Some Valvecaster Questions

[MikeH]

Bit of a back story- I recently came across a bass amp that had been pretty much gutted of all it's power section, but the preamp module (which is on it's own pot-mounted separate pcb) was there and fully intact.  According to the schem, the pre runs on 35 volts.  I don't have a 35V power supply, but I did try it with 18V ps and it kind-of worked so I'm thinking the preamp piece is still working fine.  So I was going to work up a 35V ps using a MAX1044 and 4 diodes and some caps running off a 12V wallwart, like we've seen in a couple of frequency central tube designs, with the goal being to make it into a standalone bass-pre.  Which I think just might work out well, because it was a very good sounding bass amp before.

So then... I figured I could throw a valvy circuit in there too to get some tube grit etc, and I figured as long as I've got these 35v in here, why not run the valvy with 35V on the plates?  And still run the heaters off of the 12v wallwart, of course.

So my question(s) is(are): Do I need to change any resistor values in the valvy to do this right?  Specifically R2 and R3 maybe?  Will the higher B+ effect the bias?  Or am I good to go?  In addition, I think I have enough 1/2W resistors lying around to do this, but I may not have all the values I need; will 1/4 watt resistors be ok with the extra voltage?

Referring to the schematic here: http://beavisaudio.com/projects/ValveCaster/

[GibsonGM]

Hi Mike,

As you increase the voltage, the load line for the 'AU7 will slide more to the right.  In essence, you'll have more room before clipping occurs - increased headroom.  It DOES affect the bias, but should be in a sort of 'equal' way, just with more room on either side before clipping, if that makes sense.  Might have to tweak a little, but that's 1/2 the fun!  I'd say go ahead and breadboard this baby, see what it sounds like.  Lower value plate resistors will give you a cleaner sound, higher values a dirtier one - be sure to try them.  But, you are operating at such a low voltage, you barely register on the curves, so you're in that "starved plate", non-linear, unpredictable zone. 

Even with a 47k plate resistor, at 40V you'd be pulling around 1mA, or .04W, so 1/4W should be just fine!  You might use 1/2 watters for looks (ha ha) and less thermal noise.  Plus, later, you could crank up the voltage without worrying, if you want to.  Gives you options. 

Neat idea....try other tubes, too, like 12AX7, E88C, 12AY7, if you have them.  The MarshaValve circuit might be good in this application, too, since it has a FET 'booster', IIRC.   
Let us know how it works out!   

[siore]

The valvecaster is designed around 9V operation, although people run them at 12v with good results.  That is the beauty of its design (lightweight, portability, etc..), BUT also the limitation.  If you want a tube pre-amp with higher voltages, try googling the 'real mctube'.  It does exactly what you want, correct 12v heater operation with higher plate voltages.  It uses two transformers, IIRC one to convert your AC to 9VDC, another to ramp that up to 100VDC or so for the plates.  (I may be mistaken, it's been a while since I visited the mctube webpages).  AFAIK, that's the proper way to go if you want separate voltages for a single circuit, as having two independent power supplies will most likely introduce other issues.

[MikeH]

Thanks- I have a bunch of tubes so I'll try a few out.  An yeah, I usually go a one size overkill on resistors to keep noise down.

[MikeH]

The valvecaster is designed around 9V operation, although people run them at 12v with good results.  That is the beauty of its design (lightweight, portability, etc..), BUT also the limitation.  If you want a tube pre-amp with higher voltages, try googling the 'real mctube'.  It does exactly what you want, correct 12v heater operation with higher plate voltages.  It uses two transformers, IIRC one to convert your AC to 9VDC, another to ramp that up to 100VDC or so for the plates.  (I may be mistaken, it's been a while since I visited the mctube webpages).  AFAIK, that's the proper way to go if you want separate voltages for a single circuit, as having two independent power supplies will most likely introduce other issues.

Thanks- I'm looking to make a real tube pre, just seeing if I can "do better with what I've got" so to speak.  If it doesn't sound better with 35V on the plates, I'll just run it at 12.

I know the valve caster was originally designed for 9v because it was meant to run on a battery, but isn't it more ideal at 12V because that's what the heaters are meant to run at?

[siore]

Well, I read on the valvy thread that matsumin did it because he wanted it to be run at 9v.  I dunno, probably the best he can get out of that same heater and plate voltage?  But more so, probably knowing that there's a lot of 9v power supplies (and spare unregulated walwarts that say 9v but go up to 12v actual ) on people's pedalboards.  I don't think he meant it to be run with a battery, since that much current draw will just eat it up.  And there's a note that many other 12v designs are to be found, as far as preamps are concerned.  The mctube is one such design.

But go ahead dude.  Always exciting to prototype a new design based on that 12ax7.  See what you can come up with.  Could be interesting to hear about your findings.   

[GibsonGM]

I think the whole idea here was that the OP has 35V available already, and is running his power amp off that.  So running the Valvy at 35V is still in keeping with using the same power supply, as long as it can handle the 5mA additional draw, lol. 

All that will happen is that it will get cleaner, and need to be adjusted to operate more into cutoff.  And yes, you can simply drop the 35V back to 12V if need be, it's not hard...

[MikeH]

I finally got around to doing this, and I happy to report it worked just fine.  No re-biasing was necessary.  Valvy sounds really good with the plate voltage up; more overhead for sure, more output, a little more compressed sounding.