Bluesmaster CLEAN optimized for STRAT (Blackface cap values)

[markr14850]

I wonder if putting an inductor (ferrite bead?) before the presence circuit ground would help force more of the super highs back into the NFB loop?

[greiswig]

So bypassing the whole presence circuit, including the pot, might make more sense if I'm understanding right.

it almost seems like if the goal is to keep VERY high frequencies in the GNFB loop, rather than just bypassing the first resistor with this cap, you might want to bypass directly to ground? In essence that first resistor plus whatever the presence pot resistance is set to?

Here is a good link to all the classic HiFi tube amps - no presence knobs

http://www.drtube.com/audioamp.htm[/quote]

[markr14850]

So bypassing the whole presence circuit, including the pot, might make more sense if I'm understanding right.

That depends on what you mean by bypassing.

If you mean shunting the highs to ground, then no, I'd imagine that would have the opposite of the desired effect.

Maybe you could draw what you're intending?

[greiswig]

Sure. This is what I'm trying to describe. The blue lines represent the way I'm understanding this cap bypass to be described. So it bypasses the 4.7k resistor for the D'Lite builds, and a 100k for BM builds. Total resistance to ground for the low end of this cap in a D'Lite would be 390 ohms. But you could bypass both resistors (red lines), grounding that end of the cap.

[markr14850]

Well, I'm definitely not an expert on this ringing issue.

The goal, it seems, is to get rid of the ultra highs.

My first though too was - just put a cap and resistor across the speaker terminals - like you say, just shunt it to ground.

But it seems like the idea was to get the super highs to flow into the NFB input to the PI, to be killed there.

Shunting them to ground might be sufficient to address the issue - but it's different than using the NFB to kill them. Shunting them like that will prevent them from participating in the NFB.

I don't know why the designers chose to use the NFB loop to kill the ringing. There must be some system-wide optimization happening there that gave a better result than just shunting. Maybe.

[Structo]

A bypass cap does not necessarily have to go to ground.

On the Carvin X-100 amp that Claus referenced, the shunt or bypass cap simply bypasses the resistor creating a filter.

This Carvin doesn't have the typical Presence circuit on the NFB loop.

[bluesfendermanblues]

I suggest y'all try soldering a little cap on your GNFB resistor....

If you like it - great - please share your experience
If you don't - great - please share your experience


Now its time for field work, and no more theories

[BobW]

Depending on the ferrite bead material 47, etc, it won't solve anything under 1MHz.

[bluesfendermanblues]

A bypass cap does not necessarily have to go to ground.

On the Carvin X-100 amp that Claus referenced, the shunt or bypass cap simply bypasses the resistor creating a filter.

This Carvin doesn't have the typical Presence circuit on the NFB loop.

Guys, take notice of another small detail in the Carvin amp, it has a snubber, on resistor r103, on the PI plate. IMO this is meant for reducing the usual high end artifact.

The cap across the GNFB resistor is there to suppress ringing in the OT.

[bluesfendermanblues]

Depending on the ferrite bead material 47, etc, it won't solve anything under 1MHz.

I usually put a Ferrite bead close the the input jack in order to suppress high frequency noise in to the circuit, but I don't thin that relevant in relation to swinging in an OT's coils.

[talbany]

Blues
Been doing a little reading on this intresting stuff for sure..Most of the logic behind these type mods are influianced by the HI Fi community and from what I gather so far is that NFB loops especially largs amounts of it creates a continuing loop, each time around canceling certian harmonics and other things... An accumulation of enough of these "higher order" harmonic distortion products is arguably like an accumulation of a form of noise modulation. The further out from the fundamental frequency they get, the less masked they are psycho-acoustically by the fundamental. If they maintain significant amplitude way out in frequency (which high feedback causes), they can cause a feedback amplifier to have slewing related distortions, which can be pretty bad sounding. When an amplifier is slewing, it's feedback is rendered ineffective, usually only temporarily, but it's not a pretty picture. that spreads like a cancer each time around the loop..A higher value cap like you say obviously effects tone within the audiable range but transformer ringing occurs when those higher (beyond audiable range) expierance phase shifts (now positive feedback) causing this ringing effect..The cap would in theory help to cancel these high frequency's (I assume)... I would like to know if others here agree or disagree with this rather lame theory.. Thanks for bringing this up very COOL!!

Tony

[jelle]

So now the CBS engineers that worked at Fender suddenly do not seem as dum anymore, huh?!

Those 'tone sucking' caps at the powertube grids.....

[talbany]

So now the CBS engineers that worked at Fender suddenly do not seem as dum anymore, huh?!

yeah now if they could just learn to run wire!!!.Whew..

Tony

[greiswig]

I suggest y'all try soldering a little cap on your GNFB resistor....
Now its time for field work, and no more theories

Okay. Well... PI in this build is close to a D'Lite, with the 4.7k resistor, but there's a pot there that allows me to vary it between that value and around 33k. I put a 120pF silver mica across that, leaving one end open so I could compare with and without.

It is pretty subtle...it actually reminded me of some experiments I did a few years ago comparing MP3 codecs and bit rates. The change seems to make the highs sound a bit "more analog" for lack of a better term. I end up being able to dial in more of the highs on the OD cut filter and still have it sound sweet.

There also seems to be a little more sustain, but it's hard to tell. The effect, as I said, is really quite subtle, but I can hear some difference.

[bluesfendermanblues]

I suggest y'all try soldering a little cap on your GNFB resistor....
Now its time for field work, and no more theories

Okay. Well... PI in this build is close to a D'Lite, with the 4.7k resistor, but there's a pot there that allows me to vary it between that value and around 33k. I put a 120pF silver mica across that, leaving one end open so I could compare with and without.

It is pretty subtle...it actually reminded me of some experiments I did a few years ago comparing MP3 codecs and bit rates. The change seems to make the highs sound a bit "more analog" for lack of a better term. I end up being able to dial in more of the highs on the OD cut filter and still have it sound sweet.

There also seems to be a little more sustain, but it's hard to tell. The effect, as I said, is really quite subtle, but I can hear some difference.

Interesting.

Sound and perception is hard to describe in words, but I feel the top end in my amp gets more 'relaxed' - and like you I can add more treble without the sound being harsh.

I first became aware of this phenomenon, when I searched for input and output transformers for a NEVE preamp clone.

The cheap ones from RC Components have a lots of ringing and sound annoying, whereas the Lundahl, Sowter or Cinemag don't suffer (that much) from ringing.