Thanks Roly! 

:tu: :tu:

QuoteI'd encourage you to run through this again setting the Collector load 1k to 10k

Thanks Roly, and I did just that. If interested, please see attachment. Hopefully I have it right! I had to post it as a .pdf so it spreads over three pages but it still quite readable.

Quote6mA through 1k is 6 volts, so the Collector voltage will drop by 1 volt.

I'm assuming we're using variables to understand the workings of a transistor and wouldn't really be applicable as I remember that we wanted a max. 5 volts at the collector, half the 10 v supply so we could have a clean audio amplification stage.

Quote{I had a funny feeling when I wrote that ... you ever consider doing an electronics course?

Roly so here's my story: fifteen years ago I completed an electronics certificate at the college level, had ideas of getting into the audio repair business, but kids, mortgage and other pressures put that on the back burner, way back! so now, with adult kids and at age 53 I'm gradually dusting off my knowledge and asking many questions. There are definitely holes in my knowledge and it's amazing what you can forget in fifteen years--usually the small connecting details/equations/theory but it's coming back and I pretty much spend a couple of hours  a day with my head in this stuff with an eye to start up a part-time repair business down the road. I'm having a blast and loving it!

QuoteInstead of looking to simulate circuits, consider buying a few cheap transistors and some cheap resistors and a few caps, and actually making little circuits.  Then a basic voltmeter can take readings in the circuit.

Enzo, that makes total sense. So should I be thinking about getting a DC Power Supply? any recommendations in terms of max current/voltage?

Roly thanks for taking the time, I feel like a real taker on this forum and hope that one day I can give back. :cheesy:

Some of the books I study ask me to build circuits to prove their point. I have electronics workbench from 14 years ago but it doesn't work on my operating system.  Circuit Logix Pro looks great but not cheap. Is there any good freeware/cheap software to use to help build simulated circuits to help with my understanding of electronics? Thanks.

Quotebut when we want to operate the transistor in grounded Emitter mode so we can get some voltage gain on the Collector,

Sorry, couldn't resist another question. Firstly, excellent description and I understand most of it! Great stuff and thanks Roly! So, as for the quote  above, my mind was going along beautifully with your description until I read the  bit about the voltage gain on the Collector if we grounded the emitter. Could you elaborate on how we achieve voltage gain on the collector? Is it because more current is flowing through the collector due to the removal of the emitter resistor and therefore the increased current multiplied by the Collector's internal resistance creates a greater voltage? That's what messes me up about Transistors-- I understand the base current loop creating current flow from Collector to Emitter, like a valve letting more water through, and then I think I've got this stuff, but when we start talking from Emitter to Collector I start thinking about holes filling valence shells missing electrons and conventional current. With transistors should we be thinking neg to pos/conventional current at the same time? Or should we always think neg to positive--N type material to P type material-- just to keep our heads on straight? Should we not overly concern ourselves with current flow from emitter to collector? Thanks!   

Hey Roly, thanks for going the distance with this stuff, I'm learning a ton!! So with the knowledge gained so far I'm going to dive into my transistor book and study and re-read more and then I will go back to your full model of a transistor and make more sense of it--I don't think it's fair to go further and ask more of yourself  or anyone until I've done more work on my own. (except for question at bottom of this post).

Quote... I have made use of this characteristic to fit a linear scale wattmeter to my dummy load.  It's a bridge rectifier, but the voltage is scaled down by resistances so that it operates over the early part of its conduction curve, and the result is a current of 0-1mA for 0-100W in 8 ohms (calibrated to be exact at 50 watts, leaving an error of about 5% at 10 and 90 watts.}

Sounds great, I'd like to see a picture if you have one.

QuoteI've mentioned the internal series resistance into the Base, but there is an effective series resistance with every terminal, and the one in the Emitter is a particular trap because it appears not only in series with the controlled main C-E current flow, but also in series with the controlling Base-Emitter flow (and thus results in unwanted local negative feedback within the device). {This is normally called something like r'e, emitter resistance, where the apostrophe implies an internal property.}

When you say "and thus results in unwanted local negative feedback within the device". Could you elaborate on that a little more. I find it interesting but I can't quite picture it.

Thanks again for this great information!! :dbtu:

Enzo your efforts were not in vain. I understand the signal piece, just wanted to make sure I understood the constant bias to understand that both transistors are conducting and therefore no crossover distortion. I believe that I do.
I used my scope to check the collector of TR6 and, of course, there is  signal, amplitude varied by the volume controls. Scoped all transistors and found the signal there as well.

Thanks again for your efforts!  :tu: :tu:

Great explanation!

QuoteSo we put in some bias between the bases so that both transistors are conducting just a little bit, idle current, and the transition from one transistor conducting to the other is much smoother (and much better sounding).

So this would explain the presence of DC voltages at the bases of TR6 and TR5, so that both transistors are conducting and therefore we eliminate crossover distortion:
1.3 vdc at base of TR6 (.81 VBE--1.3-.5 = 0.8v)
-1.7 vdc at base of TR5 (-1.1 VBE--1.7-.6 =-1.1v)
Measurements above confirmed measurements with multimeter.
Therefore constant current flow (over B-E cut in voltage of .7/.6/.5v) and therefore no crossover distortion. Correct?
(Not sure why the voltages aren't the same on transistor bases, maybe resistor values have drifted)

Correct?

Thanks Enzo, your OpAmp explanation plus the idea of pulling up and down helped me look at this entire circuit differently and opened up my eyes to it. Of course, a few more things as I drill down...

Quotethe voltage across the EB junction may stay the same - remember it is a "diode", not a resistor - changing current through that junction controls current through EC.  SO while you play, look at the voltage on the collector of that transistor.

I tried that: referenced to ground, on collector of TR6 I get 36.7vdc at idle. At full volume, guitar playing, 35.8vdc. So, from this info, it looks like the pos. signal excursion has pulled down the voltage a little, correct?

What is the purpose of diodes D1,D2? Separating the neg/pos bias on TR6 and TR5?

Enzo, as an electronics champ I'm glad you've taken great pains to be "shielded'. :tu:

QuoteAt idle both upper and lower output transistor "clutches" are just conducting thanks to the idle bias setting.  The half-rail is at ground potential, and the bias network is pushing the Bases just far enough apart to let through a dribble of current.

On my valvestate, TR6 B-E voltage reads .535 Volts. When I play my guitar it doesn't change. I've always known about the the turn on voltage in a transistor, and you mention that while idling there is a dribble of current going through. Is that because we're so close to .6 v so there is some current as it's about to turn on? Also, now that I'm trying to get what's in my head into my hands and the pcb, I thought for some reason that when I played the guitar the resistance of the Transistor would drop (it would turn on) and therefore so would the voltage, but I suppose if the current increases and you multiply that by the resistance you get the same voltage, no can't be right...hmmm, wondering why the .535 stays the same, I expected to see it change.
So, now that I've written this I'm thinking about your OP amp explanation and output trying to keep up with the Input for unity gain and I'm wondering if this voltage is supposed to stay the same although the current increases (enough to drive a servo motoer). But then I would also ask how does this static EB voltage stay the same when we have full output swing between the rails, don't we have one side pulling the other side down and vica versa, therefore a collective voltage drops on either the pos or neg side of the rail... P.S. Excellent responses so far and greatly appreciated!)

QuoteNow, just pretending for a moment that this circuit doesn't have the NFB blooper and in fact has proper NFB.  What would the output DC offset be in this amp?
{hint: the base of the first transistor is tied to ground via the 10k, so applying very basic transistor knowledge, the voltage on its emitter must be...  and therefore...?}

Here goes... ...I know that we want a very small DC offset or we'll get DC current travelling to the speakers and that will fry the speaker. Not sure how to calculate that, even with the hint provided. So, I can honestly say that I need to go back to transistor basics as I'm honestly not sure how to calculate the emitter voltage, wondering if it should follow the voltage somehow at the base of the transisor minus .6/.7v??  Even with this great info my brain can't wrap my head around this...this transistor stuff grips me but kind of torments me as well, it's slippery!!

QuoteThe DC point is set by the 10k to ground on the "+" input, so it basically "follows" ground for DC.

Could you possibly re-explain this, not quite sure how that works.

QuoteFor AC signals where the reactance (or AC resistance), Xc, of the cap is low then the ratio of the feedback resistors sets the AC gain

. 
Are you referring to the 10K and 4K7?

QuoteFeedback in s.s. amps is a bit of a trick because it is both DC and AC.

So what is blocking DC from entering the speaker? I see C6 which goes to ground, does that mean that it is easier for DC to go to ground then the speaker? I expected to see a DC blocking coupling cap between OT's and speaker...hmmm

QuoteThe attenuation of Rf and Rs at AC is;
Rs / Rf+Rs
1 / (1+22) = 0.04347826 times
dB = 20 log10(V1/V2)
20 * log10(0.04347826) = -27.23455689dB

Roly, looks like you are using a voltage divider equation, but not sure how you get V1,/V2 also equal to 0.04347826? Could you detail this a little more if possible?

Quote
This in turn will cause it to pull its collector more positive, and since this is also the bias point it will turn on the clutch of upper transistors, and turn off the lower clutch, so the output half rail will be pulled positive.
Now this positive-going signal is taken back to the base of the second transistor, but it has just had its emitter pulled negative, so this positive-going feedback signal will actually try and turn on the second transistor harder, and the VAS, and the output clutch.

Thanks Roly for the details, what you're saying about the feedback circuit reinforcing makes sense. But what exactly is happening when you say:

Quoteand since this is also the bias point it will turn on the clutch of upper transistors, and turn off the lower clutch

How is the lower clutch turned off? Is it all about pos relative to negative, depending on the situation? Upper Clutch being more positive, lower clutch less positive and closer to a value less than turn on voltage? Thanks. :tu:

Roly, I'll give your challenge a try. What through me off at first was seeing the NFB (22K resistor) running down the middle of the schem. I'm used to seeing them run on the outside of the schem them go to the input of an op amp or tone stack. So then I'm thinking where is the half rail? Without a half rail the system won't work or balance out (if we're to see it as an OpAmp). Doesn't the +38/-38 need a reference point?
Then I'm thinking, there is no current limiting here (but maybe there doesn't have to be if this is an older design). And should there not be diodes to separate the pos/neg swing of the signal before it heads to the speaker? Or would then both neg/pos rails be sending the full wave form and possibly it would be out of phase by 180 degrees. Hope I've made some sense here and I'm not a mile off base.


P.S. Thanks Roly, G1, Enzio, Fahey for all this great info. I'm grateful for it and will read and re-read.

I'm referring to the O/P at the bottom of Roly's current limiting diagrams at top of page. Thanks

Roly, I kept trying to insert my card into that image but no luck! I'll keep trying! haha!

Roly, thanks for posting that radio info and the schematic and the information you've gleaned from working on radios.  Also, the current limiting info and link are great as well as the Neets site. Great info indeed!!

Enzo, I have downloaded that book you suggested and have read about 70 pages, and there are sections I have to keep re-reading but it is a great book and I will finish and re-read for sure.

So Beyond the Output Transistors we have resistors and caps between the OT's and the speaker jacks, plus some caps. Are these covered in the solid state amp book? Or can you give me a basic understanding of their importance? Thanks.

Also, What does O/P stand for? I believe it's the middle rail separating the +VE/-VE.

How long has this forum been in operation? I'm tempted to print off all this info but rather than do that and save some trees I'd rather return to it electronically a month/year/decade down the road. Is there a way to bookmark this post as it will eventually descend further down the post line and be tougher to locate. Can it be saved electronically? Thanks.