Balder lightning guitar amp schematic please!

[nickb]

Q10 & Q13 collector - 46.1, emitter 0.5, base - 0.5

for what its worth voltage drop across R25 is 1.68 and R26 is 0.65

Yes 0.68 across base and emitter and 1.76 across base and collector on R26

both 33k resistors measure the same and I adjusted slightly trim pot to (equal resistance) center before taking above measurements

Thanks - all the measurements are very helpful.

That could only happen if either the current through Q6 was high or the gain of Q6 was lower than I expect. From post #81 (I assume the supply is 46.6) the drop across R27 is 1v so the current is 1/220 = 4.5mA. Q6 should have an HFE of at least 120 so making the base current 4.5/120 = 37.5uA

Now the current through R25 is 1.7/34 = 50uA, much too close to the base current. This explains why Mark gets a different result from me in the simulation and thanks for the cross check.

In my sim the base current is just 4uA, I bet his is much higher. The basic intention of Q6 is to regulate the idle current with temperature and that is the reason Q6 is thermally coupled to the heatsink. With the values we have here the idle current is going to be affected by HFE variations with temperature (and ageing). We don't know what the designer had in mind. I believe that HFE rises with temperature so the overall effect of this current starved design is that as temperature rises the Vbe lowers and so the collector to emitter voltage follows, also the rising HFE will reduce that voltage even further. This is a good thing so long as there not too much of it and we get crossover distortion. This seems a really cheapskate way of achieving thermal regulation as it's so dependant on transistor characteristics. A better and commonly seen design adds a thermistor this part of the circuit.

I did mention much earlier that the reason for leaving the resistors at 33K was complex....

So, the conclusion is to leave things as they are with 33K resistors for now. Turn the trimmer all the way towards the R25 end and then measure the voltage from Q11 base to Q10 base with no load and no Q10 or Q13 fitted. Repeat with the trimmer all the other way.

Whether we need to change R26 to something smaller depends on the result of this test.

[nickb]

I think R44,R45,R46, & R47 are tied to the power transistors output ahead of the 1.6 ohm resistor R48 and inductor L1 instead of after.
and that there is another (disk) cap labeled "683M" before the 0.22uF cap C9 going to "ground"
This "ground" is going directly to the collectors of Q11 & Q12.
Does this make any sense? That is to the voltage supply 46.6v
[ATTACH=CONFIG]45187[/ATTACH]

[ATTACH=CONFIG]45188[/ATTACH]

Noted

[bigdrums925]

Trimmer turned to R25, voltage between pins 1 (base) on Q11 & Q10 is 1.11v

Trimmer turned to R26, voltage between pins 1 (base) on Q10 & Q11 is 1.17v

[nickb]

Trimmer turned to R25, voltage between pins 1 (base) on Q11 & Q10 is 1.11v

Trimmer turned to R26, voltage between pins 1 (base) on Q10 & Q11 is 1.17v

.

Not a great range and a bit high.
It could be that Q6 has gone low gain since this worked once so try another or use a bigger value for R26 say 39K.

[bigdrums925]

Don't have a BC182 replacement on hand so changed R26 to 39k

Trimmer turned to R25, voltage between pins 1 (base) on Q11 & Q10 is 1.01v

Trimmer turned to R26, voltage between pins 1 (base) on Q10 & Q11 is 1.07v

both these voltages climb slowly with time

[nickb]

Don't have a BC182 replacement on hand so changed R26 to 39k

Trimmer turned to R25, voltage between pins 1 (base) on Q11 & Q10 is 1.01v

Trimmer turned to R26, voltage between pins 1 (base) on Q10 & Q11 is 1.07v

both these voltages climb slowly with time

Time to try a good Q10 in again. Set RV2 to the R25 end, bring the variac up carefully while monitoring the voltage across R36 and the variac current meter. If the R36 voltage gets to 35mV or you see the current surge, stop raising the voltage. Be careful things will happen very suddenly.

PS: I was wondering why Q10 blew earlier when the VOM reading never got over 0.2mV. I realised that we have a schematic but no component layout diagram. Because of that there could be confusion. So, it's best to be sure: It's very important that you monitor the current across the resistor that is connected to Q10 emitter.

[bigdrums925]

I have data sheet for the Motorola MJ11013 series and have verified that the resistors R36 & R38 are in fact tied to the emitter on Q10.
However with the VOM across R36 and slowly raising the variac, I see nothing on the VOM 00.0 on mV scale.
I too am concerned about blowing another Darlington, and more perplexing is it takes out the PS fuses as well.
This in spite of never seeing the current rise on the variac. I have 3 MJ11013's left and need 2 for this amp to work.
I raised the variac to 50% with no measureable voltage drop across R36, and less than .1 amps on the variac.
I suppose I need to raise the variac to full very slowly, but this is where I lost the Darlington's in the past.
Any suggestions before I proceed?

[nickb]

I share your worry BD. I really don't know why it blew the first time as the base voltage was not that that high. The current will rise fairly quickly but not that quick. You could replace R36 with something bigger, say 100 to 330 ohms while you bring it up an remove R38. The wattage doesn't really matter as it will just burn out if the current does suddenly rise and will give a measure of protection.

[bigdrums925]

Awesome, thanks. I'll try that when I get home tonight

[bigdrums925]

Alright then. I put a 100 ohm 2 watt resistor in R36, removed R38. Meter across the 100 ohm set on mV and ramped up the variac very slowly.
The voltage drop across the resistor slowly climbed to 15mV. Things look better as I was seeing no voltage across R36 last time.
Looks like the fuses held and so did Q10.

I'm guessing I now replace the 100 ohm with the 1.3 ohm resistors (one at a time) that were there before and try again
and if that works replace R13 and test for operation?

Awaiting your directive.....

[bigdrums925]

BTW I also checked the voltage drop across R37 & R39 but nothing.
Is that because of the lesser resistance?
Don't we need some voltage drop to indicate current in the transistor?
Anyway, seems like the amp is close to working now but I don't dare proceed on my own,
so please tell me what the next step should be without taking unnecessary risks.
Thanks again

[nickb]

OK. Change the R36 100 ohm to 10 ohm and try again. This resistor has two effects. It limits the current and also slows the rate at which the current rises. By now you should have a good feel of where on the variac you start to see the R36 voltage rise so you know to take is very slowly from that point. Leave R38 out.

PS: Did you change R23 currently 220 ohms to 22K? Do that now. If you don't the amp will probably oscillate.

[MarkusBass]

Bigdrums, I hope that you know that you can check the amp without assembling the output transistors (and without the risk of burning them again) just by using signal generator and oscilloscope? This would increase your confidence in the circuit. Do you have generator and oscilloscope?
It would be also nice if you tell us what resistors are now used. For example, what resistors are assembled in the bias circuit, 2x33k or something else. Also what resistors are assembled in the feedback loop, 22k + 220 Ohms, or something else? When you do the test are the output transistors attached to the heatsink, isolated from the heatsing and assembled using two screws (for each transistor)?
I found out the reason for the discrepancy between my simulation and Nick's simulation. It was my fault - I used output transistors that are not Darlingtons. Sorry for that. But it proves that the amp could work with standard transistors (just with one or two resistors changed).
So here are my findings again. Please ignore the previous one.
First I wanted to find out what it the reasonable value of the R26 resistor (in the bias circuit).
The following simulation assumes that the bias trimmer is in the middle and the R26 resistor is changed from 6k8 to 12k. The current through one of the output transistor is plotted.

You can see that useful range is from 8k2 to 12k. For 6k8 the current is very high and it would destroy the output transistors very quickly.
Now lets check the current range with 8k2 resistor (the bias trimmer is changed from 0% to 100%):

The maximum current is 130mA which is also to much (unless you'll be very careful when turning the bias trimmer).
The next simulation is for 9k1.

The current can be altered from 6mA to 30mA. This one looks quite good.
Here is another test for 10k.

This one is again a little bit better. The current can be changed for 6mA to 12mA. Please note that the current change is almost linear.
One more simulation - with 12k resistor.

The current can be changed from 3.6mA to 7.6mA. The current change is linear.
The last simulation shows what happens when I change the R26 resistor back to 33k (as it was originally):

The current is in a range of 50uA - very difficult to be measured. This is most probably incorrect value of the resistor, unless the amp was designed to work in class B.

So, as a conclusion, I would start with 10k as R26 resistor as see what it gives. Depending on the results, I would increase, or decrease it.

I would also considered the 220 Ohms resistor in the feedback loop. It decreases the feedback and increases the gain of the amp (to 39dB). This seems to much for me and could cause the amp to oscillate. I hope that the other resistor is 22k. Was this value originally in the amp? I would change to to at least 560 Ohms. But not now. First you have to be able to switch the amp on.

Mark

[bigdrums925]

Nick: I changed R23 to 22k.
Then changed R36 from 100 to 10 ohms. Left R38 out.
With the trim pot at the R25 end and ramped up the variac slowly.
With the meter across R36 got a reading of 2.2mV rising very slowly.
Since it was that low I tried turning the trim pot slightly and the mV went crazy so spray cleaned it and left it at R25 end.
Now what?

[bigdrums925]

Nick: I changed R23 to 22k.
Then changed R36 from 100 to 10 ohms. Left R38 out.
With the trim pot at the R25 end and ramped up the variac slowly.
With the meter across R36 got a reading of 2.2mV rising very slowly.
Since it was that low I tried turning the trim pot slightly and the mV went crazy so spray cleaned it and left it at R25 end.
Now what?