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Bell & Howell 385 (08855)

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  • Enzo
    replied
    Raja hasn't logged on since Feb of 2014, he may not get your message.

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  • airplaytheremins
    replied
    Hey Raja,
    I can't see the attached thumbnail with your schematic and parts omitted for the amp to function for guitar. Can you repost?

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  • Raja
    replied
    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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  • guitician
    replied
    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.

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  • Appaloosa
    replied
    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 Files

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  • guitician
    replied
    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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  • Raja
    replied
    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

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  • Appaloosa
    replied
    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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  • Raja
    replied
    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.

    RajaClick image for larger version

Name:	Bell-Howell-385-Layout-Revi.jpg
Views:	1
Size:	844.3 KB
ID:	821205
    Last edited by Raja; 07-19-2011, 10:55 PM.

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  • loudthud
    replied
    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, 05:05 PM.

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  • Raja
    replied
    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.
    RajaClick image for larger version

Name:	Bell-Howell-385-Layout.jpg
Views:	1
Size:	839.1 KB
ID:	821192

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  • Raja
    replied
    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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  • loudthud
    replied
    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.

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  • Raja
    replied
    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 Files

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  • Raja
    replied
    Cool, You've been so helpful.
    Thanks again.

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