Resistors Are Futile!

[thebes]

Kinda catchy play on words don't you think?

Anyways, recently acquired a Bogen mo100a mono PA amp that has

been converted from it's original 8417 tubes to El34's. I have

another one I acquired a little while ago, so I'm going to do a

monkey-see, monkey-do and copy the conversion to build a little

monoblock setup.

However, (yes I'm finally getting to the point), I made a bit of a

boo-boo on the one I already had and blew up one of the four resistors

going to each one of the power four tubes. I know what I did

wrong and while I will test other things, I'm pretty sure I'm just

dealing with the one resistor.

Trouble is I'm having a very hard time figuring out the value of the

resistor. According to the legend on my schematic "all resistors

are 1/2 watt +/- 10%" . Fine so far and when I find these

resistors on the schamatic it says "39" and then under it is written

"2w". To confuse me even more when I use the color code chart to

double-check this I can't seem to figure out the value off the chart

(never was very good at math).

Here's a pic of two of the 4 resistors, to my mind it looks like brown (or gold),green, black and silver (or white).

So what am I looking for: 2 watt resistors?, 1/2 watt resistors? is the

39 value correct? Oy yeah and should I replace this with a NOS carbon

resistor or would modern ones work just as well and what type?

Well I'm at it one more question. Besides the conversion mod

there is only one other difference between these two amps. One

has two, 22mfd, 600 VDC caps?/resistors)-see picture, and the other has

a single 47 mfd, 600 VDC. All of them have that waxy looking stuff on

the ends. I'm thinking these are replacements for the original selenium

rectifiers, but I really don't know. Doesn't matter to me but I am

wondering if both amps should have the exact same values (ie 44 or 47)

and if so what the hell are these things and what type should I buy?

[thebes]

Here's the waxy thingies.

[mike stehr]

They appear to be 15 ohm, 2 watt carbon composition resistors at 10% tolerance.

Of course measure them with a DMM or whatever to make sure if they are in tolerance.

This helps me quite often:

http://www.dannyg.com/examples/res2/resistor.htm

Ditch those wore-out Good-All Marblelites and replace them with a 47 uF value. It's a easier value to find.

[thebes]

Mike, you are a good man indeed. That calculator is really cool.

Now just what are those marbelites? Resistors? capacitors? Inquiring minds want to know.

Also should I track down NOS resistors or does it make any difference at all?

[JJkizak]

Those caps are the old standard garbage and the two in parrallel are the same value as the one, more or less.

JJK

[mike stehr]

Mike, you are a good man indeed. That calculator is really cool.

Now just what are those marbelites? Resistors? capacitors? Inquiring minds want to know.

Also should I track down NOS resistors or does it make any difference at all?

The Marblelites are coupling caps. What they are coupling is beyond me without seeing the circuit or schematic. They could be for de-coupling for all I know......

Like JJ said, they have two 22 microfarad caps wired in parallel to come up with 44 microfarads. I'm guessing the schematic claims 47 uF for that spot, and someone just swapped in a pair of 22 mic caps in parallel to come up with 44 uF. Whoever swapped them out figured that 44 mics was close enough.

A guess on my part.......

A new 600 volt 47 uF orange drop would probably work well in that spot over those ol' wore out wax caps.

You can check those Marblelites for leakage with the circuit fired up, if they pass more than 1 volt DC then throw them away.

Keep in mind I'm guessing because I have no clue of the circuit.

It's just good practice to ditch those old coupling caps and electrolytics, it saves many headaches.

If the resistors still measure within the 10% tolerance they (should) still be ok.

2 watt carbon comps tend to hold their tolerance better than 1/2 watt and one watt CC's, but you never know.......

Best thing is to measure the voltage with the circuit fired up, and see if the voltages are in spec with the schematic across those resistors.

[mikebse2a3]

thebes

I can't tell for sure from your picture but the (2) 22 mfd caps you have in the picture kind of looks in the picture to me like someone was just replacing a bad section in the (3) section Electrolytic Can Type Filter (these are usually silver looking canisters). If those (2) caps are soldered to that Electrolytic Cap section then I believe you should replace the whole Electrolytic Filter. If you look closely at the bottom of that Electrolytic Filter it looks in your picture like it is begining to leak(The Black Rubber looking stuff) out the bottom.

mike[]

Edit: Thebes any Electrolytic Filters that show signs of a black or tanish brown like material leaking out around the terminals or openings in the bottom should be considered bad.

[thebes]

Mike, you poor fool, you didn't know when to run and when to hide, now you're stuck with me for awhile![][][]

Here's a link to the manual for the Mo 100a which includes the schematic:

http://www.bogen.com/support/discontinued/

Very, very appreciative of the help so far. Now these amps also

have two more 22 value waxies, and one 47. All of them are rated at

600v. I'm thinking at this point I should just throw them all

away and put new ones in.

Which leads to several more questions:

Since they are 600 volt, should they be replaced with the same?

These are 100 watt PA speakers which were designed to be either daisy

chained or run with a variety of speakers in a lot of different

configurations. Sso I'm thinking maybe they over-speced the voltage

values in relation to what I will be using them for which is home

stereo with Cornwalls or KG series speakers. The reason I mention

this, is that from noodling around on the web, it seems that 600v caps

are not that readily available.

Which leads me to another question, what's a good source for these hard to find caps and resistors?

Also these 2 watt resistors seem to be rare. Could I get away with something else?

By the way I desoldered one end of two of these 15 ohm resistors and

they both read 17 Holmes (not Watsons) on my multimeter so I quess they

are in spec.

I quess what I'm getting at, is that if you had the bottom plate off of

these things would there be certain basic things you'd do as a matter

of course.

By the by, it's my understanding from a fellow Forum member that these

have the very same circuit used in creating the high-end Quicksilver

8417 amps, so I'm hopefull I'll get some very fine sound of these when

they are done. they will be driven with a Conrad-Johnson PV3

preamp.

I'm also doing this to have a sort of winter project and to learn a bit

about reading schematics and trying to understand a little bit more

about the technical side of tube amps. I'm also thinking that while I'm

pushing the envelope a little these are not quite a Craig rebuild-type

project and if I screw it up he can always charge me double for fixing

my mistakes.

One last question, any chance of biasing these things without a scope?

[mike stehr]

"Mike, you poor fool, you didn't know when to run and when to hide, now you're stuck with me for awhile!"

Which Mike?[]

I brushed over the schematic real quick. Strange power supply, it appears to be half-wave. Then again, maybe not. According to the manual, they are silicon diodes.

(I'm more used to tube rectifiers....)

The PS caps appear to be two 450 volt 100 uF caps in series, though I'm not too sure.

It would seem so, considering the secondary off the PS transformer is at 670 volts AC.

(That scan of the schematic makes it kind of hard to tell what voltage is printed.)

A fixed grid bias of -18.2 volts for the 8417 output tubes, 650 volts on the plates. 315 VDC on the screens.

B+ to the output transformer appears to be 860 volts DC?

Just be real careful probing around in that amp, or you'll be riding the lightning.

Have Fun!!!

[3dzapper]

Marty, You can put two 100uF 400VDC caps in series to get a 50uF 800VDC rating. Hook them up with the first one's positive where the old one is, the second positive goes on the negative of the first and the second's neg lead goes to ground. Put a 500K 1Watt resistor across the terminals each cap to balance the electrical load.

Any resistor greater than two watts will do. Check http://www.partsconnexion.com/ they should have what you need.

Mike is right, be extra careful with that voltage!!!!

Rick

[mikebse2a3]

thebes

I'm not sure but after looking at the schematic it looks like the (2) caps paralleled you were asking about might be .22mfd instead of 22mfd = .44mfd instead of 44mfd so you might want to double check those and the one you said was 47mfd could be .47mfd . These caps look to be the caps that are feeding the (-) bias voltage circuit according to the schematic.

Its hard to tell by your pictures but as I've looked at the schematic and back at your pictures it looks like the (3) section Electrolytic Capacitor that I thought you had is actually the part marked C11 and is instead a (2) section Electrolytic Capacitor.

The two Electrolytic Capacitors marked C9 and C10 on the diagram are part of what is called a voltage doubler circuit and if you notice C9 is isolated from the chassis and thus ground. I believe C9 (the middle capacitor of the 3 can capacitors) will have an insulated case (probably cardboard/paper wrapped) so be carefull around it if any insulation is off of it the case will be at about 325 volts potential!!!

Is one end of the 15 ohm resistors you asked about hooked to pin 8 of the EL34(formerly 8417) tube socket? Also is the Output tube Balance control still wired in the circuit?

Since this amp has been modified it hard to be sure what to tell you about it since we can't see it for ourselves. More pictures of under the chassis would probably help us to try to figure what has actually been changed about the amps.

mike[]

[djk]

"600 volt 47 uF orange drop " Would be the size of a beer can, if not larger.

I stored the pic and the schematic and looked at them.

C11A, B is the dual section cap(the one on the left in your photo), and is 100µF+ 10µF at 400V.

C9, 10 are the main filter caps(C9 is the one in the middle of your photo, C10 is the one at the right), 100µF at 450V. The pair of 22µF that have been added are to replace C9 which went bad. The bias cap is C8, 500µF at 25V.

The 15 ohm resistors are 1W, it won't hurt to use a 2W part.

[3dzapper]

I think Mike is correct here. Those twined caps are 0.22uFs and the other with a single cap is 0.47uF. Both values are readily available in 630V ratings.

They are C7 in the schematic. Right off the power tranny at the diode junction point.

Rick

[WMcD]

The power supply is an example of a "voltage doubler". It was not too often used even in those days.

Please note that two filter caps are stacked in series. During one half cycle the bottom one is charged. During the next cycle the "top" lead of the bottom one acts as a sort of reference "ground" to the negative "bottom" lead of the top one. Then it is charged to the same voltage. However, because they are in series, you wind up with twice the voltage.

This seems like magic. Of course you could do it yourself. Charge up one cap from a battery, set it aside. Charge up another, wire it in series with the first. Now you've got twice the battery voltage (for free). Then you run the amplifer off this until the caps discharge to some unacceptable voltage. Then you have to take them apart and start all over.

So, you ask, why is this magnificent trick used more often.

First we have to see what is good about a normal full wave bridge set up. The recifiers effectively flip the bottom half of the sine wave over so we have two positive bumps every full cycle of the sine wave. The "filter" cap is charged twice every a.c. cycle. So it only has to keep voltage up to some level for 1/30th of a second.

OTOH, in the voltage doubler, each cap is charged once during the cycle. Each has to be of a value to keep the stored voltage high enough for a full cycle of a.c., i.e. 1/60th of a second.

Behind this too is the "you can't get something for nothing" rule of the universe. If the cap has to be bigger, the inrush current is bigger, so you need a bigger transformer. Probably the same size as a normal power supply set up would require. OTOH2 you do get two voltages available as shown in the Bogen schematic.

I recall seeing voltage quadrupeller (sp) too.

Best,

Gil

[djk]

"and the other with a single cap is 0.47uF"

You're right, they're using it as part of an AC voltage divider to feed the half-wave rectfier for the bias.

[thebes]

You guys are great! Ok I'll be posting a series of pics showing

the modded (converted to el34's ) and the original 8417 setup.

My bad. Yes all the caps I was talking about are .47, .22, 600vdc

the

same with the 3 white ones you can see in the left of the picture on

the modded bogen. Your cautionary advice on the dangers of

fooling around with electricity are taken to heart and I will discharge

thecaps before working on them. Oh, can the power and filter caps be

discharged through grounding or will this hurt them?

The mods by the way are very simple. Wires are

simply connected between pins one and eight on V1,2,3,4 (the tubes

sockets) and the 10k resistor at R26 is upped to 22k. Also some wires

have been relocated off the tubes to a terminal strip installed just below

the bias pot.

I'm also including a picture of the top of the modded amp which shows

the C9 filter cap with the loose cardboard removed and some crumbly tar type

stuff on top of the can. Values off the cans for c9 and c10 are written

as 100mfd 450 vdc, operating temp 85c.

Values for the c11 caps as read off the caps themselves are given as: 100mfd at 400vdc and 10mfd at 400vdc, operating temp 85c.

I'm not so sure that filter cap c9 has been bypased because there are

still wires going into and out of it. The only difference between the

wiring on both of them is two small resistors (?) to the left of the

resistor with the orange bands on it. They read N1 on the

original and are smaller then the 2 in the modded one whose values I

cannot read without desoldering them.

So based on this any changes in recommendations? Also can I get

new filter caps and power caps for these of the same type so I can keep

it looking original? When Craig redid my Scott 299 he replaced the

cardboard covered filter caps with nice shiny plastic black ones so I'm

hopeful but need your advice.

Also any recommendations on brand names etc. I don't think I need

to put in the top dollar stuff but I do want this to sound sweet.

Also is there a simple mod so that I can bias this thing using a multimeter?

[thebes]

Modded bogen closeup.

[thebes]

Filter cap on modded bogen

[thebes]

Original bogen with 8417 tubes

[thebes]

Original bogen closeup