A quick tube (valve) fuzz box

[DrNomis_44]

Thanks John.



DrNomis_44 added it in his modifications, earlier in the thread. I've drawn up the circuit, which shows it.

Attachment 36002

That 250k pot for the midrange control really should be a 25k pot.


I've got a copy of VeeCad installed on my studio desktop pc, VeeCad is a small app for designing Vero/Strip-board layouts, anyway, I'm going to see if I can create a Vero-board layout for the modded Purgatory Pedal circuit I posted in this thread, so I can have a go at building it, I think I've got a spare 12V DC plugpack adaptor in amongst all my bits and pieces, I'm also thinking of expanding the basic design so it's like a 19 inch 1U rackmount guitar preamp with channel switching, lots of potential there.

[jonwhitear]

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You might need to substitute a different Mosfet for the 2N7000, maybe an N-Ch Power Mosfet that's capable of running on about 100V or maybe higher, since the 2N7000 is only rated to 60V maximum there's a likelyhood that running it off a +70V supply might destroy it.

Something like this?

ZVN4310A - Power MOSFET, N Channel, 100 V, 900 mA, 0.65 ohm, TO-226AA, Through Hole

https://au.element14.com/diodes-inc/...ine/dp/9524991

[DrNomis_44]

Something like this?

ZVN4310A - Power MOSFET, N Channel, 100 V, 900 mA, 0.65 ohm, TO-226AA, Through Hole

https://au.element14.com/diodes-inc/...ine/dp/9524991

Yeah, I reckon that should do the job fine.

[DrNomis_44]

For those of you wanting to build the modded Purgatory Pedal circuit, the values of the three pots used in the Marshall tone-stack that I added, are as follows:

Treble: 250k B.

Mid: 25k B.

Bass: 1M A.

[jonwhitear]

I've updated my drawing to reflect the correct tone pot values, the uprated mosfet, and your DC HV supply. I'd be grateful if you would check the supply - I wasn't clear quite how it's connected up.

[DrNomis_44]

I've updated my drawing to reflect the correct tone pot values, the uprated mosfet, and your DC HV supply. I'd be grateful if you would check the supply - I wasn't clear quite how it's connected up.

Yep, I just checked it out and as far as I can see it looks all correct to me, all the 33uF/50V caps in the DC HV power supply look like they're oriented properly, as are the 1N4007 diodes, so it should work properly, one small mod to the circuit that I would make is to put a 10 Ohm resistor in between the tube heater (Pin 4 of U2) and the +V supply for the whole Purgatory Pedal, and then change the supply voltage to +15V DC, this will enable the tube's heater to run at about 12.6V.


The 10 Ohm resistor can be a 5 Watt ceramic wire wound type.


I definitely need to have a go at building the modded Purgatory Pedal circuit one day cause I'm curious to find out what it actually sounds like.

[Simon Barden]

I may be showing my ignorance here but I thought that caps started to de-rate in value if subjected to more than roughly half the rated insulation voltage. So maybe a slightly bigger value for C7, or a higher voltage rating if you are pushing for 70+ V.

It's also not drawn for true bypass, so you'd need to modify it if you wanted that as well.

[DrNomis_44]

I may be showing my ignorance here but I thought that caps started to de-rate in value if subjected to more than roughly half the rated insulation voltage. So maybe a slightly bigger value for C7, or a higher voltage rating if you are pushing for 70+ V.

It's also not drawn for true bypass, so you'd need to modify it if you wanted that as well.

Good point, maybe use a 47uF/350V cap for C7, or maybe even a 47uF/450V cap.

[JohnH]

Good point, maybe use a 47uF/350V cap for C7, or maybe even a 47uF/450V cap.

The only downside to the 450V cap (from jaycar at least) is it's size - I'm still pondering how to get my circuit into an enclosure, ha ha

[jugglindan]

I may be showing my ignorance here but I thought that caps started to de-rate in value if subjected to more than roughly half the rated insulation voltage. So maybe a slightly bigger value for C7, or a higher voltage rating if you are pushing for 70+ V.

It's also not drawn for true bypass, so you'd need to modify it if you wanted that as well.

It looks like true bypass to me, as SW1B connects the input directly to output in the bypass position. But it's not the bypass wiring I normally use. I suggest using a 3PDT instead of a 2PDT so you can also ground the effect input (effect send) in the bypass position. From the perspective of the effect circuit this is the same as winding the guitar volume all the way down. Apparently this helps prevent popping when turning the effect on and off, although R11 should help prevent popping as well.

My other comment is that the power indicator LED current limiting resistor value is quite a lot lower than what I would use. At 12v, the 1k CLR will give 12mA through the LED which will tend towards the bright side. On my solid state pedals, I aim for closer to 3 or 4mA which I find to be bright enough. On solid-state pedals, 12mA for the LED could double the power draw of the pedal. Clearly with tubes that won't be the case, but it still might be too bright. A 3k CLR will give 4mA from 12V.

Of course, it highly depends on the mcd rating of the LED, which can be very variable. Jaycar sell 5mm LEDs ranging from 8mcd (very dim) to CREE LEDs at over 60K mcd. Which would be interesting. You could use the pedal indicator as stage lighting!