Alternate Resistor Placement

[rlyach]

I am almost done assembling my second Stereomour and I think I made a resistor placement improvement over my last build. I am again wiring the amp for dedicated 2A3 tubes, and I have replaced the Xicon resistors with Mills MAR12F resistors. On the power supply board there are two resistor placements for the 2A3. Only one is used for the 45 wiring. If you replace the two resistors per side (~277 ohms - parallel combination) with a Mills 270 ohm you can eliminate one resistor. Further, since the Mills resistor is quite small, you can place it on the side positions, thus keeping the resistor heat away from the capacitors. This could help the life of the caps. Is this correct?

[4krow]

Heat is the enemy of electrical circuits, that's for sure. What is the rated heat value of the caps in this circuit? I'm not sure it would make much of a difference if the radiated wattage of the resistor is small, and the heat rating of the cap is say, 105C.

[Paul Joppa]

In 2A3 mode the 270 ohm resistors only dissipate about 1 watt each, so that arrangement should be fine.

[Paul Birkeland]

Be very, very careful about the screw heads of the #8 screws that mount that board to the amplifier, as the resistor leads may end up being very close to them.

The chunkier resistor will have a lower operating temperature than it's 5 Watt cousin, so putting them in the slots for the 1200 Ohm resistors would be OK as well.

-PB

[rlyach]

Paul,

Thanks for the tip. Here is the board mounted. Plenty of space. By the way, I finished the second build. I took about 8 hours going slow. This does not include the base or cleaning and polishing the transformers. I also put color coded heat shrink on the STP wires. It looks nice. During system checkout the left channel cathode resistor was overheating and the cathode was at 200V. It took me a while to track down the problem. It turns out that the parafeed cap was shorted and there was DC current flowing through the output transformer into the cathode resistor. Luckily I found the problem quickly so I don't think any damage was done. I replaced the cap and the system worked perfectly. I am listening to the amp now. I did notice that with the sovtek tubes I can't get the hum below 2.5mV. With the JJ's I can achieve 0.0mV. I can't hear the hum however so I don't think my pastor will mind running the tubes that shipped with the kit.

[jimiclow]

Were you the one who bought that pre-built Steremour Doc was selling last week?

[rlyach]

No, this was the second kit I ordered last March. It finally came last week and I built it yesterday and today.
Here are the final build pictures.

[tsingle999]

Wow seriously clean build underneath. Looks like it could be used for the manual. The base is amazing too! Nice work!

[Paul Joppa]

2.5mV is more or less normal hum for an AC-powered 2A3. To get it lower, the tube itself must have some compensation, where the magnetic field of the filament wires steers the electron beam in a way that compensates for the "natural " hum. This is a real thing, I've seen it in tube design books, usually in the context of a 45 - but I've never seen any details of how it's done. I wonder if all JJ's have this ability?

[rlyach]

I need some advice now. The shorted parafeed cap caused the cathode resistor to get hot enough to smoke a little and dull the finish. The resistance still measures correctly and after I replaced the parafeed cap the voltages are now correct. Plus I listened to the amp for about an hour with no problems. Do you think the resistor is still reliable or should I replace it? If it were mine I would leave it but because I built this for someone else I am curious what everyone else thinks.

[Paul Birkeland]

Those cement resistors are meant to take a serious beating, when they fail, they actually kind of pop open and crack.  A little discoloration is nothing to worry about (tough I would measure the DCR of each plate choke to ensure that they are within 20% of each other, the last Stereomour I saw that had a short causing excessive current fried the plate choke).

-PB

[rlyach]

The DC resistance of the plate chokes are 4 ohms apart. The unloaded output transformers DC resistances are 5 ohms apart. I assume that this means that everything is still working. Also, the plate voltage of both channels is 560 when the amp is running. The Cathode voltages of the 2A3's is 57V for both tubes. I think this confirms that everything is fine. Incidentally, the plat voltages of the driver tubes are only 1 volt apart (~194V). I am just very nervous as a result of seeing smoke from the cathode resistor. The amp sounded excellent with my Orcas as well. I just wish that the capacitor wasn't shorted in the first place. Thanks for the reply.

[Paul Joppa]

I concur with PB - if the resistor is still 1200 ohms then you are fine. I would normally be more worried about the bypass capacitor because they can lose lifetime without appearing right away. But you said you measured 200v, and fortunately we used 250v caps because we had them on hand. So you are again fine.

Well, actually I knew that various conditions can seriously over-voltage the cathode bypass cap, so it's not entirely an accident that I spec'd the 250v cap we happened to have lots of on hand  :^)

[rlyach]

Thanks Paul, I just figured out what caused the short in the capacitor. The cap I had used an aluminum casing with a paper wrapping but the aluminum edges were exposed. I forgot to put insulation around the leads and when I folded them down they shorted to the casing and caused the problem. I feel pretty stupid about now. I will have to remove the coupling caps as well to head off any future problems. I just measured the resistance from the cathode to ground with the 2A3s removed and both sides read 1198 ohms exactly. Thankfully I did not destroy any components. Chalk one up for human error.

[Paul Birkeland]

Human error is the best way to learn