Headphone and DI out (pre-tone)

Any reason for using the darlington for i/p, why not a FET opamp, which would sound better imho.

The addition of a driven MOSFET output pair with variable bias to get a cross over distortion, rather than the usual clipping types that use diodes.
This would be variable between clean biased correctly and increasing offset bias for the distortion.

Just a thought based on an old design

I don't take offence at all.

Quote from: Enzo on July 03, 2015, 08:46:24 PM
I don't know what to make of this.  If the C-E short exists, you say the base must also be "shorted".

and where did I state that ?

Quote from: Enzo on July 03, 2015, 08:46:24 PM
Well, I will believe the base region may well be damaged.  But then you add that it doesn't mean there has to be a short from the base leg to the other terminals.  We do not tear transistors apart, so all we CAN do is test the relationship from one leg to another.  You might make some philosophical point about what goes on inside, but from a reasonable practical view, such as during troubleshooting, if we measure no short from the base leg, we will simply say ther is no short.  The transistor is already failed C-E, so anything else is moot.

Of course there does not have to be a short, if you care to reread the post I made, it was g1 that made that comment, and that was in respect to the different numbering of the schematic I was looking at, having different numbers to Hawks, again I explained all this.

Quoteg1 stated : A shorted Q13 could be C-E, so it would not necessarily affect the bias.

I merely pointed out that it would effect the bias. If you cannot understand this, then fine.

Same goes for the biasing of the power amp with a pair of transistors removed, if you note Hawk had decided to inject a signal, which should never be done at this stage. the four transistors are actually biased on more, than they normally would be, so the test is useless.

You must never turn a solid state amp on with no load. If the impedance is too low or high it will more than likely blow the output stages.

Think of the load as an external component of the amplifier, without which the amp will not function correctly.

Quote from: g1 on July 03, 2015, 01:17:43 PM
But this is aside from the point, you were originally speaking of it as a driver.  In which case a C-E short would have affected the bias.  So no need to argue a point you never originally intended, right?

Did you not read my post ? I did say the numbering was different duh, in other words on the diagram I have, Q13 is shown at the position of Q5 on Hawk's cct, which is a driver.

I can only go by how a bipolar transistor is physically made. For there to be a short between collector and emitter the base region of a transistor must also be shorted, this does not mean that you have to see a short between the base leg to either the collector or the emitter as the leg is connected to a small portion of the base. You should really read up on how a bipolar transistor works to understand why the base current is effected when there is a CE short. Pay particular attention to doping of the base emitter region, and the now defunct depletion layer. which is blown away when there is a short of this nature.

Quote from: Enzo on July 03, 2015, 01:06:47 PM
I need to know.  How does the removal of one parallel transistor cause the rest to fail?  Remember we already stipulated the amp would not be producing full power this way, but will work sufficiently to service.

In actual fact Hawk was more sensible than this, and mistakenly suggested he remove a pair of transistors.
What we have here is that no explanation was given when it was said it would be fine to do so, and as you should know that without checking other components within the output section of the power amp, you cannot know that powering up will be safe.
Transistors are current operating devices, which means that the bias current for the output transistors is calculated to be shared between each of the 3 transistors. See Kirchoffs current law(KCL) What you now have is a circuit designed for 3 transistors each side, being biased on harder because there is only 2 transistors each side and you still have not checked other components in a DC connected circuit.

This is in answer to Hawk's question

QuoteSo if everything else is working but I take out Q12 and Q13, the amp should work, although not as powerful as I've removed a gain stage?

No, but it could cause damage, so take your time. don't rush, and check everything. I do not mean the whole circuit as it is an output transistor that blew.
If Q3 is OK and everything after that is fine, then there should be no problems. The furthest back you really need to check is to Q1 and Q2 but these never really blow, or it would be farer to say I have never seen them blown.
Once you are happy that you cannot see any other problems, replace any blown components, connect it up with the light bulb on the mains, then take voltage readings at different points. Never inject a signal until you know all other conditions, in a DC sense are correct.

Quote from: g1 on July 03, 2015, 12:16:55 PM
  Please keep in mind that this is a guitar amp forum, and we speak in that context here.
  No one has ever seen a cap can with built in coil in any amp similar to the one in question. 

That is a fair enough statement, which I can accept, and I was speaking in terms of guitar amps, but if as you say no one has seen one, then that is acceptable, to tell me I have not seen one or am mistaken is just a misconception.

Sorry I did not mean to bruise anyone's egos, but apparently have.

I found your statement that you have not seen such a device acceptable, just accept that I have seen one, and let it go at that.

Quote from: g1 on July 03, 2015, 12:07:30 PM
  Just reading above, it sounds like Latedev is speaking of Q13 as if it is a driver, Enzo speaking of it as if it is an output transistor.
After looking at the schematic you supplied, Q13 is an output device.
Latedev, are you looking at a different schematic?  A shorted Q13 could be C-E, so it would not necessarily affect the bias.
Operating with one pair of the output transistors missing is not one less gain stage, it is less current gain.  This translates to less power available to the load, and more strain on the other output transistors at higher power levels.

I didn't see the cct diag Hawk supplied and yes I was referring to my own circuit which is exactly the same just different component numbering for some odd reason. Oh well thanks for pointing that out

QuoteA shorted Q13 could be C-E, so it would not necessarily affect the bias.

Actually it would be a miracle if it did not effect the base as the emitter and collector is physically separated by the base on a bipolar transistor, unlike an FET which you may be getting confused with.

Even more amazed nothing else went pop.
Protection circuit is CR29, Q4, Q8, CR30, R105, R106. These act as clamps on the bases of Q5 and Q9, which are the drivers, via R107 and R108 . All transistors and diodes should be checked out of circuit.
Never assume you can run the amp with one O/P transistor removed, as this could blow the rest. If you need to know why, just ask.

 I didn't say he was inexperienced at all, I merely stated he had no experience of something.

QuoteYou are mistaken. Just because you have no experience with these, don't automatically think someone else is talking of something different.

By stating as a fact I had said something wrong, is incorrect and should never have been put in the fashion it was.

QuoteYou are mixing this kind of multi electrolytic cap,

A statement of fact.

He could just as easily have stated, that his experience was of the first type only, and he had no experience of the type I had stated, or in fact warned ilyaa not to confuse a mains filter with the type of cap he showed first, which would have been more helpful, Instead he decided to state I was wrong and he was right.

As far as someone not blowing their own horn, it matters little to me what his pedigree is, just how he conducts himself. I can obviously see he wants to be helpful, but it should never be his aim to put someone else down, because of his own misconceptions.
Suffice to say I have had experience in all the areas you have stated he has, plus quite a few more in both pro audio and commercial electronics.

Quote from: Hawk on July 02, 2015, 09:57:06 AM
I removed all output transistors and Q13 was the only one shorted. Could that alone explain the fuse blowing and the amp not working?

You are quite lucky it was that transistor that blew, as it is the output driver on one half of the power amp section.
I did mention the protection system on these amps, which, on this occasion, did its job.
With a shorted Q13 the bias to the output transistors would have been pulled up to rail, turning the o/p transistors hard on. However one of the output transistors is connected to the protection circuit (this is the lucky bit), which would have clamped the bias down. It would have done this via R134 a shorted Q13 to diode CR34 and Q12 turned hard on.
This is where protection circuits can go bang.

So the transistor goes short, a big current spike and the protection circuit kicks in to clamp the bias, in the mean time the fuse is still being heated to the point of breakage from the original short. or, hopefully not in this case, the protection circuit fries and is the cause of the blown fuse.

Make sure you check every component as power amps are DC connected throughout. For all you know at this stage, Q12 could have gone into meltdown.

Bending twisting or snipping legs of defunct transistors are always advisable, just don't forget to throw then away after 

 You are mistaken. Just because you have no experience with these, don't automatically think someone else is talking of something different.

As we are talking about power supply smoothing, why on earth would you assume that a mains filter is suddenly being talked about.

A long time ago Pi filters could be had in a single can for filtering and smoothing power, I did think that Enzo may have come across these at some point in the past.

What are the Voltages around the tube.
Screen Grid
Grid
Anode

Quiescent current of 50mA @ 550V
Can you double check this ?

Did a search for an earlier Marshall for comparison, which may help.

http://www.drtube.com/schematics/marshall/1987mk2u.gif

Old chart for 6550 quiescent/idle current, for a given Voltage of 550V.
6550 SED Min 32mA Max 45mA
6550 EH Min 38mA Max 53mA

Hope info helps

This is just a number of caps in the same can for convenience and cost.

Having a look at how a cap is constructed may help you to understand more about these things. Just do a search for Electrolytic Cap Construction. then imagine an insulator around each cap. In a filter can, they just connect all the cathodes together for convenience.

Some companies specialise in different filters for different applications and may present you with 2 tabs and a ground, yet inside there will be 2 caps and an inductor between the caps, any of which can individually blow.

I would imagine Enzo knows the construction of these cans, just as I do, which is why we have both said replace the whole filter circuit. With one cap inside already going short, the likely outcome would be that the other cap may go as well.

I agree and did not see the fuzz control grounding out, so just turning that full anti cw, should be able to rule out if it is the effects side.

Fuzz control "0" gain up = hum then it is from the 3 transistors or jacks.
Fuzz control fully CW and hum its the fx circuit.

Quote from: g1 on July 01, 2015, 12:45:25 AM

Quote from: LateDev on June 30, 2015, 04:00:09 PM
I guess I will just have to be disappointed and you will have to carry on in the mistaken belief it is a test device.

I realize it is not intended to be a test device.  But when something acts as such on occasion, incidentally, why not use it to your advantage.

Yes, as Enzo pointed out, it has limitations.  But that is not to say it can't serve more than one function. 

Both statements are of course correct, but from my reading on here, the limitations of the bulb method of testing are not pointed out.

The bulb is there to stop you blowing fuses and in some cases components, however it must never be thought of either as a protection device for everyday use or a test device for the many that don't understand its limitations or even why it is used.

Yes it does have its uses in testing,providing you use it correctly, which many people don't. I even use the light bulb method for a load when testing, but I know its limitations so am aware of what to use it with.

I have seen others say that their amp distorts and the bulb lights up when they crank it up and play. They take this as some kind of fault.

For most people the light bulb must only be considered as a fuse saver, As Enzo stated earlier "My screwdriver is very handy but sucks at driving nails."

Quote from: g1 on July 01, 2015, 01:00:52 AM
  The control grid stoppers are probably not a bad idea.  Marshall's with 6550's used 1K5's, same as Fender uses in most tube amps.

LateDev:  why change the 1K screen R's?  That is the stock value for this amp with the stock 6550's, also used by Marshall's with 6550's.
The EL34's in this amp weren't stock, the schematic is for 6550's.
And which resistors did you mean to lower to 47K ?

Sorry I was just going by the Marshall cct diagrams they use either 75R or 470R depending on the model. The 47K was for the bias as is shown in the diagram.
The screen grid current is a lot higher on the 6550 which is why the resistor value was lowered from 1K5 USA which was for the EL34. If however you have more up to date data, (mine was 1990 ish) I will have to go by what you say. As we know manufacturers do modify their ideas over time.

If the circuit is stock for this model with 6550's then it should work as designed, so why change the bias ?

All things being equal and if the circuit diagram is correct, then you are indeed looking at a possible fault.
Leaky caps were common on many older valve amps, however I would check out the bias chain first. Think that is a Zener which really should be checked first.