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