Solo boost: lifting tone stack vs lifting NFB

[Gaz]

Sorry to go off topic, but what do those 220pF caps on powertube pin 5 to ground do?

It was a mod I learned from Merlin's book (highly recommended), which dumps high frequencies to ground to increase circuit stability when using feedback.

My amp was not oscillating, but I thought it would be good measure. Just as Merlin insisted in the text, there are no critical 'tone spoiling' frequencies affected by the caps.

There is an equation in the book to figure out the exact high frequency roll-off.

[DIY_Guy]

It was a mod I learned from Merlin's book (highly recommended), which dumps high frequencies to ground to increase circuit stability when using feedback.

My amp was not oscillating, but I thought it would be good measure. Just as Merlin insisted in the text, there are no critical 'tone spoiling' frequencies affected by the caps.

There is an equation in the book to figure out the exact high frequency roll-off.

Hmm I'll have to look it over, I never made it past the phase inverter section of the book.

[wizard333]

Those caps were something CBS also did on some Silver Face Fenders, and they are things that people routinely have removed as part of a "black facing" process.

They come under the category "if it ain't broke, dont fix it". If the amp were oscilating, I'd add them. IF it's not, don't.

The difference between 48khz sampling when recording and 96khz sampling should be inaudible. One goes up to 24khz; the other 48khz. Both are way beyond what humans should be able to hear.

Guess what........you can hear the difference (assuming your high end hearing isn't damaged by years of high db abuse).

So again, if its not broke, dont fix it.

[Chuck H]

+1

Also, something that is routinely ignored is that overtones and distortions above and below the 20hz-20khz standard ABSOLUTELY DO effect differences in how an amp responds within the audio range. Especially when an amp is clipping hard. So you not only may hear a difference, you may also feel a difference in envelope and dynamics.

Chuck

[Gaz]

This is all , but that's my fault now...

The difference between 48khz sampling when recording and 96khz sampling should be inaudible. One goes up to 24khz; the other 48khz. Both are way beyond what humans should be able to hear.

Guess what........you can hear the difference (assuming your high end hearing isn't damaged by years of high db abuse).

I understand your point that frequencies above human hearing may affect frequencies within the range of human hearing, but the analogy to digital recording is a poor one.

If you brought it up you should know it's one of the most hackneyed discussions in the digital audio world. There are myriad reasons why that analogy is not applicable to analog amp design, and perhaps we should go over to Tapeop to have that discussion

if its not broke, dont fix it.

It wasn't, but I did anyway, and it didn't break it. C'mon, I really don't want to have to rely on Merlin's authority, or quote the whole passage to prove this point.

+1

Also, something that is routinely ignored is that overtones and distortions above and below the 20hz-20khz standard ABSOLUTELY DO effect differences in how an amp responds within the audio range. Especially when an amp is clipping hard. So you not only may hear a difference, you may also feel a difference in envelope and dynamics.

Listen, I really respect your opinion, but I've read way too much scientific evidence to suggest otherwise. And to be honest, the change in dynamic and envelope is affected more by pick size and string thickness than frequencies above guitar speaker's reproduction range. Moreover, moving a foot or two in either direction of the speaker cab, let alone playing in a different acoustic environment, has a much greater effect on the sound than this mod.

I know one must design in vacuum for the most part, but this definitely falls well below Steve Conner's '3db change philosophy', and below what any, even attentive, listener would notice in any real live or recorded context.

Respectfully,


Alex

[Chuck H]

Listen, I really respect your opinion, but I've read way too much scientific evidence to suggest otherwise. And to be honest, the change in dynamic and envelope is affected more by pick size and string thickness than frequencies above guitar speaker's reproduction range. Moreover, moving a foot or two in either direction of the speaker cab, let alone playing in a different acoustic environment, has a much greater effect on the sound than this mod.

I know one must design in vacuum for the most part, but this definitely falls well below Steve Conner's '3db change philosophy', and below what any, even attentive, listener would notice in any real live or recorded context.

Respectfully,


Alex

Respect accepted. But consider the phenomenon of an ultrasonic parasitic oscillation, for example. An amp can seem to be virtually silent except it's working very hard at reproducing a frequency that the user can't hear and/or the speaker can't reproduce. This is an extreame example of course, but it demonstrates that what's happening above and below the threshold of human hearing absolutely can effect an amp in the audio range significantly. With all due respect I don't think there are enough "sceintific studies" to cover all the circumstances in a clipping valve amp to rule out this phenomenon at all. The effect can be subtle, yes, or more pronounced. But even when subtle it can be part of a larger picture with other aspects that combined do effect an amp in a significant way. Much of this sort of thing would be much too complex to rationalize and use as a design aspect intentionally. But some of it becomes more notable with experience.

I don't agree with Steve's 3db rule because sometimes there are several subtle differences combined that can add up to a more audible 3db difference. If you don't implement all of them you can end up chasing more dramatic changes that actually steer you away from where you wanted to go. But that's part of my personal design criteria and I do respect Steve a great deal.

So I'll just close by saying that if an amp is doing twenty interesting things above or below the audio range, These things absolutely effect the overtones and current usage within the audio range, sometimes even at specific frequencies. If you use caps (coupling or ground bleed or whatever) to remove these effects from the "not audible" areas of the amps effort, you are very likely to hear and/or feel a difference in an amp within the audio range. This is just my opinion, the readers can draw their own conclusion.

Chuck

[Gaz]

Point taken. I couldn't agree more that effect of mods are cumulative. I'd just hate to think that someone wouldn't stabilize an otherwise unruly amp in order to save some mythical frequencies

[wizard333]

If the amp were oscilating, I'd add them. IF it's not, don't.

I guess you didn't read that?

I fail to see where anyone says not to stabilize it if its unstable.

[Gaz]

I wasn't referring to your post, I'd already responded to that days ago.

[David H]

A .1 mfd cap between speaker and the switch , and the pop is gone! No effect to the sound.

[Gaz]

A .1 mfd cap between speaker and the switch , and the pop is gone! No effect to the sound.

God, I can't believe I've been at this shit for 7 years :/

[David H]

Great when footswitchable for lead as well!