Bell & Howell 385 (08855)

[Raja]

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

[loudthud]

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.

[Raja]

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

[olddawg]

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.

[Raja]

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

[loudthud]

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.

[Raja]

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

[loudthud]

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.

[Raja]

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

[Raja]

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

[loudthud]

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.

[Raja]

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

[Raja]

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.
Raja

[loudthud]

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.

[Raja]

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.

Raja