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Hackernap parallel output transistors

I think I have enough space to mount an extra set of output transistors upside down above the NCC220s on an L-shaped piece of aluminium extrusion bolted to the heat sinks of my Modushop cases. Will that be ok as far as TR5 being more directly connected to one pair of output transistors than the other?
I should be able to parallel the new transistors onto the circuit board including the low value resistors, which I assume should match the present 0.22R components.
Perhaps not an elegant solution, but hopefully practical.
Do the transistors need the hfe matching? I must have 8 suitable transistors....
Besides checking the current bias is that all I need to do? It sounds too simple!
Les and friends on the NCC300 thread seem to have spent some time on voicing...
Cheers Andy.
 
You shouldn’t have to match the hfe of the transistors, I didn’t match mine.

Mounting the transistors upside down under the original ones might make the wiring shorter and easier.

Pete
 
You could also use something like 50x25 unequal angle. Mount two transistors on the 50mm part, connecting through to the amp board on the front transistor. Then use a 25x25 equal angle to make the u shape to bolt to the case bottom
 
Mounting above is attractive because I can leave the amp board in place, everything I need to connect to is available on the top of the board. Perhaps doing this from underneath is more elegant.
Are longer connectors between the transistors an issue?
Cheers Andy.
 
Scrappy sketch. Should work if you have enough space behind original heat sink . You could fit it all to unequal angle first and wire up, then bolt equal angle to case and finally bolt the two together
489901-E2-425-E-4269-A7-E4-9-CAA2326207-B.jpg
 
What about just using the standard C channel and putting the second transistors on the bottom flange, and wire them together dead bug style

Alan
 
I have Avondale U shaped heat sinks and the below option would just involve drilling the Avondale heat sink arm opposite the present transistors and fit the new transistors with their underside facing the underside of the old transistors. Stops any worries about TR5. Less metal work. Shorter wires. I don’t remember soldering in this space being too tricky....
Cheers Andy.
 
With the help of valve-radio.co.uk, some details of the Sugden A48 mkIII (1986-1991). The parallel output transistors were a change from serial No 4200 onwards, but the circuit was always fully complementary.

sugden48mkIII by plybench, on Flickr


So, Tidying up some of the symmetry;

VT5/9 are first part of a Darlington-ish pair BC639
VT7/11 and the NPN power drivers - BDV95

BC639; emitter voltage 80V, collector current 1A, hFE 40-160, bandwidth product 200 MHz
BDV95; emitter voltage 100V, collector current 20A, hFE min 20

on the PNP side (fully symmetric, as for NPN)
VT6/10 BC640
VT8/12 BDV96

BC640 - as per BC639 except hFE 100-250, bandwidth product 150 MHz
BDV96 - emitter voltage 100V, collector current 10A, hFE min 20

resistors;
base of darlington pair drivers; R24, 25, 26, 27 - 10Ω
emitter of darlington pair to common; R28, 29, 32, 33 - 100Ω
emitter of power trans; R30, 31, 34, 35 - 0.33Ω

BugBear
 
The first board is done and we have stability at 100mv. Took a bit of time to get there. The other board is stripped out and marked ready for drilling. A job for tomorrow.
If it works it is not difficult to do. The next one will be much quicker having solved all those little bits and pieces that take up so much time.
Cheers Andy.
 
The deed is done and we have music!
I think there is more of everything that I want and no more of the bits I don’t want.
Just working through a few cds to see what has crept out of the background.
Monteverdi’s Pianto Della Madonna was first. Lovely. A bit of Pink Martini. Very good.
Mercedes Sosa magnificent in Ariel Ramirez’s Missa Criolla.
Probably a bit of KD Lang, Eva Cassidy, late American Cash, Goldfrapp and Gladiator to follow. And Floyd!
Definitely a bit better control on the bass. The Missa Criolla uses some unusual drums and there are new subtleties there.
And more detail throughout.
Strongly recommended.
Thanks for the encouragement to proceed.
Cheers Andy.
 
There is now a picture on the Qudos parallelled thread ( thanks Tim!). I would post it here but despite assistance I have failed.
The addition is dead easy.
Four transistors, four resistors, insulating bushes, mica insulators, solder tags to suit insulating bushes and Qudos transistor fixings (M3.5 and M4 respectively in my case) nuts, bolts and washers for the new transistors, heat sink paste and wire should do it.
The G version of the transistors are lead free and three or four times the price of the old leaden versions. I was going to suggest that the old lead ones sit heavily on the music but someone might believe me.
I used 3w resistors as that is what I could find. The circuit says 2.5w but the Avondale components look to be higher wattage.
Take the old board out and clean heatsink goop off. IPA seems better than meths.
Take the dimensions from the transistor manufacturer’s website and mark the locations for the extra transistors on the heatsink. Check, check and check again that the orientation of the transistor and type is correct before centre punching, pilot hole and hole drilling (4 mm for mine). The centreline for the fixing holes are offset from the external midline by the thickness of the heatsink metal. I marked this, measured the distance from the end of the heatsink to the middle of the transistor pins and transferred that measurement to the centreline. They were slightly different so measure both. The pin separation is on the data sheet and can be marked with dividers. The distance from the pins to the fixing holes are also there so can be marked with dividers.
Once the holes are drilled assemble the transistor insulators etc without heatsink goop, ensuring that the solder tag is in contact with the case. Test for shorts between the pins and case and the heatsink.If ok solder the tags to a suitable piece of wire and assemble with goop.
Test again for shorts.
Join the cases/collectors with the solder tagged wire.
Tin and solder a wire to the bases which should be facing each other, the gap between being about 1/2 inch.
Solder the resistors to the emitter pins and pcb where the other emitter resistors have been fitted on the other side of the board.
What you should have achieved is to join the collectors and bases of the new transistors to the old transistors and the new transistor emitters to the correct place on the pcb via the resistors.
Refit, not forgetting heatsink paste.
I think that covers it!
Have a look at the photo if you are unsure.
Happy to take any questions/corrections but cannot be responsible for any problems resulting from alterations to circuits!
 
Did you remove the diode across the 100ohm resistor to TR8?

Oops, I am assuming you went fully complementary, leave it in if you didn't.

Pete
 


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