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Bogen amp, 8417 tubes blow up, want my 6550 mo 100's?
I have two MO 100 that use the 6550 tubes. One is partially modified, with
LCR cap and needs to be completed, the other is completely stock. The amps need the ez 81 but have Chinese and American 6550 tubes. Any one want the pair? I would like to sell them but do not know where or how. the stock one is untouched, the other has some ceramic sockets and needs the bias circuit completed. thanks Loren "opcom" wrote in message ... My Bogen MO-200 came with no tubes. (basically eight 8417's, driving two parallel connected transformers for 200W) 8417 are expensive. I modified the bias supply for 50V, and put an adjustment for each tube, and stuck 6550's in it. 6CA7's also work fine. I split the amp into two 100W channels and put 6GH8's in the driver sockets, with mods. Mine does have a 1.0uF/100V plastic cap from filament to ground. too new to be original. Never thought about it. Patrick "George R. Gonzalez" wrote: Sometimes you wonder how some products ever made it out of the design lab. Sometimes there's so much wrong, you wonder if the item is a joke. Someday I may relate the story of the no-name amplifier, but today's story is from a quite respected company, Bogen. I aquired this Bogen PA amplifier, 50 watts out allegedly, pair of 8417's for output. Nicely built, heavy-duty transformers. No works, so I put it on the bench. I look it over. The bad news: Output tubes have white getters, don't hold vacuum anymore. The only visible good news: the pilot light is okay. I don't have any 8417's, so I go to the computer to look them up. The number doesnt sound familiar. I look up the tube specs, and whoo-boy, they have a heck of a transconductance, 23,000 micromhos! I do a Google search, and find several references like "these tubes sometimes run away". Hmmm..... let's be careful... I go to order some new ones, and no matter where I look, they're pricey-- $50-80 each. I finally find some on eBay for a somewhat reasonable price. Meanwhile I look over the amp for obvious things wrong. The cathode current balancing pot doesnt want to turn! It's not rusty, what's going on? I open up the pot and see something unusual-- it's a wire-wound 30-ohm pot with a the wire wound around a gray plastic form. The form is badly MELTED and DEFORMED. How the ^#!$%!^? Let's see, 30 ohms, maybe takes 5 to 10 watts to melt it this badly, that's----- 400 to 800 milliamps!! No wonder the tubes gave up. I replace the pot. The tubes aparently ran away to a very high current level. I check the bias circuit, looking for a bad voltage regulator, or a bad bias adjust pot. No problems there.... there is no bias regulator OR bias adjust pot. Just your basic capacitor stealing a bit of the high-voltage AC and rectifying it, then dropping it down with some fixed resistors. Hmmm, a pair of the most sensitive tubes ever made, and the designers don't make the bias adjustable! It's set by a couple of resistors, at about -15 volts. Luckily, this is just about right according to the cathode current I measure. But if the line voltage goes low, so will the bias, and it doesnt take much of a drop to make these tubes conduct like crazy.... I make a mental note to add a Zener, or more appropriate for the era, a neon bulb, to regulate the voltage. And oh yes, I make a mental note to sell any Bogen stock I have. A week later the tubes arrive, I plug them in, put a 8 ohm resistor and scope on the output, sine wave in. Time for FUN! I turn up the line voltage slowly--- cathode currents are low and stable, whew. Everything looks good. I turn up the sine wave, and at about 20 w atts out, the amp starts some ragged oscillating in the MHz on the sine-wave peaks. In about 5 seconds the tubes start running away! Good thing the Big Red Switch is nearby. I look at the circuit to see what may have gone wrong with the oscillation-snubbing components. We'll there's nothing wrong with them, because they don't exist. There's NO snubbing resistors, not on grids, screens, cathode, or plates. No rolloff capacitors on the plates either. These are some of the highest gm tubes ever made, and apparently Bogen made NO ATTEMPT to tame the tubes!! Argghh... I have no idea what the optimum value for grid-snubbers is for this tube, so I wire in a ballpark value, 2.2K. The combination "red red red" is a bit garish, but what the heck, we're somewhat peeved and desperate. With the grid resistors in place there's no more trace of MHz oscillation at 20 watts. But when I crank it up to near-clipping, the fuzzies reappear and the tubes run away again. This time I'm soo befuddled I don't hit the switch until the cathode current has passed 500 milliamps! Well, at least we know these pricey tubes have plenty of cathode emission, or at least had it. I think ahead about protecting these tubes in case they ever run away in the future when I'm not watching. The line fuse isnt going to do it, as it hasnt in the past. I ponder the desecrating effect of drilling two 1/2 inch holes to mount a pair of cathode fuse holders. Hmmm, better do it, originality be danged.. There's no easy way to add screen-snubbers without drilling holes and mounting terminal strips, so I look to taming the plates. I add a roughly in the ballpark plate-to-plate taming capacitor, 1100pf. Well actually, two 2200 pf 1KV capacitors in series. There's lots of voltage up there. Now there's no oscillation on the peaks, and the tubes don't start any spiralling current death marches. But there's something else- the leading edge of the sine-wave has a big scallop cut out of it. How can this be? A lot of probing around with scope probes and capacitors reveals nothing interesting. This is the worst of times, when everything looks okay except the output, and nothing you try seems to help. For lack of anything better to try, I decide to measure the heater voltage.. I know, I know, the 6.3 volts on the heaters can't be far off, and even if it was it wouldnt scallop the sine wave this way, but let's waste 15 seconds anyway. I should use the voltmeter to measure the heater voltage, but I change my mind and use the scope. You see the new test leads on the meter have really hard to squeeze alligator clips, while the Tek scope probe is much easier to use. Plus I can use the mental practice of trying to divide the p-p voltage by 3 in my head. So due to pure lazyness, I use the wrong tool for the job. And due to this mistake, all the answers pop out at me! The scope reveals that the heater voltage has a non-negligible amount of signal on it! I'm really puzzled now, wondering how 500mv of signal gets into the heater lines, wondering how the designers never noticed this, wondering why I'm getting steamed at the shortcomings of a 45 yr old design. I decide to attack the symptom and put a .22uf capacitor from heater to ground. The signal disappears from the heater voltage, and the output sine wave now looks perfect! Apparently there was feedback from the output tubes to the preamp tube, back thru the heater wires! What the bellepety-bleep! What the friggity-frig! How did this ever get past the designers? I solder in a 0.47 uf capacitor into the heater circuit. Lots of playing around with various frequencies and levels of sine waves, and I *think* the amplifier is finally stabilized. ( I should try it with some square waves too now that I think of it, but the square wave generator is out in the garage, which is at about 98 degrees, 90% humidity right now). So that's it so far. Had to get this off my chest. How could the designers have come out with such a shaky design? I put a nice sedate load on it, and it ran away every time! An expensive-looking amplifier, with many critical components apparently never designed into it. Any ideas out there how this could have ever worked right? Regards, George |
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