Bias ajustment and the larmar PPIMV

ignition · Post by **ignition** » Wed Nov 18, 2009 9:23 am

I have a 100w Metro kit that I added the larmar to. When adjusting the bias the larmar should be turned all the way up and Volume 1 & 2 turrned all the way down..... Is that correct?

Roe · Post by **Roe** » Wed Nov 18, 2009 10:13 am

the MV setting wont matter. but don't send a signal through the amp when you set the bias. however, play the amp at 10 afterwards and look for redplating

sixpakldp · Post by **sixpakldp** » Thu Nov 19, 2009 12:38 pm

Turn the Lar/Mar all the way up and Bias the amp. Keep your Volume knob low until you get it set right.

As you turn down your Lar/Mar your bias will also go down. So if you bias with the Lar/Mar very low, you'll end up with too high of a bias.

To check what I'm saying, turn your MV knob to a different position than when you biased it, you'll notice that it changes with respect to the position of the MV knob.

Roe · Post by **Roe** » Thu Nov 19, 2009 1:29 pm

no the larmar does not affect the bias settings

MacGaden · Post by **MacGaden** » Thu Nov 19, 2009 1:35 pm

Roe wrote:no the larmar does not affect the bias settings

+1. If it does, something else is wrong.

ignition · Post by **ignition** » Thu Nov 19, 2009 2:57 pm

I couldn't imagine why the MV would affect the bias settings, but I've seen procedures mentioned similar to what sixpakldp mentioned in his post. Just trying to make sure I had a handle on the circuit. I did mess with the volume levels after setting the bias, and they didn't move at all.

sixpakldp · Post by **sixpakldp** » Thu Nov 19, 2009 4:20 pm

Interesting . . . I only passed this on because I was told these instructions and I also noticed that when I turn the MV up to 10 my bias in mV off of pin 3 is maxed, when I turn the MV down my bias on pin 3 I see a reduction in the mV at pin 3.

I passed this info along because Structo/Tom told me that this is correct and that you should bias your amp with the MV at 10 and your Volume at 0 and also that I am correct that the MV does affect the bias current.

Let me know if this is the case because this trend is true in my amp and I am also having problems with the amp. See my post about redplating and fuses blowing, you'll see the post from Structo giving me these instructions about 3/4 of the way down page 1.

Thanks

sixpakldp · Post by **sixpakldp** » Thu Nov 19, 2009 4:25 pm

Also,

See Georges notes for the Metroamp PPMIV. Although this is a different PPMIV, he states the MV should be cranked and the amp should be rebiased.

Here are the instructions:

http://metroamp.com/wiki/images/c/c5/PP ... y_Step.pdf" onclick="window.open(this.href);return false;

toner · Post by **toner** » Thu Nov 19, 2009 5:42 pm

Roe wrote:no the larmar does not affect the bias settings

It shouldn't but does, at least in my experience.

more:
http://forum.metroamp.com/viewtopic.php ... 76#p281276" onclick="window.open(this.href);return false;

SDM · Post by **SDM** » Thu Nov 19, 2009 10:13 pm

toner wrote:
Roe wrote:no the larmar does not affect the bias settings
It shouldn't but does, at least in my experience.

more:
http://forum.metroamp.com/viewtopic.php ... 76#p281276" onclick="window.open(this.href);return false;

Should keep in mind that the meter combined with a PPIMV at various settings can be misleading when reading the negative bias voltage as opposed to bias current. Meters have a very high internal impedance when measuring DC, usually many Megs, but cheaper meters may have much lower internal resistance, and regardless it can still throw of measurements of - bias at pin 5s of the power tubes.

Example say a you have a cheap meter, specs say only 1Meg input impedance for DCV measurements. This means the meter itself looks like a 1Meg resistor and forms a voltage divider with the PPIMV pot (or just the 220Ks if no PPIMV is used) when measuring for negative DC at power tube pin 5s.

Ignoring grid stoppers, this means talking a measurement at pin 5 with PPIMV maxed (or a stock no PPIMV set up), you now have a 220K/1Meg voltage divider that otherwise wouldn't be there in normal operation (if meter was not there talking a measurement). With PPIMV off, the voltage divider disappears as the power tube grids and 1 Meg of the meter are now directly shorted to the -bias source.

Ignoring the 1Meg in parallel to the bias pot and resistor here, with PPIMV off/set at zero, say you measure -45V with this meter at pin 5s. Assuming there is no grid leakage from the power tubes, you'd then expect to see attenuation of that voltage reading when the PPIMV is then cranked to max. With this meter, attenuator/voltage divider is 220K/1000K now approx, so you'd expect the voltage reading to be reduced by 220k/1220K=18%, so would read about -36.9V now on pin 5s. So here the meter itself being in the circuit is changing your readings. If the meter was not there, the voltage divider would not be there and thus there would still be the original -45V on the grids PPIMV whether cranked or set to 0.

A better meter may have say a 10Meg input impedance. So say the same deal, measure -45V with PPIMV off, now have a 220K/10,220K voltage divider, so would expect to see the reading drop from -45V by 22K/10220K=2.15%, so about only a volt drop now to -44V. Again if the meter was not there, as would be the case when the amp is operating as normal, this drop would not be/isn't there either, voltage really is still at-45V.

All that said grid leakage can/does happen too, can follow Toner's link above to read up on that I think, but the main point is that the meter itself and quality of it can be a very misleading thing regarding pin 5 readings with different PPIMV settings. Meter specs effect how much the PPIMV setting seems to effect the - bias voltage reading. Better meters will read less change, cheaper meters more change, and that change you see well may not really there in normal operation with the meter out of the circuit - at least most of it as there could be/is some grid leakage in play.

Since grid leakage can be/is still a factor, that's the main reason you should bias with PPIMV maxed IMO, but the above is more for those of you with huge differences in - bias voltage readings like Toner had and who this response is mainly for (should have elaborated on this meter issue first time you brought up your large - voltage change). That - voltage change may well not be anything to worry about, mostly not even really there, just your meter throwing readings off. Best test is to keep an eye on the bias reading across the 1 ohmer, or whatever method you use, to check how the bias current itself changes through the PPIMV sweep. That eliminates the meter error in just checking pin 5s, and you'll know if your bias current does or doesn't really change through the PPIMV sweep, how much grid leakage plays a part.

Hope that made sense.

toner · Post by **toner** » Thu Nov 19, 2009 10:27 pm

Great info as usual Steve. To be fair and just so others know, I have a fairly cheap meter (not rat shack level but also not a high-end Fluke, etc.). I just don't have a need for an expensive meter with how little it's used.

sixpakldp · Post by **sixpakldp** » Fri Nov 20, 2009 6:39 am

That was an excellent explanation.

I also have a cheap meter. Prior to owning my amp, I mainly used it around the house when doing electrical work.

Structo · Post by **Structo** » Sat Nov 21, 2009 2:27 pm

sixpakldp wrote:Interesting . . . I only passed this on because I was told these instructions and I also noticed that when I turn the MV up to 10 my bias in mV off of pin 3 is maxed, when I turn the MV down my bias on pin 3 I see a reduction in the mV at pin 3.

I passed this info along because Structo/Tom told me that this is correct and that you should bias your amp with the MV at 10 and your Volume at 0 and also that I am correct that the MV does affect the bias current.

Let me know if this is the case because this trend is true in my amp and I am also having problems with the amp. See my post about redplating and fuses blowing, you'll see the post from Structo giving me these instructions about 3/4 of the way down page 1.

Thanks

Pin 3 is the plate or output of the power tubes. This is where the output transformer primary winding is connected.

You measure plate voltage there, not any current or bias voltage.

for bias measuring you measure in DC volts across the 1 ohm resistor on the cathode of the tube. Pins 1 and 8 should be joined and the 1 ohm resistor from there to a ground lug on the mounting bolt of the tube socket.

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