VVR Install Problems

[bclick]

I have just installed Dana's nice VVR2 kits in my Metro Plexi 50. This is a fixed-bias, power tube only implementation. I just fired it up and something is wrong.

When I plugged the guitar in, it sounded normal. Settings were: MV 10, Vol1 2, Vol2 2, VVR 10. As I turned the VVR pot up, there was no volume change, but a loud buzzing appeared in the middle of the pot range, then went away as the pot reached the other end of it's rotation. I turned the amp off then to prevent any possible damage.

Voltage levels to power tubes look ok. V to preamp tubes is low, and I am wondering if I need to bypass the 10K and/or 8.2K resistors in the B+ line that dropped the voltage after the feed to the power tubes. Attached is my schematic with voltage readings. All in VDC unless noted.

<incorrect schematic removed>

[UR12]

Are you sure your schematic is correct. You usually don't have a center tap on your secondary when using a bridge rectifier. Hum like you describe is usually caused by noise getting onto wires between the mosfet and the board.

[bclick]

Hi Dana - No, I am not 100% positive my schematic is correct. I have posted on Metro forum for vetting there as well.

I believe the rectifier circuit is correct. The Marshall/Metro circuit only uses 2 of the diodes in the bridge and there is a center tap.

This is a "temporary" install until I am sure I like the sound. Was not sure where to place the VVR pot, so for testing I just pulled the Presence pot out of it's hole and pushed down out of the way, and installed VVR pot. Perhaps the proxitmity of VVR pot/wires to Presence pot is causing buzz?

But should I not be hearing a volume reduction any way?

Some pics attached. Thanks!

[jaysg]

re: the PT taps

If you poke around a pile of schematics with a FWB and a center tap, you'll see that they'll connect it to the middle of the first PS caps if they're in a totem pole arrangement. I'm not sure why. I've got one and I disconnected it just to see what would happen. I couldn't hear a difference and voltages all stayed happy.

[Jana]

The reason why the CT is connected to the middle of that totem of caps is to equalize the voltage across the caps. With it disconnected there could be 500 volts on one cap and 10 on the other (or any other numbers you choose). Bclick is not using a FWB configuration, only using two of the diodes in the FWB to create a Full Wave, hence the center tap is grounded.

[bclick]

Thanks for the replies. Jana is correct: both the original 1987 and the Metro version only use 2 diodes from the bridge, and ground the center tap.

Should I bypass one or both of the 10K & 8.2K resistors in the B+ line that feeds the preamp tubes? B+ to these tubes is lower since installing VVR and I only am regulating B+ to power tubes.

- Bil

[UR12]

Thanks for the replies. Jana is correct: both the original 1987 and the Metro version only use 2 diodes from the bridge, and ground the center tap.

Should I bypass one or both of the 10K & 8.2K resistors in the B+ line that feeds the preamp tubes? B+ to these tubes is lower since installing VVR and I only am regulating B+ to power tubes.

- Bil

before you go unhooking a bunch of stuff can you do some voltage measurements? Measure the voltage to the power tubes with the dual pot set to full power and check it again with it set to minimum power. Also check your bias reading at full and minimum.

[UR12]

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[bclick]

OK - I see where I goofed up. Doh! I connected the bias supply directly from the bridge to the VVR instead of farther into the bias circuit - as per your drawing!

Is the grey rectangle the 'symbol' for the board in your directions? Then the junction of the 2nd bias cap and resistor goes to Bias IN.

Is the dual gang pot turned full CW for maximum voltage?

Revised schematic included. Ignore voltage readings - I have not physically made the changes yet. I will delete the drawing in my 1st post so no one copies it and makes the same mistake!

[UR12]

I have attached a revised schematic showing the changes you need to make. You may have blown your mosfets but it is too early to tell. You should get max voltage when you turn the pot full clockwise. Measure the voltage as you turn the pot and see if your B+ is varying and let me know what your voltages. By the same token you can measure the bias voltage and see if it is changing also. If they aren't changin there is a good chance you need to replace your mosfets.

[bclick]

Thanks Dana. I made the changes and the voltages seem to be tracking.

VVR Pot____V5 Cathode____V5 Bias
Max________431V________40mV
half________243V________18mV
Min_________40V________3.6mV

Guitar volume changes very slightly with the VVR pot, with more distortion at min VVR pot.

And there is a loud buzzing noise between 40% & 60% on the VVR pot, almost sounds like an electrical arcing noise. I made a shield with alum foil & cardboard to shield the presence pot from the VVR pot wires. Made no difference.

[UR12]

Thanks Dana. I made the changes and the voltages seem to be tracking.

VVR Pot____V5 Cathode____V5 Bias
Max________431V________40mV
half________243V________18mV
Min_________40V________3.6mV

Guitar volume changes very slightly with the VVR pot, with more distortion at min VVR pot.

And there is a loud buzzing noise between 40% & 60% on the VVR pot, almost sounds like an electrical arcing noise. I made a shield with alum foil & cardboard to shield the presence pot from the VVR pot wires. Made no difference.

Ok, Sounds like we may be getting some where. You stated "v5 cathode" but I think you meant v5 plate. The cathode should be grounded. Your Bias voltage should be like -40v not mv.

How and where are you measuring your bias?

How are you setting your bias?

It may not be a problem but you have your board mounted right on top of your power tranny windings and they eminate 60hz hum. I wouldn't worry about that yet, lets see if we can figure out why your vol isn't changing when you change the setting of the guanged pot.

[bclick]

V5 plate - you are right. Measuring at pin4 on V4 & V5. Measuring Bias at pin 1 and it is definitely 40mV. There is a 1 ohm resistor between pins 1 & 8 and ground on each power tube, so measuring 40mV here = 40mA, which is upper range for EL34. (I need to add these R to the schematic)

I have adjusted both board bias pot & VVR bias pot and both can affect the bias. I am setting it with VVR dual gang pot full CW.

[UR12]

V5 plate - you are right. Measuring at pin4 on V4 & V5. Measuring Bias at pin 1 and it is definitely 40mV. There is a 1 ohm resistor between pins 1 & 8 and ground on each power tube, so measuring 40mV here = 40mA, which is upper range for EL34. (I need to add these R to the schematic)

I have adjusted both board bias pot & VVR bias pot and both can affect the bias. I am setting it with VVR dual gang pot full CW.

With the 1 ohm resistors you are actually measuring current not voltage. You need to check the voltage on pin 5 and see what that is to make sure it is tracking properly. I just picked up some more heat sinks and can get you one in the mail today if your still interested.

[mlp-mx6]

With the 1 ohm resistors you are actually measuring current not voltage. You need to check the voltage on pin 5 and see what that is to make sure it is tracking properly. I just picked up some more heat sinks and can get you one in the mail today if your still interested.

bclick is correct, he measures the voltage drop across the resistor and converts directly to current. He is not measuring current across a resistor.