anyone ever played an amp...

[drew_half_empty]

with the negative feedback loop taken out?

I played an orange halfstack with the negative feedback loop on it taken out, and the thing was a monster, tremendous increase in volume, sounded real real buttery, but was also incredibly percussive. So percussive, in fact, that it set off a few cymballs

leaves me wondering if there's sort of mod you can do to an amp to have variable compression, so you can back it off a bit & just get an incredibly buttery loud tone

there'd be no need for 100 watt heads, unless you play clean

anyhoo, any similar expiriences?

[ES350]

Re: anyone ever played an amp...

This is a pretty common mod on BF/SF Fenders---a 5k linear pot in series with the NFB resistor (value varies from model to model) lets you dial in a lot more grease, typically mounted in the ext speaker jack. Depending on amp layout/design, zero NFB may get you into some trouble with oscillations and microphonics, especially if you are running boost/distortion pedals into the front end. I have a few amps set up like this and it works really well...

[drew_half_empty]

Re: anyone ever played an amp...

did it have the same percussive sound or is that exclusive to orange's?

also, anyone have any info about the variable compression mod i was talking about?

I didn't start this thread because of this but it does give me some ideas for the amp i've been planning to build

[Vasshu the humanoid typhoon]

Re: anyone ever played an amp...

Waits for the amp to go into howling feedback

[Xeromus]

Re: anyone ever played an amp...

the negative feedback loop should be taken out of this forum.

muhahahaha

had to make a funny

[drew_half_empty]

Re: anyone ever played an amp...

Waits for the amp to go into howling feedback

if it's controllable feedback then who cares?

[Rich_S]

Re: anyone ever played an amp...

if it's controllable feedback then who cares?

No, this isn't controllable feedback Rid's talking about. It's oscillation internal to the amp. Negative feedback is designed into amps to make them more stable and not oscillate. Reducing or removing negative feedback is an easy mod which can push an amp "closer to the edge". Of course, too little NFB can push it over the edge.

On the other hand, too much NFB turns an amp into a late 70's Fender, after those dratted CBS engineers started mucking around with Leo's designs.

There are amps around with NO NFB from the get-go: my 18 Watter clone is one example.

In The Guitar Amp Handbook, Dave Hunter has an excellent discussion of NFB and modding it - what it can do to an amp's sound (good or bad). He does a great job of avoiding generalization, making it clear that NFB is but one component of the amp's overall sound, and mods that work in one amp may not in another.

Experiement!

[the guy who invented fire]

Re: anyone ever played an amp...

Drew: The Fender Custom Vibrolux Reverb started out as an existing design (63 reissue Brown 2x10 Vibroverb) and one of the mods done to the Vibroverb circut was completely removing the negative feedback loop...makes a HUGE difference!

[Scott_F]

Re: anyone ever played an amp...

On jeremy's new amp I'm building, the NFB will be switchable in or out.

[Curly]

Re: anyone ever played an amp...

just FYI, most, if not all of the Fender tweed amps had no negative feedback

I feel that this, along with cathode bias on some of the tweeds, made them richer harmonically, as well as softer feeling

however, Fender was kind of on a mission to make amps louder and cleaner over the years, and adding negative feedback was part of that process

[ES350]

Re: anyone ever played an amp...

check the schematics...by 56-57, most of the big and medium power tweeds (and a few of the smaller ones) had NFB. FWIW, lower plate voltages, PI design, softer rectifiers, tone stack, lower insertion loss and a few other things seem to make the big differences...

[drew_half_empty]

Re: anyone ever played an amp...

cool, thanks everyone

any info about a variable compression dealy?

[Lake Placid Blues]

Re: anyone ever played an amp...

The amount of NFB is one of the big differences between the tweed bassman and the early Marshalls. A JTM45 has about 4 times more NFB than the 4-10 bassman. This must be an important part of the trade mark more violinesque tone of an overdriven Marshall, compared to the more purcussive spank of an overdriven tweed.

NFB is variable to a degree on most amps. This is the presence knob. The presence control reduces NFB as it is turned up. 0 on the Presence knob is more NFB, than the presence knob on 10, as I understand this. This control is part of the power amp behaviour and is quite a different thing from "compression" involved with overdriven preamp tubes in a high gain amp.

It's interesting to compare the NFB set ups of the Marshall power amp through the years, as it's illustrates concisely the effect of NFB. Once they went to EL34's, they began hooking up the NFB loop to the 8 ohm lug of the OPT secondary. The NFB resistor was IIRC 47K. With the change to 6550's the NFB loop was changed to the 4 ohm lug and the NFB resistor was increased to IIRC 100K. This reduction in NFB was needed because of the cleaner nature of 6550's. So we see that reduced NFB results in a dirtier power amp, but also a brighter and more purcussive one too. The 6550 was brighter and more percussive by nature, so the diffrence between 6550 and EL34 Marshalls is fairly stark as far as power amp compression. On the more modern examples, with EL34s, Marshall went back to the 8 ohm lug, but AFAIK retained the higher value NFB resistor, striking a compromize.

I have experimented on my Marshalls by putting the purple wire on diffrent OPT lugs, and changing the NBF resistor. It does make a noticable difference in response. Generally the greater the NFB to the PI; the rounder and smoother the response. The less; the more purcussive and aggressive.

Perhaps one could make the range of NFB greater (than traditional or typical ), from 0 to lots, in a circut design to obtain the range of variation you seek?