Mesa Bass 400 - Snap, Crackle and Pop

[christarak]

>> post #4

I am sorry, but I am not sure what your comment above refers to.
Certainly, you were correct in suggesting the 33K. I did reflow all of the solder connections in the V2A to V2B area as you suggested, but I found the culprit before I disconnected the 0.047 cap. I appreciated your input and learned from your discussions with the other members. I appreciate your invaluable time in supporting my queries. All the best from Sydney. :-)

[vintagekiki]

There is definitely an error in the schematic around the Treble shift switch location in the tone stack.

Schematic around the treble shift switch is OK.
When the treble shift switch is in the on position, a 750 pF cap is connected in parallel to the 250pF cap.
The 10M resistor prevents transients when switching the treble shift switch.

[Dave H]

Looks like a kind of borderline operation where the strong cathode degeneration stabilizes the grid current at a low level (45µA here).
Just wondering what defines the grid voltage (or current).
Shouldn't there be considerable variation between tube samples?

What was the signal source impedance when simulating the plate voltage swing?

Signal source was a 12AX7 gain stage.

Here are the small and large signal plots

[Helmholtz]

I put the 1M in series. The voltages are still lower than the original now with Cathode @140.1v and Grid @138.2V. The faulty 33K cathode resistor measures 60K which will have affected the current and voltages. My voltages are lower than the schematic, but it works and is quiet. The LF hum is better, but that may be my wishful thinking.

Now the voltages changes make sense: With 3M3 the voltages are lower than with 2M2 because of lower grid voltage and lower cathode current.
With the 60k cathode resistor the voltages were higher than with 33k because of increased voltage drop.

I don't think the resistor values, resp.the operating conditions of the CF can have a noticeable influence on the hum.

[Helmholtz]

Signal source was a 12AX7 gain stage.

Here are the small and large signal plots

Thanks Dave H. Very interesting.

Happy having learned something new.


Circuit operation requires some grid current because without it both grid and cathode voltages would essentially be at B+.
I think there must be an equilibrium grid current (and Vgk), where two opposing effects mutually compensate.

Maybe like this:

Higher than equilibrium grid current would require a more positive Vgk, while the increased grid current would lower the grid voltage (via the 3M3),
thus counteracting the Vgk getting more positive.
Also a more positive Vgk would increase cathode current and cathode voltage, likewise counteracting a more positive Vgk.

[Helmholtz]

My voltages are lower than the schematic..

Your voltages look good and should allow for more than ample signal swing according to the simulation resuts of Dave H.

I think that actual circuit voltages will depend on individual tubes.
Grid conduction mode is not covered by tube datasheets and I would expect considerable variation.

[vintagekiki]

The faulty 33K cathode resistor measures 60K

Definitely fault is in the resistor.
It happens that the resistor changes its value after some time during operation. This is due to the unsafe contact of the leadout wires with the resistance material, so measurements can show one value at "cold" and another value at voltage. In the sound it can be manifested as a transient phenomenon, squeaking, crackling ...
The change in the value of the resistor is more pronounced at >MOhm resistors especially if it at voltage.

[vintagekiki]

My voltages are lower than the schematic, but it works and is quiet.

Your voltages are within limits
On many MB schematic says that DC voltages may vary 20%
https://schematicheaven.net/boogieamps/boogie_mkiv.pdf

[christarak]

Definitely fault is in the resistor.
It happens that the resistor changes its value after some time during operation. This is due to the unsafe contact of the leadout wires with the resistance material, so measurements can show one value at "cold" and another value at voltage. In the sound it can be manifested as a transient phenomenon, squeaking, crackling ...
The change in the value of the resistor is more pronounced at >MOhm resistors especially if it at voltage.

This is exactly what I saw. Thank you.

[christarak]

Definitely fault is in the resistor.
It happens that the resistor changes its value after some time during operation. This is due to the unsafe contact of the leadout wires with the resistance material, so measurements can show one value at "cold" and another value at voltage. In the sound it can be manifested as a transient phenomenon, squeaking, crackling ...
The change in the value of the resistor is more pronounced at >MOhm resistors especially if it at voltage.

That is VERY interesting. I will remember this for future high M scenarios. I really appreciate your trouble.