Ampeg PF-50t… NFB Mods?

keyofnight

keyofnight

Jun 3, 2011
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seattle, wa
#1
Hey folks!

Last year I performed the legendary @mnats/@Bertr "humbuster" mod on my PF-50t. This was such a great mod, and I'm glad I did it. Now, I'm trying to figure out if there are other mods I can do. I've been really interested in mods that add high frequency content to the signal… so I'm wondering: is it possible to do a "presence"/"resonance" mod on this amp?

That is: has anyone thought much about the negative feedback loop on this amp and tried to add/remove signals from it? I know this is a popular mod for other amps, but I haven't looked at the schematic for this one to even know if it's even possible. ;)
 
#2
I'm sure someone who knows what they're doing could wire you up a few mods provided they'll fit, including presence. No idea what resonance is, but the ultra-hi switch kind of acts like a presence switch already. But I have no idea what or how they'd do it.
 
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#3
Resonance usually affiliated with adjustable Q of filter.

Guess you could call presence that.
Doesn't control Q in any way
Just filter over feedback loop.
Technically you could name it whatever heck you want.

Not sure how they did ultra bright.
Just cap over volume knob. Or other Ampeg circuits use set resistor so brightness isn't effected by volume settings. Good enough to not care for presence

Either way simple multi band EQ pedal in front or in loop. Would give you plenty highs without farting around with trying weasel in presence.

Even pre tube change can wake it up.
Large plate Sovtek LPS about clear as it gets.

Increase negative feedback is way to reduce hum. Kinda cheating and poor way fix ripple. Hum balance on heaters works too. Board layout or using inductors in power supply real way do it. Less feedback better. Not so much issue with 2 tubes. With 4x or 6x beam tetrodes to much feedback is asking for trouble

Did they bother to even use a inductor for the pre . That reduce hum
 
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#4
You need a schematic of the Power section (PI+Pwr valves).
Then: https://www.aikenamps.com/index.php/what-is-negative-feedback

NFB is tricky as it will usually make the amp have a more abrupt change in sound when hitting distortion.
In my opinion the PF50T with its modest 50w is not suited to NFB as it may often be pushed at the limit.
It is (to me) more suited to a 200w Hiwatt/Mywatt where one has enough power without hitting distortion.
 
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#7
coreyfyfe said:
On tube guitar amps typically the presence and resonance controls are both modifiers in the negative feedback circuit. Presence acts on the highs, resonance on the lows.

Here’s some background in the case of fender/fender like amps and mods thereof.
Click to expand...

This is what I had in mind—an NFB limiter for the bass frequencies.

I appreciate y'all being cautious about what these controls are called, because sometimes it's hard to know without the schematic handy.
 
#8
Bertr said:
You need a schematic of the Power section (PI+Pwr valves).
Then: What is Negative Feedback?

NFB is tricky as it will usually make the amp have a more abrupt change in sound when hitting distortion.
In my opinion the PF50T with its modest 50w is not suited to NFB as it may often be pushed at the limit.
It is (to me) more suited to a 200w Hiwatt/Mywatt where one has enough power without hitting distortion.
Click to expand...

Interesting. I guess I was under the impression that it would be a good fit because of all the lower watt Fender amps I see people mod with presence controls. That said, I definitely loved the presence control on my Sound City B120.
 
#9
I remember a post years ago where someone took all these ambiguously named amp controls, listed them, and wrote a description of what they do. He wrote "mid scoop" for all of them except presence, which he described as "treble boost."

Yeah, that pretty well sums it up :D
 
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#10
In "traditional" understanding both controls act in feedback loop just do different things: presence control allow more hights, resonance control cut the hights. Why should be mention "hights" ? Cause both control are used for voicing just modifying the nfb ratio in treble region, keeping generally global feedback at the same. Keeping the amp to perform as same from a point. The damping factor of the amp is changed insignificant and just for the frequencies which are affected. On the other hand, changing the global nfb of an amp is a serious thing starting with stability concerns and finished with the way the whole amp will perform. I have not to much experience testing global nfb effect in extreme substantial amounts over bass guitar amplifiers but have to admit it will not do an bass amp more tight , "cartboard sterile" is a proper word to use it. Voicing just on treble range will not affect completely the amp.
I don't know to exist a schematic of pf-50t available.:whistle:
 
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#12
BogeyBass said:
Resonance usually affiliated with adjustable Q of filter.

Guess you could call presence that.
Doesn't control Q in any way
Just filter over feedback loop.
Technically you could name it whatever heck you want.
Click to expand...
Again, I'm going to hammer you for CONTINUING to provide bad information, since that seems to be your field of expertise on TB.

Decreasing feedback will increase system resonance since output impedance of an amp serves to damp out the electro-mechanical resonance of the speaker, and thus the SYSTEM Q of the amp-speaker combination.

Changing the feedback with respect to frequency is a way to change the effective resonance within a range of frequencies. This is what the presence control (when used in the global feedback loop) does.

Bear in mind Paul's cautions with regard to stability. It's quite east to turn an otherwise perfectly fine working amplifier into an unstable, unusable mess.
 
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#13
Oh I see. Lol

Cool hence why I said adding high percentage of feedback is asking for trouble. In other words like I said before Paul. Especially with beam tetrodes in 4x 6x application. Pretty well known for self oscillating


The resonance statement was a joke. Most call it presence. The joke was various manufactures changed the name make it sound cool.

Resonance, range expander etc etc.

Mesa has no legacy tube bass amp. They have all been discontinued.

Nothing to say about any of the other information. Just get on your high horse when I make a joke on purpose lol
 
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#17
agedhorse said:
Again, I'm going to hammer you for CONTINUING to provide bad information, since that seems to be your field of expertise on TB.

Decreasing feedback will increase system resonance since output impedance of an amp serves to damp out the electro-mechanical resonance of the speaker, and thus the SYSTEM Q of the amp-speaker combination.

Changing the feedback with respect to frequency is a way to change the effective resonance within a range of frequencies. This is what the presence control (when used in the global feedback loop) does.

Bear in mind Paul's cautions with regard to stability. It's quite east to turn an otherwise perfectly fine working amplifier into an unstable, unusable mess.
Click to expand...

Thanks for chiming in. This is exactly why I want to try this mod: I'm most interested in changing the frequency response / gain structure of the amp itself. I'll heed your and Paul's warning and I'll be careful in the planning phases of this mod, if I go through with it. (I might not, since I can get close to sounds like these with the Fairfield Circuitry Four Eyes.)
 
#18
keyofnight said:
Thanks for chiming in. This is exactly why I want to try this mod: I'm most interested in changing the frequency response / gain structure of the amp itself. I'll heed your and Paul's warning and I'll be careful in the planning phases of this mod, if I go through with it. (I might not, since I can get close to sounds like these with the Fairfield Circuitry Four Eyes.)
Click to expand...
A way better way to go if you ask me.
 
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