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[Amps] Russian GU5B: anyone used this tube?

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Subject: [Amps] Russian GU5B: anyone used this tube?
From: Dave white <>
Date: Mon, 29 Jun 2009 11:50:47 +0100
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Hi all
Has anyone used this tube at all? Anyone got experience of building or 
operating an amp with this particular bottle?
Dave G0OIL

-----Original Message-----
From: Angel Vilaseca <>
Sent: 29 June 2009 10:26
Subject: Re: [Amps] Sweep tube amp by DL9AH, made by HB9AWI

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.

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