The Huffer: schematic inside.

[timo]

Hi, I know this is an old thread, but worth resurrecting, IMO. I built one of these yesterday. The basic tone seems nice but mine distorts pretty badly if not played with an insanely light touch or having rolled the volume WAY back. This is with a MIJ standard passive Fender Jazz Bass. At first i didn't have a heatsink in  the 7812 but even after adding one (granted, it's held in its place by thermal grease, loose metal wire and holy spirit, but it still warms up) the distortion didn't go away. It's been fed by a 1A unregulated 12v wall wart giving about 15 volts idle  and after the reg the voltage is 11.98v.

Has anyone else built one?

[soulsonic]

I haven't built one, and I'm not really familiar with the characteristics of that particular tube, but if I have distortion issues with a tube circuit, I usually start adjusting the cathode resistors. This circuit doesn't have a cathode resistor, so maybe you could try adding one..... but the triode amplifier and the two diodes share a common cathode - this might be a problem for the anodes of the two diodes being returned to ground and the cathode having a resistance. As the circuit is now, they are held at the same potential, so I don't believe there is any interaction between the elements, but adding the cathode resister would cause a difference of potential and a current flow between the elements. Hmmmm..... try disconnecting pins 5 & 6 (the diode anodes) from ground and attach them to pin 2 (the cathode). Then connect a 1K resistor between pin 2 and ground. The 1K resistor value is just a guess, but this should at least let you know if adding a resistor there can remedy the situation. Better yet, stick a 5K pot between pin 2 and ground and see if the distortion goes away as you increase the resistance.

[timo]

Thanks for the tips! I'll probably try a different tube first, just in case. when I recorded some noodling with it it the waveforms seemed to be clipped from the upper side. Would this hint that the tube is misbiased?

[Steben]

A 12U7 would work fine too (=low voltage 12AU7). More of a booster then; like in the Sopht amps.

[soulsonic]

Yes, that kind of asymmetrical clipping does give an indication of what's happening with the bias. If you wanted the thing to normally be clean, it should be biased "in the middle", which means that if it were to clip, it would clip both the top and bottom symmetrically. Not every tube stage is biased like that, but it's usually the best starting point.

+1 to the 12U7. I need to get some of those to play with.....

[moosapotamus]

Is it possible to get more than unity gain out of the 12AJ6 circuit? I can't seem to swing it. 

~ Charlie

[brett]

Hi
the data sheety says a voltage gain of 16, but I seem to recall getting 5 to 12 out of one of these some time ago.  It will depend on the health of the tube and the bias and plate load being right.
cheers

[Caferacernoc]

Also try lowering that 220k plate resisitor for a cleaner sound. 100k is a more normal value.

[moosapotamus]

Thanks for the suggestions. I'll try tinkering a little more... 

~ Charlie

[mth5044]

Anybody save the schematic for this? Seems to have disappeared!