31-07-2015, 05:33 AM
Just a quick note, my post regarding calculations as regards a rheostat, they were posted to illustrate a point (see post #81)
Lawrence.
Lawrence.
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A Simple, Valve Curve Plotter
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31-07-2015, 06:00 AM
Regarding using DC for the heaters, would feeding directly directly heated valves with AC for the heaters affect curve plotting, eg: a directly heated rectifier with AC heaters doesn't put the heaters AC waveform on to the HT line because it isn't referenced, or am I wrong.
Lawrence.
31-07-2015, 08:16 AM
(31-07-2015, 05:33 AM)pwdrive Wrote: Just a quick note, my post regarding calculations as regards a rheostat, they were posted to illustrate a point (see post #81) Speaking personally, I know almost nothing about valves, so wasn't really across the the range of heater currents and voltages that needs to be covered. Therefore, I thought it was very interesting, and I think it demonstrates nicely why something more sophisticated than a simple rheostat is required 
31-07-2015, 09:31 AM
Maybe one of these:
http://www.vvttransformers.co.uk/vth1221...asheet.htm Not sure how much they are. Secondaries: HT: 250-0-250v Heaters: A) 0-4-5-6.3v " : B) 6.3v center tapped. 1.4v and 2v valve heaters could be done via pot via from one half of the 6.3v winding (3.15v) If Ac can be used for normally DC heater valves then this transformer might do...Maybe. Not trying to convince anyone, just putting ideas into the hat. Lawrence.
31-07-2015, 09:55 AM
(31-07-2015, 09:31 AM)pwdrive Wrote: Maybe one of these: Crunching some numbers on the above: 1.4v heater at 25 ma, R = 70 ohms, Pd = 175 mw 2v heater at 250 ma, R = 4.6 ohms, Pd = 287 mw A 100 ohm pot might do and the derating might be ok, failing that a couple or so of switched resistors and a lower value pot, that would give an even better chance regarding derating the pot. All other heater voltages would be straight off the transformer, the two 6.3v windings could be switched in series for valves with 12v heaters. Just another thought. Lawrence.
31-07-2015, 10:27 AM
Not sure if this one's been mentioned before. Get a small toroidal transformer, the sort with a hole in the middle. Then wind your own secondaries through the hole. A bit of experiment needed but it won't be more than a few turns for each volt of output. Bring out windings to clearly marked sockets and voila, Robert is your aunt's husband
 If you're rectifying and smoothing the output this will need fewer tuns per volt than if you're using the AC direct.If I wanted a set of fixed voltages rather than fully variable I'd likely go for this approach. I can probably let you have a toroidal transformer for the cost of postage.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
31-07-2015, 10:56 AM
VVT Transformers are closing down.
 Lawrence.
31-07-2015, 06:52 PM
I have just started work on the circuit design. I will rectify and smooth the outputs, but I'm wondering if the slight ripple will have a detrimental impact on the valve tests. Does anyone know?
If I have to use a regulator for one or more of the circuits (heater, plate, control grid or screen grid) then I'm going to need anything from 30v to 240v. Is there an IC that can manage 240v? The highest value IC I know of is the LM317 and that can only be turned up to 57 volts! Thanks Nick
31-07-2015, 07:43 PM
According to an article an IRF740 with a few components should do the variable job but it won't be regulated, there's a bit about it here:
http://www.eleccircuit.com/the-variable-...ly-0-300v/ I wouldn't have though a small ripple would affect much. Just for something to do I did a quick calc for a 4 volt heater at 1 amp running via a 20 ohm 50 watt pot divider across a 6.3 volt AC source, at first glance the pot derating seemed to fit ok but maybe someone else can check to make sure just in case I got it the wrong way round. Lawrence
31-07-2015, 08:08 PM
Yeah, that circuit looks like it'll work Lawrence. Thanks
I have a variac, which I bought some time ago and which I could use for this curve plotter, but I have it in mind for another project downstream. For simplicity too I am going to have a go at making this little unit with a 50W 50ohm wire wound ceramic pot in the heater circuit and we will see how it copes during a soak test on a breadboard. If it doesn't work then no harm done, I can move to plan B. Then C. Then D. Then...... LOL Nick |
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