Yeah I found it Don't take it as gospel but it will likely be close to what is in front of you.From memory give or take a few brain cells the rails where around 35 VDC.Not a rock concert level of power but a big 12 inch driver makes it OK for a fold back Amp.My guess at why it blew?? Likely someone added a dodgy external speaker jack and did not bother to check the internal speaker was already 4 Ohm and adding another driver would definitely blow it up.The owner who knows me well has had no problems since we fixed it and removed the ext socket.(over a year ago now)Phil.
Transistors are always measured in the diode setting, that's what it's for.Google how to measure transistors with a multimeter, as not to repeat the same time and again.Good luck.
Yes, all analog meters had a single resistance setting for everything. Oh well.Believe it or not, I sometimes still use my old analog (needle) meter , both to keep trained with it, and because some things are best shown by a wiggling needle than by rapidly changing numbers.
Hi Noddy,Well done you worked that out well just need to flip Q5,, which was my booboo. Others here will know more than myself ,, but I'd be a bit worried about that *220R* off the base on Q2.In my real world testing Destruction of perfectly working transistors. I did realize that if that is set around 1k it seems to stay safe but depends on circuit/voltages and much other stuff which is mostly beyond my ability to understand let alone explain clearly.One quick Q?what is the DC voltage at the base of Q2?Circuits like this are a very good learning tool for understanding DC coupled stages as they are very basic and in my experience can often sound better for guitar work.From what I've understood (while blowing things up) the idea is to keep the base of Q1, Q2 and the speaker out should be all close to Zero Volts.Phil.
Hi Phil,Thanks for the reply and the comments. My redrawn cct does indeed have Q5's emitter amended! Also I'm pretty sure that the resistor is 220ohm... and as for the voltage on the base of Q2... well the amp is now all reassembled and I am loathed to open her up, in case I disturb something again. I was fixing this amp for a friend and have told them it's all done! Success...etc... Hahaha!I wish you'd have asked these questions a little earlier, I could have checked it all for you. It has been quite a learning curve for me... and I have now also built a Variac/Light Bulb station which I shall be using more in future, I get the feeling...;c)Can anyone else comment... about the 220ohm... or even what volts there 'should' be on Q2's base...?CheersTomQuote from: phatt on April 24, 2012, 09:52:39 AMHi Noddy,Well done you worked that out well just need to flip Q5,, which was my booboo. Others here will know more than myself ,, but I'd be a bit worried about that *220R* off the base on Q2.In my real world testing Destruction of perfectly working transistors. I did realize that if that is set around 1k it seems to stay safe but depends on circuit/voltages and much other stuff which is mostly beyond my ability to understand let alone explain clearly.One quick Q?what is the DC voltage at the base of Q2?Circuits like this are a very good learning tool for understanding DC coupled stages as they are very basic and in my experience can often sound better for guitar work.From what I've understood (while blowing things up) the idea is to keep the base of Q1, Q2 and the speaker out should be all close to Zero Volts.Phil.
1) The voltage on Q1's base will be a few mV DC, because its base current flows through R2 to ground.The voltage on Q2's base *should* be the same, because it also flows through a 10K resistor to ground : R7 .And where is that ground connection?: through the speaker.I didn't even mention its value because 4 or 8 ohms are *nothing* compared to 10000.In fact it won't be *exactly* the same because Q1 and Q2 do not have the exact same current gain, but anyway they will be only a few mV from each other.So , in a nutshell, Q2's base will also be a few mV (less than 100mV ) from ground.Exactly what Phatt said from his considerable experience, only I added the *why*.2) those 220 ohms shown are necessary so the amp has gain, otherwise you would need to hit it with over 20V RMS.The AC (signal) gain is= (R7/220)+1=(10000/220)+1=46.5 ... call it 50.Which in practical use means , since it can put around 20V (20000 mV) into that speaker, it will need to be driven with = 20000mV/50=400mV .Easy for any preamp, even a 9V battery powered one.The 47uF capacitor in series with the 220 ohm resistor blocks DC, so the amp has AC gain around 50, but DC gain of 1 , which is good because otherwise any DC at the input would produce a speaker burning disastrously high DC voltage at the output.This amp uses a classic design, time tested and used by the Millions , basically for one reason: it works ... and very well. Long live SS !!!!!!
Quote from: J M Fahey on April 24, 2012, 11:54:22 PM1) The voltage on Q1's base will be a few mV DC, because its base current flows through R2 to ground.The voltage on Q2's base *should* be the same, because it also flows through a 10K resistor to ground : R7 .And where is that ground connection?: through the speaker.I didn't even mention its value because 4 or 8 ohms are *nothing* compared to 10000.In fact it won't be *exactly* the same because Q1 and Q2 do not have the exact same current gain, but anyway they will be only a few mV from each other.So , in a nutshell, Q2's base will also be a few mV (less than 100mV ) from ground.Exactly what Phatt said from his considerable experience, only I added the *why*.2) those 220 ohms shown are necessary so the amp has gain, otherwise you would need to hit it with over 20V RMS.The AC (signal) gain is= (R7/220)+1=(10000/220)+1=46.5 ... call it 50.Which in practical use means , since it can put around 20V (20000 mV) into that speaker, it will need to be driven with = 20000mV/50=400mV .Easy for any preamp, even a 9V battery powered one.The 47uF capacitor in series with the 220 ohm resistor blocks DC, so the amp has AC gain around 50, but DC gain of 1 , which is good because otherwise any DC at the input would produce a speaker burning disastrously high DC voltage at the output.This amp uses a classic design, time tested and used by the Millions , basically for one reason: it works ... and very well. Long live SS !!!!!! Arrh,, Mr Fahey you are very kind about my experience Thanks for the maths as it does help explain all those transistors that sacrificed there guts to help an Amateur learn.Re discrete Pwr Amps,, I have one of my first ever attempts at hand drawn PCB somewhere in the shed and the schemo from a Electronics mag, So I'll try and find it,, might be an interesting exercise to analyze the merits or lack off. Oh Yes the first one did actually work but the second one blew instantly.Oh Well 50 % success for a beginner was not bad ay?Cheers, Phil
