Low BIAS voltage A15MK2

Hi,

can't you adjust the bias trim pot to a (absolutely) lower voltage than -19 V?

Are the cathode voltages symmetrical? That means is V(R51) = V(R52)?

Cathode current and so the voltage across the cathode resistors will rise when the bias voltage gets smaller (i. e. -17 V).

Try it with caution - observe the cathode voltages while doing this.

Bernd

Hi,

may be there some misunderstanding?

The bias voltage - that is what you adjust by the trim pot and feed to the grids of the power tubes - is never "OK". It's a servant to the cathode (exactly anode) current, which is adjusted by the bias voltage to the right amount.

And the 31,5 mV aren't the bias voltage, too. The 1 Ohm resistors are means of measuring the cathode current: 31,5 mV across 1 Ohm indicate 31,5 mA of cathode current (Ohm's law).

So my first question from my former posting isn't still answered:

Can you lower (absolutely) the voltage with the trim pot and rise the cathode current by this?

For getting exactly the same current in both tubes you have to use seperate adjustment facilities. But sometimes sound will get poorer by this because of lower harmonic distortions. So you'll find that circuitry more in (hifi) audio amps than in guitar amps.

You really mean noise and not hum?

That noise rises when switching to the channels with higher gain ist quite normal, it's more a question of how much it rises. The simplest thing to reduce noise is testing different tubes in the first position of the noisy channel.

Bernd

Low bias voltage A15Mk2

Hello,
one question about the voltages measured so far.
According to chapter 2.4.1 of the manual I did the measuring
at pins 10-12: result 257 Volts.
At pins 13-15: result 23,5 Volts.
At pins 16-18: result 7,2 Volts.
Looks Ok to me right?

Than, following the measurement instruction in chapter 2.4.2
I measured 327 Volts at the (+) rectifier,
323 Volts at (+) C26,
-16,9 Volts at pins 2/3
and 7,2 Volts for the heaters.
Although most are within tolerance, the bias voltage seems to be
rather low. Adjusting the biascontrol one way doesn't increase the voltage,
moving it the other way around does decrease the voltage to zero.
Is this a reason to worry or is it still acceptable?

Greetings.

Hi,

why don't you tell us the voltages across the 1 Ohm cathode resistors (R51/52). That's what you really need to know about the right biasing.

and

Zitat

-16,9 Volts at pins 2/3

where do these pins belong to? :confused:
A tube? Which tube?

Bernd

Again the bias voltage

eBerd,
if you are not familiar with the manual of the A15Mk2 then why are you answering as if I was some kind of a moron? You must have noticed that I was just following the instructions in the manual , step by step, and in chapter 2.4.2 (page 18) there is the part that is referring to the bias CONTROL. And, as this appears to be a variable resistor, provided with 3 PINS, I assumed these pins 2 and 3 were referred to. And as I like to limit potential failures I posted my question. I'm sorry I've bothered you. Won't happen again, i'll post my questions somewhere els.

Hi Outsider,
no reason to get angry. Bernd didn't treat you disrespectful in any way, he was just trying to be helpful.
Biasing those powertubes means setting their idle CURRENT. Nothing else matters.
To make things easy, your A15 has a 1Ohm resistor from each powertube's cathode to ground. Just set your multimeter to mV and measure the voltagedrop across these(each at a time, of course). Ohm's law is our friend here - mV directly translate to mA.
If you're able to adjust the idle current to the desired value, you're done.
It really doesn't matter what negative voltage your tube grids get to see, as long as your idle current won't go overboard, you're fine.

cheers,
Thomas