Doxa 70 SE Amplifier

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Hi All!

I am about to change a defective transistor inside this amlifier.
One channel has 6pcs BD246C and 6pcs BD245C.
Those six are coupled in paralell.

I need to replace one piece of BD245C.

I have at hand TIP35C,BD249C and some 2sc**** 2SD****

Can they be used as replacement in this case?

Thanks for Your comments.,
ManUtd.
 
What do You mean?
TIP33C is a direct replacement for BD245C.
This amp was Originally built for BD245C and BD246C and it sounds well
with those transistors as well.
Why is will it sound better with TIP35C,TIP36C ???
Please advice why.

Regards,
ManUtd.


PMik said:
Im sry to say, but i think its a really sad event :bawling:
when you spoile :smash:
a really nice amplifire, by saving very! few bucks :dead:
 
What i mean is to use two different Types, brands of semiconductors, will for sure raise the THD

I have heard the amp you got....its one of the better ones, and afir ....its uses low global feed-back .....you have to make it as symetrical as possible to keep or better improve specs, this is crucial point.

Other types of power-transistors besides the TIP/BD's could be used ....But the TIP's are the cheapest of them all :)
 
Thanks Pmik,

I think i understand.
If i change to TIP33C instead of BD245C i should also change BD246C to TIP34C in the same time?
Then there is a matching pair again?

Reg,
ManUtd.




PMik said:
What i mean is to use two different Types, brands of semiconductors, will for sure raise the THD

I have heard the amp you got....its one of the better ones, and afir ....its uses low global feed-back .....you have to make it as symetrical as possible to keep or better improve specs, this is crucial point.

Other types of power-transistors besides the TIP/BD's could be used ....But the TIP's are the cheapest of them all :)
 
Yeah! :) keep it "clean" by replacing parts symetrical, if you cant get a part .....you have to replace in both channals...this is not a cheap amp...it was made for wanna be/have high End gear users ;)

Ps. Audio 60 made by the same engineer, was also a nice "bang for bucks" amplifire ;)
 
Thanks Again,

Could You help me a bit with some recommended replacement transistors for this unit?
I have pairs of MJL3281A/MJL1302A and 2SC2837/2SA1186
also i have MJL4281A/MJL4302A and MJL21193/MJL21194

Could i Use some of this pairs for this amp?

Reg,
ManUtd.



PMik said:
Yeah! :) keep it "clean" by replacing parts symetrical, if you cant get a part .....you have to replace in both channals...this is not a cheap amp...it was made for wanna be/have high End gear users ;)

Ps. Audio 60 made by the same engineer, was also a nice "bang for bucks" amplifire ;)
 
Thanks again,I will wait for Your advice.
I have a lot of 2SC2837/2SA1186 at hand,no problem changing both channels.



PMik said:
ill be back with a reply a little later on....busy for the moment Also have to look at the datasheet (transfer curves), afir. the 2SC2837/2SA1186 will be best choice...but ill have to check on that ;)
 
Thanks for helping me.

Seems like You know a lot about this amp.
How do i know if i turn the bias-pot in correct direction?
It normally should be opposite of clockwise?
Measuring more resistance?
The voltage measured across emitter resistor should be the same as for the BD245C? After adjusting and When amp is warm.

Thanks,
Regards
ManUtd


PMik said:
Whoops! almost forgot :xeye:

Remember to set the bias to close to the minimum, before you start it up with the Sankens and then set the idle current ;)
 
Hournest...i dont remember the detail of which way to turn the bias pot ... been some years since i last had a Doxa or Audio60 in my hands.

If they made it the correct fashion.... it will increase the current clockwize

you should be able to start the amp with the functioning channal attach only ...this to make a notice of wich way to turn the pot. ;)

Regarding idle current...yes it should be the same as with the BD's, if we assume its the factory setting of idle current.

If not sure if its correct ?

make sure you have min. 50mA each transistor to maintain good calm, warm and articulated treble....more is better.


Cheers ;)
 
Do NOT replace the original BD254 and 246 with "better" japan versions!!!
Keep the originals!!
Remember that the constructor of DOXA, Knut Lauvland, have done a lot of testing to find the best possible transistors for the amp. And this does not always meen the ones with the best spec.

In early versions of the DOXA amps, Lauvland used fast and linear transistors like 2sA1227 and 2SC2987. But these were changed to BD245 and 246 by the ear-test!!
 
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