Mark V 90 hum in lead channel, 45 and 90 watt modes only.

[woodbutcher65]

I've noticed that my Mark V 90 has developed a bit of a hum in channel 3, only in 45 and 90 watt modes. And only when the rear panel triode/pentode switch for channel 3 is in triode mode.

10 watt mode has no hum, and no power mode hums in either channels 1 or 2.
Neither does tube/diode switching cause hum in channels 1 or 2.

It's the same whether in variac or full power mode.

Reverb, EQ, and solo boost do not cause any changes.

Setting the voicing switch on channel 3 to Extreme slightly increases the hum level.


Got any ideas?

I'm going to check power tubes, it could be one. I'll do it by first pulling the outer pair of power tubes, and seeing if the hum goes away,
and if not, put them back in place and then pull the inner pair and check again. If that identifies a tube problem, I'll replace that pair.

 

[woodbutcher65]

Update: Did the tube pair swap check, narrowed it down to a pair of STR 447s. Put in a well matched pair of spares, the hum is still there.

I guess I'm going to have to pull the chassis and check to see if any resistor values around those tube sockets (screen resistors) may be off value and thermally damaged. That could account for it.

 

[mace]

Channel 3 in 10W mode is pentode. So maybe there is something up with the channel 3 triode circuit, since that’s the only time there is hum?

 

[woodbutcher65]

It was actually a tube. It checked fine on my tube tester but that doesn't cover all possible operating modes. Strange that it would hum in triode mode, and not pentode mode, but it did.

So, the 10/45/90 front panel switch is only correct in its labelling if the tube is set to pentode mode, is that correct?


Even though I've had a Mark V:90 for more than a year I'm still discovering things about it.

 

[rarebitusa]

The way I understand it, which may be incorrect, is the front selects the mode. 90W mode is always Simulclass with diode rectification. The 45 mode on chans 1/2 have the tube/diode option. Ch3 45W mode is the only one that has the triode/pentode selection option. The 10 W mode always uses the tube rectification.

 

[mace]

Glad you’ve got an easy fix with tube replacement only.

I think ‘rectification’ and ‘pentode/triode’ are being mixed up.

For the other points:

The ‘pentode/triode’ selector for channel 3 only works in 45W and 90W modes. Channel 3 10W mode is hard wored pentode. Mesa confirmed this in an email as the manual isn’t clear. You can test yourself by hitting low chords and moving the pentode/triode switch. It does nothing in 10W mode. This is all channel 3 only.

Rectification with ‘tube’ or ‘diode’ is something else entirely. Per the manual channels 1 and 2 only at 45W allow you to choose between tube and diode rectification with the toggle. 10W mode is always tube and 90W mode is always diode for channels 1 and 2. However, channel 3 is always diode rectified at all power levels 10, 45 and 90W.

 

[mace]

So, the 10/45/90 front panel switch is only correct in its labelling if the tube is set to pentode mode, is that correct?

The 10/45/90 panel switches are always correct. The difference between pentode and triode on channel 3 45W and 90W is a tone and ‘feel’ changer. Pentode has more lower mids on the tone and triode feels a little different. Power output is the same for either pentode or triode.

 

[rarebitusa]

Glad you’ve got an easy fix with tube replacement only.

I think ‘rectification’ and ‘pentode/triode’ are being mixed up.

For the other points:

The ‘pentode/triode’ selector for channel 3 only works in 45W and 90W modes. Channel 3 10W mode is hard wored pentode. Mesa confirmed this in an email as the manual isn’t clear. You can test yourself by hitting low chords and moving the pentode/triode switch. It does nothing in 10W mode. This is all channel 3 only.

Rectification with ‘tube’ or ‘diode’ is something else entirely. Per the manual channels 1 and 2 only at 45W allow you to choose between tube and diode rectification with the toggle. 10W mode is always tube and 90W mode is always diode for channels 1 and 2. However, channel 3 is always diode rectified at all power levels 10, 45 and 90W.

Good explanation there... yea the manual is a bit misleading, they talk about the IIC+/MkIV, but the Pentode/triode works on all 3 voicings in 45 W mode. Now I never knew that 90W on Chan 3 also was switchable, had never tried it but sure enough this is true. Ch 3 Triode mode with 90W is actually very cool, thanks for outlining that fact.