Mesa Bass 400 - Snap, Crackle and Pop

[Helmholtz]

No, the cathode is low impedance.

Sure, but the cathode voltage follows the grid voltage (it's an AC as well as a DC follower).
But the double voltage measuring error would have required measuring cathode and grid voltages with 2 meters at the same time, which is rather unlikely.
So I think the 3.9M is there.

[Dave H]

I still think the amp has a 3.9M resistor from grid to ground which is missing in the schematic - an Ohmmeter could tell.

The cathode is sitting at 172V without the resistor and it can manage 100V p-p output with only a little rounding on the positive peaks (not what I expected)

[Helmholtz]

The cathode is sitting at 172V without the resistor and it can manage 100V p-p output with only a little rounding on the positive peaks (not what I expected)

Confused.
In post #10 you said 272V ?

[g1]

No, the cathode is low impedance.

But while the grid measurement is made, the cathode voltage would drop, but this would not appear when measuring the cathode?

[vintagekiki]

The measuring instrument (> 10M) does not load the measuring point.

[christarak]

Thank you for the replies. It's now 630am. Coffee#1 is making its way to my brain cell. I will now measure those voltages, check the grid of the CF for unnoted resistors and report back before I try the key suspects suggested. Thanks, guys. I'll be back...

[vintagekiki]

Coffee#1 is making its way to my brain cell.

Good morning.
In addition to morning coffee, be sure to have a small cup of schnapps for better circulation.

[christarak]

Well that's more or less what I predicted in my earlier posts (post #3 ff) The only way to drop a voltage across the 3.3M resistor is grid conduction, causing tube saturation and clipping of positive grid signal halfcycles.

In grid conduction mode the grid input is low impedance - at least for positive going signal.

I still think the amp has a 3.9M resistor from grid to ground which is missing in the schematic - an Ohmmeter could tell.

It's odd that there is nowhere for that current to flow, other than into the grid. I can see no other resistor referenced to ground. My benchtop meter measure 7M to ground, but my Fluke measured your predicted 3M9. I assume the difference is the input Z of the 2 meters.

[christarak]

[QUOTE=Helmholtz;n937504]

Bias CF (V2B) is OK (2VDC)
CF without Rg1 according to gnd is used by almost all amplifiers.
QUOTE]

Sorry, no.

The -2V bias as shown in the schematic can only be achieved with an additional resistor to ground, forming a voltage divider with the 3.3M.

In a DCCF (as in the Vox) things are different, as the grid voltage of the CF is defined by the plate voltage of the preceding stage.

In normal operation (i.e. without grid conduction) there is no grid current which could drop voltage across the grid series resistor.

So if the grid is only connected via a single resistor to B+, the grid would sit at full B+ / or go into grid conduction and saturation.

Without a current through the 3.3M resistor, voltages at both ends must be the same, .i.e. 320V at the grid.

The impedences are so high that I am sure the meters load down the circuit. Referenced to ground, the voltage on V2BK is 178V, and on V2BG is 176V, giving us -2V and the sun is shining. However, measuring directly between pins 7&8 produces -0.38V, and the Mesa clouds come to obscure the sun once more.

[christarak]

Good morning.
In addition to morning coffee, be sure to have a small cup of schnapps for better circulation.

HAHAHAHA! Normally I wouldn't this early, but if my more learned colleague suggests this as a way of better coping with Mesa amps, then I have no other option than to comply.

[christarak]

I have measured the actual voltages and attached them for you. I was unable to physically find the large resistor to ground for the V2B bias that Helmholtz was logically suggesting. See updated schem. There is definitely an error in the schematic around the Treble shift switch location in the tone stack. I will now begin following your advice(s) for finding this damned crackle. I'll report back shortly...hopefully with good news. THANK YOU GUYS!

[Helmholtz]

But while the grid measurement is made, the cathode voltage would drop, but this would not appear when measuring the cathode?

Find the "hidden" voltage divider .
(Loading down always means voltage dividing between source resistance and load resistance.)

[Helmholtz]

My benchtop meter measure 7M to ground, but my Fluke measured your predicted 3M9. I assume the difference is the input Z of the 2 meters.

Meter input resistance matters with voltage but not with resistance measurements.
Compare your meters with a known megohm resistor.

But you need to lift the 3M3 resistor from the grid before measuring grid to ground resistance.
Otherwise there will be a DC path through the power supply.

[The Dude]

[FONT=Calibri]Have I mentioned that I dislike Mesa amps? They almost always cost me money......

(Diverting the thread momentarily) This shouldn't be the case. I've found that you have to charge what the repair is worth. And frankly, MESA amps should cost more to repair. They are more of a pain to fix and cost more to replace. IMO, if you're losing money on MESA amps, you're not charging enough to fix MESA amps. Again, keep in mind the replacement cost of this amp. The customer should just be glad you're not charging by the pound.

[Dave H]

But while the grid measurement is made, the cathode voltage would drop, but this would not appear when measuring the cathode?

Yes, that's what I was meaning