Enough with Pedals, what about amps

[Satan Of Saturn]

And the schematic shows choke. Never worked with chokes before. Hence have no idea about them (m still a learner  ) What should be the voltage, resistance and inductance rating of the choke?
The choke I think is an important part which shouldn't be ignored here. Choke will reduce hums in circuit with better bass response, right?

[R.G.]

But I think there's something wrong with 'earth' line at bottom right... it shows that it has been connected with +40V :-D that is definitely wrong i think.

I don't seen a connection between the earth line and +40V. Where is that?

wont be a big problem in designing a PCB for that. coz its a basic transistor type amp.

Good luck.

Just a bit confused with the power supply.. I mean what should be the transformer rating? should it be 40-0-40 with 3A.

You need a transformer that produces +/-40Vdc after rectifying and filtering. Read "Power Supplies Basics" at geofex.com for some of the background, but to shorten up the typing, you need a transformer with either two secondaries both of 30VAC rms, or one 60VAC rms with a center tap. With two secondaries you'd wire them to be one 60Vct winding. The secondary needs to be rated for three to four amperes minimum for continuous use.

Should I use a regulated voltage source to get exact 40V? or just rectifier output with heavy filtering would do?

Regulated is not needed.

Again, good luck. There's a lot of learning to do here.

[njkmonty]

Posted by: Greenmachine
Insert Quote
Build a light bulb thingy.  Its a 100 watt bulb mounted on a box with a plug and socket. Plug this into the wall. Plug the amp into the bulb unit.  If there's a short in the amp somewhere, the bulb will light up 100%.  Shut off the amp. Unplug from wall. Discharge filter caps and begin troubleshooting

[PRR]

> I think there's something wrong with 'earth' line at bottom right... it shows that it has been connected with +40V :-D that is definitely wrong i think.

It is correct, but incomplete, and the drawing does not follow the essential symmetry of this circuit.

Drawings like this can lead you down a wrong path.

And with EXPENSIVE parts like in a POWER amplifier, mis-understanding can lead to blow-ups, out-of-cash, and disgust with the project.

It IS hard to draw this circuit *clearly*. I tried below and am not happy with my drawing.



In drawing it does help to do the Bridge Rectifier as a "box" with wires coming out at draftsman's convenience, NOT how the parts could be (and never are) laid out (as that cute diamond shape). You also have the CT-Gnd-Earth common wire going "past" the rectifier when the other wires must go "through" the rectifier.

[Satan Of Saturn]

Yeah this is what I was trying to explain 

I wish my English was better 

need some details about 'choke' in the circuit...

[R.G.]

need some details about 'choke' in the circuit...

20 turns thick copper wire 25mm diameter

Must be air cored.

Parallel with 10R 5W wirewound resistor.

Keep a fire extinguisher and a helper to turn off the power near by during bringup.

[PRR]

> Choke will reduce hums in circuit with better bass response, right?

Not this one.

A hum choke is Big Heavy.

Smaller chokes reduce highs.

This choke "disconnects the load" for frequencies above maybe 50KHz.

Why? Because feedback amplifiers can be fussy about their load, especially at high frequencies. And loudspeaker amps get all sorts of junk connected. A basic bass speaker is not too bad. Some hi-fi speakers are full of "corrective filters" without regard for the load on the amp. 300 feet of speaker wire is a lot of capacitance for some amps. And then there are electrostatic speakers.

I've never (yet?) started a fire with a power amp project. (I have repaired many smoked-up amps.) Mostly what I do is replace 4 out of 5 blown transistors, turn-on, blow them all up too fast to smoke. Power amp is NOT forgiving of small mistakes, there's too much POWER ready to run amok.

[R.G.]

I've never (yet?) started a fire with a power amp project. (I have repaired many smoked-up amps.) Mostly what I do is replace 4 out of 5 blown transistors, turn-on, blow them all up too fast to smoke. Power amp is NOT forgiving of small mistakes, there's too much POWER ready to run amok.

Me neither. But the OP is a bit too resistant to advice. He'll do whatever he wants. Scares me a bit for him.

[Satan Of Saturn]

Ok so I'm probably taking a big risk but I'm ready for it... hopefully I can manage things if I can get proper things. In the schematic, no such specification of choke given... what should I do? If I replace the choke with some small resistance with high watt say 1 Ohm 5 watt thn what would be the result?

[R.G.]

Ok so I'm probably taking a big risk but I'm ready for it... hopefully I can manage things if I can get proper things.

It's your responsibility. I'm a big believer in giving people facts and any adivce that they need to be able to make their mistakes be non-fatal, then letting them choose what to do. You have been given the facts as I see them and as other people see them, and advice about where the dangers might be. What you do is up to you.

In the schematic, no such specification of choke given... what should I do? If I replace the choke with some small resistance with high watt say 1 Ohm 5 watt thn what would be the result?

There is no specification of the choke on the schemo. The chokes used in that position in power amps is almost never specified because it's not a stock, buy-from-a-supplier part. It is almost always something the amplifier maker either makes up for themselves or has made up specially for them.

And the specifications I gave you are well within the workable range. You can use them as stated:
20 turns thick copper wire 25mm diameter
Must be air cored.
Parallel with 10R 5W wirewound resistor.

The copper wire should be solid core so it will be self supporting, and should be no smaller than about 1mm diameter. Get some thick magnet wire, and wind it on wooden dowel for about 20 turns, using some extra wire length for the leads that go into your circuit board.

This choke serves the function of isolating the amplifier from loads which may be too capacitive and could affect the stability of the amplifier's feedback The exact value is not highly critical, but having a choke there is critical. It also needs a low-value resistor in parallel to damp any possible resonance of the choke with the stray capacitances that may be there.

If you replace that with a high power, low value resistor, you do two things. (1) You lose the effective protections against unpredictable speaker loads and crossovers that the choke gave you and (2) you waste a lot of the audio power you worked so hard to get in that resistor, burned off as heat.

[teemuk]

...the specifications I gave you are well within the workable range. You can use them as stated:
20 turns thick copper wire 25mm diameter
Must be air cored.
Parallel with 10R 5W wirewound resistor.

The required inductance is in the range of only few microhenries and you basically achieve that with just few turns of transformer wire. And as said, exact value isn't critical. It doesn't make a noteworthy difference whether you actually wrap 18 or 22 turns. This circuit works way above any audible range.

Air core is a must only in practice: You do not want to saturate this choke and it will be passing a lot of current at moderately high voltage. In practice most metal-cored chokes would saturate in this particular application and the ones that wouldn't would be excessively bulky and expensive without offering any additional benefits from having a core whatsoever. For just a few microhenries air core is entirely adequate.

Another common construction method is using the parallel 10R power resistor as a body for the choke. Simply wrap the transformer wire around that resistor.

[anotherjim]

Just to clarify, "Transformer Wire" is plain solid copper with a clear coating of insulating enamel/varnish. You don't want the turns of the coil shorting to each other. You need to scrape the coating off at the ends in order to solder it.

Would solid core pvc insulated domestic wiring cable work just as well? So long as it can't develop enough heat to melt the PVC.

[Canucker]

I haven't read all of this thread...just the first few postings ad then skimmed the ones that started getting specific (fifty hour work weeks have shortened the amount of time I spend online drastically).... any time I've looked up or read anything about building tube amps it starts with the obvious warnings and then branches off into the "its not cost effective".... thank gawd the warnings are there but as far as the cost effective part goes people can always re-purpose things...build something into an old stereo speaker cab and even use that speaker (if its the right amount of ohms)....that saves you any wood working and if you give up on your project at least you didn't drop a hundred bucks on a speaker...test things out with the stereo speaker and then if all goes well, upgrade to a proper instrument speaker. I've seen people (online) use cake baking tins as a metal chassis to build tube amps into...$2 at the dollar store....
There was a tutorial on building one out of a desktop computer casing that required airplane snips (sheet metal cutters), a scratch awl, a pencil and a ruler....the intentions being to use a computer that was headed for or picked from the trash.
Essentially some cost cutting measures and time savers so you can spend time on the guts and don't rush and become sloppy on the safety side of things.

[Canucker]

As far as it being a really complicated undertaking....how complicated can this really be?.....


I have a few of these little amps that I've picked up from yard sales over the years and every time I play one I think why am I not building one??? I believe they have 19 electronic components plus three tubes and they sound pretty good....probably a lot better with an upgraded speaker!
Can't imagine it being a costly thing to build either....

[tca]

After making a few pedals, i was kinda interested in making a proper bass amp of say 60-100 watt (solid state)... Anyone having any reference or PCB layout for that? Interested in something similar to Hartke or say Bassman. Found a few schematics but they were just like a tangent on my head :-D

Start small.

Make a 1W (class A) with a mosfet and a large heatsink (with a power resister as load), make it 5W! Burn your fingers in the process and try a CCS version to get up to 10W... you can do all this with a laptop powersource (18V/6A), no chance of getting electrocuted and dead. Play with the amp a few months... and then think if you *really* need/want/can build a 60-100 Watt amplifier

Cheers.

P.S. (edit)
I forgot... get a laptop powersource with an electronic fuse. A short circuit WILL happen, eventually.

[Satan Of Saturn]

Hmm...that sounds easier... actually I was afraid of 40VDC.... and 100 watt would literally burn my skin... I'll open up few amps, would check them and then will make my move :-)

[teemuk]

As far as it being a really complicated undertaking....how complicated can this really be?.....

The main issue lies in devil being in the details.

Just because amplifiers can look very simple doesn't mean a novice knows the right design and layout practices forehand and can likewise produce an equally sound design from scratch. There's a fine line where desings / layouts become noisy or inherently instable. One can build "simple" with good practices or bad ones, but amps tend to be way more picky about the good ones than battery-powered pedals operating at low voltages and currents. A pedal can be very forgiving towards bad building practices while an amplifier probably says "Boof!".

[Satan Of Saturn]

Last day I opened up my frnd's Marshall 100 watt stack (though not JMP2195)
saw something like

http://i01.i.aliimg.com/photo/v0/371724225/air_core_inductor.jpg

with a resistance fitted in the middle of the choke!!! Does the schematic tries to explain something like that?? A choke with R37 in the middle ??

[PRR]

> a resistance fitted in the middle of the choke!!!

Did you read teemuk's post?

Another common construction method is using the parallel 10R power resistor as a body for the choke. Simply wrap the transformer wire around that resistor.