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Old 17th August 2013, 08:20 AM   #1
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Default Transistors/rail voltage

Got an amplifier to repair that from nature needs slow transistors in the output to operate properly

Rails are 53+53 DC and there is 4 transistors per amplifier

Problem is that i choose to go with ST TIP 3055 -2955 F to preserve the "'slow "" of the thing while my semis seem to be rated 100v

Anyone thinks that i am playing with fire ?

Kind regards
Sakis
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Old 17th August 2013, 08:26 AM   #2
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A bit close to the tolerance voltage limit.
Are they TO220/239 case?
There are lots to choose from dependent on power requirements. Without further detail I cannot comment further.
What do you mean by "Slow" type? That will have little effect on the performance as the amplifier will have compensation in the feed back loop to stop the amplifier from taking off at high frequencies.
Why not replace with the original types?
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Old 17th August 2013, 08:40 AM   #3
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case is TO 247

The specific amplifier is unique world wide regarding the output stage ...Analyzing the concept and what the Brit had in mind will raise a big discussion

Originals was factory stamped secret style BD249-250 closed TO247 type plastic

many replaced with 2SB 817 2SD1047 combos and run in to stability problems (15MHZ device )

Kind regards
Sakis
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Old 17th August 2013, 08:57 AM   #4
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Quote:
Originally Posted by sakis View Post
case is TO 247

The specific amplifier is unique world wide regarding the output stage ...Analyzing the concept and what the Brit had in mind will raise a big discussion

Originals was factory stamped secret style BD249-250 closed TO247 type plastic

many replaced with 2SB 817 2SD1047 combos and run in to stability problems (15MHZ device )

Kind regards
Sakis
That would explain the failure. 106volts rail to rail and the best version BD pair is maximum C-E 100volts.
The 2SB/D equivalents are more suited voltage wise and can be easily slowed down. Fit a 47pF cap C-B to each output transistor and a 22R Base feed resistor to stop any parasitic oscillation.
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Old 17th August 2013, 08:59 AM   #5
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Here is a useful link for supplies; 2SD1047 / 2SB817 NPN / PNP Planar Power Transistor Complimentary Pair | eBay
Unless you have an account with Farnel, RS or Mouser etc.
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Old 17th August 2013, 09:40 AM   #6
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yet again ...

Measures like that already factory exist ( and also responsible for the sonic signature of the specific amplifier )

As about the failure mode i don't think that has much to do with rail voltage versus transistor specs . In the specific amplifier this has only to do with a mechanical construction ERROR that doesn't allow huge heatsinks to work properly and efficiently enough .

Obviously in the design the error exists and the arrogance from the UK manufacturer to allow the machine in the market including a fatal error ...

here is a close up of how wrong trs are mounted while one encapsulated bolt ( as seen in the second picture ) forces the transistor out of the heatsink which is 15cm long and 1.5mm thick to cool 100W amplifier and includes curves which in combination with the bolt make things even worst

... Still in the "frame'' of the case a huge heatsink stands but has no actual contact with the transistors and only with the frame of the case which is the "'real" heatsink

Picture one :close up

Click the image to open in full size.

Picture two : encapsulated bolt detail


Click the image to open in full size.

Picture three useless heatsink


Click the image to open in full size.






Kind regards
Sakis
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Old 17th August 2013, 09:51 AM   #7
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I must say, I have seen better construction!
Are the Allen screws loose or just not fitted correctly do you think?
A bend in the legs of the trs to allow heat movement would help.
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Old 17th August 2013, 09:56 AM   #8
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no it will not ...screws are as tight is possible but what creates the problem is that the encapsulated bolt is actually "'sitting "" on the curve and that makes it misplaced versus the vertical axis of the "heatsink"

Tightening the screw will force the transistor away from the surface of the heatsink
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Last edited by east electronics; 17th August 2013 at 10:00 AM.
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Old 17th August 2013, 10:00 AM   #9
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If you ever produced a consumer amplifier you will say that this type of construction and especially with that level of accuracy and even for massive production as seen in the pictures now days and generally will be impossible to produce since costs so much .

means that people had the guts and the money to produce a modern engineering and metalwork miracle but just made a serious design error ...classic British case !!!

Kind UK regards
Sakis
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Old 17th August 2013, 10:01 AM   #10
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Quote:
Originally Posted by JonSnell Electronic View Post
That would explain the failure. 106volts rail to rail and the best version BD pair is maximum C-E 100volts.
If you look at the datasheet, you'll see that at 100Vce the allowed current is very, very small, much less than what the Pc would suggest. Keyword here is Safe Operating Area.
So those transistor are very unreliable at the max Vce especially if they need to source some current!

jan
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