• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Load For PSU

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Just wondering if someone could help me out with some advise regarding correct loading of a power supply.

I am building a push pull amp circuit around 40-50W with 1 x 12AX7 1 x 12AT7 & 2 x 6L6GC tubes but first up I want to test my power supply with the correct loading.

Plate voltages for the 12AX7 & 12AT7 will be 250-300v & 450-500 for the 6L6GC's

Could someone be kind enough to run me through the proceedure to calculate my load resistances etc as I would like to load & test my power supply correctly & learn how to do it first before I proceed any further.
I understand what heater voltages & current I require.
Thanks aardvarkash10,

Yes I have seen the PSUDII program, unfortunately my power supplies are very unconventional & more suited to LTSpice sims hence the reason to learn about the correct loadings required. It is something I want to learn more about if you or anyone else can help me with it.

I have attached the basic Guitar Amp circuit that I am going to use although it is incomplete at this stage as I have to add Reverb & a few other things.
The Reverb etc will be run from another small power supply though.

I have only pieced this circuit together from other Idea's etc I have found on the Net, I think the original schematic was from Grant Wills.
I'd appreciate it if you could have a look & see if you can find any errors etc in it.
Take no notice of the tone control stack as I haven't drawn the Potentiometers in correctly at this stage as I have been working on my power supply as mentioned first.
One step at a time!



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Ok let me see how I go here,


Just from the figures I get in the LTSpice sim at this stage with a 500Hz signal through the circuit I get:

480v DC at the centre tap on the OPT Primary the "Average plate voltage" is now 472v or 531v RMS, so I am unsure which figure I should use for the Impedance--Average or RMS values.

Average current is 128mA or 189mA RMS, again not sure which one to use.

For Average Voltage & Average Current I get Z=V/I = 472v/128mA = 3687 Ohms

Or for RMS values I get Z =V/I = 531/189 = 2809 Ohms.

If I have that correct.

This is for one output valve & one side of the Centre Tap of the OPT so I guess you just double it for both, this I am unsure of also.

By Bias of the output tubes do you mean the negative voltage bias which is -50v.

Sorry for all the confusion on my part, just learning but I want to do it correctly!

Thanks for you time.

nope - you are complicating it. Figures I am looking for are at idle - no input. Not too interested in what Spice calculates.

You should by now have drawn loadlines all over a graph of the 6L6 anode curves. The loadlines will be based on the impedance of the OPT, the voltage across the 6L6's (anode to cathode), and the bias voltage of G1.

If I'm being a bit cryptic here, go take a look at valvewizard and follow the links to push-pull output. Merlin explains it well (for me anyway!)
Thanks aardvarkash10,

I appreciate your input as it has made me aware of various things, I have only just been reading up on Load Lines etc & have started plotting lines etc.

Being cryptic in this situation has also helped me out as I have read a lot today & learnt heaps.

I will have a look at the site you supplied the link to & read some more, it's all very interesting.

I'm sure i'll have some more questions but i'll read up first & then get back to you.

Thanks for the advice so far.

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