Simple curve plotter for directly heated triodes - UK Vintage Radio Repair and Restoration Discussion Forum

SurreyNick · 19th Oct 2014, 12:26 pm

I am pretty sure I have got the circuit right for this simple curve plotter for directly heated triodes but I would appreciate if someone would just cast their eyes over it before I wire it up. I don't want to risk damage to one of the meters.

The intention is to apply a maximum 200v HT and +/- 50v GB and employ pots with meters to adjust the voltage accurately and then read off the anode current (mA) to plot the characteristic curves.

Thanks
Nick

KeithsTV · 19th Oct 2014, 1:03 pm

The Grid circuit looks OK but you probably only need a negative supply of up to 20 to 30V. I wouldn't use the pot across the HT supply for the anode as the anode voltage will drop as the current increases. The pot would also be dissipating 4W with a 200V supply.

You need a low impedance source for the anode voltage. Have a look at the Sussex Valve tester circuit where the anode voltage supply uses a FET whose gate voltage is controlled by a series of fixed resistors giving predefined voltages.

I've plotted the characteristics of a triode using a variable HT supply for the anode and a variable low voltage supply for the grid. Takes a little time but fairly easy to do.

Keith

Al (astral highway) · 19th Oct 2014, 5:30 pm

Hi Nick,

You need to have a load in your anode circuit.

Also, yes, take note of the the previous comments about HT supply regulation.

And you've done a slightly odd thing there as you have placed the full R of your variable resistor right across your battery power supply, drawing a fixed current even on standby.

Cheers,

Restoration73 · 19th Oct 2014, 5:58 pm

I do not know of the power supply, but may I suggest one or two fuses in series just in case the valve has an internal s/c

SurreyNick · 25th Oct 2014, 11:26 pm

Hi. Thanks for the feedback on the circuit and my apologies for not acknowledging them sooner. It's been a tough week and my attention has been elsewhere. I will make the suggested adaptations.
Nick

kalee20 · 25th Oct 2014, 11:33 pm

It'll work - the points mentioned by previous posters are valid improvements - but you need to be careful when using it.

As shown, it's quite possible to apply full HT and zero bias, just by twiddling the controls. You can easily exceed maximum anode current and dissipation by doing so, which will not do your valve any good. While valves are by no means as fragile as transistors, at the higher currents I'd suggest you have a switch in the HT supply. Set the bias, switch on the HT, take the current reading and HT voltage, and switch off again - half a second only of high current.

SurreyNick · 26th Oct 2014, 2:15 pm

Will do. Thanks kalee20.
Nick

19th Oct 2014, 1:03 pm	#2
KeithsTV Nonode Join Date: Aug 2007 Location: South Bradford, West Yorkshire, UK. Posts: 2,573	Re: Simple curve plotter for directly heated triodes The Grid circuit looks OK but you probably only need a negative supply of up to 20 to 30V. I wouldn't use the pot across the HT supply for the anode as the anode voltage will drop as the current increases. The pot would also be dissipating 4W with a 200V supply. You need a low impedance source for the anode voltage. Have a look at the Sussex Valve tester circuit where the anode voltage supply uses a FET whose gate voltage is controlled by a series of fixed resistors giving predefined voltages. I've plotted the characteristics of a triode using a variable HT supply for the anode and a variable low voltage supply for the grid. Takes a little time but fairly easy to do. Keith

19th Oct 2014, 5:30 pm	#3
Al (astral highway) Dekatron Join Date: Nov 2006 Location: London, UK. Posts: 3,496	Re: Simple curve plotter for directly heated triodes Hi Nick, You need to have a load in your anode circuit. Also, yes, take note of the the previous comments about HT supply regulation. And you've done a slightly odd thing there as you have placed the full R of your variable resistor right across your battery power supply, drawing a fixed current even on standby. Cheers, __________________ Al

19th Oct 2014, 5:58 pm	#4
Restoration73 Nonode Join Date: Jun 2013 Location: Surbiton, SW London, UK. Posts: 2,801	Re: Simple curve plotter for directly heated triodes I do not know of the power supply, but may I suggest one or two fuses in series just in case the valve has an internal s/c

25th Oct 2014, 11:26 pm	#5
SurreyNick Heptode Join Date: Apr 2014 Location: Wigton, Cumbria, UK. Posts: 728	Re: Simple curve plotter for directly heated triodes Hi. Thanks for the feedback on the circuit and my apologies for not acknowledging them sooner. It's been a tough week and my attention has been elsewhere. I will make the suggested adaptations. Nick

25th Oct 2014, 11:33 pm	#6
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,082	Re: Simple curve plotter for directly heated triodes It'll work - the points mentioned by previous posters are valid improvements - but you need to be careful when using it. As shown, it's quite possible to apply full HT and zero bias, just by twiddling the controls. You can easily exceed maximum anode current and dissipation by doing so, which will not do your valve any good. While valves are by no means as fragile as transistors, at the higher currents I'd suggest you have a switch in the HT supply. Set the bias, switch on the HT, take the current reading and HT voltage, and switch off again - half a second only of high current.

26th Oct 2014, 2:15 pm	#7
SurreyNick Heptode Join Date: Apr 2014 Location: Wigton, Cumbria, UK. Posts: 728	Re: Simple curve plotter for directly heated triodes Will do. Thanks kalee20. Nick