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[2E0ILY]

Any tips on a bench test set up for this simple circuit? Whilst I am confident it should just work I am curious as to how I might check it out before installing it in the amp cabinet.  It's a cathode bias circuit for a valve (tube) linear RF amplifier. I tried with what I thought was a usable method, but the pot control suddenly reduces voltage as it approaches 1.85K and goes lower in resistance. I am using the add on 15V zener to achieve greater than the standard 3.5 to 35V range. Looking for a max of 45V. Thanks.

Schematic at htpp://www.gatesgarth.com/biasschematic.jpg

[2E0ILY]

Whilst the US was asleep I was given an idea to compare it with a GM3SEK Ian White Tetrode Board schematic, which has a very similar bias circuit built in.  I have a working GM3SEK Triode Board, so I dug out the manual. There are subtle component value differences between my home built one and Ian White's circuit for higher bias voltages. He too adds a 15V Zener and cap for this, to his "standard" schematic. On the home brew just built to the W4ZT schematic it has R5 at 100 Ohms and R2 at 680 Ohms. Ian White has R5 at 10K Ohms and R2 at 1K ohms. Substituting those values and it works fine with no sudden change at the end of the potentiometer low resistance end of the track. Happy as I am to have it working as expected I would like to learn from this, can someone say why it might not have worked as expected before please? Thanks.

[W9GB]

Triode Amplifier Bias Boards
by Tony King, W4ZT (SK, Feb. 2009)
http://bias.gs35b.com

W4ZT Amplifier Bias Board Documentation
http://bias.gs35b.com/docs/index.html

On the home brew just built to the W4ZT schematic it has R5 at 100 Ohms and R2 at 680 Ohms. Ian White has R5 at 10K Ohms and R2 at 1K ohms. Substituting those values and it works fine with no sudden change at the end of the potentiometer low resistance end of the track. Happy as I am to have it working as expected I would like to learn from this, can someone say why it might not have worked as expected before please?

Chris -

Tony had 3 different versions of this Triode Bias board in 2006.  
You can tell difference by numbers (design date) etched on foil side of his assembled PC boards.
http://bias.gs35b.com/docs/index.html

I provided the links (above) from the GS35b / W4ZT web pages maintained by
Tony's good friend, Ron Wilson, K4POZ (Marietta, GA).
http://w4zt.com
GI-7b sockets still provided by Ron, K4POZ.
http://gi7b.com/gi7b.html

Tony stated the following, about the last design of the W4ZT Triode Bias Board (060923 etched on foil).
http://bias.gs35b.com
This board is built using the TL431 Voltage reference chip and the TIP147 PNP power darlington transistor.
This circuit is right from the data sheet on the TL431 and is basically the same circuit used by Ian White, GM3SEK and Paul Hewitt, WD7S on their Triode Control Boards.
I built this board primarily as a retrofit for amps that already have a Zener diode in them or for the builder that isn't interested in a full blown control board. --Tony, W4ZT (2006)

R1 (10K pot) is in series with R5 .... permitting field adjustment of bias, as required for retrokits in older HF tube amplifiers.
http://bias.gs35b.com/docs/060210a.html
The 10K potentiometer, R1, provides a bias adjustment range of approximately 3.5 to 39 volts. For initial installation the potentiometer should be set for maximum bias to insure you start your adjustment at the lowest plate current. Fuse the circuit just above the maximum expected plate current.

[N8CBX]

It can be bench tested (and pre-adjusted) by using Ian's procedure in his Triode board notes.
Jan N8CBX

[2E0ILY]

Thanks to you both. I have the manual for the GM3SEK board and following Ian's method (which was actually what I was doing, thank God), it now works fine with revised R5 and R2 values.
I don't have to waste a complete Triode Board on just an adjustable bias now! All the best.

[N8CBX]

Just to throw out another comment, I'm designing a Triode control board (much like Ian's) using a small Arduino to do all the logic and alarm controls. It's a reduced version of Ian's board, lets say. I think I can get it down to a 3" x 4" board size.
Don't have any prototypes yet, and don't know how long it will take. Its slow going.
Jan

[GM3SEK]

 


73 from Ian GM3SEK