I'm kind of embaressed now... It turned out that the problem was in power transformer which I had recently upgraded (this is rated 600VA). It blew my flat's fuse even with no load connected at all. I didn't know that I have to design a soft start for a transformer bigger than 300VA...
Made a soft start circuit in primary winding and it's fine again.
Thanks everybody for help and comments.

Quote from: joecool85 on February 28, 2012, 11:09:50 AM

Quote from: J M Fahey on February 26, 2012, 10:58:39 AM
*Many* (most?) amps can not be turned on with load connected, they tilt.

What?  I must be misunderstanding because it sounds like you mean that amps can't have the speaker connected until the amp has been powered up.  Unless it's disconnecting electronically somewhere that I don't know about, any amp I've had works fine leaving the speaker connected all the time.

I think what he meant was that amps tilt for a short period of time when turning on and off but normally they recover from that. Mine doesn't
And yes - there are special circuits that try to mute the amp on power ups and power downs.

Is it normal that voltage on ballast resistors is so small - it means that trannies are biased really really cold.

Quote from: J M Fahey on February 26, 2012, 10:58:39 AM
*Many* (most?) amps can not be turned on with load connected, they tilt.
Yours *seems* to be already repaired , but let´s recheck for safety.
1) turn it on with bulb, without speaker.
Check that you have close to 0V DC at the output (less than 100mV).
Then that you have less than 30mV across the ballast emitter resistors R4/13/20/23
If all fine:
2) without turning off connect the speaker.
You should hear *something* in it: slight hum, hiss, that show it´s alive.
If so, plug yourBass and play a little, at very low power.
If you play loud, it might tilt again.
3) If all normal, plug the amp straight into the wall, no bulb.
Play sometime and check with your digital temperature sensor (your finger) that, say, most transistors are just warm but one sizzles or something like that.
Post results.
And ... good luck  :dbtu:

Thanx for your help!
Output DC voltage - 20mV
voltage on ballast resistors <1mV
It plays alright if I attach the speaker when it's already on (as I said on my first post).
When I tried it straight from the wall - it was ok the first time. Than I turned it off for a while and it blew my apartment's fuse (sorry - I don't know the correct word in english. probably you don't call them "Fuses"). I'm starting to wonder - may be it's because my fuse is 10A B type. May be C type would not blow?

Check the attachment.
I'm pretty sure that I've checked all the trannies and all the jumper resistors (they are not shown in the schematics)

Hi guys!
I'm having problems with my band mates bass amp head.
It stopped working one day.
I found that one of the driver transistors was dead (it was not short circuited but hFe could not be measured) so I changed that. The fuses were blown and some 0R resistors as well (they are used as jumpers on the PA pcb).
This far it was straight forward but now the frustrating part comes.
If I turn it on (with a light bulb in series with 220VAC cable) without any speaker load and attach speakers later - it works fine. If I turn it on with speaker connected - it trys to jump to positive rail voltage (the light bulb limiter does not allow it to do that). Judging from the burned 0R resistors it has been run with only one rail connected for a while. All the trannies are fine now (desoldered and measured hFe for all of them).
Has anyone expirenced such a strange PA behaviour?

Quote from: Casey4s on May 25, 2011, 10:52:02 AM
Thanks for the kind words guys.  Working in that little box was a challenge in itself for my 63 year old eyes, my hats off to you regular pedal builders.

Nice sound, although not quite what I'm used to hear from a tube circuit.
The box is quite big actually - you shouldn't complain. I have to handle electronic components that ar 0.02" by 0.01'" small at work - not that's a challenge (although not connected to guitar electronics)
Not sure if these are 0201 or 0402 but still - just to give you the feeling:


Just a curiosity - how long does the battery last - it's having a rough time heating the filament. My guess - 2-4 hours of playing, depends on battery itself.

I did a little upgrade - now it has 16 steps and tap tampo feature.
Here it is with synth sound HERE
And here it is with distorted guitar as input HERE
Videos have awfull sound and in no means I call it music. They are just to show how it works. However - beautiful music can be created with it.

If it is not a million channel/million feature amp then most probably you will need latching switches. In most amp designs if you put a LED in series with resistor paralel to the switch - you will get a footswitch with LED.

Ofcourse you can run guitar through it. Or vocals... or any other sound signal.
It's hard for me to make a video with guitar but you can search for Ooh wah demos in youtube - this does the same thing but with more filter adjustment possibilities.
Useless for "classic" styles like rock/metal/pop, but really nice tool for any experimental player.
The guy whom I made it is really happy with it. The only thing is - for playing live with band it's pretty hard to control it and adjust speed. So I have to make it "midi clockable". Hope to finish on this soon.

Hello everyone!
Just wanted to show my new creature.
I have made a Meatball clone for a friend of mine (McMeat layout) and he was interested in Z.Wex Ooh wah. I thought it would be much more useful to make just the "LFO" generator and feed it into Meatball. This gives more features than regular Ooh wah.
Here's a little sample video. I have to admit - no guitar sounds in this video. Sound is created by a distortion pedal with feedback (the signal line is      MT2-->TU2 -->Meatball  while bypass output from TU2 goes to input of MT2)
http://www.youtube.com/watch?v=4WKh_BytKuY

Thanks for reply!
Well I'm aware of alpha's 9mm production

I've used them in some of my designs, but still - it is hard to stuff more than five of them in a row in a box like my first post shows. And they are quite expensive as well. I think I'm gonna go with trim pots as they ar not directly in audio path for my project. Other option would be to use one pot, rotatory switch and a uC to control it all, but that's too complicated to operate for user.

may be those are just trimpots with handles?

Hi everyone!
I'm thinking if any one has an idea where to get these really small potentiometers. For example like some of Z.Vex pedals use

I was also wondering if KMG would be so nice and post some more info about winding those nice output trafos for his FET power amps...