Tube EQ Pedal

[blown240]

Can someone help me with this schematic?

I want to build a VOX based tube EQ pedal that will also give me a bit of a volume boost.  But I dont want it to break up at all.  Just nice and clean.

In this schematic I basically took a valvecaster and stuck a Vox Berkeley tone stack in it.  I know this isnt exactly what I want, but thats where you guys come in.  Will this work, and how can I make it clean.

[Quackzed]

Will this work, and how can I make it clean.

it looks like it'll work. though i'm used to seeing cathode resistors and cathode bypass caps...
as for it being clean...
i think your gonna need higher voltage to get any kinda clean out of it, like minimum 35v-50v even then it may not be clean..at 200-300v it'll be clean.
maybee take a look for smps or voltage doublers...
also check out the 'alembic f-2b' its basically very similar to this, but its a blackface fender input stage with a bf tonestack!

[amptramp]

Some tubes work perfectly well with contact potential bias as long as the input voltage is strictly controlled.  If you ran this circuit at 100 volts on the plate supply, the grid resistors could be up to 2.2 megohms and there would be no problem with insufficient grid bias (the 12AU7 is rated for a maximum of 2.2 megohms with cathode bias) with no cathode resistor.  Contact potential is the negative grid bias caused by the grid intercepting electrons emitted by the cathode.  As Quackzed says, the higher the plate voltage, the cleaner the signal and the more signal voltage will be premssible.  With 250 volts for the B+ supply, you can add a 820 ohms in series with the cathode to get the necessary bias.  The 12AU7 is usually operated at about 10 mA on the plate, so the 100K plate resistors (which would be OK for a 12AX7) are a bit high.  Typical plate resistance for the 12AU7 would be about 15K to 30K.

[iccaros]

would it not a cathode follower be better for the first stage, push through the EQ and then get a little boost from the last stage?

[Ronan]

I would forget it unless you want to use 200 to 300V B+. For clean sound and a 9V supply you are probably better off using solid state circuits.

If you are determined to use tubes, use a 10 or 12V AC wallwart, and run it into a small transformer to step up the voltage, rectify it to get the 200 - 300V. Why not just copy exactly the Vox circuit? Other than that, you are learning what don't work well, IMO. The only reason I'm replying is because I've been in a similar situation. I learnt the long way is the short way, in the end.

So here are some some pics of what I ended up with, a Marshall preamp used as a stompbox that I built maybe 18 years ago, I still have it and it still works. The step-up transformer is under the aluminium sheild which stopped mains hum getting into the rest of the circuit. 300V B+

Also the 12AU7 tubes don't add any character IME, except for some sort of dullness. 12AT7 (or 12AX7 might sound better.

[stm]

I can add the following:

1) Using a pair of back to back 220:12V transformers to get 12VAC and 300VDC is a great clever solution if you want tu make a "normal" tube preamp, however you might want to stay away from it if you are not comfortable working with high voltage

2) Talking about starved-plate tubes, I found a very enlighting article a few weeks ago, so you might want to take a look at it if you haven't:

Type "triodes at low voltage" in GOOGLE and you'll see the first outcome is a PDF article by Merlin Blencowe.

He characterized the curves of several samples of four popular tubes running at low plate voltages. Suitable ways of biasing at low voltage are proposed.

From the article, some of the drawbacks of a starved 12AU7 tube are:

a. Very low input impedance, in the order of 20k instead of the 1M you would expect in a HV circuit. This certainly loads the guitar and dulls it.

b. Biasing needs to be reconsidered for low voltage.  Different schemes are proposed.

c. Input dynamic voltage range is quite reduced (+/- 0.25V) so a guitar will easily overload the tube and it will be problematic if you want to get clean sounds.

The bright side is that 2nd harmonic distortion is still there, even in excess, and may need some taming in order to be within what is considered "musical", which is easily achieved as shown in the article.  For instance, refer to Fig.4 and replace R4 (4k7) with a 1k + 10k pot to adjust the harmonic content from a good deal of 2nd harmonic to little harmonic distortion where odd harmonics are predominant when clipping levels are achieved (note that having or not having 2nd or 3rd hamonic is neither good nor bad; we are talking about different flavors with may please different people or suit different situations).

If I were to design a 12AU7 low voltage tone circuit, I would consider the following starting point:

a. Use a 12 VAC (not DC) wallwart adapter for the supply
b. Run the two heaters in series from the 12 VAC, thus respecting their nominal voltage
c. Full-wave rectify the 12 VAC which would roughly produce 16 VDC, which I then would use as the B+ or main DC supply for additional dynamic range
d. Use circuit of Fig. 4 as the input stage driving the tonestack (Note: don't be discouraged by having to use an opamp)
e. Add a second circuit as in Fig.4 after the tonestack to buffer it (Note: a TL072 opamp would cover the two triode stages)

Good luck!

P.D.  I do not discard ROG will come up with an original tube project at some point in the future.