Cold biasing

[jnew]

So lately with all the variac ing down to use plexi's without attenuating or re-amping, it got me thinking about the effect this would have on the biasing. So I took my amp and buried the bias. It plays a huge role in how the amp sounds and behaves (in the right ways). But going to cold moves it into a sterile and sort of lifeless place. It all makes perfect sense now regarding cranking the bias and then variac ing down to useable club volumes.

AFAIK, the jury is still out on just how low the heater voltages can go.

[Krinkle]

I mentioned this on the other thread but I think you posted about it before I did and I just missed it. I LOVE the volume I get when using the variac at 60VAC. But I do NOT have enough gain. I put my blue MXR 6 band in front of it and I just could not get it to sound right. I'm getting better results with my Mooer Solo, a Riot clone. I biased to 50mA at 90VAC. I saw that you changed your resistor to get more bias voltage, what did you end up using?

[jnew]

I think on my Small Box, it's either a 47K or a even a 33K. Can't remember and I'm offshore right now. On my newer 68 Iskra build, I had a 68K but had to lower that to 51K for it to bias at 90 VAC.

[dirtycooter]

I fiddled with my unlit tube-it didn't die and just wasn't makin good contact with the probe between. All is good now.
I wanna reference check my meters and calibrate them with a snap-on dmm before I raise mine up more.

So this is starting to make sense with the accounts given. Jacked up bias but lowered AC input. I am probably not gonna have enough swing either and will have to change a bias resistor. But this is another little golden thread right here proving the method to the madness. Thanks for the confirmation
I will let you know where mine maxes out in mA vs V

[Krinkle]

That's good to hear, but scary at the same time. I hate intermittent problems. I agree, I'm getting ready to jack my bias up at 60VAC. I'll calculate and see where I land, I changed the bias resistor so I think I should have enough.

[jnew]

Keep us posted. This should be interesting if you intend to bias at 60 VAC. It's going to call for a lot of current.

[Krinkle]

I'll look at the data sheets to see if there is a max current spec. At the very least I'll increase the bias current a little at a time to see if I can get my "oomph" back, instead of just jacking it. I may not need to go all the way to the calculated value.

I dug out my homemade EP3 pre and if I add that when running at 60VAC it is a LOT closer to what I want to hear. I'm hoping that I can jack the bias and get there.

I have to go look through your posts to see what you are/were running, I really liked your clips.

[jnew]

I'll save you the trouble. All the clips are at 90VAC and biased at 70%. The Small Box has a 330uF on V2. The new amp doesn't.

[vh junkie]

Keep us posted. This should be interesting if you intend to bias at 60 VAC. It's going to call for a lot of current.

I don't understand this concern? A less negative bias voltage will make the bias hotter... as you turn the variac down, the bias voltage becomes less negative... it just doesn't get hotter fast enough as you head down towards a plate voltage of 300-350v. If you are in the 300-350v range, do the appropriate calculations. I think a little cold will sound good(do the 70% calculations to see the upper end for your B+)... try between 50ma to 43 ma, respectively... I don't see any current issues here...

[jnew]

With the Variac at 60VAC, the plate voltages are real low. Like 230 if I remember on one of my amps. So, for example, 17.5 divided by 230 = .076. Doesn't this mean this calls more more current to maintain a bias of 70%? Or am I not understanding this properly?

[vh junkie]

Current from where? From the bias supply? No...
Cathode current yes... as you lower the plate voltage, the cathode current needs to increase to maintain the same plate dissipation. None of this is a strain on the PT though...

[jnew]

Correct. That's the current I thought I meant. So less cathode current ='s a colder bias. Right?