Mesa Dual Rectifier Maverick -- DC on input

[xtian]

Well, I read through many Maverick threads here and elsewhere, but did not see any similar info to my issue.

I am seeing 0.2vDC on the input jack when in Lead mode, 0.1vDC when in Rhythm mode. I have tried three different 12AX7s in V1. The ground connections measure correctly, and chopsticking makes no change.

The amp is noisy with hum and buzz--turning the Volume pots also sounds like DC on the pots, though I read only 0.020vDC on the pots.

How can any DC leak onto the input jack?!?

Mav.pdf

[Pedro Vecino]

Yesterday exactly the same thing happened to me with a DC3. The volume of the guitar creaked a lot with the lead channel selected. I took out the jack, lubricated the toothed washer and the contacts, and then reassembled it. The problem disappeared. However I have pending to put a new jack.

[alexradium]

secondary emission is a byproduct of fairly low anode voltage ,especially on modern production tubes,if its annoying try add a 100n and another 1M to ground to block any leakage.
I've seen several MB that leak because of bad boards and defective or bad designed switching systems.

[Helmholtz]

secondary emission is a byproduct of fairly low anode voltage

Not much known about "secondary emission" in small signal triodes. What's your source of information?

[Helmholtz]

I am seeing 0.2vDC on the input jack when in Lead mode, 0.1vDC when in Rhythm mode.

Is the DC positive or negative wrt ground?

I guess the voltage drops to zero without a plug inserted?

[xtian]

Well, still can't explain the stray DC on the input, but my main issue, nasty hum, was coming from a bad solder joint on the Lead channel's Master Vol pot's connection to the PCB.

[xtian]

DC is positive WRT ground, and yes, vanishes when no plug inserted (input jack is properly grounding the tip).

[Helmholtz]

DC is positive WRT ground, and yes, vanishes when no plug inserted (input jack is properly grounding the tip).

Strange. Have you also tried different tubes in V2?
Does the instrument connected have active electronics? Just excluding the DCV comes via the instrument.

[drewl]

I've seen bad cathode caps do weird stuff like that.
Can't hurt to try.

[pdf64]

Some 12AX7 have a higher than desirable degree of (usually positive) grid current, an operating point with very low plate voltage seems to exasperate the issue. Try some tubes from different manufacturers.

[Helmholtz]

Some 12AX7 have a higher than desirable degree of (usually positive) grid current

Normal grid leak current is caused by electrons hitting the grid and charging it negative. This is how and why grid leak biasing works.

Positive grid leak would mean a gassy tube that must be considered defective. I don't think it would produce the specified circuit voltages (at plate and cathode). Also I would expect it to be noisy.

[Chuck H]

A lot of the above is indicative of trying a different input stage tube. This is a good idea. I've seen tubes do some weird things and what you describe would be the least of it.

[xtian]

I have tried a variety of tubes in V1...and the evidence is getting WEIRD.

I inserted a 12" instrument cable in the input jack and attached my multimeter to tip and sleeve. I turned the Output Level (master master volume) to zero and powered up. In Rhythm mode I see -12mv (thanks, @Helmholtz, for pointing out the importance of pos/neg). And in Lead mode, I see -170mV.

I tried several different tubes, and got similar readings.

Now the weird starts. I put the amp in standby while swapping V1...and I noticed I still had -ve DC on the input jack! And then I pulled V1 out, and STILL had -ve DC on the input! With no V1, in run mode, I see -15mV in Rhythm and -76mV in Lead, and in standby, I see -10 and -28mV.

Looking at the schematic, assuming no V1 is installed, wouldn't you have to suspect LDR1? I don't see any other path for stray potential.

[Chuck H]

Looking at the schematic, assuming no V1 is installed, wouldn't you have to suspect LDR1? I don't see any other path for stray potential.

That's what I get.

[Helmholtz]

I have tried a variety of tubes in V1...and the evidence is getting WEIRD.

I inserted a 12" instrument cable in the input jack and attached my multimeter to tip and sleeve. I turned the Output Level (master master volume) to zero and powered up. In Rhythm mode I see -12mv (thanks, @Helmholtz, for pointing out the importance of pos/neg). And in Lead mode, I see -170mV.

I tried several different tubes, and got similar readings.

Now the weird starts. I put the amp in standby while swapping V1...and I noticed I still had -ve DC on the input jack! And then I pulled V1 out, and STILL had -ve DC on the input! With no V1, in run mode, I see -15mV in Rhythm and -76mV in Lead, and in standby, I see -10 and -28mV.

Looking at the schematic, assuming no V1 is installed, wouldn't you have to suspect LDR1? I don't see any other path for stray potential.

Now this is confusing as all voltages are now negative and inconspicuous regarding level. What does it take to restore the higher positive voltages?

Again my earlier questions: What changes with a different tube in V2? Can you exclude positive DC being introduced by the (active) instrument?