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

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kvdmeulen900:

--- Zitat von: Laurent am  3.02.2021 06:02 ---Hi,

With about 280-290V plate voltage on the plates of a EL84 in SE, that will work without troubles. If you go higher, as Dimashek says, the dissipation will rise so biasing colder will be needed. Moreover if you increase the plate voltage too much, you will need to have a higher reflected impedance on the primary of the output transformer, which may make the sound muffled compared to less reflected impedance.
So keep plate voltage around 250-285V and everything will be fine. In the shop, the small RKT 30VA of the shop should be your best bet.
Cheers,
Laurent

--- Ende Zitat ---

I understand, thank you! So, if I calculate correctly, the RKT 30VA will output about 1.4 * 200 = 280ish volts @  85mA / 1.4 = 60mA if it were ideal. However, as dimashek stated it's closer to 200 x 1.3 = 260 volts @ 85mA / 1.3 = 65mA.
Since I'll be drawing very close to that amount of current, I'm still a little worried the small 30VA will get too hot or fail... would it be possible to bias the power tube on the cold side to make it consume less current? So I could, say, incrase the cathode resistor a bunch so it draws less current?


--- Zitat von: iefes am  3.02.2021 09:41 ---I didn't realize you're planning a SE amp, apologies for that!  :facepalm:  I should have read your entry post more carefully.
In this case I'm with dimashek and Laurent  :topjob:

--- Ende Zitat ---

No problem! Thank you for your advice nontheless  ;D

Laurent:
 ???
The 30VA used to be rated at 200V@100mA...
With 85mA that will indeed go onto the safety factor.

kvdmeulen900:

--- Zitat von: Laurent am  4.02.2021 06:26 --- ???
The 30VA used to be rated at 200V@100mA...
With 85mA that will indeed go onto the safety factor.

--- Ende Zitat ---

Indeed, I remember them being 100mA as well! I do still have one of those laying around, however the 6.3V taps can only supply 1,5A. This means I’d be close to the max rated heater current instead. All tubes combined I believe I’ll be drawing around 1360ish mA. So that’s another potential problem :’)

However my guess was that if I enlarge the value of the cathode resistor on the power tube, current draw would diminish. Is this assumption correct?

Laurent:
No issue with your heater current as long as you don't implement DC heaters for the preamp. The "old" RKT 30VA was designed with that in mind. I think it is also used in the small Jim amp.

If you raise Rk, the current will decrease as you will bias the tube colder. But the sound may sound more "steril". And you will reduce the Headroom.

kvdmeulen900:

--- Zitat von: Laurent am  4.02.2021 08:20 ---No issue with your heater current as long as you don't implement DC heaters for the preamp. The "old" RKT 30VA was designed with that in mind. I think it is also used in the small Jim amp.

If you raise Rk, the current will decrease as you will bias the tube colder. But the sound may sound more "steril". And you will reduce the Headroom.

--- Ende Zitat ---

I understand! Thank you very much for your advice! :)

I think I’ll go for that old 30VA I still have. I was just a little concerned since the heater current is so close to the maximum current the 30VA can supply. I read elsewhere that a safe margin of about 20% is advisory.

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