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Thread: Bell & Howell 385 (08855)

  1. #1
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    Bell & Howell 385 (08855)

    I'm rehabbing a 385 with 6v6's and have a lot of questions. I sure hope some of you can answer them.
    I'm replacing the pots, the schematics call for a 1 meg audio on the volume and 3meg dual on the tone, which seems to be linear. Can anyone tell me why this amp would need a 3 meg on the tone pot and what is the purpose of the dual pot on the tone?
    I'm going to move the on/off switch to the rear of the chassis to simplify things.
    I've removed the oscillator coil circuit and replaced the filter caps.
    When I was turning the tone knob I got some motor boating and static, the reason for replacing the pot.

    I was able to stop the motor-boating by moving some wires.

    It seemed that when the knob reached 3/4's of the way the sound would cut out and/or start farting.
    I want to thank you in advance for any and all help you can give me.

    Here is a link to the schematic,

    http://acofs.org.au/part_5_files/Bel...%20diagram.PDF

    Raja
    Last edited by tboy; 06-21-2011 at 11:06 PM. Reason: fixed link

  2. #2
    Supporting Member loudthud's Avatar
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    It looks to me like the amp uses the "James" tone network (sometimes incorrectly called Baxandall). The front section is the Bass control and the rear section is the Treble control. Typically 10% audio taper pots are used so that flat response is when the pots are set to mid rotation. Normally I would expect a dual stacked pot with concentric shafts. Did someone repair the amp an install a simple dual pot?

    You may not be able to find a 3 Meg pot. A 1 Meg will work, some of the caps around the controls may need to tweeked to get the affected frequencies adapted to guitar.

    As for the cut-out/farting issue, look for a filter cap that is not connected or possibly a bad part was installed.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

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    Thanks for your response Loudthud.
    I found that Gretsch and the Fender TBX tone control's are similar to this one.
    I have pulled the pot and have taken it apart to clean. It looks to be original to me.
    The rear section is 3.3 meg and the front is 2.7, and both are linear. I'm going to put it back in but while it's out I'm going to replace any caps and resistor's that have drifted too far out of range. I have found some resistors that must have been marked wrong. One has 2 red and one orange stripe but measures 2.2 K. I don't know if a 22k compared to a 2.2k would cause too much of a difference.
    Someone must have messed with the amp at some point because I found that the R40 was set to around 470 V DC when it should be 325.
    Have you ever seen an amp with a wire-wound resistor to set the B+ voltage? I'm not that knowledgeable about these amps so forgive my ignorance. Also on the 6.3 heater circuit there is a variable resistor to fine-tune the setting for that voltage. A neat idea if that is what those are for.
    Again thank you for you input.
    Raja

  4. #4
    Old Timer olddawg's Avatar
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    I would gut this thing and start over. It would make a great 18 watt Marshall clone with tremelo. You have plenty of heater current to have both channels, even a extra gain stage if you wanted. If you want to raise the B+ you could use an SS rectifier.

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    Hey Olddawg, Well I already have a Lite IIb, which I really like but this amp does sound pretty great as is. It just needs a little work to get rid of some hum and there isn't very much room in the chassis right now.
    Raja

  6. #6
    Supporting Member loudthud's Avatar
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    Quote Originally Posted by Raja View Post
    Someone must have messed with the amp at some point because I found that the R40 was set to around 470 V DC when it should be 325.
    Have you ever seen an amp with a wire-wound resistor to set the B+ voltage? I'm not that knowledgeable about these amps so forgive my ignorance. Also on the 6.3 heater circuit there is a variable resistor to fine-tune the setting for that voltage. A neat idea if that is what those are for.
    Again thank you for you input.
    Raja
    The pot on the 6.3V is a Hum Balance control. Adjust for the lowest hum.

    R40 might not have enough range because you removed the Oscillator circuitry. Less load on the B+ lets the voltage rise. You see this type of control on really old high quality audio gear where they adjust the B+ right up to the maximum ratings on the tubes' data sheet.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

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    Oh, That makes sense.
    I was looking at some Ampeg schematics and saw that hum balance control and a lot of the Ampeg's used the James tone control with very similar values. I'm learning a lot with this project. Thanks again for your help, very much appreciated.
    Raja

  8. #8
    Supporting Member loudthud's Avatar
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    I was just looking at the schematic and noticed they want you to measure the screen voltage of the 6V6s and set it to 325V. The main B+ will be about 348V.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

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    Cool, You've been so helpful.
    Thanks again.

  10. #10
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    I have highlighted the areas of the schematic that I think can be omitted and still have the amp function for guitar.
    Could someone have a look and let me know if I have it correctly marked.
    Thank you in advance for your help.
    Raja
    Attached Thumbnails Attached Thumbnails bell-howell-385-schematic.jpg  

  11. #11
    Supporting Member loudthud's Avatar
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    Everything looks good except what's between the input jack and pin 1 of V1. In a guitar amp you would normally expect a 33K or 68K where the 1 Meg is now and a 1 Meg from pin 1 to ground.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

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    Well thanks again Loudthud!!
    I'll add those two resistors and remove the other parts from the amp.
    That will give some room to work with, this amp is a bit cramped.
    Thanks again.
    Raja

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    I've drawn a layout for the Bell & Howell 385 amp and was hoping some of you might take a look and let me know if everything was as it should be. Any advice for improvements would be appreciated.
    Thanks for looking.
    Rajabell-howell-385-layout.jpg

  14. #14
    Supporting Member loudthud's Avatar
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    Nice diagram. Did you draw it or find it on the web somewhere? What software was used to create it?

    The grounding of the amp may cause a high level of hum and other problems. There is no way of knowing how bad it will be until you get the amp up and running. One possibility is there may be oscillations because the input stage, volume control and output transformer are all grounded at the same point and then there is a common wire back to the filter caps. These oscillations can be next to impossible to track down and eliminate without the aide of an oscilloscope.

    Is this the original ground scheme? One thing that puzzles me. There are two terminal strips that support all the filter caps. The terminal strip that is closest to the tube sockets has a gray line between the two terminals that look like they have screws holding the strip down. What goes on there? Are those two points tied to the chassis with screws?

    Edit: V5 pin 8 has no connection. It needs a wire to V4 pin 8.
    Last edited by loudthud; 07-19-2011 at 06:05 PM.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

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    Hey Loudthud,
    I did the drawing with Adobe Illustrator. I created all of the images used in the drawing.

    The grounding scheme is how the amp was built except for the input section. The input originally grounded on one of the unused pins of the PEC tube which I removed. I drew it connected to the star ground and can change that.
    I removed the original filter cans which were grounded to an aluminum wall which is attached to the chassis and I followed the same scheme with my replacement filter caps. As you noted there was also a grounding connection to the star ground point from that same wall. I thought that might create a grounding issue and was going to remove it if I had hum issues.
    I'll go ahead and remove that wire, relocate the input ground and make the changes to my drawing then re-post.

    They way this amp was grounded was probably causing the hum when I first played it. Also the heater wiring wasn't consistent. On V4 the wiring went from pin 2 to pin 7 on V5.

    Thanks for catching the connection I missed on V5, you've got some sharp eyes!!!!

    I've attached a revised drawing with corrections suggested by Loudthud and some fixes to errors I found.

    Rajabell-howell-385-layout-revi.jpg
    Last edited by Raja; 07-19-2011 at 11:55 PM.

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    B&H guitar amp project

    Hi, I just stumbled onto this thread. I realize its a couple months old, but I was curious how your B&H project turned out? I also have a couple of B&H 08855 that I would like to turn into guitar amps. Your drawings look great, and I was wondering how your amp sounded if you got it together. thanks for all the help.

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    I got busy with other projects and haven't put it back together.
    I just about have it gutted and will have some time to work on it soon.
    Have you played through your amps? How do they sound?
    I found some errors on mine that I think were causing the hum. The heaters were wired incorrectly and there were some areas of concern with the grounding of the amp.
    My drawing hasn't been proven yet so it's not to be trusted. I would leave your filter caps alone for the time being and look at the filament wiring and grounding on the amp and clean those up first, if they need it.
    Raja

  18. #18
    Senior Member guitician's Avatar
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    The first stage pentode could be eliminated by using just the three gain stages of V2 & V3. Increasing the plate resistors on the first couple of 12AX7 stages would be required and if more gain is needed, just add cathode bypass capacitors. It would reduce noise, and may sound better too. IMHO

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    Thanks for the response. I have not yet plugged my amps in. they are still in their "original" state. I'm somewhat of a newbie to this and I was wondering the best way to attack them? I was thinking i would just plug one in and see what happens, but I don't have the original power cord or the right size speaker out jacks. So I figured if I needed to get in their just to replace the power jack and spk. outs I might as well chop out whatever else is not needed for guitar use. Then I saw your schematic and thought about going deeper while i had it apart. do these amps sound good left original? thanks for the help and info!
    Attached Images Attached Images

  20. #20
    Senior Member guitician's Avatar
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    I would say by looking at the pics that they are not good for newbe's. They're way too compact to work on. If you just use the transformers and chassis as a start, then re-arrange the tube placement and component layout adding terminal strips as needed per the redesign, you may end up with a really nice guitar amp. The filter capacitors will need reforming if it hasn't been powered on in years. Two 6V6GT, 5Y3GT, and a pair of 12AX7's are the ingredients of some great amps.

  21. #21
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    Appaloosa,
    I would do the minimum necessary to just hear what you've got to work with. In your photos the back amp has a note on it that said it was blowing a fuse and if in the other photo of the gut shot the back amp is the same then someone has made changes to the amp. Two of the caps are not original but they are in the section of the amp not needed for guitar, but the brown variable resistor (10 W/500ohm) has been changed. This resistor should be set so pin4 @ V4 reads 325 VDC. If you remove the unnecessary components from the amp that resistor will need to be adjusted.
    If it was me I would start with the amp to the front in your photos and find a power cord that will work or change it out and rig a speaker up to it and see if it still works. V6 is not needed for guitar amp but I would leave it in and I would also leave the PEC tube in but would blind it by wrapping black electrical tape around it.
    When I bought mine I also got an extension speaker with it so I was able to adapt a 1/4" plug to it easily.
    These amps were meant to be used with a loud film projector and hum was not an issue.
    Let me know if I can be of any more help.
    I'll update when I get further along with my amp.
    Raja

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