Circuit changes for the Pignose 7-100

[Roly]

but if the voltage across the resistor is less than 0.6VDC then it can be regarded as almost non-existant as it ceases to bias the output stage on. Thus it serves no purpose.

The bias divider is shown as 1k and 220 ohms in parallel with the forward biased diode, the supply is 9V nominal.

9V * 220/(220+1000) = 1.62295082 ...oh gee, 1.6 volts, so the diode certainly is in forward conduction and provides a thermally compensated bias to the output pair.  The parallel 220 ohms works in conjunction with the dynamic resistance of the diode to shape the thermal characteristic.

[mark]

Thanks for the reply Roly.

The parallel 220 ohms works in conjunction with the dynamic resistance of the diode to shape the thermal characteristic.

Please explain how?

The effective resistance would seem to be 81 ohms including the 220 ohm resistor. It doesn't really seem to perform much of a purpose to me. The current draw is about 7mA which is well within their specification.

It would seem to me the purpose of the diode is to turn off the output pair. I'd imagine the output pair conduct and then a voltage becomes present across the 2.2 ohm resistor, as that becomes large enough to exceed 0.6VDC the output pair switch off.

Mark

[J M Fahey]

*Very* simplified answer (01:42 AM here, very sleepy, going straight to bed):

1) Assuming a diode has a "fixed" 0.7 or 0.6V drop is a convention; actual diode drop depends on current .... and also has a temperature coefficient.
Diode drop varies *a lot*    if current varies also a lot.
From, say, 0.5V or less to more than 1V    :loco
Download and look at the V/I curve of a 1N400x.

2) since a transistor has huge gain, a "small" base current will produce a big collector current.

3) please accept this rough explanation, don't want to fill the page with formulas, just concentrate on the basic concept:
a) suppose the "diode" diode and the TIP41 BE diode are exact twin brothers.
b) if so, idle current (say, 5 mA) through the bias string will produce , say, 0.62 V across the diode (values totally made up)
c) since they are twin brothers *and* in parallel, said 0.62V diode bias voltage applied to the TIP41 will make 5 mA go through it. Fine.
But that's not "any" diode, it's a BE junction in a transistor ... which has current gain ... lots of it.
Say: Hfe 40X (also made up).
Then that collector will pass (*idle*) 5mA x 40= 200 mA 
Too much (unnecessary by the way) , will eat the battery and overheat the transistor.
I would need to add a trimmer somewhere , or get a programmable diode (yes, they do exist) or .... lower the current passing *through* the diode.
Advantages?
I have a "less than 0.6V" diode .... but I keep the thermal coefficient.
Sounds crude (ok, it *is* crude ) but it works and is a very popular solution.
Peavey and Fender use a resistor in parallel with 1 diode in the string to get "less than" 3 or 4 full diodes which are needed in theory.
You have to waste some minutes *once* selecting empirically the proper resistor and then churn out 1000000 amps without trimmer pots nor need to adjust them .
Savings add up.

Of course, *the other* solution is to use a transistor connected as an "amplified diode" which lets you easily get "3.5 diodes" , or whatever you need.

[mark]

Decided to get the CRO onto this thing. I inputted 1Khz and the output with a 7.5 ohm load and a 12VDC supply was roughly 2 watts before distorting. I guess it comes as no surprise the touted spec of 5 watts is no where near the mark.

Prior to distorting the waveform looked smooth without any crossover distortion. I suppose this should come as no surprise as the issue I have is with bass frequencies.

I think I'll need to stop using RG Keen's quick and dirty oscillator and use one that can input lower frequencies. I'm expecting to see the output to drop off with the lower frequencies.

Mark

[Enzo]

Hifi guys, and others like to state that output power MUST be measured before distortion.  And with that bit of reasoning they decide some 50 watt amp is really only a 20 watt amp.   But what if an amp produces distortion at all levels? Isn't designed to bhe distortion free at all? Do we claim it has zero watts output, even though it is screaming loud?  Not in my world.

I see guitar amp specs as a measure of how loud they are - how much power they can put into a load.  They are not, and are not designed to be, hifi, or even particularly clean.  So the question becomes:  how much power can the amp put into its speaker - damn the distortion and full volume ahead.


In hifi, your rule makes some sense, since hifi amps are not supposed to distort anything.  SO when you turn up a "50 watt" hifi amp, but it sounds like crap over 20 watts, sure, call it a 20 watt amp and scold the maker.  But guitar amps are not compared on the basis of clean, they are compared on the basis of loud.

[Roly]

As JM indicated, you can't understand a thermal compensation diode using a simplistic first-order model.  The diode has two resistances, a bulk resistance which is generally assumed to be fixed, and a dynamic resistance that changes radically around V-gamma, the cut in voltage.

For large signals we use a bodgy value for V-gamma of around 0.65V, but it is not a switching function, and when you look closely enough you see that the diode goes progressively from non-conduction to conduction over a range of voltage.  Moreover V-gamma has a significant temperature coefficient generally taken as -2mV/°C (at 1mA), which also depends on the forward current.  I actually use this in my Thermo-Fan design.

The traditional way of rating guitar amp output power is sinewave into a dummy load at the onset of clipping, and is known by the (inaccurate) name of "watts RMS".  "Watts RMS" are identical to watts thermal and can be measured calorimetrically (mass/time/T-rise).  While this may not satisfy Hyper-Fi and mathematical purists it provides guitarists with a dependable yardstick that became necessary when marketeers started with all sorts of misleading power ratings such as "music power", "peak music power", "peak-to-peak music power", and latterly PMPO - "Peak Music Power Output" where one watt ("RMS") computer speakers are rated at "400 watts PMPO".

Hi-Fi amps are generally rated by power at a given distortion level, say 0.1% THD, and at 1kHz.  With guitar amps the onset of clipping will be more like 5 or 10% THD, and more usefully at 440Hz for a tenor amp, and perhaps 100Hz for a bass amp.

[J M Fahey]

Yes, 2W just before visibly clipping, (without actual % indication) is reasonable and about what I measured some 40 years ago on the original, germanium powered one.
By the way, 2W is a lot for a bedroom amp.
1W even better.
1 , 2 or 5W are not enough to practice with a drummer  anyway, and the higher the power, the shorter the battery duration.

[mark]

Thanks for the replies.

The watts angle should be approached conservatively, this Total Guitar review of the Lunchbox Juniour is a good reason why; http://www.musicradar.com/gear/guitars/amplification/instrument-amps/guitar-combo-amps/lunchbox-junior-571263

"You've got a good 35 watts to play with, but solid-state power is a very different thing to valve power, and this combo won't fill a room like the 15-watt Eagletone Raging 15R we looked at a few months back, or even a five-watt valve combo, for that matter."

I dare say the whole development of the notion of SS watts and valves watts is another thread in itself. It did giving the guitarists the notion that SS amps are inferior to valve amps, noting down classic SS amps such as the Roland JC-120 again is another thread in itself.

The question I am now asking myself is why does the bass response improve with more bias current?

Mark

[J M Fahey]

That review is biased, big way.
35 WRMS are *MUCH* more than 5 W RMS , even if tube.
The notion that you can play a Club or similar job with 5W RMS, even if tube , is ridiculous.
To put names on it: a Peavey Envoy 110 , 35W RMS with a generic (Eminence made?) 10" speaker is louder than any AX84 or similar 5W single EL84 amp.

The big handicap the Lunchbox has is that it uses a ... lunchbox sized cabinet and to get *some* lows, they are forced to use a 6" acoustic suspension speaker, which by design is low eficiency.
But that's another problem.

[Roly]

Ouch!

The Lunchbox review hits a number of painful nerves, not least that to my everlasting shame I used to actually write such breathless "reviews" - first hyperventilate until you got the room spins, then rave (but never under my real name).  "Sausage journalism" I call it - you write a long and glowing blurb and the editor cuts off as much as they need to fill the space - and satisfy the advertiser.  What you don't write is that the first two sent for review smoked before you could plug a guitar in, the one that actually worked sounded like shite, and everyone who heard the claim of "35 watts" collapsed on the floor laughing.

"Reviews" like this are a kind of down market wannabe advert copywriting and have about as much substance as a cloud of steam.

Slightly more seriously, size matters.  When I hear "small", "tiny" or similar I know that what follows is likely to be some shade of horsefeathers.  "Fantastic bass in a shoebox" is simply attempting to rewrite the laws of physics, and in Days of Yore it used to be said of loudspeaker systems that "There is no substitute for cubic feet".  After fifty or more years of the march of technology, guess what, there is still "no substitute for cubic feet".

A while back I saw the release of a bass rig with eight 10's in small boxes being simply thrashed by 650 watts.  It appears to have sunk without trace, because the physical reality is that it doesn't matter how many watts you sink into an isotropic radiator, it's isn't going to make any sound, much less reproduce bass.

Now we try the opposite.  Many years ago I worked for a company making small rockerbox AM radios that would fit in your hand.  These had a two inch speaker in a plastic case that was about 3x4x1 and full of holes.  If you jammed it against your ear you got tolerable reproduction and battery life, but otherwise they sounded 'orrible.  I took an 8-inch speaker and glued it into the top of a large coffee can.  The difference was amazing.  I had a "party trick" where I would take one of these, tune it to a local rock station where it sounded like an Aardvark with a bad does of the flu, then connect it to my 2x12 guitar cab and enjoy the astonished expressions.

There was nothing wrong with the electronics, in fact they were rather hi-fi, but when it came to the speaker and baffling it all fell apart.

Take a 12-inch speaker, box it in 50-60 litres, and it doesn't matter what you drive it with - it will sound okay even if it lacks the "legs" to keep up with a drummer.

A great deal of stuff is sold on the unstated premise that you can beat the physics.  Well you can't.

Valve and transistor amps behave differently in overload, but up to the point of clipping valve watts equals transistor watts equals (bottom line) heating watts ("watts RMS").

[J M Fahey]

Here's practical proof:

1) the crummiest amp out there, the Smokey, with even *less* parts than datasheet recommended "minimum" configuration.
Just an LM386 (which is a "crippled" amp for simplicity) + 2 47uF caps ..... and nothing else, not even a Zobel.
Mind you, one of those 47uF caps is in series with the speaker !!!
So it kills everything below 400 Hz.
Ouch!!, tinny as h*ll, isn't it?
And ... SS of course!
Let's hear it onto it original 2" plastic speaker mounted inside a paper cigarette box  :duh and then into a real guitar box:
http://youtu.be/RLm0-kPvRlM

2) a Tube icon, now in an SS version, the Tiny Terror:
first into a *tiny* cabinet with an 8" speaker, then into a real 4x12"
http://youtu.be/XzDohmkXeJs
To add insult to injury, it's powered from .... a wall wart  :loco

[mark]

Speaking of memorable SS amps.

http://www.youtube.com/watch?v=Z1SEDuWLtd4

The great unknown.

http://www.youtube.com/watch?v=OPpmzLoPwAM

Mark

[mark]

The outcome of the Pignose was that it was fixed and returned to my friend. I decided to replace the TIP41 transistors in the output stage with TIP41C transistors.

This helped the overall tone of the amp though it didn't have as much gain, I found the trade off was it was much more touch sensitivity which was a great trade off. I can't explain why changing the transistors helped so much, my friend was very happy with the amp.

Here is a link to the various Pignose circuits I found during the repair process.

http://www.ssguitar.com/index.php?topic=2985.0

Mark

[Roly]

The difference between the TIP41, 41A, 41B, and TIP41C is the Collector-Emitter breakdown voltage, being 40, 60, 80, and 100 volt respectively.  In all other parameters they share the same data sheet, so it's hard to understand why there would be any difference between them when operating on lower voltages.

[Enzo]

Oh come on, The TIP41C will better handle the 100v transients than the TIP41.   Improves the stereo soundstaging.