[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]

30VA toroidal transformer[/url] which provides 200V @ 0,085A.

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]

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

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...

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!

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?

[iefes]

I wouldn't worry too much about the max-voltage of the EL84s plates. There are many amplifiers out there which run at anode voltages way above 300V. For example, the Mesa Boogie Studio .22 runs the EL84s at 390V on the plates, Fenders Blues Junior at 330V. My Carmen Ghia inspired amp also runs over 300V without problems.
So if you're not dependent on the lower plate-voltage (soundwise or for whatever reason) I'd just use the larger transformer and run the tubes at these higher voltages. 

[kvdmeulen900]

I wouldn't worry too much about the max-voltage of the EL84s plates. There are many amplifiers out there which run at anode voltages way above 300V. For example, the Mesa Boogie Studio .22 runs the EL84s at 390V on the plates, Fenders Blues Junior at 330V. My Carmen Ghia inspired amp also runs over 300V without problems.
So if you're not dependent on the lower plate-voltage (soundwise or for whatever reason) I'd just use the larger transformer and run the tubes at these higher voltages. 

390V, hot damn. I bet that Mesa burns through tubes rather quickly then.
Is there anything I should take into account regarding the higher voltage? For instance, should I up the power rating on the screen resistor from 1W to 2W?

EDIT:
I also wonder what effect the higher voltage will have on the sound of the amp. Will the poweramp behave differently in terms of overdrive?

[dimashek]

Higher Voltage makes sence for push-pull amps to get more power out of 2 small tubes, biasing it very cold
For SE amp, like yours - higher Voltage will give a bit more output, but will increase the power dissipation as well. So you'll need to low down the bias current. = less output on other halfwave.

[Laurent]

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

[iefes]

I didn't realize you're planning a SE amp, apologies for that!    I should have read your entry post more carefully.
In this case I'm with dimashek and Laurent 

[kvdmeulen900]

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

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?

I didn't realize you're planning a SE amp, apologies for that!    I should have read your entry post more carefully.
In this case I'm with dimashek and Laurent 

No problem! Thank you for your advice nontheless 

[Laurent]

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

[kvdmeulen900]

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

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]

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.

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.