rather poor.
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!
Vy 73 de Angel Vilaseca HB9SLV
Felipe Ceglia - PY1NB a écrit :
> Hi Angel,
>
> I one recent CQ DL magazine, may 2009 (the one with the CW op YL on
> cover), there is a project for a funf-tube PL519 amplifier. If I
> recall it correctly it does not use a HV trafo, but for sure the
> output tank was PI.
>
> If anyone have any information about the Fuchreis tuning network, I
> would like to hear about it.
>
> 73,
>
> Felipe - PY1NB
>
> Angel Vilaseca wrote:
>> Gentlemen,
>>
>> I just bought an old amp that uses 5 PL 519 TV sweep tubes in parallel.
>> As it reads on the box, it was home made on June 1983 by HB9 AWI.
>> The amp does not work anymore, but it is superbly built. The builder,
>> HB9AWI, obviously put a lot of effort into it.
>>
>> I would like to restore it. It is so well made and so typical of ham
>> state of the art at the end of last Century.
>> No inexpensive Russian power tubes were obtainable at these cold war
>> times! On the other hand, old color TV sets with sweep tubes still
>> alive were plentiful. The PL519 had the reputation of being quite
>> rugged. Most frequently, it was the line output power treansformer of
>> the TV set that failed, not the tube.
>>
>> Unfortunately, there was no schematic included with the amp..
>>
>> The seller told me the design of the circuit was by DL9AH. It is
>> quite a non-typical design:
>> For one, the tank circuit is not a Pi, nor a Pi-L! It is called a
>> Fuchkreis. It seems it was invented by an Austrian ham called Fuchs.
>> I never saw anything like it.
>> The power supply is a transformerless design. Apparently, it delivers
>> + and - 300 Volts. So needless to say, the cathode circuit of the
>> five PL519s is quite unconventional too. The bias circuit uses two
>> pentodes, a PL 504 (a b&w TV sweep tube) and a smaller one. Never saw
>> anything like this either!
>> There is a separate 10-turn potentiometer for each PL519 to adjust
>> the current through each tube. Quite sensible when you use five
>> second-hand tubes, that may have already lived a number of years as
>> sweep tubes in TV-sets.
>> The power supply, the bias circuit, the relays circuit all have lots
>> of VK200s and decoupling caps everywhere.
>>
>> As I just said, in a true ham spirit, it may use cheap tubes, but
>> obviously a lot of care was used to design and build this piece of
>> equipment.
>>
>> So, really, I would like to restore it. I tried finding the DL9AH
>> design on the web, but no luck!
>>
>> Does the phrase " Sweep tube amp, DL9AH design" ring a bell?
>> Especially for european hams?
>>
>> Any information about the design, or schematic welcome.
>>
>> Vy 73
>>
>> Angel Vilaseca HB9SLV
>>
>>
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>
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