I'm in WAY over my head...

[R.G.]

Well... I figured that RG would write up a rational and kind explanation encouraging Gary to share with the community, rather than just taking driveby shots. Then Gary would reply with some paranoia about someone stealing his work. More rational replies would be written, and ignored. We'd all be forced to wonder whether said circuit really exists. So, I just skipped all that, and resolved it by claiming "golden ears". Now we can get back to real life and not illusion.

Egad! It's so ... predictable!

This approach is indeed very efficient; however, I can see some potential problems with it. Gary has enough problems with being afraid someone will steal his goodies if he posts them. What WILL he do if he thinks someone can read his mind if he just mentions them? ARGGHH!!!!

[Chuck H]

+1
Classic presence control = active circuit.

[Chuck H]

What WILL he do if he thinks someone can read his mind if he just mentions them? ARGGHH!!!!

Tinfoil hat?

[Chuck H]

I'm guessing you're familiar with the story of Dean Markley and the JMF Spectra amps. I was told that compromises in build quality resulted in many of those DM amps breaking down before they left the showroom floor. Of course, that was 30 years ago, and they may have learned from the experience.

Yupper. The amps were built as per the stock design with the exception that the transformers were upgraded to larger units. This combined with small screw heads and no washers for mounting the transformers caused the new heavier units to rip through the mounting holes. I'm sure there were also some typical failures you would find with any run of amps too. But the heavy iron and small screws were the real problem. Sometimes, if handled rough, the amps would come out of the crates broken. They really were big transformers too.

The new transformers are more typical of similar amps in size. The mounting problems have been addressed. Also, just because the new design guy doesn't want any problems, a heavier circuit board with heavier cladding was also spec'd for the reissue. The new amps are really very good. But I fear that the gain channel, while very good sounding, isn't up to modern tastes. Another thing is the goofy effects loop. I told Dean to do away with it. He thought that was a good idea but I think I read that they included it anyway. It was a series loop with send and recovery level controls. So it's really easy for the average guitar player to get confused. Or for the amp to end up out of adjustment just because some kid was tweaking it on the show room floor. Really hard to sell an amp that isn't working properly.

But I digress

[jamesmafyew]

Pete is right, the presence control really does interact with the speaker. I've measured similar results on the bench, and I also feel that boosting treble earlier in the circuit, without increasing the power amp output impedance, doesn't sound as good.

In the light of this, I think calling the presence control "passive" is a little unfair. Surely it's an active tone control, using the power amp as its gain block.

Totally agree with this -- and two examples come to mind of the effects of output impedance.

One is the Hiwatt presence control on the DR amps, where the wiper of the pot is connected to ground. I had a hard time understanding this until KO'C explained that it's like a normal presence control when it's clockwise, but when counter clockwise is actually loading an earlier preamp stage at high frequencies. In the middle, you get a bit of both... and the combination of the "active" part of altering the output impedance at high frequencies, with the passive part of attenuating at an earlier stage, makes for a really interesting and memorable sound.

The other example is the "damping" control on the Ampeg VT-60 and VT-120 amps. All that does is cut the NFB and simultaneously drop the input to the PI by 6 dB. So overall gain doesn't change, but output impedance goes up. The power stage's frequency response and perceived output level don't change. The feel COMPLETELY changes.

One recent job I did was adding a switch to a 50W Marshall 800 to change the NFB resistor. The guy wanted the option of more NFB for a "tighter" sound, stock NFB value, and then even less NFB to get a wild out of control sound. Didn't make the amp louder or change the "tone" he said, but he really liked the different feels for different kinds of rooms he was gigging in.

Edit: Also, the point about NFB in a tube amp not really changing the amount of distortion, but the harmonic content, is well-taken. Clarifies some real-world experiences.