New build Low volume and LarMar issue

[yalesmith]

I just finished an express (my second) with a Lar MV i have the following issues.

Low Volume - This is the second amp I have built that have low volume, all the voltages are good but low volume. The other amp (rocket) has the same issue....Both amp sound like 12 watt amps rather than 30 watt amps. No clue what it can be.

LARMar - It works except when I turn it all the way up the bias goes crazy. The Tubes begin to red-plate and to flicker. I originally made the mistake of not cutting the leads on 1 of the 2.2m resistor and it went into the body of the pot, wonder if I destroyed the pot but I don't think so. Also i connect the swamper resistor at the pot and not the tube if that makes any difference.

Any Ideas?

[JazzGuitarGimp]

You have the CCW lugs of your dual-gang pot connected to the negative bias voltage, and not GND, correct?

[yalesmith]

CCW? I am not familiar with the term but i have the left lugs (looking from back) connected to bias and not ground. I am able to bias the amp perfectly , i have bias jacks to measure tube dissipation. I also have the back of the dual pot grounded but that should not make a difference I think.

[M Fowler]

counter clock wise (CCW)

[Darrell minx]

I have had a few issues with that design myself. one thing is it is not grounded... I switched over to the style you can see on the Weber amps site "java" model.... it is quieter, safer and grounded as well... I belive you can trace this MV design back to early 70's Orange amps or maybe even as far back as matamp...... One thing I know for sure with either design is the coupling caps coming off the phase inverter have to be very strong at least 600 volts did not work right with 400 volt sozo. I would change the caps first and see if that helps.

[Darrell minx]

to adress the low volume problem... could be impeidence mis-match with the OT. what kind of transformers are you running? also double check values of every resistor.

[M Fowler]

Are you connecting left pot pins to bias?

I've used this LarMar MV in amps without any problems?

Mark

[yalesmith]

I am double checking everything and i realized i was using 1k ohm swamper resistors instead of 1.5k....which should not make much of a difference i think.

- I am also running Pacific OT transformer (the one RJ use to sell).
-The coupling caps are rated at 630 volts, they are mallory 150's
- The bias is being routed off the left pins as well

It is about 2 am in NY and as i mentioned i swapped the swamper resistors from 1k to 1.5k so i will not be able to test until later.

Thanks

[Smokebreak]

I would look for a cold solder joint. How does the amp function without the MV, wired stock with the 220K's?
This may help : http://forum.metroamp.com/viewtopic.php ... 0&start=15

[yalesmith]

also does anyone have the schematic for RJ's pacific express OT? more specifically the primaries, I think i may have mixed up the color codes but am not sure.

[martin manning]

RJPOT

[yalesmith]

SH$$T thats my issue, I did it by memory I will hook it up tomorrow. such a dummy mistake! I cant believe i hooked up the CT as one of the primaries.

Thanks

[jjman]

If the bias current goes way up at max master vol the contact in the pot is probably bad at that point on the pot's surface. Assuming the wiring is correct. "Safety resistors" from the wiper to bias lug should avoid the issue but would be masking the real problem in the pot.

[yalesmith]

I switch the OT primaries and bam all good. I am getting oscillation and feedback when I turn it past half way but I have read it may be NFB phase issue, which I think is a fancy way of saying switch the grid wires coming from PI.

[JazzGuitarGimp]

I switch the OT primaries and bam all good. I am getting oscillation and feedback when I turn it past half way but I have read it may be NFB phase issue, which I think is a fancy way of saying switch the grid wires coming from PI.

it might be easier to swap the green & violet OT primary wires.