Category Archives: GU32

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!

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Using IKEA Näsjö candle sticks for housing a 6C33C tube

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.

An added benefit of the sphere meeting the square socket is that there is an air gap which is necessary for cooling the 6c33c tube. I am playing with the idea of mounting a computer fan in the larger hole in the bottom to provide forced air cooling for the 6C33C.
So there it is. The beginnings of a forced air cooled 6C33C tube holder that can be mounted out side the chassis for max cooling and cool looks.

How to bring down the voltage on the GU32 amp

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?

Breadboard of GU32 Amplifier

I have built the amplifier with the values specified in the schematic (http://www.audiohobbyist.com/images/832/832sch.jpg), but I have a power transformer that is rated 230VAC to 200V, and the grid voltage is up to 238Vin my neighborhood. This gives me B+ unloaded of 302V and loaded 283V which is to high it should be 250V. I´ll get back to this problem in a later post. 
For now I´ll describe the circuit as built. First on the far right a custom Edcor transformer XPWR146: Pri: 230V 50Hz  Sec. 200V 250mA  and 6,3V 4,5A, this is rectified by a  KBU8M 1000V, 5A rated solid state rectifier in the upper middle of the picture. Filtering is by a single 800uF 385V electrolytic capacitor.
This B+ power is then fed to the center input of an Edcor CXPP25-MS-8k where Plate 1 and 2 of the GU32 is connected. The B+ is also connected to a turret board containing the plate supplies for the 12AT7, the star ground points, and cathode bias of the GU32. Here is the schematic for the turret board:
The colors of the wire follow no particular system.

832A tube amp prototype finished today

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.

A new fixation

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.

The 832 proved to be suitable for a small Push Pull amp, so within a matter of days the 6c33c was shelved and GU-32 tubes (the 832A s Russian cousin) were ordered from Ukraine and septar sockets were scoured from China. The septars, by the way are the same sockets used in the 6c33c.
Suddenly I had a kind of direction to the projects I am doing:
I want to build one of each of the three main types of tube amp. the Single Ended Triode, The  Push Pull, and the Output Transformer Less. I want to build designs that have a B+ voltage of as far under 400 volts as possible, and they shall be proven and good sounding, cheap amplifiers.
So for the 832 amplifier I have chosen to build from this schematic:
 

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.