Another valvecaster :P

[marcos_s_p]

Hey guys!

I have a 12AU7 not being used here and as I only have a solid state amp for now I was thinking to build this to give a bit of valve fell to the sound. Is it worth? I actually also have a 12AT7 if not mistaken....could even do a duo...

In such case, I should have everything for the build in spare parts...except the right pot values. I should have by now 1M, 500K (only one) and 25K (maybe a 50K as well). Could those work instead of the 100K and 50K?

Also, I've found quite a few schematics, but I'm not sure which one is a definitive tested and good to build... ?

Many thanks in advance,
Marcos

[anotherjim]

You can use the 25k for gain  - it will give the louder portion of the 50k pot it's meant to have. You can always put a fixed resistor in series with the pot, up to 27k, if the gain is too much for you. If you do find a 50k, use that there.
The Tone pot could be a 500k and you can put a fixed resistor across it to bring the maximum effective tone resistor closer to 100k. It will work leaving it at 500k, only that most usable control range will be all at one end of rotation.
The 1M can go as the output volume. It will raise the maximum output level compared to the intended 100k which you can counter my simply turning it down more than you would for 100k.

[marcos_s_p]

Thanks anotherjim,

I´ll check what I have!

What about a tested schematic? I´ve also heard that It can get bassy mostly with Les Pauls...Is there a workaround?

In case I want to make a duo... I saw a schematic where there are a volume, tone and two gains pot (one for each valve, if I´m not mistaken). But always both are on or off....could I have a 3PDT for each valve?

Thanks

[marcos_s_p]

Btw, I think the name of the duo is twincaster, but I only found schematics outside the forum....(that are actually from the forum....lol ).... I used the search toll with no success...

So I was wondering if there is a way to built it with two 3PDTs, because the schematic that I found was with only one....

Thanks again,
Marcos

[marcos_s_p]

Any insights??

[rankot]

I have this one in my files:

[anotherjim]

These starved plates designs have low input impedance (Valve Wizard has a pdf article). You may see high as 1M grid resistors, but the actual input is much less than 1M (20k measured!). You would expect humbucker guitars to be very dull as a result. Maybe the 47nF interstage caps (C2) could be smaller for less bass (try 22nF), but a better solution might be a MOSFET or op-amp clean buffer in front.

[rankot]

I've built my Valvecaster with two additional features: B+ is 27V and there is JFET buffer/preamp before it. Really kicks!

[marcos_s_p]

Thanks guys...

Yeah, I'm worried that building that it will be dull with my Led Paul...

Do you know any other valve based overdrive project that wouldn't be? And that I could use the 12au7 or AT7?

Thanks

[Marcos - Munky]

Go high voltage

[PRR]

> These starved plates designs have low input impedance

Not invariably. (FWIW, electrometers often worked with 30V batteries and gave super-high impedance.)

But that's awful starved. If you bias-up to not-so-starved (even just reduce cathode resistor way small) you do get into grid current and low-Z.