Tube pedals... plate voltage?

[Pierre]

The torroidal transformer in my Madamp G2 is pretty small in size, and carries a secondary of 6.3V (1.4A) and 185V (0.04A). A similar transfo could be used in a medium size chassis, along with a single 12AX7 for a straightforward boost, but I'm wondering if 185V would be optimum for a 12AX7. If it's too much, that should mean more headroom and punch right? If it's too little though, then it seems like a pretty impossible idea, unless other pretubes run on less voltage.
By changing the tube's bias (with a switch), you could get easy added versatility as well.

The idea is a one or two knob 'overdrive', while in fact just basically another preamp stage before V1. It could also be used as a volume boost for the poweramp in the FX loop, though it may overload it a bit. I don't quite know enough about theory to figure that bit out yet.

If it's a stupid idea and I'm way over my head, say it :laugh2:

 

[Rich_S]

Re: Tube pedals... plate voltage?

185 is definitely usable for a 12AX7.

Have you seen Fred Nachbaur's The Real McTube project? Fred died a while back, but his webpage lives on. Also, if you search for "McTube", you'll get a lot of hits for people who've built it, then made modifications. It has some interesting ideas in it, including the use of small back-to-back transformers to get plate voltage for preamp tubes.

Also, do a web search for a schematic of the Matchless Hotbox (I think Schematic Heaven has it, if not, PM me). Both of these are intended as tube overdrives, but by using only parts of them, or simply taming the gain, you can use them as the starting point for a boost.

 

[Pierre]

Re: Tube pedals... plate voltage?

Thanks for that dude!!

The idea is actually for a as simple as they come boost. My main concern now would be the level jump from non boosted to pedal on.

As far as controls go, would anything go against a 'gain/drive' for the first triode, and a 'level' after the tube?

EDIT: I found the hotbox. That'd help me a lot! Thanks a lot!

Oh another question... on my Madamp, the heater supply is not rectified, only the plate. On the hotbox schematics, the heater is rectified. Which would be correct? Whether the transfo is DC or AC isn't written on it, but since heathers need to be in VDC, I'm guessing I'd have to rectify the voltage only if the transfo stays in AC right?

 

[Rich_S]

Re: Tube pedals... plate voltage?

All amp power starts out as AC - that's what comes from the wall, and transformers only work on AC. The transformer usually has multiple windings on its secondary side - 6.3V for the heaters and high voltage for the plates. The high voltage is rectifed to DC and filtered.

None of the Hotbox schematics I have show DC heater supplies. Heater supplies are usually AC: simply by twisting the heater wires and grounding the heaters circuit at some point they can be kept quiet. (It takes a LOT of filtering to quiet DC heaters.) Even better is the "elevated" heater circuit - I think it's used in the AX84 P1. There's a description of how it works their P1 Theory document, and AX84 is a good place to ask questions about it.

 

[Evan Skopp]

Re: Tube pedals... plate voltage?

Sounds like you guys need an SFX-03...

 

[Pierre]

Re: Tube pedals... plate voltage?

Sounds like you guys need an SFX-03...

Maybe, but the SFX03 is still about 150 bucks more expensive than my current previsions Sorry man.

 

[nebucanazza]

Re: Tube pedals... plate voltage?

Real good info in there..now I know why my Rocktron Silver Dragon has the oddball 13V AC adaptor..plan to open up that baby soon,I suspect that article will go a long way in understanding the circuitry

 

[drpietrzak]

Re: Tube pedals... plate voltage?

I have, and like, my Silver Dragon. But it is a starved plate approach to the use of the tube. It is not the same as the Duncan item or the other approaches discussed here.

 

[nebucanazza]

Re: Tube pedals... plate voltage?

My bad:smack: ..didnt read the article fully..But why use an AC power supply then?
anyways, I'll find out the exact design once I open it up..

 

[drpietrzak]

Re: Tube pedals... plate voltage?

My bad:smack: ..didnt read the article fully..But why use an AC power supply then?
anyways, I'll find out the exact design once I open it up..

They daisy chain them and they are cheap.

 

[Marcel]

Re: Tube pedals... plate voltage?

I heard that people got very good results using this small mic pres as a boost though sounds like a cool project!

 

[Pierre]

Re: Tube pedals... plate voltage?

Mhmm...
I found some hotbox schematics. One has a rectified heater supply (?) while two others don't.

I can't use the transformer I already have, but I'd need one with secondaries of 6.3V/300mA and 200 or 250V/6mA

 

[Pierre]

Re: Tube pedals... plate voltage?

Ah my local electronics store maplin sells the transformer I'd need for 12 pounds. That's good news ^_^ I'll try to get started on this as soon as I can. The problem of course is the casing, and I have no way of doing it or getting it done. But we'll see. Also, no PCB, so I'll have to improvise on perfboard it seems....

 

[Marcel]

Re: Tube pedals... plate voltage?

So you jump into the cold water?
Have look!