Making IBAM work with Sheldon Stokes Power Supply

PShuff

Active Member
Hi Guys! :banana:
Could someone please give us some tips on how to use the IBAM with the Sheldon Stokes little cap board?

Right now we have the little board's rectifier bridge and caps feeding the Fisher stock bias resistors which in turn are running to the grid resistors. We currently have -19.17v at the grid resistor's junction.

We've been looking at this thread to get an idea how to build it. I think we can do that. Just need to know the correct way to hook it up! :yes:
http://www.audiokarma.org/forums/showthread.php?t=394064&page=2

Do we just build it like in post #23? We are puzzled because we don't see for certain that the .1uF caps stay hooked up the same, like just free up the ends of the grid resistors and run the 4 wires to those and leave everything else the same?

Please help if this question makes sense!

Phyllis
 
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Phyllis.

Take the bias supply (voltage lead) from the stokes board and delete the original bias resistors. Then extend the lead to the IBAM INPUT. The Outputs from the board (with a 200K resistor on the end) tie in to each tube at the junction of the 1K resistor and the .1uf cap.

The board replaces everything between the BIAS power cap and the 1K resistor at the tube.

If you scroll down to post 29 in the above named thread, look at the LAST Picture. You'll see the O.D.'s I used for Coupling caps, and a blue resistor just above each. They are encircled in a yellow oval. Those are the 200K-220K tied into the junction between the 1K resistor and the coupling cap.

The ORANGE Wire on the board comes from the BIAS Supply. And the Black wire on the board is the chassis ground.

Have fun. You'll like it if your tubes are old stock and can get them all to match. Plus they'll run easier with the 220K/.1uf.

Larry
 
Phyllis.

The Outputs from the board (with a 200K resistor on the end) tie in to each tube at the junction of the 1K resistor and the .1uf cap.Larry


The part I left as your quote is where we get confused. :scratch2:

When you refer to the 200K resistor are you talking the grid resistor that's already there for each tube or will we be installing 4 more?

Our grid resistors are set at 182K as per Mr. McShane.

Thank you Dear! :)

Phyllis
 
There should only be 4 total,where you install them is up to you. I removed mine that were down on the stand offs and installed new ones on the board.

You can see the board in the pic. I also had to remove the 2 bias resistors that were connected to the paper cap in the holder. V14 also gets fed by the bias wire before the board. This was a 500-c , I suspect either other models are similar.

img5647gp.jpg
 
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The grid rsistors that are already there.

Attach the end of the BIAS power wires to the ends of the 182K resistors that are NOT soldered to the terminal strip with the 1K resistor AND the .1uf cap. Your NOT adding another grid stop resistor.
 
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The grid rsistors that are already there.

Attach the end of the BIAS power wires to the ends of the 182K resistors that are NOT soldered to the terminal strip with the 1K resistor AND the .1uf cap. Your NOT adding another grid stop resistor.

So the 200K resistors pictured in that nice drawing in post #23 were that value simply because that's what the poster used for his grids? If I had drawn that picture it would have been 182K?

Sorry to ask everything twice, just want to be absolutely sure! :yes:

Thank you Larry! ;)

Phyllis
 
The grid rsistors that are already there.

Attach the end of the BIAS power wires to the ends of the 182K resistors that are NOT soldered to the terminal strip with the 1K resistor AND the .1uf cap. Your NOT adding another grid stop resistor.

So, you just "split" your grid resistors at the lug where Fisher attached the white wire?

I did look at your pictures but they were so dark, or our computer monitor stinks, that I couldn't make out the fine detail.

I looked again and I think we're getting it!

Thanks again sweet pea!

Phyllis
 
Phyllis.

Take the bias supply (voltage lead) from the stokes board and delete the original bias resistors. Then extend the lead to the IBAM INPUT. The Outputs from the board (with a 200K resistor on the end) tie in to each tube at the junction of the 1K resistor and the .1uf cap.

The board replaces everything between the BIAS power cap and the 1K resistor at the tube.

If you scroll down to post 29 in the above named thread, look at the LAST Picture. You'll see the O.D.'s I used for Coupling caps, and a blue resistor just above each. They are encircled in a yellow oval. Those are the 200K-220K tied into the junction between the 1K resistor and the coupling cap.

The ORANGE Wire on the board comes from the BIAS Supply. And the Black wire on the board is the chassis ground.

Have fun. You'll like it if your tubes are old stock and can get them all to match. Plus they'll run easier with the 220K/.1uf.

Larry

Larry,
When you can, please tell me the minimum voltage value all the little parts need to be and what value the pot needs to be too.
10K?
We are putting together another parts order with Mouser for our second project and will get the stuff from them while we're at it.
We're addicted now and have an attic full of old tube stuff, mostly old table top radios from the '50s and '60s. They look simple now after the Fisher, as long as we don't have any major issues.
We want the best stuff we can afford. Bourns the best pot choice for this? What physical size pot is best? I know they are very small but is there a standard value to help us weed through those massive listings?

Thanks again and hug and kisses!
Phyllis

P.S. How's the back?:worried:
 
So the 200K resistors pictured in that nice drawing in post #23 were that value simply because that's what the poster used for his grids? If I had drawn that picture it would have been 182K?

Sorry to ask everything twice, just want to be absolutely sure! :yes:

Thank you Larry! ;)

Phyllis

Exactly. Substitute your 182K's for the 200K in the drawing and you're all set.

Larry
 
Exactly. Substitute your 182K's for the 200K in the drawing and you're all set.

Larry

Hi Larry!
We take it too the the little 33uF caps are of the polarized variety?

And not to bug you but what wattage should the resistors be?

The ones in that thread are like big power types is why I ask.

Thank so much again! :banana:

Phyllis
 
The 33uf's are standard radial polarized caps.

1/2 watt are fine.

Look at my picture. the resistors are actually 1/4watt on that one and is running right now without problem. In your case I'd probably go with a 1/2 watt just for piece of mind.

attachment.php


Larry
 
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The 33uf's are standard radial polarized caps.

1/2 watt are fine.

Look at my picture. the resistors are actually 1/4watt on that one and is running right now without problem. In your case I'd probably go with a 1/2 watt just for piece of mind.

attachment.php


Larry

Thank you Larry! That picture is one of the reasons I asked because there are definitely several different versions!

The pots all 10K?

Thank you Larry! :banana:
Phyllis
 
The pots are standard 10K pots.

You can get all the parts at Radio Shack for under $15.00 including the pots, caps, resistors, and the board. The caps might be hard to find at RS as they don't normally carry 33uf or any that start with a 3.

IIRC the caps on this one are 33uf/50v UPW series Nichicons from Mouser. Mouser # 647-UPW1H330MED $0.43 each
 
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The pots are standard 10K pots.

You can get all the parts at Radio Shack for under $15.00 including the pots, caps, resistors, and the board. The caps might be hard to find at RS as they don't normally carry 33uf or any that start with a 3.

IIRC the caps on this one are 33uf/50v UPW series Nichicons from Mouser. Mouser # 647-UPW1H330MED $0.43 each

The actual value of the caps is not critical. Anywhere from 10uf to 33uf would work fine.
 
I listed the caps as those are what i used in the IBAM's I built for my 400 and the 800C. The values were based off DREW's schematic. Just making is simple for Phyllis to follow.

Larry
 
Phyllis. There are as many variations (builds) as there are guys here doing it. But they all use the same schematic. You can use a board like I did, or do it point to point like Bhamham does. Or anything inbetween. Depends on how comfortable you are at scratch building a circuit. You may be better off with a board.

The cap values you have 10uf to 33uf/50V . Resistors are 12koHm 1/4W, and the Pots can be 1 turn or multi turn 10KoHm 1/2w.
 
The actual value of the caps is not critical. Anywhere from 10uf to 33uf would work fine.

Hi Mr. Bolce! (and everyone else!) :banana:
Please help us pick the best pots; we want the best quality within reason.
We were hoping for Bourns if possible (but it doesn't have to be we just know they are good ones).
These little trim pots are some of the weirdest listings we've come across at Mouser.
Here is a sample search:
http://www.mouser.com/Passive-Compo...cv7?P=1yzuue5Z1yzs2l9Z1yzmno7Z1z0z5h6Z1z0x6d8

Some of them don't even have spec sheets, some are only rated at 10 VDC..........AAAAAAAAAHHHHHHH! Too many choices but not enough specific choices with clear, consistent information :no:

Please help if you have time.

Thank you,
Phyllis
P.S. We want to stay with Mouser if possible
Thanks!
 
Hi Mr. Bolce! (and everyone else!) :banana:
Please help us pick the best pots; we want the best quality within reason.
We were hoping for Bourns if possible (but it doesn't have to be we just know they are good ones).
These little trim pots are some of the weirdest listings we've come across at Mouser.
Here is a sample search:
http://www.mouser.com/Passive-Compo...cv7?P=1yzuue5Z1yzs2l9Z1yzmno7Z1z0z5h6Z1z0x6d8

Some of them don't even have spec sheets, some are only rated at 10 VDC..........AAAAAAAAAHHHHHHH! Too many choices but not enough specific choices with clear, consistent information :no:

Please help if you have time.

Thank you,
Phyllis
P.S. We want to stay with Mouser if possible
Thanks!

The pots you show are good, stick with the 200 volt variety. I would choose a single turn pot that works for your board layout. Doing a search on the IBAM will show several implementations that myself and others have done. The best have a measurement point that can be reached without disassembling the amp.

Here are my thoughts on the pots. Avoid multi-turn pots, they encourage excessive adjustment and often do not have defined zero and full turn stops. The IBAM is designed for a single turn pot. Do not use stemmed pots, they can be inadvertently adjusted, like the blue pot in the MPX. These pots will be left unadjusted for years, so wire types and exposed track types are to be avoided. The best pot IMHO will be a recessed screwdriver adjustment, single turn, sealed, carbon film type.
 
I do have a suggestion for firing up a newly installed IBAM. There have been several instances of output tube red plating. I have had this happen myself. Usually the culprit is a mistake in the IBAM creating a zero volt bias. In a negative voltage bias larger negative volts is less power while "0" volts is full on. Zero volts (or open connection) is fatal to output tubes almost instantly. The 10 ohm "fusistors" protect the output transformers, do not count on them to protect the output tubes.

The usual causes of zero volts bias are bad solder joints and/or miss-wiring.

I strongly suggest testing the IBAM before installing the output tubes. Powering up the unit without the output tubes in place stresses the power transformer so a variac is the best way to do this test. The bias is a separate supply so there is no warm up time involved in these tests, just do them as quickly as possible. Even thermistors installed to slow start the unit should not interfere with the test since we are looking for a zero volt condition. If you do not have a variac (or even if you do) try to limit on time to less than 15 seconds.

First install a 1 amp fast blow fuse. This will help safeguard the unit. You should always install a 2 amp fast blow while doing work. This can save you from yourself.

Naturally pull all the output tubes, and only the output tubes, leave all other tubes in place.

Turn all of the IBAM pots fully counterclockwise. This should generate maximum negative voltage if the circuit is good.

Hook up a known good voltmeter positive lead to pin 6 of the first output tube the negative lead to chassis ground. Turn the variac up to 60 volts and verify negative voltage. Turn the unit off and connect the positive lead to pin 6 of the next output tube. Repeat for the rest of the output tubes.

Without a variac be sure to minimize the test time to avoid stressing the power transformer. Not a big risk but still a real one.

This test should verify that the IBAM is generating negative bias and will not red plate those precious output tubes.
 
The pots you show are good, stick with the 200 volt variety. I would choose a single turn pot that works for your board layout. Doing a search on the IBAM will show several implementations that myself and others have done. The best have a measurement point that can be reached without disassembling the amp.

Here are my thoughts on the pots. Avoid multi-turn pots, they encourage excessive adjustment and often do not have defined zero and full turn stops. The IBAM is designed for a single turn pot. Do not use stemmed pots, they can be inadvertently adjusted, like the blue pot in the MPX. These pots will be left unadjusted for years, so wire types and exposed track types are to be avoided. The best pot IMHO will be a recessed screwdriver adjustment, single turn, sealed, carbon film type.

Hi!
We figured out the single turn part. Were hoping you'd show us what you use specifically! :yes:
Half these listings don't give lead length. That frustration is one of the things that led to the post.
And almost every one are listed as "cermet", and now you are recommending carbon film.
So, please show use what you would buy, exactly.
Please! :banana:

Phyllis
 
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