This amp doesn't sustain like others. I have tampered with everything except the presence routing, and possibly using a shield on it.
It's a basic 50wt circuit (1987) with no real lead dress difference than others I have.
It has a PPIMV that was routed (in) under the board. The original grid feeds were still in situ (!!!) (clipped at board) so I removed those, and routed the MV to grids away from the output selector, which helped a bunch.
I am curious about the best route for the presence. I am guessing in the 1.5" void between the end of the board and the power.. looks like no-man's land through there.. so remove the NFB resistor from the board, and put it right on the 8ohm out.
Tell me no!
Tell me no?
Yeah, it's more fun to talk about than actually getting out the screwdrivers again.
Signatures have a 255 character limit that I could abuse, but I am not Cecil B. DeMille.
Parasitic oscillation stopping notes. You know how it sounds when it's half the signal.. well, imagine that it's not particulary audible, but still there just enough.. dead like there is nothing under my fingers... no push back, never does a note take off, never does a chord bloom into feedback. They just kind of sit there and fade.
My original heads all play fine without a right hand at all. Just touch the note and away goes the horsey. This amp is a chore to play.. like Jeff Beck would start sweating and begging, ya know?
I have made a bit of progress with all I have done, but something is still holding it back from being really good... or good, really. All great parts, and an unexpected outcome.
Awww.. I suck, so far. I want to be good at this. I assure you that I am not!
Signatures have a 255 character limit that I could abuse, but I am not Cecil B. DeMille.
Parasitic oscillation stopping notes. You know how it sounds when it's half the signal..
I don't have an answer, but I have a question. Maybe I am aware of all of the effects of parasitic oscillation. I know it can sound like motorboating and it can seem to rob the amplifier of power since it is spending power to reproduce the oscillation even if it is not audible. I was not aware of parasitic oscillations stopping notes. How does that work? Also, I don't know what you mean when you say "it's half the signal". Thanks.
Yes, so any tone added along to the actual played note has an harmonic effect with the played note. This can be as blatant as squealing, or heavy ghost notes in the lowend.
Ever play with a real hot octavia?
I detect, but can't hear directly what I imagine might be 3rd and 4th order harmonics piling on each other, and with my extremely low filtering, the harmonics are ghosting to some extent. It sounds brash. Trashy. It robs the efficiency of the amp, and makes it COMPLETELY un-smooth, dead like a hammer, or something like a buzz pedal instead of blooming. So, like playing a steel drum through a Marshall.
I play a note, the pure note has a duration of about 2 seconds, then it starts sounding as if the lows fade, and the high crap takes over. No WHUMPA no chance of blooming.
BAWW.... eeeeeeEEEHHHHHHHHhhhHHHHHHHHhhHHHH.. This is not a description of an electrical buzz, it's high throated midrange hash.
If it ends up at the studio before I work it out I will post a sound clip. It is much better with my rerouting, but not even close to my old stuff. Not even.
Oh, I meant over the top noise as half the signal, and we all have at least momentary experiences with something like that, probably.
This only happens when the amp is close to fully driven. With the volume at a setting lower it sounds like 10 different ac/dc songs depending on where the knob is parked. It's unplayable as a lead amp at this moment. Unsustaining, inarticulate.
Last edited by Reeltarded on Thu Dec 15, 2011 6:46 pm, edited 1 time in total.
Signatures have a 255 character limit that I could abuse, but I am not Cecil B. DeMille.
If I understand it correctly, it doesn't actually stop the notes, but the parasitic oscillation (note) spends so much of the amplifier's energy trying to produce frequencies that the speaker cannot produce, that the audible part of the signal is reduced or even stopped altogether. To be honest, I have never heard it or had to tackle this issue myself. Before I got hip to Merlin Blencowe, Randall Aiken and others, I started out by reading Gerald Weber's books (I know, don't say it). He talks about it quite a bit and claims that it is a real PITA to fix, but I don't remember him going into detail on exactly how to track it down and fix it.
...and you just haven't had it prepared correctly. You have to bread it, chicken fry it, and drown it is home made brown gravy with lots of sweet Texas yellow onions that have been cooked down in the gravy.
I would think if you have a parasitic but ultrasonic oscillation combined with audible frequencies you will get harmonics that are sums and differences of the frequencies. Some of these could be in the audible range, and they might not be pleasant harmonics...
So those of us who actually like Arby's are what ... chopped liver ?
Signals combined with extra unwanted stuff (parasitics) can suffer from "Intermodulation Distortion", or IMD as HiFi guys abbreviate it.
It's not a pretty sound either like the low orders of harmonic distortion that we are used to in a good tube amp.
