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Help with Bogen DB10 amp making clipping sound

Sam Hopkins

Sam Hopkins

New Member
Hello everyone,

I recently purchased a Bogen DB10 tube amp which looks like it's been taken very good care of and rebuilt at some point. According to the manual, in order to run a turntable through the high or low mag inputs, a cable must be connected from the pre-amp out to the amp input. When I do this and run the TT through either high or low mag inputs (I'm using a Stanton 500 V3 cartridge), I start getting a fast clipping sound when the volume is increased only at about 20 percent. When I back the volume off to just right below the point it starts clipping, if I then increase the treble it will also start making this same sound when it gets to a certain point. It used to give me a slower chirping sound when I first got it but now it is a long steady but fast clipping noise. The patch cable I'm using between the pre-amp out and amp input is brand new and tested.

If I plug a receiver straight into the 1/4" amp input bypassing the pre-amp stage, it plays clear and very loud. If I plug my turntable the same way directly into the amp input it also sounds fine, but of course the volume is very low even turned up to maximum without the pre-amp.

Any clues what the problem might be? Supposedly the tubes were all tested. I have very limited experience in electronic repair so please bear with me.

Thanks in advance for any advice!
 
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I would try moving the jumper cable away from all other signal sources, especially the speaker wires,output transformer and power transformer. If you could make a short solid wire jumper from the input to the output it would be best.Many integrated amps use a jumper like this. Does it also have this problem with no TT connected?
 
Thanks for your reply. I'm using a new patch cord which is only 10" long so it's a very short loop. Each end has the proper connection and no adapters. I've also tried two other cables with the same results. When the cord is patched through and the turntable isn't plugged in, I start getting a static,scratchy hum when I turn the volume knob up a little past where it usually starts clipping when the TT is plugged in and playing. I've also noticed the pre-amp tube is not completely stable on the base and the whole thing wobbles slightly. It was this way when I received it. The tube is a metal 6SC7.
 
Can you post an underside pic of the chassis around the 6SC7? does the noise change when the tube is moved around? Check for DC at the amplifier in and preamp out jacks.

If C2 has a bad section it could oscillate at higher volumes, it would sound like clicking, and clipping
 
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Here is a photo of the entire chassis and one of just the upper left section under the 6SC7 tube. If I move the tube slightly with the volume up a little it causes static. I'm not sure how to check for DC at the amp in and pre-amp out jacks. Sorry for my ignorance. I do have an ohm meter. Thanks for taking a look. The clipping sound I'm getting sounds like someone imitating a helicopter and it starts when the volume reaches about the level of a normal conversation, maybe slightly louder.
 

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Wiggling the tube and causing static simply sounds like dirty or loose tube pins and/or the socket.

Clipping is distortion. A putt putt type of sound is called motor boating.
 
It looks like it was properly rebuilt, there will be no DC on the input / output, (you would check by setting the meter to volts and measuring from the center pin to the outside of the jack).
The output transformer was replaced.If it was not wired correctly it could cause problems.
You could check the - side of the caps to check for good connection to ground. Meter set to continuity( with the amp unplugged) There are lethal voltages inside the amp when plugged in, and over 300v when turned on, Be Careful! The caps could store a charge for a little while after it is on, there is a bleeder in the circuit,
 
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s-petersen said:
It looks like it was properly rebuilt, there will be no DC on the input / output, (you would check by setting the meter to volts and measuring from the center pin to the outside of the jack).
The output transformer was replaced.If it was not wired correctly it could cause problems.
You could check the - side of the caps to check for good connection to ground. Meter set to continuity( with the amp unplugged) There are lethal voltages inside the amp when plugged in, and over 300v when turned on, Be Careful! The caps could store a charge for a little while after it is on, there is a bleeder in the circuit,
Click to expand...

Thanks for the recommendations and safety warning. I'll try what you suggested when I get some time later tonight. I'll be certain not to do anything with it while plugged in. It has been unplugged now for more than 12 hours now.

I was hoping that since it plays fine when the pre-amp section is by-passed, it would be much easier to pinpoint the problem.
 
s-petersen said:
The way it sounds is that some of the c2 caps are not grounded, if the amp is wired like the schematic.
Click to expand...

Would the amp still play clearly at lower volumes like it is doing if some of the c2 caps are not grounded properly? I'll watch some videos to try to learn how to test for grounding on the caps.

I uploaded a video I made a few weeks ago of the problem. The patch cord shown here is different than the new short one I recently replaced it with, but all still produces the same noise.

 
If it plays fine at high volume with the phono preamp out of circuit, and motorboats with the preamp in circuit, problems with the C2 caps seems likely. If some of the coupling caps were replaced with much larger values, there could be some similar problems.
 
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