Valvecaster Tube Preamp Gain Problem.

[nguitar12]

So I successfully build the Valvecaster Tube Preamp according to the following schematic:



Everything is working as expected but I would sorry to say that the sound of this pedal is pretty crappy as what I have heard on youtube. When I turn the gain all the way up it will produce a fuzzy distortion which I don't think it is usable.  It sound ok as low gain setting so I am planning to use it as a booster but the problem is I can't seem to get a complete clean sound out of it even at lowest gain setting. Can someone please suggest a possible solution on this?

[SuzukiScottie]

I built one from the same schematic and it's fantastic. Pretty similar to what you find on YouTube. Plenty of gain, or none. It works ok on 9v but better on 12v. If you are sure your build is ok, have you tried a different tube?

[vigilante397]

When I turn the gain all the way up it will produce a fuzzy distortion which I don't think it is usable.

"Fuzzy" is not usually a word I hear used to describe valvecasters, and makes me think something is wrong. Even with the gain fully cranked it should be more of a moderate-gain overdrive. What layout did you use (i.e. vero, perf, point-to-point)?

[nguitar12]

When I turn the gain all the way up it will produce a fuzzy distortion which I don't think it is usable.

"Fuzzy" is not usually a word I hear used to describe valvecasters, and makes me think something is wrong. Even with the gain fully cranked it should be more of a moderate-gain overdrive. What layout did you use (i.e. vero, perf, point-to-point)?

I do the layout myself. It should work properly as this is a pretty simple circuit. It sound like most demo on youtube which I will describe as "Fuzzy", especially the bottom end.

I lowered R2 and R3 and the gain is now turned down.

[GibsonGM]

You're using a 12AU7, not a 12AX7, right?  Just checking...

I like the circuit, but have run them at 12V, 18V (gotta remember to tweak the heater supply...) with better results.  Cleaner, of course.

[nguitar12]

You're using a 12AU7, not a 12AX7, right?  Just checking...

I like the circuit, but have run them at 12V, 18V (gotta remember to tweak the heater supply...) with better results.  Cleaner, of course.

I use 12AU7 as schematic suggested. I use 12V but than tried higher voltage. I find the sound improved with voltage. The max voltage I try is 25v just wonder what is the max voltage this circuit will take (my boost module can go up to 60v)?

[GibsonGM]

The real limit to how high you can take the voltage are your coupling caps, C1, C2, C3.   Usually those will be 50V or 100V units, take a look....

If you used 1/4W resistors, you are ok up to a couple of hundred volts.

Thing is - if there are no errors on your end, this should not be 'very fuzzy', really.  Look closely again, ok?   If you put 25V into it, you should be sounding pretty clean, which suggests that SOMETHING isn't connected right.   

Are you overdriving your amp so much that the AMP is causing the bad sound??

[PRR]

> The max voltage I try is 25v

If the tube HEATER (pins 4 5) is given 25V, tube life drops from >5,000 hours to under 1 hour (with possible instant-death).

The original plan showing 9V heat on a 12V tube is "wrong" but does no harm. The under-heat may be part of "that sound".

Going by-the-book, heater gets 12.6V. Going by experience, 11V and 14V don't make a lot of difference.

With heater at "sane" voltage....

At 9V and even at 25V plate supply, there can be a LOT of difference between "identical" tubes. Tubes are processed for 100V-300V uses. There are several 1V "imperfections" which can go different ways. 1V grid-cathode is 20V at 12AU7 plate. So the whole zone 9V-25V depends a LOT on tube processing.

Voltage checks may give a clue of something very wrong. However the voltages are sure to be wonky anyway, and unlikely to "match" any other build, yet still work. Maybe.

I would check for <1Meg each grid to ground. A tube can "work" without a grid-leak path, but way off from what you expect, and unstable with dynamic signal.

[amptramp]

You can always put the heaters in series and run the heater supply at the nominal 25.2 volts that way.

You are going to get "gulp distortion" or "blocking" in the second stage because without a cathode resistor or negative bias on the grid, the grid will act like a rectifier and collect electrons when it goes positive.  These electrons will then bias the grid negative when the signal voltage goes down, possibly enough to cut off the stage.  The recovery time constant will be C2 * (R4 + R2||rp) where rp is the plate resistance of the first stage.  Since this is a bass turnover time constant, the sound can be blocked for a noticeable time.

A cathode resistor in the second stage would eliminate some of the distortion.

[PRR]

> put the heaters in series and run the heater supply at the nominal 25.2

The posted plan has one 12AU7. This can be run at 6V or 12V.

If he had *two* 12A_7 then a 25V connection is trivial.

However my guess is that running two bottles (four stages!) of this barely-alive type amp may be just too much to listen to. And certainly need extended tweaking just to get something not-awful.

[amptramp]

Then go with the 20EZ7 in place of the 12AX7.  Same tube with a 20 volt 100 mA heater and you can get the barely-alive sound to come a little more alive.  But still add a cathode resistor for the second stage or it will sound like a radio with a grid-leak detector from 80 years ago.

[PRR]

My feeling is that 12AU7 is more appropriate than 12AX7 (or 20EZ7). At super low voltages we get super low current. Too low to make useful signal. 12AU7 passes more current at any voltage, may be less liable to starve-to-death at very low voltage.

Also there's "contact potential" which runs tenths of a Volt to maybe 1V. The old-timers' rule was never use a Mu higher than your supply voltage; really meaning don't let your cut-off voltage get into the contact potential range.

[amptramp]

There is always the 12U7.  It is designed to run at 12 volts on the plate with 1.0 mA plate current and a µ of 20.  Same pinout, but these are rare devices.