Microphonic pickup, guitar volume and amp interaction

[Chuck H]

If your amp is high gain and you stand close to the amp with your guitar, the guitar directly talk to the tubes or OT. I tried using the THD Hotplate to lower the speaker level to room level and still feedback.

Actually, in the case of an attenuator, your pickup is probably feeding back with the components in the attenuator. This is very common. Since the attenuator is subbing for the speaker, I'd say that the speaker part of the circuit is still in play. And without an attenuator it certainly IS the speaker.

Don't believe me, if you take a Mesa and crank the high gain up, then turn down the volume on your guitar to get clean sound. You'll notice it absolutely sounds like $hit. It's like you turn down the tone pot all the way, all muffled. Why? because they use so many caps in the signal chain to drown out the feedback.

Nope. It's because when a typical guitar volume pot is turned down it creates a low pass filter as well as changing the resonant peak of the circuit. The amp will have the same EQ regardless of input signal level. Clipping not withstanding. Read on. Some amps are much better than others about rolling off some top end when clipping. In the case of an amp that DOES roll off top end when it clips, you get a nice fat distortion tone and then when you roll back the volume for clean the top end returns. Nothing about a confounded signal path in the amp will make your guitar sound comparably muted when you turn the guitar down.

For the record, I think Mesa make the worst amp amount all the big names. They put so much crap in the signal chain there is no more tone left.

I agree. The best thing you can do for most Besa Moogies is take out bunches of high pass circuitry, little feedback loops and bleeders. But this usually requires some restructuring of gain at some stages. Not all that tough if you know what your doing. And always worth the effort.

[trobbins]

I agree with Chuck that the feedback loop is most likely influenced by the high Q character of the pickup output at the squeal frequency. The other major high Q influence may very well be the speaker, as cone breakup is very prominant in guitar speakers around that same freuqency region, and so some speakers may be much more susceptible as they could easily be many dB higher at specific frequencies, which may align with the pickup's SRF.

The impact of tube microphonics at that squeal frequency, especially if the squeal is very sensitive to guitar vol pot position, is likely less, but certainly an additive component. A tube can have very specific high Q microphonic resonances of 20+dB above a nearby frequency, but all frequencies can contribute to the signal output of the tube, as an independant signal generator which may only be randomly phase aligned to any squeal frequency generated by separate loop feedback. If a tube is strongly generating output at the same frequency as the pickup resonant loop then the onset of squeal could well be influenced relative to a not so microphonic tube, or different physical setup of the amplifier that suppresses tube mechanical acceleration.