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Power Transformer for AX84 Hi-Octane with LND150 FX-Loop

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kvdmeulen900:
Hello dear people of Tube Town!

First off, I apologise for posting in English instead of German. I'm from the Netherlands and I could attempt to write a bit in German but I fear it would be a mess - and I'd probably cause a laugh or two among you :') Anyway, on to the issue at hand.

I'm in the process of assembling an AX84 Hi-Octane and wanted to implement an LND150 FX-Loop from the wiper of the treble potentiometer (send) to the 1st lug of the master potentiometer (return). I'm wondering if the power transformer I selected for this project is too small or maybe just on the safe side. I chose a 30VA toroidal transformer which provides 200V @ 0,085A. If I do the math, counting current consumption of all the tubes and two LND150 mosfets, the total current consumption would be:

12AX7, four stages = roughly 4,8mA
EL84 = roughly 53,5mA (screen current + plate current)
LND150 Idss = 2 x 3mA
Which makes a total of 64,3mA.

Since 64,3mA is a bit close to the maximum current rating of 85mA, I'm a little worried if that poor little transformer is going to give in when I turn the amp on eventually, due to inrush current feeding the filter caps etc. I'm also worried it might run hot after running for a while, since it'll constantly be supplying that current because it's an SE amp.

I've looked at an alternative, which would be a 50VA toroidal transformer but I noticed that the secondary voltage is 250 volts. I did some research and I concluded that after bridge rectification the DC voltage would be 1.4 * 250 = 350 volts. I figured this is a bit too high and am unsure if the voltage dropping resistors would drop enough of that voltage. I'm afraid the voltage might still be too high and that it'll potentially be harmful to the tubes (particularly the power tube). I'm planning to use a TT PSU Mini for rectification and filtering.

Can I safely use the 30VA toroidal or should I opt to upgrade to the 50VA toroidal? If I upgrade to the 50VA toroidal, would the dropping resistors in the PSU Mini drop the voltage sufficiently? Or would I have to increase resistance there as well?

If one of you could enlighten me on this issue, I'd be delighted :)
Thank you in advance!

Best regards,

Kerin

dimashek:

--- Zitat von: kvdmeulen900 am  2.02.2021 17:21 ---30VA toroidal transformer[/url] which provides 200V @ 0,085A.

--- Ende Zitat ---
The 200V 85mA are AC!
If you rectifier it, (assuming perfect tranny and diodes) you will have 200*1.41Vdc  and 85/1.41mAdc...
Voltage goes up, current down.  = energy conservation law  :)
So about 60mA

In real world, the voltage will not rise to 200*1.41, only to about 200*1.3.

So this small tranny could work.. but could get very hot
 

dimashek:
Dropping 50*1,4 Volts with 60mA current drow will dissipate 4.2Watt.
big >10W Resistor mounted on chassis will do the job quite well

You will benefit from better filtering of Uanode in this case!

kvdmeulen900:

--- Zitat von: dimashek am  2.02.2021 17:39 ---The 200V 85mA are AC!
If you rectifier it, (assuming perfect tranny and diodes) you will have 200*1.41Vdc  and 85/1.41mAdc...
Voltage goes up, current down.  = energy conservation law  :)
So about 60mA

In real world, the voltage will not rise to 200*1.41, only to about 200*1.3.

So this small tranny could work.. but could get very hot

--- Ende Zitat ---

Ah darn, you're right!! How could I not have thought of that? Thank you for your insight :)
I suppose it would be a LOT safer to use the bigger tranny in that case. However, I still fear the rectified voltage would be too high...



--- Zitat von: dimashek am  2.02.2021 17:46 ---Dropping 50*1,4 Volts with 60mA current drow will dissipate 4.2Watt.
big >10W Resistor mounted on chassis will do the job quite well

You will benefit from better filtering of Uanode in this case!

--- Ende Zitat ---

In that case, could I simply drop the 50 volts using a simple voltage divider? Or place another resistor in series in the PSU Mini after say, R3? And then feed the anode of the power tube (through the output tranny) with B2? And feed the screen with B2 as well (of course including the current limiting resistor..)?

EDIT:

After using this calculator, I determined the 470 ohm 5 watt resistor of the PSU Mini should be replaced with a 1k ohm 10 watt resistor in order to drop the voltage by approximately 65 volts.

kvdmeulen900:
Actually, I'd rather drop the voltage a little lower to be on the safe side for the poor EL84 which will be operating quite close to it's limit at 285V...
If I'd drop the voltage to ~260V, I'd need to replace R3 of the PSU Mini with a 1.35k resistor. Since these don't exist comercially, I could parallel two 2,7k ohm 5 watt resistors, correct?

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