Simple Tube Buffer: along the right track?

[brad]

I reckon part of the fun of DIY is the ability to make pointless things that aren't really necessary...just for the fun of it.  The Tonebone thread got me thinking about how cool it would be to add a tube buffer to a regular old circuit instead of using a transistor buffer.  This isn't a quest for "tube tone" -it's just fun excuse to use a tube!  I'd just be happy that it conducts electricity.  

So anyhow, I don't know crap about tubes, but I DO know that I don't want to get electrocuted.  I noticed the Tube Driver uses 9v DC, so I figured it'd be cool to have a tube buffer that uses a 9v wall wart too (so what if starved plate sounds crap).

I don't know enough about tubes to be able to fill in the values, but isn't this basically what a buffer would be like using a 12AX7?

[R.G.]

It is one way to do a tube buffer.

Some comments on that circuit:
- it's an inverting buffer with gain
- the gain is quite high, maybe 40-50
- the input voltage range is quite low, perhaps under 50mv; it may distort for good or for bad, at less than guitar level inputs, almost certainly with humbuckers
- the input impedance is about equal to the grid resistor, but you'll be in positive-grid clipping with quite small grid voltage; this is just another way of saying the same thing as the last item

If you want lower gain and more immunity from distortion (which is a common requirement for buffers) put in an unbypassed cathode resistor, cathode to ground. This raises the grid-cathode bias and lets you have some more grid voltage range as well as lowering the gain. You can also bypass the cathode resistor to get some of the  gain back if you like.

If you want NON-inverting, connect the plate to the + power, connect two resistors from cathode to ground, and connect your grid resistor from the grid to the junction of the two series cathode resistors. Take the output from the cathode. You'll also need a series capacitor at the input now, as you have raised the grid above ground with this biasing scheme.

This circuit has a high input impedance, a "gain" of just under one, and will drive moderate loads on its output.

[JHS]

IMHO a 9V supply voltage is way to low for a ECC 83. At 9V it will act more as a limiter, that's good for smothing out diode-clipping but will kill all the headroom and the dynamic response of the circuit.

For a good tube OD you can use low voltage tubes like the ECC86, EF95 and PCC88 and they are even better for emulating tube amp distortion than FETs.

The ECC 86 nominal supply voltage is 6V DC.
The EF95 pentode is great too, and nominal supply voltage is 12V .
The PCC 88 is another good low voltage tubes acting at 40-80V DC.

They were used a lot in the '60 for car radios and in headphone amps.

JHS

[puretube]

input-C is there already, in above schem...

[brad]

I understand what you're saying R.G.  I was able to use your tips to pull up some more info, and have found some really helpful stuff!  (For example:
http://www.angelfire.com/electronic/funwithtubes/Amp-RC.html )

I'll have to read up on some of the tube basics and also look into those low voltage tubes!

Just three small addtional questions about tubes in general;
a) If you wanted to use a 12AX7 as a clipping diode, would one simply treat the plate and cathode as if they were a regular diode, and then just apply power to the heater?

b) Would a simple low voltage tube opamp actually be possible to build for use in traditional diode clippers like the Distortion+?  All the documentation I've seen for tube opamps have been for high performance designs (i.e. complex and high voltage!).  

c) I noticed that this schematic for a "Tube Distortion" pedal uses 12v for the plate and 6v on the cathode.  Does this simplify matters?  Here's the link:
http://www.montagar.com/~patj/tubedist.gif

Thanks in advance if anyone has some pointers.  I don't expect to be handed answers on a silver platter, I'm just trying to find some direction!

[brad]

I just happened to revisit the 12v, all tube "Sopht Amp" page, and it's first stage has a clean gain of 4 using a 12U7 tube:
http://www.sopht.ca/index.php?s=26

Perhaps that's the ticket?  After all, a trimmer could be used to attenuate the volume a touch before the next stage if the gain is still too high.

[puretube]

search by Author = slajeune ;(=Stephane)
to find the evolution from ruby tuby to sopht amp
IIRC, it all happened in this forum...