Mesa Bass 400 - Snap, Crackle and Pop

[christarak]

Have I mentioned that I dislike Mesa amps? They almost always cost me money. The only reason I am not pulling out tufts of hair is that Sydney is in lockdown again and I have few other repairs. This time I am doing battle with a Bass 400. The schematic looks simple enough, but this is where Randall Smith’s evil genius lures in the innocent, then slams the trap shut.

The noise is that typical crackling plate resistor sound. The noise is affected by the Master but not the individual Volumes. The noise is still present when V1 is pulled, but removing V2 brings blessed silence. Easy, me thinks. Just change the 100K plate resistor on V2. No luck. Better clean the valve sockets, input and Send/Return sockets. Nope. Still crackling.

I made a little sound probe to plug into a battery-powered amp to pin down the noise. It works a treat. In the attached drawing “N” means “noise at this test point” and “S” means “Silent, except for the usual valve noise”.

The schematic didn’t quite match up with reality, hence my little sketch. I also replaced the 100K and 10M resistors and the 250pF in the tone stack. I hope I’m wrong, but it's starting to look like the Treble pot with the Switch piggy-backed. That would be a major PITA requiring the removal of a Mesa PCB and tracking down a part from the new Mesa distributor, so I am very much hoping that you have another suggestion.

[Jon Snell]

I would suspect/replace the two components I have marked.

It's probably only the 3.3M at fault.

[Helmholtz]

The V2B wiring looks wrong. I would have expected an additional resistor of 3.9M from grid (pin 7) to ground.

Otherwise the voltage drop across the 3.3M of 150V can ony be explained by permanent grid conduction/current, resulting in tube saturation.

[vintagekiki]

Is there a crackling when .047u (pin7) is removed from the treble pot.
First check that the solders are correct, that there is no cold solder somewhere.
If V2B voltages are correct check or replace V2, 33k, 3.3M, .047u, .1u

Schematic is OK. It is similar to Mesa Buster Bass 200.
https://sites.google.com/site/guitartubeamprepair/mesa-buster-bass-200
https://www.talkbass.com/threads/mesa-200-buster-club.965576/ ¹⁾

[Helmholtz]

Schematic is OK. It is similar to Mesa Buster Bass 200.

CF won't work properly without the missing resistor. Would heavily distort by clipping positive CF input signals. Also tube voltages would be different.

[vintagekiki]

CF won't work properly without the missing resistor. Would heavily distort by clipping positive CF input signals. Also tube voltages would be different.

Bias CF (V2B) is OK (2VDC)
CF without Rg1 according to gnd is used by almost all amplifiers.
Rg1 in CF is used only in combination with Rk (1k) in serie with Rcf (33k)
_{Translation is is not a literary, technically quite well understood.}

1)
http://www.valvewizard.co.uk/dccf.html
The DC Coupled Cathode Follower

http://www.valvewizard.co.uk/accf.html
The AC-Coupled Cathode Follower

[Helmholtz]

[QUOTE=vintagekiki;n937498]
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.

[vintagekiki]

Sorry, no.

You are talking about fixed biased cathode follower.
1)

The factory is certainly not wrong.
It would be interesting to find a layout or pcb for Mesa Bass 400

[Helmholtz]

You are talking about fixed biased cathode follower.

All cathode followers are essentially auto-biased, but they need a (typically positive) grid reference voltage to set the cathode voltage to desired level/operating point.
The cathode voltage follows the grid voltage within ~ 2V.

The factory is certainly not wrong.

Maybe, but schematics have errors.

[Dave H]

CF won't work properly without the missing resistor. Would heavily distort by clipping positive CF input signals. Also tube voltages would be different.

That's what I thought too but LTSpice says the cathode sits at 272V meaning there must be grid current and a positive grid to cathode voltage. The grid can't be at -2V wrt the cathode as on the schematic. I suspect those are measured values and the grid voltage is being pulled down by the measuring instrument.

[vintagekiki]

All cathode followers are essentially auto-biased, but they need a (typically positive) grid reference voltage to set the cathode voltage to desired level/operating point.
The cathode voltage follows the grid voltage within ~ 2V.

You are talking about AC-coupled cathode follower.

Maybe, but schematics have errors.

It is unlikely that there are errors at 3 Boogie models (MB Bass 400, MB Bass 400+, MB Buster Bass).
Boogie would notice it and correct it.

https://forum.grailtone.com/
The Boogie Board

[Helmholtz]

That's what I thought too but LTSpice says the cathode sits at 272V meaning there must be grid current and a positive grid to cathode voltage. The grid can't be at -2V wrt the cathode as on the schematic

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.

[Helmholtz]

You are talking about AC-coupled cathode follower.

Yes, it IS an AC coupled cathode follower.

[vintagekiki]

I suspect those are measured values and the grid voltage is being pulled down by the measuring instrument.

The measuring instrument pulled down voltage on the grid and on the cathode at the same time.

[Dave H]

The measuring instrument pulled down voltage on the grid and on the cathode at the same time.

No, the cathode is low impedance.