SWR working pro bass amplifier driver transistors  - Page 1
Forum section: electronics / repair

author: Anks
author_rank: Frequent Contributor
timestamp: December 21, 2015, 05:49:42 pm
content: I've got an amplifier (SWR working pro) repair that is pretty urgent and I can't get hold of the driver transistors before that it is needed. I generally as a rule do not used different species parts than the original designer used unless it's necessary but musicians around this time of year really can be a bit pushy.The original drivers are mje15034 and mje15035 (the ones fitted are mje15032/33 and are defiantly factory) and the only issue was that one of the legs as snapped off one of them. To confirm this I've filed away at the case and reattached and the amplifier works as expected but the lead won't hold for long as the vibration when play tested breaks it loose. It's output stage uses 2sc3264 2sa1295 outputs 3 of each and if I remember correctly +-80v rails.Now I've seen very similar amps use 2sc4793 and 2sa1837 as drivers with the same output transistors and even higher rails but these don't have as high collector current as the mje15034/35 I see no reason for this to be an issue but would like someone else to have a look make sure I'm not missing something obvious. I'll attach the schematics and relevant data sheets.Thanks for any help it will be much appreciated.For some reason the attachments won't work I'll try again.

author: Anks
author_rank: Frequent Contributor
timestamp: December 21, 2015, 06:18:15 pm
content: Here is a link to the schematic over forum size limithttps://www.dropbox.com/s/ju75lp3clgqyhlp/WorkingPro%20400%20%26%20700%20schematic.pdf?dl=0Also ill attach the main output data sheetsIf anyone as any ideas of other transistors that are common in amplifiers that better suit the application let me know Thanks

author: singapol
author_rank: Frequent Contributor
timestamp: December 22, 2015, 01:56:42 am
content: Best advice is to use original types as in schematics ( sorry i'm unable to see it). You claim mje15032 as factory installed but I think it is not. If you look at 2SC3264, it's maximum base current is 5A and mje15032 maximum collector current is 8A so it's risky to use it as it could pump more current dependening on how much base voltage or current there is. Mje15034 maximum current is 4A and is more likely the correct driver.If you can't get original components then it 's your responnsibility to find equivalent types with similar specsand package mounting.I can't comment on the other power output transistors as I can't see the actual schematics. If you do these for a living then it's your responsibilty to know your electronic basics. Do things baesd on correct knowledge/facts.

author: Brumby
author_rank: Unknown
timestamp: December 22, 2015, 04:05:44 am
content: The power amp circuit for anyone interested... (I cut it in half for better readability)

author: vk6zgo
author_rank: Super Contributor
timestamp: December 22, 2015, 05:31:45 am
content: It is nice to be virtuous,& always use the original device,but sometimes,in the big,bad,world,substitutesdohave to be used.If the other amp has the same,very similar,or better,specifications,I would tend to go with it.On many occasions in decades in Electronics,from necessity,I have made such substitutions & they have,in the overwhelming number of cases,been completely satisfactory.You are often better off using the components used by anotherreputablemanufacturer in a similar circuit,than to trust some of the so-called "equivalents lists".

author: Kleinstein
author_rank: Super Contributor
timestamp: December 22, 2015, 11:45:35 am
content: A higher current rating for the driver transistors is not a problem at all - this number is a rated current for low voltage operation. The transistor can delliver more current and possibly fail, especially at the rather high voltage - The SOA limit is actuall quite low and the transistors might fail because of this. Especially the 2SA.. and 2SC47.. are even worse there.I would prefer the largest ones from the list (MJE15034).

author: Anks
author_rank: Frequent Contributor
timestamp: December 22, 2015, 12:25:48 pm
content: In the end I went with the mje15034/35 as I had some in another amplifier that I won't miss till after New Years.