Mesa Bass 400 - Snap, Crackle and Pop

[Dave H]

Confused.
In post #10 you said 272V ?

Sorry my mistake. It should be 172V as the schematic. I didn't expect it but it's what LTSpice says.

[christarak]

(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.

Dude, have I told you lately that I love you! Reading your comment has just lifted 12kg (the weight of this boat anchor Bass 400) from my shoulders.

[christarak]

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.

Of course, you are correct. The fluke measured a 10M resistor at 10.1M and the ET3240 Benchtop (cheap Chinese...it's very slow compared to the Fluke, but I like it) at 10.13M. I lifted the 3M3 from the grid, and the grid measures open circuit of both meters. That 52 microamps of current must be flowing into the grid. Mystery.

[Dave H]

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.

There is grid current flowing. The real grid to cathode voltage can' t be -2V I'd expect it to be positive. Are you sure the grid isn't at +0.38V wrt cathode?

[Helmholtz]

Sorry my mistake. It should be 172V as the schematic. I didn't expect it but it's what LTSpice says.

Amazing, thanks.

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?

[Dave H]

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.

I was thinking it's a single meter and a single measurement error. The cathode voltage measures correctly (low impedance) but the grid measure low (high impedance) the cathode has followed the loaded down grid but it's not being measured at that time.

[The Dude]

Just skim read the thread and may have missed something. You said pulling V2 gets rid of the noise, but did you try a new tube at the V2 position? Maybe the noise is just arcing in the tube.

[christarak]

Victory! The problem has been found.
I replaced the 3M3 with a 2M2 (no stock on hand of the 3M3). The voltages changed to cathode 145v and grid 143v, with a -2v bias. The crackle remained.
I then replaced the 33K on the cathode of V2B...and BINGO! Silence returns.
Should I replace the 2M2 I used with a 10M resistor...or just order another 3M3?
Joy. And learning. Two common results of this forum. Thank you all for your expertise.

[vintagekiki]

>> post #4

[Helmholtz]

I was thinking it's a single meter and a single measurement error. The cathode voltage measures correctly (low impedance) but the grid measure low (high impedance) the cathode has followed the loaded down grid but it's not being measured at that time.

Understood.

My post was written under the wrong assumption of a considerably higher cathode voltage.

[christarak]

Just skim read the thread and may have missed something. You said pulling V2 gets rid of the noise, but did you try a new tube at the V2 position? Maybe the noise is just arcing in the tube.

yes I tried 2 different tubes in V2...just because I don't trust 1. The fault ended up being the 33K cathode resistor.

[christarak]

There is grid current flowing. The real grid to cathode voltage can' t be -2V I'd expect it to be positive. Are you sure the grid isn't at +0.38V wrt cathode?

I just checked it again Dave. Keeping in mind that I now have 2M2 from grid to the 344V supply (no stock of 3M3) the voltages referred to ground are now: Cathode @149.1v and Grid @147.2V. With the Fluke negative lead on pin 8, I am reading -0.260v at pin 7.

[Helmholtz]

I replaced the 3M3 with a 2M2 (no stock on hand of the 3M3). The voltages changed to cathode 145v and grid 143v, with a -2v bias.

Voltages getting lower with the 2M2 is even more surprising.

[christarak]

Voltages getting lower with the 2M2 is even more surprising.

Wouldn't using a lower resistance increase current and in turn lower voltage? it works out to 88microamps with the 2M2 resistor compared to 50microamps with the 3M3. I can now hear a 50Hz (Aussie) hum that was not audible with all that crackling noise. I have not changed the 13(!!!) filter capacitors, but now you have me thinking that the V2 bias may be out by me using the 2M2.
If my local (30 mins each way) electronics supply store has the 3M3, I may go for a drive so I can put this to bed.

[The Dude]

You got 1M resistors? Just put a 1M and your 2.2M in series and call it good. Close enough. Save a trip to the store.

[christarak]

You got 1M resistors? Just put a 1M and your 2.2M in series and call it good. Close enough. Save a trip to the store.

Yes! Of course, Dude. This amp has fried my brain cell.
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.