Finally the chassis for the 832A amplifier is starting to take shape. I have spent the summer thinking about how to manage heat, pull wires out to the spheres, and what the hum and interference will be like. Having decided I need forced ventilation, everything loosened and I mounted most of the chassis yesterday and today. I still have all the connectors to mount an some holes to drill. But you can see where it is headed. My youngest child named it Rudolph!
As promised in a previous post I will show how to use the larger of the two IKEA Nässjö candle holders for making a chassis for a large power tube. I am planning on using the 832A tube or the 6C33C tube. Both of these use septar sockets.
These candle holders have a large and a small cup press fitted into the molded sphere. I chose to drill a hole in the smallest sphere, and use this hole for sticking in a skrew driver and tapping out the larger cup first.
However, the screwdriver went right through the bottom of the cup. The material is quite thin and fragile. So I had to tap them out by hitting the edge of the cup, not the bottom. In order to do this it is best to start by drilling a hole through the largest cup and driving out the smallest cup first. This gives more room for aiming and driving out the larger and more fragile cup last.
The sphere is to big to mount securely, so I did this while holding it. (had to set it down to take the picture)
Here are both the cups, out of the candle holder. Unlike its smaller sibling, which I mounted a socket in here, There is no internal structure that needs to be drilled away.
However the large candle holder is thinner in cross section, so in this case. I had to drill holes and mount the socket with screws and nuts.
In order for the screws to fit snugly with the spherical surface I mounted them in a vice and pounded them to a suitable angle. Mild steel electroplated panhead screws were used. I let the socket but up against the sphere, although spacers cut at an angle may be better.
I measured the B+ without tubes in to 303V and with the tubes 283V. This is a fair bit above the Audiohobbyists recommended value of 250V, which is also the recommended value from the data sheets. Here is the schematic for reference:
From DIYaudio I was recommended to use a hybrid rectifier to bring down the voltage the required 23V. This is a combination of solid state and tube diodes. Here is an example of such a circuit in a 300B amp Introducing a tube rectifier in the circuit has the added benefit of delaying power up on B+ so the tubes have a chance to heat. I have researched several rectifier tubes and have found the following may be suitable: 6AU4-GTA drops 25 volts, 5V4-GA drops 25 Volts, and 6CJ3 drops 25 volts.
However there are a couple of problems with this. The existing filter capacitor is very large (800uF). With a rectifier tube I have to build a pi filter. The first capacitor should be to about 50-100uF, then a choke of 2-3 Henries and I could keep the old 800uF capacitor I have chosen.
This strategy gives me the reduction in voltage from the rectifier tube. However the Pi filter with the choke is potentially quick reacting, and should not need such a big second capacitor since this capacitor essentially acts as a reserve power supply that can take care of transients when the transformer and damping circuit is not able to follow up.
So this leads me to the second idea. What if I use a inrush resistor in stead?
The colors of the wire follow no particular system.
I bread-boarded the amplifier design from Audiohobbyist.com today. It is a 5 Watt push pull design, with the 12AT7 double triode as driver and phase splitter, and the GU-32 (Russian 832A) as the power tubes. It is a double tetrode, so I have both the push pull tubes in one package. I had Edcor build a special order power transformer for it. With the specifications Primary 230V 50Hz, Secondary 200V 250mA and 6.3 V, 4.5A, and I bought Edcor´s standard 800ohm/25W push/pull output transformers.
The building process went smoothly, the only problem was loose turrets on the turret board I used to build up the different B+ supplies. Once that was fixed it played beautiful music.
My only worry is that I have slightly higher B+ than the design calls for. I need advice, and I need to work my way into loadlines (this is a PDF) to understand if this is a problem.
This amplifiers debut was with Ali Farka Touré.
While surfing around studying the 6c33c tube thinking about building an alternative to my lethal Audioromy FU13 amp I came across a tube that looked like some part from a UFO. The 832 tube.
And of I go again, collecting parts and trying to decide on a cabinet design. In the mean time a pair of horn speakers I built for the MINIBLOK project. have been tested out and then rebuilt into a Boom Bench. But that’s a story for the next post.