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Vintage Maplin 50W Amplifier and Pre-amp Kit
Vintage Maplin 50W Amplifier and Pre-amp Kit
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Old 18th May 2017, 12:33 PM   #11
mt490 is offline mt490
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It always seemed to be whatever was easiest to manufacture.

Actually I have a System/360 power supply sitting outside under a garbage bag and the main currents run on a busbar and the heatsink; the base is just a fly wire. I had thoughts to upcycle it to a mono amplifier but never got to it. It wouldn't be hard if the devices on the control card are still in working condition as the diff amp is already there.
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Old 18th May 2017, 12:36 PM   #12
jaycee is offline jaycee  United Kingdom
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Ugh, connecting those transistors with wires like that has probably caused a load of problems... not to mention that Q6 is not in contact with the heatsink at all, so the amplifier was probably going into thermal runaway.

The correct output transistors are BD711/BD712 which are now obsolete, but you can still get them if you look around. Failing that, it looks like D44H11/D45H11 from OnSemi should work very well.

I'd definitely build some sort of DC fault protection kit as well though!
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Old 18th May 2017, 12:48 PM   #13
roclite is offline roclite  United Kingdom
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Vintage Maplin 50W Amplifier and Pre-amp Kit
I don't think the boards were ever powered up and the components are definitely vintage and obsolete. The flying-lead transistor madness was probably because the constructor didn't have skills to mount the pcb and output stage transistors onto the heatsink, as per the designers intention. Speaker protection circuit definitely a good idea. At least I can test the re-built boards on the bench first with 8 ohm test load, to check performance and stability.
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Old 20th May 2017, 10:14 PM   #14
MikeVou is offline MikeVou  United Kingdom
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"...flying-lead transistor madness"

Was it madness? Have you ever seen the arrangement Quad used in the 33 power amplifier (circa 1968?)? 2N3055s mounted about 10 - 14 inches from the PA boards, and connected over that distance with surprisingly thin flex. Didn't cause problems. Fact is that those earlier 2N3055s had a much lower frequency response than later versions - probably only thing that prevented it turning into a 50W oscillator.
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Old 20th May 2017, 11:32 PM   #15
prairiemystic is offline prairiemystic  Canada
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Quote:
Originally Posted by MikeVou View Post
"...flying-lead transistor madness"
Was it madness? Have you ever seen the arrangement Quad used in the 33 power amplifier (circa 1968?)? 2N3055s mounted about 10 - 14 inches from the PA boards, and connected over that distance with surprisingly thin flex. Didn't cause problems. Fact is that those earlier 2N3055s had a much lower frequency response than later versions - probably only thing that prevented it turning into a 50W oscillator.
+1

Do the math and it's nH of inductance, pF's stray capacitance and possible transformer-action from mutual inductance in flying-lead power transistor wiring.

It looks sloppy and bad but I haven't seen it cause problems with old fT a few MHz transistors. I believe AM radio needs much larger parasitics.
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Old 21st May 2017, 12:22 AM   #16
MikeVou is offline MikeVou  United Kingdom
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Indeed. I believe the later epitaxial 2N3055s had higher fT of about 5MHz. Don't forget that "crazy flying lead" arrangement was also being used with the 33's Triple output stages- always tricky for stability. I've read a number of reports of problems when the original 2N3055s were replaced with "modern" ones, but the originals worked fine in the 33.

Last edited by MikeVou; 21st May 2017 at 12:26 AM.
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Old 21st May 2017, 01:08 AM   #17
roclite is offline roclite  United Kingdom
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Vintage Maplin 50W Amplifier and Pre-amp Kit
Yes, I agree with the fact that with suitable bandwidth-limited or slow transistors, the flying lead arrangement works with no real issues. My point was more that the constructors wiring arrangement was unnecessary and he didn't follow the assembly instructions to make a safe and reliable connection to the heatsink.
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Old 8th February 2018, 11:45 AM   #18
blu_glo is offline blu_glo  England
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I did myself cage a pair off eBay too; these were with the original mounting brackets... renovated and replaced electrolytics with NP where appropriate..not run through the mill yet though....
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Old 9th February 2018, 11:16 AM   #19
roclite is offline roclite  United Kingdom
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Vintage Maplin 50W Amplifier and Pre-amp Kit
Very neat job, hope they sound as good as they look.
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Old 9th February 2018, 01:01 PM   #20
blu_glo is offline blu_glo  England
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Thanks, I'll get to test them someday, when I get my lab set up again..... Just to note I looked at your photo, and your R22 - is it wirewound? You should really have 2W or so Carbon or Metal film in this position.
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