Whats wrong with my volume control?

[iainpunk]

Hey,

Im building a tube based circuit and it works great, until you turn down the volume.
When you reach about the half way point, you start to lose almost all bass and a chunk of trebble, and it sounds like crap. Im currently using a 500k log pot, and the load on it consists of a 100nf cap in to both an op amp input and a 60k to Vref to change the dc bias. The op amp is unity gain and the output is directly connected to the circuits output jack. Any help?

[PRR]

I think we need a circuit drawing.

[iainpunk]

Hey, heres the schematic


Any idea why it loses bass and trebble? Losing trebble is somewhat understandable, but bass?

The vref is for the next stage of the power amp, the op amp is fed with both ±12 volt

[GibsonGM]

Hi Iain,

Your power supply ("B+") is really, really low for a 12AX7.  It WILL work, to some extent, but over 50V will give good operation, and over 200V is normal.  You can experiment at 12V, though, for learning purposes.

Need a cathode resistor to elevate the grid to the right voltage WRT cathode to be biased properly, for one.   500R to 1K will 'work' for experimenting but should be calculated.

Need a grid stopper, right in series with the grid, say 33K.   Needs a grid leak resistance just BEFORE the grid stopper, from input to gnd, 1M is good.
Try those, then see what your guitar pot does.    Look up "Fender Bassman Input Stage" to see a pic of this type of gain stage in operation.
We'll worry about bypass cap after     And perhaps an input cap as this will be a 'pedal' and may see other pedals' DC at the input...but ok for now.

I'd try bypassing the output buffer and see if you can get the gain stage going, then worry about buffering - at this point, just to see if you get sound.  The output impedance of the triode will be so high, it WILL sound like crap without the buffer.    At that point, you would decouple that opamp with caps from its power supply pin to gnd....perhaps you'd insert a 100R resistor in series with its power, as well.  This filters the power to keep the gain stage from affecting buffer's operation.

All issues with running tubes at low voltage (Matsumin Valvecaster is another good example to look up). 
 

[Ben N]

What ^he^ said. Also, 100k for a single tone control seems really low (although I don't know if that is scaled for low voltage operation). In high voltage  preamps, that is typically 500k or 1 meg.

[anotherjim]

Scaling down of filter resistance is only done to take advantage of the lower source impedance of the preceding stage that usually, but not always, happens when tube circuits are adapted to lower voltage work; and then only for the lower self noise of lower resistances.
Are you sure R1 isn't 56k? An anode resistor at 560k is pretty high making the source impedance high, so it don't need re-scaling. With the 500k volume control, and if you ignore the op-amp stage for the moment, there is a 6dB loss right there, so if you made R1 that high to get maximum voltage gain, you won't be able to keep much of it unless it's followed by a buffer stage with at least 3M input impedance.

The way the volume pot acts like a tone control may due to the low impedance it is driving from the op-amp stage and the variable load it's putting on the tube stage & tone control. Your R4 could be 1M without much noise penalty and in turn C3 can be 10nF. But to maintain frequency response and not lose much signal level, R4 needs to 3M or more as I said above.

[iainpunk]

Hey guys,

First of all, thanks for the replies.

The biasing and grid network has been left out for simplicity and the fact that the app im using (in om my phone) is a pain in the a$s.

The tube im using is the 12au7, not the 12ax7. The au works fine in "space charge" mode, wit this lower voltage. The high resistance R1 is to get distortion from the circuit. And yes its loosely based in a valvecaster, but modified to within an inch of its life to work as a preamp in a hybrid amplifier (everything runs on 12v)

It works just fine, like i want it to, except when the volume is turned down below half way. Im going to change R4 to 4M7 (thats what i have at hand right now) an check how it sounds.

Thanks for the advice.

[iainpunk]

Owkay, i changed R4 to 4M7 and the trebble problem is resolved, but the main thing the volume control does is cutting out bass until its half way and then starts takeing the trebble an mids with it. Any suggestions?

Funny how it is the opposite with guitars when you turn down the volume. 

[PRR]

If you don't want tone-change when you turn the Volume, use a LOW value volume pot in a low impedance part of the chain.

[iainpunk]

Hey, i dont have a 10k log pot at the moment, ill try it tomorrow with a 10k 10 turn pot when the neighbors children aren't asleep ( the amp is quite loud). Thanks in advance.

Iain

[Tubebass]

Tube pin numbers are incorrect.