Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => The Newcomer's Forum => Topic started by: cbg Rick on April 30, 2015, 06:57:48 PM

Title: Popular electronics 22 watt amp
Post by: cbg Rick on April 30, 2015, 06:57:48 PM
I found the plans for this amp in the December 1967 Popular electronics. It is rated at 22 watts with a 45 volt power supply. I'm sure the transistors are long out of production and I plan on using a 24 volt power supply. I'm using this amp as a project to dust off 30 years of cobwebs that accumulated on the knowledge I used to have.
I did build this amp on a breadboard using 2n3904 and 2n3906 for the first two stages and I used TIP 31 and TIP 32 as the output transistors. I used the resistor values called for in the schematic. I powered it up with 12 volts and it sort of worked with a very distorted output. Next step, I built a 24 volt power supply and powered it up again and the magic smoke escaped from a couple transistors  :o I had rebuilt the circuit and may have made a mistake in wiring.
I decided to regroup and start over fresh. Since this is a learning project I'm going to recalculate the bias resistor values and see what I come up with.
Title: Re: Popular electronics 22 watt amp
Post by: cbg Rick on April 30, 2015, 08:24:28 PM
I have a couple questions about calculating the resistor values for this circuit.

1. How does one decide what Ic (collector current) value to use for each stage of an amp? The information I'm using says to use an Ic of 5ma for the first transistor and an Ic of 10 ma for the second transistor. Just wondering how these values were arrived at  ???

2. In the schematic above I was able to come up with values for the collector and emitter resistors for the first transistor and for the resistor from base to ground..... but then I noticed there is no direct DC current path from the base to the positive rail. How do I calculate the value of that resistance?
Title: Re: Popular electronics 22 watt amp
Post by: Roly on April 30, 2015, 11:54:35 PM
(http://www.swtpc.com/mholley/PopularElectronics/Dec1967/PE_Dec_1967_Cover.jpg)

Found a cleaner print;
(http://www.swtpc.com/mholley/PopularElectronics/Dec1967/PE_Dec_1967_pg30.jpg)

(http://www.swtpc.com/mholley/PopularElectronics/Dec1967/PE_Dec_1967_pg32.jpg)

Sadly the thread on tdpri.com doesn't have much to offer, only a long rambling debate about transistors, distortion, and negative feedback.

(http://www.swtpc.com/mholley/Catalog1971a/Page03.jpg)

SWTPC - Southwest Technical Products Corp. (a.k.a. "sweat pack") had a good reputation in the day, did a lot of very early personal microcomputer stuff, S-100 buss.

(http://www.vintage-computer.com/images/swtpc6800custom.jpg)


The "22 watts" is IHFM or "music power", the actual rating of the amp is 18 watts RMS (which is typical of modern small gigging combos and generally more than enough).

It claims a very high bandwidth, 20Hz to 100kHz, which is fine but would need to be confined for guitar work. (or excessive hiss)

The raw input sensitivity is 1.5V for full output.  A guitar will make a noise but not drive it fully.

The raw input impedance is more of a problem at only 5k.  This is fine for a power amp but far too low for an unbuffered (passive) guitar.

So you are going to need a preamp of some sort to provide;

- an input impedance of 1Meg or more
- voltage gain for sensitivity
- EQ/tonestack for sound colouration
- a volume control

The very simplest thing you can do is put a FET source-follower buffer in front of it, but it really needs a preamp that is a bit more full-blooded.  But later.


On closer consideration...

Uh oh ... "one silicon and one germanium output transistor" (and no emitter resistors either)

That does not bode well.

Sorry, but I don't think this circuit makes a good starting point.



PS.

Quote from: cbg Rick
1. How does one decide what Ic (collector current) value to use for each stage of an amp? The information I'm using says to use an Ic of 5ma for the first transistor and an Ic of 10 ma for the second transistor. Just wondering how these values were arrived at  ???

I go about it backwards.  For 50W in 8 ohms we need about 5 amps peak.  Assuming a venerable 2N3055 output transistor, it has a current gain, Hfe, of as low as 20 at this current.  This means that it will require;

5/20 = 0.25A into the base.

Now the driver has to deliver 250mA peak, and assuming an equally venerable BD139 as the driver it has a minimum Hfe of 40, so it will in turn require;

250/40 = 6.25mA of drive at its base.

And so it goes.

For preamp and small signal stages one can assume a current of about 1mA which is about where most small signal transistor parameters give their best, gain, noise, but this is a massive generalisation and the datasheet (http://www.digchip.com/datasheets/search.php) always rules.


Quote from: cbg Rick
2. In the schematic above I was able to come up with values for the collector and emitter resistors for the first transistor and for the resistor from base to ground..... but then I noticed there is no direct DC current path from the base to the positive rail. How do I calculate the value of that resistance?

The base current for the first transistor is derived from the output "half-rail" (emitters of Q4 and Q5).

At power-on Q3 and Q5 start to conduct until the half-rail rises to where Q1 brings the rise under control.

This bias is also therefore a DC (and AC) negative feedback path from the output.  The pot R5 is used to adjust the half-rail so it idles at half the voltage of the supply (and the AC gain ends up being whatever it is at that setting - a runcible arrangement IMO).
Title: Re: Popular electronics 22 watt amp
Post by: cbg Rick on May 01, 2015, 06:38:17 AM
Working backwards makes perfect sense to me. It seems simple now.  :dbtu:

The mention of germanium transistors was referring to other amplifiers, this amp uses all silicone transistors. I did a quick search and data sheets are available for all the transistors in the parts list.

I realized at about 3:30 a.m. this morning that the first transistor doesn't use a typical voltage divider to bias the transistor. Thanks for explaining how it actually works.

I'm looking at a couple FET preamp schematics and will start with building one and get it working while I study the lil tiger amp.

Thanks for the help, things are getting clearer  :cheesy:
Title: Re: Popular electronics 22 watt amp
Post by: cbg Rick on May 02, 2015, 11:02:58 AM

The base current for the first transistor is derived from the output "half-rail" (emitters of Q4 and Q5).

At power-on Q3 and Q5 start to conduct until the half-rail rises to where Q1 brings the rise under control.

This bias is also therefore a DC (and AC) negative feedback path from the output.  The pot R5 is used to adjust the half-rail so it idles at half the voltage of the supply (and the AC gain ends up being whatever it is at that setting - a runcible arrangement IMO).

I'm  seeing how this might not be the best circuit to learn on. I'm going to forge ahead with it for now.
In the mean time I have built a FET preamp on my breadboard and gotten it to work :dbtu: it's funny how little accomplishments like that can be so inspiring.
Title: Re: Popular electronics 22 watt amp
Post by: g1 on May 02, 2015, 01:53:24 PM
a runcible arrangement IMO).
Sorry but I will sidetrack here a moment.
I'd never seen that word "runcible" before and had to look it up.  Now I know how Roly meant it, but I also see the word has been hijacked for a very interesting product.  They call it an "anti-smartphone" so I guess that makes it a "dumb-phone"  ;).
 The ideas and philosophy behind it are quite interesting and inspire a little bit of optimism for me as far as tech for future generations go.  Of course it may just be a way for someone to make lots of money, but "dreaming is free" as she said. 
http://mono.hm/runcible.html
Title: Re: Popular electronics 22 watt amp
Post by: Roly on May 02, 2015, 11:29:28 PM
"Runcible" is a nonsense word invented by poet Edward Lear and first used in "The Owl And The Pussy Cat", but Lear never defined what it meant and used it in other places where the meanings were inconsistent, just for the sound.

In my use it means an arrangement that could be better and more refined, not impressive, fairly ordinary or lacking.
Title: Re: Popular electronics 22 watt amp
Post by: Loudthud on May 03, 2015, 11:41:51 PM
I have found that the more runcible a circuit, the more it sounds like a tube amp.

In the Poptronics circuit, the AC gain is not affected by the bias Voltage set pot R5 because that pot is bypassed by capacitor C3. One little quirk of the circuit is that setting R5 doesn't track the main power supply. As the power supply sags, the Voltage at the + side of C5 doesn't move much. This works to your advantage if you use a soft power supply. You get more compression and touch sensitivity.

I have attached my version of a similar circuit. No global AC feedback puts the gain up around 100. The output impedance is not as low as you might think, but it depends on the beta of the output transistors you use. In the one I built, it was around 5 Ohms. Power the preamp from a decoupled supply from the main Voltage rail. Add enough capacitance to stop the low frequency motorboating.
Title: Re: Popular electronics 22 watt amp
Post by: Roly on May 04, 2015, 06:07:33 AM
Quote from: Loudthud
AC gain is not affected by the bias Voltage set pot R5 because that pot is bypassed by capacitor C3.

Yep, sorry, overlooked that.   :-[
Title: Re: Popular electronics 22 watt amp
Post by: cbg Rick on May 04, 2015, 04:55:58 PM
I have attached my version of a similar circuit. No global AC feedback puts the gain up around 100. The output impedance is not as low as you might think, but it depends on the beta of the output transistors you use. In the one I built, it was around 5 Ohms. Power the preamp from a decoupled supply from the main Voltage rail. Add enough capacitance to stop the low frequency motorboating.

I appreciate you taking the time to do this. I will breadboard this amp and see what happens.
I got a bit sidetracked from this project, I built a Fender Blackface FET preamp   https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&cad=rja&uact=8&ved=0CCsQFjAE&url=http%3A%2F%2Fwww.redcircuits.com%2FPage120.htm&ei=ztpHVfmmGMKNNuWVgNAL&usg=AFQjCNG6A16LMLO7h7v6qNx0MGwYvWLn3g&sig2=aGc3cA9fmjTxmGkb-utV_Q&bvm=bv.92291466,d.eXY (https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&cad=rja&uact=8&ved=0CCsQFjAE&url=http%3A%2F%2Fwww.redcircuits.com%2FPage120.htm&ei=ztpHVfmmGMKNNuWVgNAL&usg=AFQjCNG6A16LMLO7h7v6qNx0MGwYvWLn3g&sig2=aGc3cA9fmjTxmGkb-utV_Q&bvm=bv.92291466,d.eXY) and in a rush to hear it work used it in front of a TDA 2030 amp I had built from a kit.
The Runcible Lil Tiger or "Loudthud" is moving back to the top of the todo list now. Thanks again.
Title: Re: Popular electronics 22 watt amp
Post by: Loudthud on May 04, 2015, 05:51:22 PM
A little over a year ago that preamp was dissected here.

Link: http://www.ssguitar.com/index.php?topic=3373.0 

It should work great with this power amp, but it will want to oscillate (low frequency motorboating)through the power supply. Resist the temptation to use a Voltage regulator or zener diode. You will have to experiment to get the right Voltage and enough bypass capacitance to make it behave.
Title: Re: Popular electronics 22 watt amp
Post by: cbg Rick on May 04, 2015, 07:42:02 PM
I had issues with motorboating when I ran it through the TDA2030 also. I'm in the process of soldering the preamp onto perfboard now. I think I read the thread you linked to, but I'll read through it again.
Title: Re: Popular electronics 22 watt amp
Post by: Roly on May 04, 2015, 11:40:15 PM
I have to say again that the redesign required to turn the original Ge+Si "Lil Tiger" into a workable Si-Si "Runcible Lil Tiger" is not inconsiderable the child will be a hardly recognisable child of the parent.  This is about as far from a kit as you could get.

As an amp designer I would, and I suggest you also, seek out a design that is closer to your desired end point to reduce the amount of modification required.

I have been searching for a suitable circuit on line and I'm a bit surprised that I can't find what I'm looking for; a bit of refinement without too much complexity.

My biases lead me to;
- Long-Tiled Pair, LTP, (diff amp) input stage, say 2x BD139
- Voltage Amplifier Stage, VAS, say a BD140
- quasi-comp output stage, say BD139+2N3055, BD140+2N3055
- simple split-rail supply.

This basic transistor selection could be improved but mainly depends on what is locally available.

Many amp designs are scalable, mainly by changing the supply rails, and may work over a wide range of supply voltages and thus powers without modification (e.g. a "50 watt" design should scale down to 15 watts).  At the same time you can scale/adjust the transistor and heatsink specs.
Title: Re: Popular electronics 22 watt amp
Post by: teemuk on May 05, 2015, 10:46:14 AM
Ok, I'll take a bite...

The PE magazine design undoubtedly works to some degree. To some that might be enough and they may be the same people who appreciate the seeming simplicity.

But then there are inherent issues...

Bias. Well, the bias string probably tracks thermal changes to some degree if diodes are thermally coupled to output devices. But it probably has to be somewhat "cold" bias, though, because... well... in all its simplicity it's a sub-optimal design and more susceptible to thermal overruns. This issue is enhanced by the lack of emitter resistors in the output stage. So we can expect more crossover distortion and compromised reliability.

Gain. As is, the stage is configured as inverting amplifier. Very similarly to opamps (this is a generic NFB circuit after all), gain is determined by feedback impedance and total input impedance. So, consistent performance will require an additional buffering stage to keep source impedance to power amp stage constant. Added complexity.

Reliability. Obviously this amplifier is not protected against exessive load currents in any efficient manner. A momentary short circuit or a complex load may destroy the entire amplifier in less than an eyeblink.

Overload performance. If this was a small signal stage, operating at low currents, similar circuit might provide rather musical, though somewhat hard and asymmetric, clipping. Being a high current stage it's much more susceptible to transistor "deep saturation", which leads to much less musical momentary "rail sticking", which will sound nasty. In worst cases rail sticking may become permanent and destroy the amplifier instanteniously.

Stability. As is, it looks poor and is highly affected by layout and overall design. A practical circuit may require local high frequency negative feedback in VAS (and possibly drivers), and the output probably should have a Zobel filter (remember those complex loads?). Added complexity.

I agree with the opinion that addressing these issues will require a complete redesign of the amplifier (I built a similar circuit myself and vouch for that). In the end, picking a more adequate design as a starting point may prove more effective. Unless you want to use this as a learning experience of course.

Anyway, as I said, to some degree it works. It's a moderately powerful power amp, makes noise and all. I would just be constantly worried about its reliability. No thanks.
Title: Re: Popular electronics 22 watt amp
Post by: cbg Rick on May 05, 2015, 07:08:36 PM
I have to say again that the redesign required to turn the original Ge+Si "Lil Tiger" into a workable Si-Si "Runcible Lil Tiger"

I'm sorry but, you keep referring to this amp as having Ge transistors, I checked the transistors in the parts list and I'm not seeing any Ge transistors listed. Am I missing something?
Title: Re: Popular electronics 22 watt amp
Post by: Loudthud on May 05, 2015, 07:45:34 PM
The output transistors 2N4921 and 2N4918 are (were) silicon. The bad news is they are only rated at 30W and 3A collector current (if you get Motorola parts). A  different version, MJE4921 and MJE4918 are a little beefier at 40W. But no matter, these parts haven't been around for probably 30 years. Note that those parts have a reverse pinout compared to most TO-220s. Like many parts of that era, the beta falls off substantially as maximum collector current is approached, less than 20 at 3A.

The two RCA diodes that mount on the heatsink were kind of cool in their day because you didn't have to insulate the package from the heat sink. They are long gone too I suspect. I just use a TO-92 VBE multiplier face down with an improvised clamp.

SWTP amps weren't known for their reliability. They would usually burn up if subjected to any serious use. Today we have much better parts and know how to beef up the weak points. But this amp does not need a Zobel, it doesn't have enough gain or bandwidth to need one.


Title: Re: Popular electronics 22 watt amp
Post by: cbg Rick on May 05, 2015, 08:32:51 PM
I'm going into this project knowing that there are better ways to build an amp. I realize the transistors are probably no longer available and substitutions will have to be made. Its a learning exercise.
The points you guys are making about this circuits short comings are not falling on deaf ears, I understand it is a far from perfect amp. I don't think any children or small animals will be harmed if I build it, so I am going to forge ahead. :cheesy:
In case I didn't mention it before, it has been 30 years since I did any real electronics work so I have forgotten a bunch of stuff. It is slowly coming back as I study the replies and google the stuff that I don't understand. I need to look into this VBE multiplier that you mention..... Would it also work to use the BE junction of a TO-220 transistor to replace the diodes and clamp that transistor/diode to the same heatsink? 
Title: Re: Popular electronics 22 watt amp
Post by: Loudthud on May 05, 2015, 09:58:15 PM
The VBE multiplier is the circuit around the 2N3904 transistor in the schematic I posted in reply #7. For simplicity, replace the 68 Ohm resistor with a short. The circuit multiplys the Base to Emitter Voltage of the transistor so that it acts like several diodes in series. The pot adjusts the amount of multiplication so that the power transistors turn on just enough to eliminate crossover distortion. As the power transistors heat up, they need less Voltage to conduct the same current and the VBE multiplier (hopefully) reduces the Voltage applied to the Bases.

A power transistor could be used to make it easier to mount to the heat sink. You will need an insulator. This circuit works better with a high beta transistor, the beta interacts with the resistor values to some extent. It takes a little experimentation to get the thermal compensation just right. But this is a guitar amplifier, it doesn't need to be perfect.
Title: Re: Popular electronics 22 watt amp
Post by: cbg Rick on May 05, 2015, 11:05:03 PM
I vaguely remember needing to add a small resistance to help clean up crossover distortion, I just don't remember it being called a VBE multiplier. The posts here are helping a bunch and I appreciate you guys taking the time to add your thoughts and sharing your knowledge. I've downloaded LTSPICE and am working on figuring out how to use it. Seems like a great tool.
Title: Re: Popular electronics 22 watt amp
Post by: Roly on May 06, 2015, 07:09:27 AM
Quote from: cbg Rick
I'm sorry but, you keep referring to this amp as having Ge transistors, I checked the transistors in the parts list and I'm not seeing any Ge transistors listed. Am I missing something?

No, you may well be right, but as teemuk points out above, this is the least of your redesign problems.  This is still a case of "jack up the hubcaps and put a new car in-between".


The VBE multiplier is only one way of getting a temperature sensitive bias, such as a string of diodes, but only the transistor circuit operates as a "base-emitter voltage multiplier" (thanks to the gain of the transistor).

Quote from: cbg Rick
Would it also work to use the BE junction of a TO-220 transistor to replace the diodes and clamp that transistor/diode to the same heatsink? 

It not only works, it's my personal favorite.  BD139/140's for example have a very handy mounting hole through the middle.


Quote from: cbg Rick
so I am going to forge ahead. :cheesy:

Popcorn?  Check.
Hard hat?  Check.
Smoke mask?  Check.

 ;)
Title: Re: Popular electronics 22 watt amp
Post by: cbg Rick on May 13, 2015, 10:09:37 PM
No smoke yet, but it was close. I followed the original schematic and breadboarded the circuit. I used 2n3904 and 3906 transistors for Q1,2 & 3 for the output transistors I substituted NTE 152 & 153. With a 35V supply I adjusted R5 for half the supply voltage at the emitters of the output transistors as per the instructions, after about 5-10 seconds the output transistors were to hot to touch. Not wanting to let the smoke but wanting to hear it work I put a 50 ohm transistor between the collector and supply, collector and ground of the output transistors, readjusted for half the supply at the emitters. Everything was cool  8) and the amp does amplify.
So my question is what is the proper way to get the current under control in the output transistors? Would lowering the values of R9 and R10 put less bias voltage across the BE junction of the output transistors and in turn lower the CE current?
Title: Re: Popular electronics 22 watt amp
Post by: Roly on May 14, 2015, 05:55:36 AM
{here starteth the redesign...  8| }


Quote from: cbg Rick
what is the proper way to get the current under control in the output transistors?

Okay, assuming that your output transistor are mounted on a sufficient heatsink (and by "sufficient" I really mean "excessive" - such as a couple of redundant CPU coolers o.n.o.), you are discovering why almost all amps are fitted with emitter resistors in the output pair.  These could be just about any value from 0.1 ohms to a couple of ohms, but typically 0.22/5W.

{the bigger the heatsinks the greater their thermal inertia and the more time you have to hit the "off" switch before your output transistors melt.}

What you are seeing is thermal runaway, what these resistors are there to prevent.

The reason for thermal runaway is that the VBE for each output transistor depends on temperature, falling with increasing temperature.  If the bias voltage is fixed then as the transistor gets warm and its VBE drops, it turns on more, and gets hotter.  This is a positive feedback loop that quickly leads (as you found) to thermal runaway, and if not quickly caught the destruction of the output pair.

With an emitter resistor between each emitter and the half-rail the result of more current is more voltage drop across the resistor, and this is in opposition to the drop of VBE.  If the resistor is large enough, and it doesn't need to be very large, the voltage across the emitter resistor will rise faster than VBE falls, reducing the thermal loop gain below unity and making the output stage thermally stable.

So the first thing to do is add a couple of emitter resistors.

Another thermal feedback loop exists between the transistor cases and the three bias diodes and ideally at least two of these should be tightly thermally couple to the output transistor cases (to increase the loop frequency response/reduce loop delay by greatly reducing the delay between the transistors getting hot and the diodes following).

Purely for physical reasons transistors in cases such as TO-220 are a whole lot easier to mount in intimate contact on the (TO-3) output transistor mounting bolts (thermally coupled but electrically isolated).  My personal favorite is to mount a couple of BD139's (or BD140's, doesn't matter, just get the polarity right) on or very near the output devices, and use the E-B junctions as two of the bias diodes.

In some stereo amps you will find a 3-way tagstrip mounted on one of the transistor bolts, carrying a normal diode bent over so it rests on the top of the transistor case and with a glob of thermal paste to improve coupling - a bit agricultural, but effective.


Quote from: cbg Rick
lowering the values of R9 and R10

It might, but at the expense of increasing the dissipation in the drivers, and it doesn't address the basic problem of currently having a thermal loop gain above unity, and runaway - the output transistors get hotter faster than the thermal compensation diodes in the bias chain can pull back the base current injection.

A way to reduce the bias is to reduce the value of R6/100r, but that alone won't reduce the thermal loop gain, just start it off at a lower value.  This might make the amp thermally stable at idle but won't stop runaway once it is driven.

It may seem strange but this thermal loop between output transistors and their compensation diodes is subject to the same stability criteria (http://en.wikipedia.org/wiki/Barkhausen_stability_criterion) as an electrical feedback loop.

If the loop gain remains well below unity, say x0.7, the output stage will finally settle to a steady state, but the closer it is to unity the longer it will take until it settles, and may significantly over- and under-shoot before it does.

As it comes to unity (e.g. x0.99) the whole amp may go into a slow thermal oscillation, continuously ramping up and down but never settling.  {in a marginal case you can even get an oscillation on an oscillation, an idle current wobble that dies away only to keep reappearing.  This can be over periods of tens of seconds to minutes, and can depend on the ambient temperature.}

As it comes above unity the output stage will be thermally unstable, what you have now, and may well self-destruct if not caught in time.

The acid test for thermal stability is a half hour run into a dummy load at about 2/3rds full power at a high ambient temperature (heat lamp), and you are looking for the heatsink temperature to settle under drive, then for the output pair current to ramp down to idle as soon as drive is removed.

HTH