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Author Topic: Peavey Bandit 112 Dampening Adjustment  (Read 15304 times)

mojah

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Peavey Bandit 112 Dampening Adjustment
« on: November 04, 2011, 09:00:51 PM »

I looking at adding a dampening factor adjustment to one of my SS amps. I'm looking at loosening up the feel a bit and before I put it under the soldering iron I thought I would ask around here.. My first impression would be to add more series resistance in the neg feedback loop at R108 any other thoughts?
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J M Fahey

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Re: Peavey Bandit 112 Dampening Adjustment
« Reply #1 on: November 04, 2011, 09:32:32 PM »

"Normal" voltage feedback makes amps with high damping and tight, dry bass.
"Current" feedback amps do the opposite.
Yours has (like most modern SS guitar amps) a mixture of both, set to a point the designer liked.
To have a taste of the different flavors, you can add a couple switches which do the following
1) Shorting R110 will kill current feedback, increasing damping.
2) Opening R109 (lifting one end from ground) will increase current feedback, decreasing damping.
Good luck.
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mojah

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Re: Peavey Bandit 112 Dampening Adjustment
« Reply #2 on: November 05, 2011, 05:32:11 PM »

Thanks for the explanation  :)  I'm used to tube amp power stages. I didn't know current amps behave in the opposite. I think I'll add some switches and see what happens..
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J M Fahey

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Re: Peavey Bandit 112 Dampening Adjustment
« Reply #3 on: November 06, 2011, 08:08:19 AM »

I noticed that effect on my own a ***long*** time ago.
I started making guitar amps in 1969 , all tube of course.
In 1972 Argentina defaulted on its external Debt (what Greece is about to do now) and, not having U$$ available, imports were impossible.
Tubes dissappeared from the shops, just like that .... or were worth their weight in gold.
Started using Transistors, which were still affordable, but noticed that the sound was not the same.
Part of it was that SS amps had too high damping (approx. 100 ) which caused "dry" bass; I measured my Twin Reverb type tube amps, and damping was around 1.
I added current feedback (straight from SS design books) until I got the same value.
It helped a lot.
I guess I invented "Valvestate" on my own, about 15 years earlier than Marshall, go figure.
Necessity is the Mother of Invention.
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mojah

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Re: Peavey Bandit 112 Dampening Adjustment
« Reply #4 on: November 18, 2011, 09:19:15 AM »

I finally got back to that amp. We lost power here for a week.  I put 1 meg audio taper pot in series with R109, routing the wiring through the low input jack. I'll try it out at my next rehearsal and report back. I have another amp with a different topology I want to try this with next so I may have to pick your brains some more. Thanks for the help, J M

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mojah

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Re: Peavey Bandit 112 Dampening Adjustment
« Reply #5 on: November 24, 2011, 07:43:31 PM »

The pot worked but I think I'll put a much smaller value one in and mount it on the back. After about 10k of resistance there isn't any change in feel. My other amp a PV Special has a discrete transistor output driver stage. Looking at the schematic I think raising R79 would increase the current feedback. R96 is the current sensing resistor? There dosen't seem to be any voltage feedback?  Does that look right to you guys? 
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J M Fahey

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Re: Peavey Bandit 112 Dampening Adjustment
« Reply #6 on: November 24, 2011, 09:57:55 PM »

Voltage feedback is R77/R78 and R79 in parallel.
Varying R79 will have a *slight* effect, not much.
Current feedback is Zspeaker/R96.
Both voltages are added because R78 is in series with R96.
The switching I suggested earlier worked by killing either voltage or current feedback, leaving "the other" alone, and has appreciable effect.
Adding pots works, in theory, but real world, store available values make adjustments subtle.
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scmitche

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Re: Peavey Bandit 112 Dampening Adjustment
« Reply #7 on: May 17, 2012, 12:34:09 PM »

The standard Peavey circuit shown gives a voltage gain without speaker of ~41 but with a nominal 4 ohm speaker this varies approximately between 19.2 and 36 because speaker impedance is greatest at resonance (typically 38-40 ohms at ~110Hz with a closed back amplifier) and also increases from the nominal value as frequency increases from about 600Hz. Now damping characteristics of the amp also change with frequency and speaker impedance. At 4 ohms speaker impedance damping factor will be 0.9 and amplifier output impedance 4.5 ohms (a reasonably good match) while at speaker resonance, i.e. 40 ohms the amplifier output impedance will only rise to 5.5 ohms and damping factor will increase to about 7.3, which is a good thing as it stops the speaker flapping about at resonance.
Look at Rod Elliot's stuff from ESP, Australia for more info on how to calculate all this. I've set up a spreadsheet to convert an Award Session Rockette 30 and am getting quite good results for a 'tube type' sound.
Hope this is useful.
Steve Mitchell
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Roly

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Re: Peavey Bandit 112 Dampening Adjustment
« Reply #8 on: May 26, 2012, 03:54:51 AM »

Quote from: J M Fahey
I guess I invented "Valvestate" on my own, about 15 years earlier than Marshall, go figure.

No JM, you invented it on your own, but Marshall ripped it off.

Attached is from "High Fidelity Sound Reproduction", 1958, Newnes.  I tried this on a KT88 bass amp build in about 1968 but, not understanding that the Celestion 18 was already in a grossly overdamped cab, found it didn't make any difference.  I've only started to reconsider this idea since reading Rod Elliot's stuff and thinking about trying to make SS amps sound more valve-like.

It's cruel that nobody gets credit for nutting out something second, but I have no doubt at all that Jim Marshall would have been very well aware of the prior RCA work.
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teemuk

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Re: Peavey Bandit 112 Dampening Adjustment
« Reply #9 on: May 26, 2012, 05:07:52 AM »

The utilization of current feedback - first in positive format - to alter output impedance and damping characteristics dates back to about early 1950's and tube technology. At first it was used to straighten out the amplifier's response. There were several articles about the invention in various magazines of the art and eventually the design was even patented by a certain Bogen designer who was also responsible for writing various magazine articles about the idea.

Aside a few applications the idea never caught much fire in a large scale. From that point on the scheme was used now and then, in guitar amps at least Seymour Duncan utilised it. They pretty much carbon copied a certain magazine circuit.

Current feedback in negative phase form, to "unstraighten" the response of a solid-state amp, was introduced already in 1965 in Triumph's solid-state amps. That's at least the first reference I've seen of it so far. From then on various manufacturers have claimed they have "invented" it and that it is something entirely new, groundbreaking technology. They usually have their own market-appealing pseudoscientific name for it - likely because the guys who market these amps realize it's more wortwhile to give an impression that the amplifiers feature some super exciting proprietary feature than to simply state that their amplifiers are copying the exact same idea as 100 other amp makers.

A short list of where I've "first" seen negative current feedback used to mimick tube amps is somewhat along these lines:

- Triumph amps (1965)
- Ampeg SST/SBT series (1969)
- Jordan 120 & 140 (1970)
- Polytone amps (1975)
- Rickenbacker TR series (1977)
- Randall amps (ca. 1977)
- Risson amps (ca. 1978)
- Barcus-Berry amps (1979)
- Dynacord GS-series (1980)
- Peavey (ca. 1980, possibly earlier)
- Fender "CIP" (1981)
- The famous Carver's "Challenge" to HiFi magazines and following series of amplifiers (1985)
- Crate G-series amps (ca. 1986, possibly earlier)
- Rickenbacker RG-series (1989)
- Rocktron (1991)
- Marshall Valvestate series (1991)

...so at the point "Valvestate" amps appeared the technology was already well-established and pretty much a standard feature in guitar amps. If I remember right, someone tipped me of a german book about transistor guitar amps written in early 1980's and the book already discussed the principle. The patent of Dynacord's tube emulation from 1980 just briefly mentions the featured current feedback, and basically makes no big deal about it since at the point everyone making guitar amps already seemed to use the scheme. And like J M Fahey states, it can even be discussed in good analog electronics reference books. So, wake me up if you can actually point a reference prior 1965 of "inventing" this thing.

Did Marshall "invent" it? Well, obviously not. Were they aware of that type of circuit? Most definitely, those guys ain't stupid. Would their amps have sold like hot cakes without the "Valvestate" hype...? Hell No! Like said, those guys ain't stupid.
« Last Edit: May 26, 2012, 05:40:08 AM by teemuk »
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phatt

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Re: Peavey Bandit 112 Dampening Adjustment
« Reply #10 on: May 26, 2012, 08:57:24 AM »

I did research this trick long time back using discrete's on breadboard.
Tested some of ESP ideas along with others but I never found it did much.
But hey I'm only the hobby geek here so I may have missed the point :lmao:

My point being maybe it only works well when big Amps are cranked up through a stack of speakers.
Through small combo rigs of say 30 /50 watts it's hardly worth the effort.

A local chap has a shed full of Peavey Amps and I've had the chance to hear quite a few Early and later Bandit type rigs.
And None with the fancy FB have seriously impressed me.
Phil.
« Last Edit: May 26, 2012, 09:23:45 AM by phatt »
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teemuk

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Re: Peavey Bandit 112 Dampening Adjustment
« Reply #11 on: May 26, 2012, 09:26:44 AM »

I think it really falls down to how each individual perceives certain characteristics in tone.

Personally, I hear the effect only as slight boost at low and high frequencies. I usually have to struggle to hear it as well.

Then again, I've encountered people who say it makes an astounding difference. Last one saying so wasn't even talking about current feedback per se but about differences of running a 100% tube amp to either purely resistive dummy load, or to a dummy load that mimicked speaker impedance. The effect / difference is essentially the same.

For him the resistive dummy load was too sterile, the reactive was "squishy" and responding to playing dynamics making a "Night/Day" difference. Personally, I had to struggle to hear the slight difference. ...as usual.

I once angried someone by stating that only thing the "Reactance" control in his Rocktron Velocity did was basically equivalent to diming bass and treble controls of a generic HiFi -style EQ - nothing else. He got mad insisting the control turned his amp to touch-responsive dynamic setup that sagged like a real tube amp.  ...all that from a generic boost of low and high frequencies. The control didn't even try to mimic the unique response of a poorly damped amp driving a loudspeaker. It just introduced a basic treble and bass boost.

It's all in how you perceive things, and perceiving can be based 99% on imagination and 1% on "real" auditory information. It's always more or less subjective. Therefore I wouldn't jump to definite conclusions too quickly. Yes, objectively viewed the damping barely has a slight effect on frequency response... but so far I never encountered anyone who would sense things 100% "objectively".
« Last Edit: May 26, 2012, 09:40:13 AM by teemuk »
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J M Fahey

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Re: Peavey Bandit 112 Dampening Adjustment
« Reply #12 on: May 26, 2012, 10:53:07 AM »

Weeelll, it's easy to do "research" today, specially thanks to Google, or Internet in particular. ;)
No such thing in 1972 !!!
Nor widespread sharing of schematics either.

I got my Marshall, Hohner, Farfisa, Fender , etc. schematics, by drawing them with pencil and paper, out of an open chassis on my bench.
No other way.

Not forgetting that up to that time *most* amps were still tubed.
And no widespread "buildable" 100W amps schematics either.

The 2 most powerful ones which *were* available (schematics "on paper" using commercially unavailable transistors such as Delco's DTG110 do not count) were the "RCA 70W" with 40411 output transistors and Philips/Fapesa/Miniwatt transformer driven 100W amps with 2 x 2N3055H outputs, driven by a 3rd 2N3055, class A.

Later they offered a 200W schematic of a bridged amp, using 2 silicon transistors and 2 germanium, go figure.
And no short protection either.
All with voltage feedback, of course.

Triumph??? Jordan??? Maybe .... but *WHO* knew them?
Even today !!!!
Never seen any of their schematics, only "Jordan BossTone" which is a pedal.

I was lucky to have one copy of Jack Darr's great book, probably the 1968 edition, with 3 Ampegs: BT15, the 5 channel PA100 and a crazy ST something guitar amp.
All 3 shared the same Power Amp, with 4 40411 output transistors, 80V single supply (so needed output capacitors).
No output short protection !!!!
They relied on modified XLR connectors to avoid shorts (which are inevitable with standard plugs) and one of them had a *lamp bulb* or a 4 ohm 50W? resistor in series with the extension speaker jack. Go figure.
And, of course, *none* of them used current feedback!!
Nor anybody else which was widely known, such as Peavey. ;)
Besides, as I said earlier, schematics were *not* available or circulated to the general public.
Years later I was the Technical Consultant for Import Music, which among many other brands, represented Acoustic, and I got a huge folder full of schematics, many of which unreadable photocopies of photocopies of .....

So I stick to my guns about having independently invented and applied mixed feedback technology in 1972.
All others mentioned (besides the 3 first: 2 unknowns and one which did not visibly use it) came later, as per the amp list posted above.

In fact, I did not take the idea from any amplifier, guitar or otherwise, but from reading about regulated (discrete) power supplies, which was a widely discussed subject.
Most were constant voltage (of course, that's the point); a few were "exotic" (by 1972 standards) current sources, meaning current feedback.

I understood and could design feedback very well (was doing 4th year Engineering by then) and had the idea to make an amp with 4 ohms internal impedance, something I had *measured* in the Twin clones I was building .

Since voltage feedback made a PSU (and an Amp) behave as having zero impedance, and current feedback made it behave as having infinite impedance, surely a combination of them should behave as having a definite intermediate number !!!
In this case the desired 4 ohms.

With those parameters in mind , it was easy to calculate values which I use even today .... and by the way my 1972 network is much more efficient and simpler than that used by , say, Polytone, Crate, or most others, even today. ;)
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teemuk

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Re: Peavey Bandit 112 Dampening Adjustment
« Reply #13 on: May 26, 2012, 02:29:29 PM »

Quote
Triumph??? Jordan???

Well, at least those Triumph amps were the main amps in Keith Richards' rig (not to mention the company built amps for Jennings/Vox) and I'm sure some other people were acquinted with them too...





I'd imagine Marshall was more than willing to do some some research with whom and what they compete with.
« Last Edit: May 26, 2012, 02:36:03 PM by teemuk »
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Roly

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Re: Peavey Bandit 112 Dampening Adjustment
« Reply #14 on: May 26, 2012, 11:28:36 PM »

Quote from: J M Fahey
So I stick to my guns about having independently invented and applied mixed feedback technology in 1972.

No argument from me JM, quite a few things have been invented independently in more than one place, Calculus for example.   :cheesy:

Just to note that the RCA scheme I posted above is a bridge that actually allows the current feedback to be varied through zero to either phase.
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