Pimp (and help fix) my Transtube Bandit!

[EdGrip]

Hello everyone!
I've taken the board out of my Peavey Bandit, because I've been meaning to have a look at the dodgy input jack and tame the effects of the bright switch. Honeyiscool from the OSG forum recommended I replace all the stock 4560 opamps with newer, lower-noise NE5532s - an easy thing to try, since Peavey socketed all of the opamps! Good old Peavey. (I also emailed them about 2am before I went to bed, asking for a schematic - the next morning there was one waiting in my inbox. Great.)

I will socket the resistor-capacitor line that the bright switch engages and try some different values. I'll probably replace the earlier signal-passing carbon film resistors with metal films, cos why not? Turns out the tank is a Fender-style 4-spring Accutronics model, so that's staying. The only problem is:

The reverb circuit is noisy and troubled by a strange problem: After a while, a whistle/whine/tone appears, and rises in pitch like a jet engine, and then settles somewhere annoying. Its level depends on where the reverb knob is set. What is this? What can I do to make it stop? Any other general suggestions?

ALSO: I cranked the clean channel the other day, and hit it with an SHO pedal in front. After a while I think there was a volume drop. It's hard to say when you're thrashing around at that volume in a little room. I also think I noticed an effect where there was a sort of volume flutter, like a fast-ish tremolo pedal, under the sound when I hit big chords. Is this output transistors, or my abused inner ear?

Thanks all!

(I will post gut shots in a couple of days - I don't think there's a Transtube Bandit dissection thread anywhere! They're great amps.)

[EdGrip]

N.B. - I have, in the past, seen people asking about the Bandit, and its circuit, and people turn up and say, "It's a Bandit. It is what it is. There's no point changing anything". Well, maybe. But that's a boring answer, and not really in the FSB spirit. I'm inside this amp because I'm hoping it will teach me more about audio electronics (it already has!) and because I have a real affinity for the sound of the clean channel. Just because it cost £50 and "Is what it is", doesn't make it less worthy of exploration than an old mojo Fender Showman or something.

[devastator]

"It's a Bandit. It is what it is. There's no point changing anything"

that's right , it's way better to copy a fuzz face or a tube screamer

Could you post the schematic ?

with it i'll be easier to see what you can modify.

[phatt]

I agree with devastator,,
Peavey made some good stuff but most bandits are not on that list.
Many models exist and there are differences so yep post the schemo.

The ones I've had the miss- pleasure of tweaking where horrible bright sounding Amps ,, but some may love that sound.
Bright sharp in your face loud suckers but no real soul.
A lower spl speaker may help to smooth over the harshness.
Phil.

[EdGrip]

Well, I wasn't posting it because of copyright hoo-hah.

But, here is a link to a heavily-corrupted photograph of Hunstanton beach that just happens to look almost exactly like a schematic for a Transtube Peavey Bandit -

https://img21.imageshack.us/img21/5112/ ... ematic.jpg

I thank you. ^_^

[devastator]

hi

thanks for the schematic !

intersting stuff , if you look closely it has the same kind of signal path that a tube amp has (adding stages to increase saturation) .

what do you want to mod now ?

for the bright cap the values is maybe a little bit high . You can try 470pf (instead of 1n in the original circuit) even lower.

don't bother you with changing all resistors with metal film , is gonna be a pain and not sure it will improve the sound .

[EdGrip]

Yes indeed - the gradual adding of gain and clipping was Peavey's intention with the Transtube idea. For anyone interested, Hartley Peavey himself has written a paper on it, which you can read here: http://www.peavey.com/monitor/pvpapers/Chapter3.pdf

I was only going to replace the first few resistors that are directly in the signal path, where incoming signal is smallest and presumably, therefore, noise floor is most important. It's a laugh, innit?

The main thing I'd like to do is simply make the reverb circuit behave itself (see above for my description of the problem with it.)
I don't know enough about audio circuits to know what would cause this kind of effect, so I was hoping someone here could point me in the right direction.

Interesting that the T-dynamics control is sending signal in one of the later opamps to ground... The paper says:

Most people ASSUME that our TDynamics control is nothing more than “master volume”… WRONG! The T-Dynamics variably engages a clipping circuit in the output stage that effectively lowers the power amp’s headroom, thus emulating a lower powered amp. When the TDynamics control is turned down, it is not just acting as a volume control for the amp, it actually decreases the available voltage swing just as a smaller/less powerful amplifier would do while retaining all the vital TransTube overload characteristics that so effectively emulates a tube amp’s sound and feel.

Also, the Presence knob and the Resonance button - the paper suggests these are to emulate negative feedback and speaker/transformer interaction - are actually basic filters, very late in the circuit.

I'm actually less fussed about the bright switch now that I understand (again, thanks to the schematic - this was meant to be an educational exercise!) that a bright switch is only effective at lower volumes. So I will probably socket both cap and resistor and then tune it to be pleasing at the kind of volume I most often practice with at home.

Fun fun fun.

EDIT: Hartley's paper also credits the Ibanez TubeScreamer as introducing asymmetric clipping. So. Y'know. Pinch of salt, and all that...

FURTHER EDIT: Pictures WILL follow when I can be bothered, but I just wanted to say, this amp (and the board inside it) is very nicely - and robustly - put together.

[phatt]

Hi Ed,
The reverb may need a larger series resistor between tank and circuit. You could try decoupling it from tank with a Cap.
Also the cap across can actually cause problems,, try lifting it out of circuit for a bit and test.
grounding paths in rev circuits can also cause many issues.
Phil.

[KindaFuzzy]

I had one of the crappier versions as my first 'good' amp. Used to run the neck humbucker on my squier supersonic into it with a danelectro daddy-o for years, and that was the only amp that pedal sounded good through. I bet a woolier sounding speaker would tame the harsh.

[EdGrip]

Probably reassembling this today. Thanks for your responses!

Here's the reverb circuit, in case anyone browsing has any further thoughts RE the 'verb oscillation/whine thing.

[phatt]

Hi Edgrip,
Check the tank is plugged the right way, easy to plug them in backwards which may cause issues.
Usually,, The output earth is also the case common, the input common is often left floating.

The driver circuit uses current drive (I think) which floats off ground,,,So if plugged wrong way it could cause odd things to happen.
Phil.

[blackbunny]

+1 what Phil said.

Also, clean the RCA plugs on the leads to and from the reverb tank, preferably with a solvent that doesn't leave any residue (ie, contact cleaner).

Spray contact cleaner onto and into the RCA sockets on the reverb tank, and plug & unplug the leads into them several times.

Remove each white nylon connector from the PC board one at a time, and spray contact cleaner onto the pins, then plug and unplug several times.

Corroded PCB connectors are a common cause of problems on Peavey amps - and other brands too. For example, Marshall JCM2000 DSL / TSL.

[teemuk]

The reverb circuit is noisy and troubled by a strange problem: After a while, a whistle/whine/tone appears, and rises in pitch like a jet engine, and then settles somewhere annoying. Its level depends on where the reverb knob is set. What is this? What can I do to make it stop? Any other general suggestions?

My Crate combo amp does the same thing and in it it was acoustic feedback. Starts with low amplitude and frequency "hum" that escalates to very loud higher frequency "whine".

In it's case there was really not much that could be done: The reverb feed point was taken post master volume control, basically allowing any input signal with just the right resonance to push the tank into oscillation via acoustic feedback. Naturally tank's padding / shock mounting against vibration could be improved but the amp was a combo so you kinda have to live with heavy vibrations occuring in it. In it's case the only solution was never to dial reverb to max. I could live with that, never needed that much reverb anyway.

Not saying it is what's happening in your amp but it sounds suspiciously like it.

I'd check if the tank's mounting is broken or corroded in a manner that it doesn't isolate vibrations anymore as effectively. If it's that it could be a simple repair to do.

ALSO: I cranked the clean channel the other day, and hit it with an SHO pedal in front. After a while I think there was a volume drop. It's hard to say when you're thrashing around at that volume in a little room. I also think I noticed an effect where there was a sort of volume flutter, like a fast-ish tremolo pedal, under the sound when I hit big chords. Is this output transistors, or my abused inner ear?

The "T-Dynamics" power amp emulation of TransTube amp does some pretty neat "tube power amp -like" things when pushed hard. Maybe you are hearing it's effects? Soft clipping, dynamic crossover distortion and compression, negative feedback decaying on power amp's tone controls, etc. ?