Mkiic+ problem: STILL can't get a clean tone

[GeoBull]

I have an '84 Mkiic (now a '+'): 60/100, EQ, Reverb, EVM 12". I recently sent it to Mesa for Mike B. to mod to '+' (GREAT customer experience BTW). Since I got it back I haven't been able to get close to the clean headroom I was used to. Because one of the power tubes was bad when I got it back I switched them all out with new Mesa top-of-line 6L6. After that for a test I switched out all the 12ax7's. Still not much real clean headroom.

It seems unlikely that something mechnical would be wrong: the amp was in Mike B's capable hands for a few weeks. Any thoughts on what might be happening here? Is the '+' mod just a dirtier amp?

BTW...recently bought a Lonestar 2x12, 2007 with 10w class A option. Great amp.

Thanks in advance for your help.

 

[GeoBull]

Thanks for the reply.

It starts breaking up at 6 on channel, 2-3 on Master. This is a setting that used to be clean and strong. It's now strong but slightly dirty.

 

[beaurawls]

Since you replaced the outputs, you must check and probably adjust the bias. If your bias is such that the output tube idle current is less than it should be, the amp will break up earlier and have less power out.

Regards,
beaurawls

 

[GeoBull]

Since you replaced the outputs, you must check and probably adjust the bias.

Thanks for the reply. I was under the impression that Boogies don't need bias adjustment? [1] Is that incorrect?

[1]
From the Mkiic manual:

Negative bias voltage has been permanently set during amplifier burn-in and should never need
adjustment. Correct bias voltages varies according to amplifier model: Measured at pin 5 of each 6L6, the
correct bias for Standard Sixty watt models should be -47, Hundred Watt = -52 (or -47 for Hundred
Export) Simul-Class: -56 and -46 (-46 and -37 for Export Simul-Class) Series 300: -56.)

 

[Boogiebabies]

I'm really wondering if you have a preamp plate load resistor going south.
It may be adding higher than normal voltage to your V1 or V2 and clipping. I run my volume 1 at 8. With a humbucker it will bark with a decent pick attack, but with a single coil it is clean and sweet.

 

[GeoBull]

Thanks for the reply, BB. Would a good amp tech be able to fix this? Or diagnose it? Or will it need to go back to Mesa?

 

[beaurawls]

Geobull,
Under NO circumstance try to set bias by a voltage reading of bias voltage. Bias can ONLY be set by measureing, either directly or indirectly,the plate current.

Output tubes have a designated plate power dissipation, The plate dle current in milliamperes times the plate voltage gives you the idle power in watts. This must be within a critical range. To little and a thin weak amplifier power out will result. Symptoms are lower power than you used to have and earlier breakup. Setting to specs will result in a fuller fatter tone and more punch. Setting too high will cause the output tubes to overheat and glow red on the plate structure. This will eventually short the tube and amplifier damage may result.

Even with selected tubes from Mesa-or any other distributor-the variations are to great to be able to plug and play. The variations in the current they will draw with any given bias voltage is too great to risk.

A change as small as 10% in the idle current produces a notable change in tone.

As an example, I have one customer-Vasti Jackson, a Mississippi bluse musician, with an ear so trained he can discern a 3% change in idle current. He must be present as bias is set to achieve his tone. He likes the fatter cleaner tone achieved on the hotter (more current) end of the range. The tubes do not last as long at this setting, but there is a trade off between tone and tube life.

As Boogiebabies said, other components could cause similiar symptoms.

I suggested bias because you had to change output tubes. I am sure Mike checked the bias current at the shop and had it within specs when it was completed, but changing the output tubes could have altered this.

If you have a local tube amp tech, he should be able to check this. Most tech's have a bias checker that plugs in between the output tube and the amp. This is a very fast and inexpensive test and can be done with you present and would tell you if you need amp repair or bias adjustment. I would avoid an audio only type shop. Guitar amps are a different beast and require additional skills and knowledge to maintain.

Did you turn the amp on with the bad tube in it? If so, it is possible it damaged a screen resistor on the tube socket it was in. A bias check would also check this as bias in the tube now in this socket would not balance with the other tubes.

 

[GeoBull]

Geobull,
Did you turn the amp on with the bad tube in it? If so, it is possible it damaged a screen resistor on the tube socket it was in. A bias check would also check this as bias in the tube now in this socket would not balance with the other tubes.

I did turn the amp on and play for a while with a bad tube. That could be part of it. Looks like time for a trip to a good local tech

Thanks as always for the invaluable help.

 

[Restless Rocks]

I would also avoid amp techs who are not aware that all Mesa Boogie amps are manufactured with a fixed (non adjustable) bias. Unless modded with a potentiometer which has been improperly set you will be able to use mesa spec tubes without bias readjustment.

 

[beaurawls]

Have an experienced tech show you how different bias levels affect the tone of your amp, have it set from cold to hot as you play through it and observe the tone changes, they are very noticable. Try this and you will be suprised at what your ears tell you.

Boogie's need their bias set just as any other class AB1 output stage. Fixed bias-nothing more than a selected resistor divider-is a good attempt to aleviate this if you selected tubes. It may still vary over 10% which is not acceptable to discerning musicians. Most settle on a setting between 60 to 75% of the tube's plate dissipation. A range far too great to accomodate by tube selection.