Palomar 450HD 2879?

[sanjosebay21]

ok i have a 4 pill palomar 450hd all transistors are gone my brother took them out and used for another amp originally had sd1446's now i have 4 new 2sc2879 can i use them and if so what needs to be changed or modified in amp to handle them i know its probably cheaper to buy new amp or new sd1446 i just want to put this back together and put to use just thought some of you might have the know how to change out the sd1446 to 2sc2879 thanks.........

 

[9C1Driver]

That amp came with 1446 finals, best bet is to replace them with the same finals. If you do want to swap to 2879's you will have to have a good amp builder re-work it to take the larger finals, they are not a direct swap.

 

[sanjosebay21]

Do u know what's parts need changed?

 

[ExitThirteen]

They require re-wrapping the input and output transformers, some capacitance changes, and a change in the combiner circuit. IMO, not worth the hassle. You're better off going with the original SD1446's and going that route.

~Cheers~

 

[Shockwave]

If you look at the datasheet online for both of these transistors and compare their input and output impedance at 30 MHz (not 50 on the SD1446) you'll see they are not far off. Around 1.45 ohms for both ratings on the 2SC2879 and the SD1146 is around 1.4 for input and 1.25 for output. The biggest difference is that the 2SC2879 has more than twice the collector current rating and would be a much more rugged replacement being they both cost about $35 now.

Impedance can be tweaked for a better match by replacing any fixed value dipped silver mica caps across the input and output transformers with variable compression trimmers such as the 464 trough 468 series. Adjusting them for maximum power transfer should show more output than the stock finals. The 2SC2879 also has noticeably more gain than the SD1446. The specs show the SD @ 10db and the 2879 @ 13db. They are closer than this at 30 MHz but the 2879 is still ahead.

 

[Robb]

Think this page can give you an idea of what has been discussed:

http://www.cbtricks.com/Amp/txstar/dx250_dx350/graphics/ts_dx250_dx350_sch.pdf

Although one amp is using 2290's and the other amp is using 2879's, it still points out that changes would still need to be made.

 

[dejaskysdad]

THANK YOU SHOCKWAVE AND ROB!!!! that's what I needed this information is helpful