Van
Loenen 300B Amplifier
Sad news. Ton passed away on 19 August, 2005. Rest in
peace my friend.
Here are some photos of the amplifier I built for my good friend Ton Van Loenen of Oirschot, Netherlands.
This is another version of the famous WE91A theater amplifier circuit. This amp was a challenge because
it had to be built for the 230VAC 50Hz power used in the Netherlands. The filament transformers were hard to find.
This is a head-on shot of the amp on the bench. The tube lineup is, left to right:
5Z3, 6SJ7, 300B (EH), 6SJ7, 300B
I've included a selector switch on this amp providing for four sources (sorry, no phono preamp)
The power and output transformers are Hammond as is the chassis. I've had very good luck with the
Hammond line, so I keep coming back.
Above is a shot from overhead with the amp on the bench. You can see the hum pots behind the power tubes.
The filter capacitors are 370VAC motor run capacitors made by Amrad. I parallel the 50uF primary filters
with a 100uF LCR and the amp is plenty quiet. Only 1.6mV of hum on the speaker terminals, my best yet.
Here's how it happens:
I've gotta cram all that stuff in there? Maybe I can make it fit.
Chassis and transformer bells have been powder coated. Nice finish. I have a machine shop cut the oval holes
for me. I don't have a tool for it. I cut the rest of the holes with a set of Unibits.
I have three sizes that cut all the holes you see here. They're cut with a 1/2" hand drill, not a drill press.
Here's a shot with the transformers, motor run caps, tube sockets, cathode resistors and hum pots mounted. Lots of wires!
Here's the topside with all the major components mounted. 'Scuse the messy bench. It gets that way when I work.
Here we are with a few more parts mounted and some wiring started. The first thing I do is tie off all the transformer leads.
Here's a shot with the chokes mounted and the filament wiring in place. The filament transformers are toroids
with 230VAC 50Hz primaries providing two 6V 4A secondaries on each toroid. I mounted them on a long
bolt with padding between. I used resistors to lower the filament voltage as required. You'll notice them
in the later photos. I bought the toroids from Aqua Blue in Antwerpen, Belgium. Thanks Benny. It was a
challenge to find your place and I really enjoyed hearing your 845 amps.
Here's everything in place. You can see the input board and the speaker connections behind the blue LCR cap.
You can also see the resistors for the filament wiring. The volume pot is an Alps 22 step attenuator from Parts Connexion.
The aluminum angle you see serves to clamp the motor run capacitors in place and to stiffen the chassis. It tends
to sag a bit under the weight of the transformers unless I use these stiffeners. For appearance, I polish them on a buffer
after I cut them and drill them for grommets.
Here's a shot of the Business End of the amp. You can see the input board, speaker terminals, IEC plug and fuse holder.
I only had one "oops" on this amp. As I was mounting the pilot lamps (neons) I realized, I need 230V neons! I can't
get those here, so I wired a second red and green lamp in series with the ones that show and tied them neatly under the
chassis. Works fine.
The amp runs very cool. After three hours, the power transformer is slightly warm to the touch and the outputs are cold.
It may run a little warmer on 50Hz power. For testing purposes, I bought a 200 watt step up transformer from ebay for $12.
Here's a link to an article from Sound Practices magazine that inspired me to tackle this project and others like it.
I really miss that magazine and many thanks to Joe Roberts for making it available again on CD.
Visit Southern Electric Audio and see what else Joe has to offer.
Please email me with any questions or comments: rkoonce@dsityler.com