Tube Based (McTube) Pedal Design Power Supply Question

[bungusbeefcake]

Hi guys,

I'm building myself a McTube type pedal and basing it on the schematics I have found on here. However, I note that the schematics are designed for use with US power in mind, i.e. 110-115 V AC. Unfortunately I live in Europe, Ireland to be precise and our wall supply is 220-230 V AC.

I have attached the original McTube schematic I found and am basing my design upon. So basically I have found a transformer which I think might work and I'm thinking of putting it in a similar manner to the one in the schematic. The transformer will hopefully do the following: I'm looking to use mains input i.e. approx 220/230V AC and get out 2 different supplies, one at 12V AC and one at around 110/115V AC. The 12V supply would preferrably be at 0.5A and the 110/115V, well I'm not sure what that would come out as regarding current but I know the 12AX7A tubes and the previous schematic brought the tube input current to 0.3A.

Attached is the data/spec sheet for the transformer.What do you think, is the wiring diagram I made about right? and using that 12V supply would I get it to come out as 0.5A since I'm making the output to 2 sources in parallel? Or would it be smarter to just feed the top 12V supply directly to the 2nd transformer and keep the 2nd 12V supply for the other circuit, as in the 2nd diagram.

Thanks

Bungus

Triad FS24-250 Transformer Data/Spec Sheet
http://www.mouser.com/catalog/specsheets/FS24-250.pdf

McTube Schematic


Transformer Wiring Diagram 1


Transformer Wiring Diagram 2

[amptramp]

It looks good, but I would be inclined to go a step up in voltage rating for C1, C2, C3 and C4.  The peak voltage they will see is 176.75 for C1 and C2 and about 14 volts for C3 and C4 which is below the device rating but is not really derated enough for good reliability.  R5 seems a bit high - you usually see a 12AX7 biased with 1500 ohms on the cathode.

Transformer wiring diagram 1 is the one I would prefer because with diagram 2, you do not get equal sharing of the currents in the first set of 12 VAC windings.  I presume they can get the turns ratios identical.

[bungusbeefcake]

Thanks very much for the advice amptramp.

I didn't actually check those so you just saved me some hassle later on! The C1 and C2 caps I was thinking about are: https://ie.mouser.com/Search/ProductDetail.aspx?R=REA330M2EBK-1325Pvirtualkey21980000virtualkey140-REA330M2EBK1325P

and the C3 and C4 caps are: https://ie.mouser.com/Search/ProductDetail.aspx?R=RGA222M1EBK-1625Gvirtualkey21980000virtualkey140-RGA222M1EBK1625G

I was thinking about throwing a negative feedback loop in the circuit as well just to see if it would do much. Basically throwing a wire with a on/off switch in after C6 and going straight to after C8. On the wire I would have a 330k Ohm resistor. What are your thoughts??

[psychedelicfish]

TBH, I would just go with running the plates at higher voltages and just use two 240V:12V transformers. If you do this, C1, C2, C6 and C8 should be rated probably at 350V or higher.

[amptramp]

TBH, I would just go with running the plates at higher voltages and just use two 240V:12V transformers. If you do this, C1, C2, C6 and C8 should be rated probably at 350V or higher.

Usually around here, the next size up for most film capacitors from 200 volts is 400 volts.  I have seen 250 and 350 in electrolytics and either value would work, depending on the derating rules you are using.

[bungusbeefcake]

psychedelicfish: Originally it was my intention to run it at the higher voltage, but I was thinking I might run into problems regarding the current going into the tube. Maybe I'm just confusing myself, but will the current change if I use the higher voltage with the same resistors?

[amptramp]

psychedelicfish: Originally it was my intention to run it at the higher voltage, but I was thinking I might run into problems regarding the current going into the tube. Maybe I'm just confusing myself, but will the current change if I use the higher voltage with the same resistors?

There will be some change, but the 12AX7 takes 1.89 watts on the filament, so that will swamp the ~0.3 watts per plate.  Even if the tube was a dead short, a 330 K resistor in parallel with a 680K resistor gives you 0.90909 mA for the first stage and 0.441 mA for the second stage, so you get 0.405 watts for the plate supply.  Depending on whose tube manual you read, the 12AX7 can take a maximum of either 300 or 330 volts on the plate.  There will be a change from maybe 0.5 mA for the first plate and somewhat less than that for the second to the values above.  This is not a major circuit load.  There should be no problems.

[bungusbeefcake]

Amptramp: Ok that sounds good! So, just to be sure, I should run the circuit as is laid out but I can put the 230/240V into it.

Also, sorry to be annoying you, but I just want to know about the transformers again, the one I'm looking at is rated at 6VA so using it to transform 230V-12 @ 0.5A and then transforming that back to 230V would be at 0.026A. Is this right? Can that be used and will it be brought up to the required current? Or would I have to get a transformer at around 75VA to provide me with 0.3A?

Sorry for all the questions, I just don't want to ruin anything or myself!

[amptramp]

6VA should work.  You should be able to get wall warts in that size and if they meet the standards (like CE), then you have an amplifier that is both functional and legal.  I have seen people use wall warts inside an enclosure with connections made by terminals.

[bungusbeefcake]

Yeah, I was my calculations were based on the wall warts, but as I'm in Ireland its difficult to source particular ones so I was just thinking of getting those transformers since I'm ordering all my resistors, capacitors, pots and diodes from there. As I'm ordering the transformers separately I can order ones specific to the purpose, so would it be better to order one that outputs a higher current?

[amptramp]

Your total power should be less than 3 watts.  A 6VA transformer should be adequate.  You can always go for higher current in case you want to add more stages.