Biasing tubes in the EHX Hot Tubes pedal...

[triskadecaepyon]

Can you bias the tubes in the Electro harmonix HOT TUBES pedal?  I would really like to bias the tubes to make them sound better.  Has anyone biased them before (if at all possible)?  I think I do see a pot to bias the tubes with on the PCB, but I don't feel like risking it if it's not the biasing pot.

[aron]

Well, what makes you think that the tubes are not biased correctly?

I'd be a careful with that pedal since it does use high voltages inside. If you aren't sure it's a bias pot and how to use it, I wouldn't fool around with it.

[triskadecaepyon]

they don't even get warm at all when I use the pedal for 1 hour plus.  And no, the pedal is not broken.  I am thinking that I am not taking full advantage of my tubes, so that's why I wanted to know.  Would you think the design would incorporate a bias pot though?

[The Tone God]

they don't even get warm at all when I use the pedal for 1 hour plus. And no, the pedal is not broken.

Well if you hear a sound and the tubes glow then it shoud be ok. Do you mean physically warm ? If so then how hot the tube gets is not neccesarily an indication of how hard the tube is working.

Would you think the design would incorporate a bias pot though?

Depends. I would guess that the circuit is class A therefore would probably not need a bias pot.

Puretube, any insight ?

Andrew

[puretube]

nothing to bias in there...
trimpot inside is to set max. Q of the tonecontrol to prevent sqealing.

to "sound better": probably you find it too harsh (?):
turn down the gain knob!
do the distortion with the drive knob.

[triskadecaepyon]

yeah I guess you are right.  I asked EH and they said it was "self biased" already.  I was looking for a "tube saturation", or subtle overdrive, but not as a distortion pedal.

[bruce bennett]

the 12ax7 tube doesn't need to be "hot" to have great distortion. your thinking about power tubes, not pre-amp tubes.

the last thing you want is to have your 12ax7s melting down your PCB in that pedal. :lol:

12ax7s can be re-biased for higher gain factors, BUT that doesn't mean they will physically run hotter.

I have not looked into that circuit... but I'm sure if you check the cathode connections (probably a resistor and bypass cap) on pins 3 and 8 you can find charts that will tell you what gain factor the circuit is shooting for.

I know the Dan Torres book shows that type of chart.

[triskadecaepyon]

yeah I guess I forgot it was the power tubes.

[mlabbee]

If you figure out how the circuit is configured, this: http://amps.zugster.net/tools/triode.html is an awesome tool for figuring out your gain quickly - you can then play around and see how different configurations affect gain.  I'm currently building a tube preamp and this thing has been really useful in helping me figure out what kind of parameters I'm going to vary in my breadboarding stage (now if I can just get some TIME!!!!!)

[Mark Hammer]

yeah I guess I forgot it was the power tubes.

Sometimes a little knowledge is a dangerous thing.  Sometimes it's just an embarrassing thing, eh?  :wink:

S'okay.  You learned something in the process, even though you had your knickers around your ankles to do it.  It happens.

[puretube]

overall gain can be reduced by increasing RC or decreasing RA of V1A & V2B;
or by disconnecting their C C.

(or by turning down the gain knob, which is absolutely no sin!)

don`t mess with V1B & V2A; don`t use other tubes;

[Gringo]

(or by turning down the gain knob, which is absolutely no sin!)

Oh yes it is!!  :twisted:

[puretube]

you ever messed with a HOT TUBES ?