question about change this demeter DI box to 220v

[paulster]

There is very little chance you'll have damaged the other transformer since this is connected to the 1/4" output jack and is a balancing transformer to drive the XLR jack. It isn't wired as an output transformer as you'd get in an amp so it doesn't get to see the high B+ voltage at all, only the capacitor-coupled output signal.

I think you'll have to go back to Demeter to get a replacement PT but you should find everything else intact since the area that would usually fail would be the power supply, and that would fail (usually) by caps popping because of over-voltage.

[kwm488]

thank you.
i will call demeter and buy new pt.
if not, maybe i pay for rewind the pt.

as the cap, i tested. all cap uf value correct. is it mean cap no problem.
how abot resistor? all resistor look good. should i need to desolder every resistor and test

[paulster]

What's the voltage rating on the 3 blue caps on the power supply board? These are the only ones I'd be concerned about, if you massively exceeded their voltage rating. They may well be 400V or 450V rated caps though and this circuit doesn't use a high B+ voltage since it is based off a 150Vac winding, so even 300Vac for a moment when rectified would give you no more than 424V. That would be fine on a 450V cap and is unlikely to kill a 400V cap, but would shorten its life a little.

One other thing to note is that whatever value the mains fuse is for 110V, you should have replaced it with one of half its value for 220V, since the current drawn will be half. Not that this would have necessarily saved your transformer, but for the ongoing safety of the unit you should make sure that it's been done.

[kwm488]

the blue cap is 400v . and it is use 0.5a fuse for 110v . when i plug 220v to this unit. i use 0.25 fuse. but i keep to use 0.25a fuse for 110v input now. the fuse not blow. it is strange.

i plan to upgrade this unit cap before. i will use wina and black gate. i think sound will better , do u think so. but black gate no 47uf/250v cap. can u recommend good brand to me.
so if the csp is hurt. it dosent matter. i just worry ot and my mullard 12ax7 hurt. this unit can sound ,and u mention no b+ to ot. so the ot should no problem,right

[paulster]

so if the csp is hurt. it dosent matter. i just worry ot and my mullard 12ax7 hurt. this unit can sound ,and u mention no b+ to ot. so the ot should no problem,right

If you imagine this unit without the XLR socket then you have a 1/4" input jack, a preamp, a line driver and then a 1/4" output.

The Jensen balancing transformer is connected with its (unbalanced) primary to the 1/4" output jack and its (balanced) secondary to the XLR jack, so the only way it could fail would be if a DC voltage had managed to get onto it and damage its primary winding. This would mean that the output coupling capacitor on the board would have to have failed.

You'll certainly have given the Mullard a hard time, particularly its filament. Once you get the transformer sorted out I'd try it with a new production tube and compare and contrast against the Mullard. Your ears will tell you whether it's good or not.

[kwm488]

i learn a lot from you .
the ot should fine. at least it xan sound, and output cap no fail.

i bought a one transformer today, it is 220v in , out is 12v ac 0.5a. i hope to use it to try to replace the heater wind before i get new pt. is it work? also i can try the sound result, i remember what sound of this unit before i demage it.

also, can recommend some good 47u 250v cap. in fact, i dont like xicon much.

as the mullard , i put it to my jcm800, i will get result.

thank this forum and you . you help me build marshall amp and this di unit

[paulster]

i bought a one transformer today, it is 220v in , out is 12v ac 0.5a. i hope to use it to try to replace the heater wind before i get new pt. is it work?

The drawing you did suggested that the Demeter transformer had a 150Vac (HT) and 6Vac (heaters) winding, so you can't use a 12Vac transformer instead.

They use 6V because they then rectify it and drop the voltage a little with the resistors between the black caps on the power supply board to get a smooth 6.3Vdc supply to the tube.

Whilst it's possible to run a 12AX7's heaters off 12.6V it's not going to be practical to do this because the heater connections are PCB traces.

also, can recommend some good 47u 250v cap. in fact, i dont like xicon much.

Rubycon, Nichicon and Panasonic all have those values in similar sizes that you'll be able to fit onto the board.

As it's a Class A device which will be always be drawing the same current I'd be more concerned with getting the level of filtering as high as you can rather than worrying about esoteric caps for the power supply.

Less ripple with a no-name capacitor will sound better than lots of ripple insufficiently filtered by Black Gates, so I'd try to fit the largest value caps you can within the real-estate you have for the power supply if you want to upgrade. Look at how much vertical space you have, how big a diameter caps will fit on the board and also the lead pitch and then see what you can get that will fit in neatly and safely.

[kwm488]

The drawing you did suggested that the Demeter transformer had a 150Vac (HT) and 6Vac (heaters) winding, so you can't use a 12Vac transformer instead.

They use 6V because they then rectify it and drop the voltage a little with the resistors between the black caps on the power supply board to get a smooth 6.3Vdc supply to the tube.

Whilst it's possible to run a 12AX7's heaters off 12.6V it's not going to be practical to do this because the heater connections are PCB traces.

i get confuse now. i do research, if 12ax7 tube pin 4 and 5 short and pin 9 to ground. so you need to use 12VDC to be heater. this DI box pin 9 is go to ground. so i guess my PT should provide more voltage and rec to 12VDC.

now, i just measure 6VDC. so i plan to cut my PT heater 6VAC wire. and use another transformer 12VAC instead. so it go to rec , it will be 12VDC. is it work for me?

[paulster]

On a 12AX7 there are two 6.3V heaters, one is wired pin 4 to 9 and the other is wired pin 5 to 9.

If you want to use 6.3V then you want the heaters in parallel so you connect pin 4 to 5 and supply 6.3V between pin 4/5 and 9.

If you want to use 12.6V then you want the heaters in series so you ignore pin 9 completely (i.e. leave it unconnected) and supply 12.6V between pins 4 and 5.

When you run off AC you usually have the heater centre-tap connected to ground so that the heaters are effectively +/- 3.15Vac referenced to ground, but you could also reference pin 4, 5 or 9 to ground instead.

The same is true for DC, and in this case Demeter has chosen use pin 9 as their heater ground reference.

It is possible that they've gone for a centre-tap ground reference and are effectively supplying +6.3V and -6.3V to pins 4 and 5 from a 12.6Vdc supply, but if those pins are tied together on the PCB then they'll have to be using a 6.3Vdc supply. This would also agree with the wiring drawing you posted showing voltages too.

Check whether pins 4 and 5 are tied together on the PCB or whether they connect back to the power supply board independently. That should give you the answer.

[kwm488]

hi
12ax7 pin4 and 5 are connect together. so 6VDC go to tube pin 4 and 5 is correct or wrong? if it is correct, so my PT no problem?


and i find out one thing. i replace all W06M. two brown wire 6VAC to W06M. 18vDC is come out (i just test, i haven't connect the W06M to the PCB). but after, i connect the W06M to PCB. i just measure 6VDC. this section is the heater DC come from W06M to first 3300uf/16v and connect to 1R8 resistor and to next 3300uf/16v.

why the voltage is different before and after i install W06M to PCB? voltage will drop after connect to 3300uf/16v cap??????????

[paulster]

If pin 4 and 5 are connected together then you definitely need 6.3Vdc to be supplied to the tube, so you can't use your 12V transformer.

The voltage differences you are seeing when you connect the bridge rectifier just to an AC supply and read the DC output voltage are for two reasons:
1) the transformer has no load, so the voltages will be higher than under load
2) you have rectified AC and not DC unless you have any filter capacitors, so your waveform looks like this:

which won't make sense to most multimeters set on either AC or DC.

Does the Demeter transformer get hot when you have wired it correctly, or did it only do that when you wired it wrong? With the secondaries disconnected from the rest of the circuit, what AC voltages are you seeing on them?

If they are around 6Vac and 150Vac then there may not be anything wrong with the transformer at all.

[kwm488]

i learn more again. lucky, i ask you before i put 12VAC transformer to the unit.

anyways, voltage to tube pin 4 and 5 is 6VDC.


i really don't understand why voltage come out from W06M is 18V, but if i put the W06M back to the PCB. it will 6VDC?

i dsconnect all PT wire and measure again. red wire is 150VAC, brown is 6VAC.

the PT just get very hot when i rewire wrong and run 2 min. i try to turn this unit yesterday overnight, the PT is not hot, just warm.

so you almost sure the PT no problem?? if not, what wrong of my unit? i just think the bass guitar sound warm little bit if i add this DI unit.

do you mind i record some bass sound with and without this DI box and let you hear the different?

should i need to change all resistor? if the PT no problem, and Ot you think it is almost impossible get hurt. the rest is just cap and resistor.

but all cap the uf is correct from my cap measure. but is this point can prove the cap no problem. i have experienece of marshall amp, the 50+50uf/500v cap show correct uf. but the cap get smoke.

[kwm488]

and i find out one more problem. i use multimeter to test , the both brown wire is short. and both red wire is not short. is it mean my PT wrong or correct? my multimeter will sound if two wire is connect.

also, the heater W06M, + side and - side are short too even i don't plug 12ax7. is it correct?

i seem to understand how this unit work. but i need you help me to prove.

you are the only person reply me and help. i will not forget forever.

[paulster]

i really don't understand why voltage come out from W06M is 18V, but if i put the W06M back to the PCB. it will 6VDC?

See the diagram I included in my last post. A multimeter doesn't really know how to deal with a rectified AC signal that is neither AC nor DC so it will read incorrectly. As soon as you have a filter capacitor connected you'll be reading a genuine DC voltage.

i dsconnect all PT wire and measure again. red wire is 150VAC, brown is 6VAC.

the PT just get very hot when i rewire wrong and run 2 min. i try to turn this unit yesterday overnight, the PT is not hot, just warm.

That sounds like the transformer is intact then since those voltages are correct.

so you almost sure the PT no problem?? if not, what wrong of my unit? i just think the bass guitar sound warm little bit if i add this DI unit.

You've put twice the heater voltage onto the 12AX7 and twice the expected B+, so it might have suffered a bit, as might have the filter capacitors. Ignore the XLR connection for the moment and compare the unit in and out of circuit using the single-ended output, so you go bass -> amp/passive DI and compare that to bass -> Demeter DI 1/4" in -> Demeter 1/4" out -> amp/passive DI.

Before you plug the unit into your amp or passive DI check for voltage (carefully) on the output jack in case the coupling caps have put DC on the output.

Testing it like that won't be using the Jensen transformer at all, and you can see what effect swapping out the 12AX7 for another tube sounds like. If it sounds bad then swap the 12AX7 and see what happens.

If it sounds better then you've probably cooked the 12AX7.

If it still sounds bad then you probably want to replace the filter caps.

Once you've got it sounding good using the single-ended output you can try the XLR output and see how it sounds in balanced mode which reintroduces the Jensen transformer into the equation. It shouldn't have been possible to damage this unless the last coupling cap failed and put DC onto the transformer and cooked its primary winding.

[paulster]

and i find out one more problem. i use multimeter to test , the both brown wire is short. and both red wire is not short. is it mean my PT wrong or correct? my multimeter will sound if two wire is connect.

That's fine. The heater winding is low voltage and high current, so it has a very low resistance and can appear as a short to a meter.

The opposite is true of the high voltage winding so it often won't show as a short.

also, the heater W06M, + side and - side are short too even i don't plug 12ax7. is it correct?

That's also fine. Depending on which way round you connect your meter leads to the + and - terminals there is a current path through the rectifier diodes between + and -. This test doesn't tell you anything useful really.

More useful would be to measure the voltages at pin 5 (should be 6.3V), pin 1 and 6 without the 12AX7 in to make sure the plate voltages are sensible before putting another tube in there.