Latest amp in the workshop is this Plinius 8150. If you look at the photos you can see that there has been quite a severe short circuit in the amp, leaving a charred section of the circuit board, and a black mark on the top of the case. The amp was also oscillating badly. On disconnecting the pre section the oscillation stops so it looks like the cause is the damaged preamp section. Damaged resistors and capacitors will be replaced and everything else checked over.
This amp had seen better days. One of the EL34s had blown, taking out the cathode ‘fuse’ resistor and the main power supply caps were running hot, a sure sign they had dried out and aged. In addition to new psu caps and replacement cathode fuse resistors, the opportunity was taken to upgrade the coupling caps, replace some loose input sockets and swap the valves to Sovtek military spec 5881s, which to my ears are better sounding, and which have also proven to be highly reliable.
The amp is beautifully designed and a joy to work on. As always, care must be taken desoldering and soldering to the pcbs, as the glue holding the tracks onto the board tends to dry out over time allowing the track to become loose.
Coupling caps were swapped for Mundorf ZNs, a great sounding cap, and a new find Leclanche Swiss caps. Both are tin foil /film caps and I prefer the Mundorf ZN tin foil film types in sound quality to Mutdorfs more expensive /exotic metallised film variants.
‘About 55 lt bass reflex cabinet from a dismessed couple of RCF speakers which I was not able to make singing as good as I liked, they always had too loud bass with the original RCF dome mids too laid back…
Generic 30″ woofers with generous bass (until I combine them with the 4 ovals)
Dismessed oval RCF wide band “speech” speakers from classrooms, mounted on open baffle
plus Philips 1″ dome tweeters
Additional back Peerless 1″ dome tweeters, opposite polarity, still to be fixed
Will probably change the Peerless tw with another couple of Philips tw, a bit more efficient
Woofers feeded through the original filter inside the RCF cabinet (?) abt. 500 Hz. Plan to try an home made 2nd order filter, 5mH + 50uF, maybe more direct to increase a bit sensibility.
Mids RCF trough pass band 1st order, 80uF PP + 0.39 mH coil, 5.6 Ohm heater for damping down. In this way the ovals are very transparent and direct. Voices are very realistic.
Tweeters 1st order, 4.7 uF-100V, wima caps.
The D’Appolito style configuration helps creating an extended vertical stage.
Sound is quite engaging and live. Soundstage is big. The bipole radiation is very nice.
Only thing I need to gain some bass. Half rotating the bass knob on the Rotel amp balances well.
After trying with the xover mod, I will try opening and playing with a wider bass reflex port, various lengths, thus sacrificing the bass extention but gaining few dbs.Made with all second hand parts’