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Troubleshooting 12B4/RH84/Akai project

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pjanda1

pjanda1

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Joined 2005
wildburroaudio.com
#1
I've got most of my 12B4 pre and RH84 amp (SS rectified) into the Akai M8 chassis. I'm trying to fire them up, but I'm having problems. I'm using an isolation transformer>variac>150W incandescent bulb in series. Without tubes in the amp, the bulb is getting awfully bright around half throttle on the variac. So, it seems to me like I've got excessive current draw/some sort of short. But, I can't find any problems. I've got nearly infinite resistance (well, my meter thinks it's infinite, but there are 470K bleeder resistors across the caps) from B+ to ground. No shorts in the heater wiring. Everything seems as it should.

Both mono amps behave the same, so I don't suspect a parts failure. Both worked fine before I tore them apart. I hope I'm not being crazy, but I've never had that giant bulb glow much before. If I burn up one of these PT's, I'm up a creek.

I've never had an amp not work at this stage before! I attached some pics in case fresh eyes see something I'm missing.

thanks,

Paul
Wild Burro Audio Labs - DIY Full Range Speakers
 

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#2
It's difficult to see much of value through that jumble of wires. But here's what I would do. Start by disconnecting wires from the power transformer one at a time. Begin with the red ones (one at a time) and see if that relieves the load. Then work forward to localize the problem.
 
#3
I assume the problem is with the B+. I guess maybe that isn't a safe assumption, but I don't see where I could possibly have gone wrong with the filaments. I was going to start disconnecting stuff from the B+ one tube at a time, but I want to be sure I'm not missing something obviously stupid. Is my wiring a little too messy? I do try and keep everything short and non-parallel at the expensive of tidy looks. This is my first attempt at doing something on a terminal board like that. Hindsight is always 20/20, but I'm not particularly thrilled with the layout.

I'll start pulling it apart. I just received a chip amp (in partial trade for another chip amp) that somebody built and sold on ebay (I got it from the ebay buyer). It lightened my spirits a bit to see the ground path go in a big circle. No wonder it hums. It is always good to know that you aren't the dumbest kid on the block.

Paul
Wild Burro Audio Labs - DIY Full Range Speakers
 
#5
In the picture it looks like the center tap of the high voltage winding is grounded. If you are using a bridge rectifier you may not want the CT grounded. It's very hard to say for sure without seeing the schematic or know what the circuit is.
 
#6
This may be a stupid question, but how does one know whether or not to ground the CT? The stock Akai (schematic) had a fullwave rectifier (6X4) with the CT grounded. So, I followed suit. Here is an RH84 schematic. I'm using SS rectification to try and get the B+ close to spec and ease to load on the PT. It probably doesn't matter, but 1st cap is 22uf, 6H choke, 330uf second cap, 5uf on top of the 'T7 (what I had). I've got a resistor and diode (for decoupling) leading to a 33uf cap for the 12B4 (I'll adjust the resistor value once I know where my B+ is at). The 12B4 pre is just a very simple resistor loaded stage (for now).

Paul
Wild Burro Audio Labs - DIY Full Range Speakers
 
#7
With a full wave rectifier, the CT of the high voltage winding is grounded. You used the word "bridge" which is different then a common full wave circuit. Full wave is two diodes. A bridge is four diodes with one junction grounded, usually the negative. (doubles the voltage)
 
#8
It seems as if I built a bridge when I needed to build a full wave center tapped rectifier. Then I grounded the CT, thus shorting each secondary to ground half of the time. Sometimes I think clearly. Othertimes . . . At least I had a variac and a bulb to limit current. And the power transformers and I live to see another day.

thanks for your help,

Paul
 
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