Trainwreck express - negative feedback off...

[riscado]

Anyone ever disconnected the negative feedback loop in their express clones?

If so did the amp become unstable or was it still under control?

Thanks

[ODwan]

Hi,
I did that and my amp was unplayable with squealing if I turned the Vol pot beyond 9o'clock. At that time it was total blowout distortion! The loop when right back in!
Timo

[Shermock]

Hmmm... Did you reverse your OT connections when you tried this? Just curious....

Shermock

[ODwan]

No. Should I?
Timo

[riscado]

shermock reversing the OT wires has no effect if the negative feedback has been disconnected as far as I know... you only do that if the amp is squealing and has the negative feedback connected.

ODwan thanks for sharing, removing the Neg. Feedback is something which is sometimes cool sounding in some amps, however I was almost counting on the express getting a little crazy without it given the amount of gain.

[lastwinj]

a good rule of thumb. any amp over about 25w will most likely need some kind of feedback. so removal of feedback on one over 25w will probably produce undesireable results.

[edit]
but, as with all amp builds, anything is possible. there really are no set rules.

also it was early, i meant to say 40w. i forgot about the ac30, but it is cathode biased, so it gets some local feedback off the cathode, IIRC.


jeremy

[ODwan]

a good rule of thumb. any amp over about 25w will most likely need some kind of feedback. so removal of feedback on one over 25w will probably produce undesireable results.

jeremy

Sometimes you just have to adjust PI cathoderesistor values. One time I modded a Plexi reissue for a fellow guitarist and removed feedback which made it unstable, but the sound was desired.So I changed the PI values to 820R / 22k and all was good.
Timo

[riscado]

Sorry but I'll have to disagree with that rule of thumb...

I've actually seen many amps quite above that power rating with their negative feedback loops removed... like odwan said it's a matter of adjustments if necessary.

One amp that has an interesting design and no negative feedback loop is Dr z's route 66, for example, rated for 32watts but actually is pumps out quite a bit more...

[klingo]

RT66...yes no feedback from the OT into a gain stage, but Ultralinear and a 47K tail resistor.
I've read that tail resistor in Schmidt phase inverter doesn't introduce feedback, because currents of both triode are increasing/decreasing in opposition through it .
i hope translation to be reliable and understandable
Is that true?

[riscado]

well yes, but then that was point... rules of thumb don't apply!

The ac30 is another example, I've also seen 50watters without one so they would "crunch" earlier, no instability issues, I've also seen it done to 28w tweed fenders such as pro/bandmaster/super circuit with the cathodyne PI... the list goes on and on, that was my point

[ODwan]

NFB subtracts from the overall voltage gain of the poweramp (i.e. splitter and power tubes) in order to increase bandwidth and to linearize the PA. A poweramp with NFB needs more signal comimng in to reach full output than one w/o NFB, because the latter has more inherent gain. NFB does not affect the rms wattage of a PA. Removing the NFB loop does not only free the gain and "limit" bandwidth (mostly has enough bandwidth left for guitar applications) but also the PA has less control over the speaker load (less damping). This brings into play the characteristics of the speaker, mainly high and lo pass filters of 2. order, creating response spikes at high and low frequencies. This for example creates the "scooped" response of the Mesa Rectifier in Modern mode, which removes NFB.
Timo

[riscado]

one thing I always seem to notice about non-negative feedback amps is that the notes seem to feedback/sustain with lots of harmonic complexity, but than again maybe this is just me...

[Shermock]

Hello all!

Just read all these responses. I, too, tend to like the sound of no neg. feedback if I can help it, though sometimes the neg. feedback just does the right thing, like in an Express!

I have had amps howl and squeal with positive feedback when the neg. fb/presence circuit is removed, and I've always cured it by reversing the OT leads.

Just my $.02--

Shermock

[Johnhenry]

Cool Mod is to put a Linier pot out the back so you can adjust how much NFB you want or need, and when i do this mod i hear the sound tighten up with more NFB and looser when i use less NFB, i notice the note's will bloom more with less NFB,
Just an idea for you to think about !
Johnhenry

[klingo]

Hi Schermock

My poor old brain doesn't understand the way you get positive feeback by removing negative feedback.
i've experienced squealing when you feed a phase reversed signal in the feedback input(which i cure by reversing PT grid leads),but if you don't fed any signal in....please could you explain it to me?

Cheers