SET amp causing woofers to pump

V

vinnie2

New Member
Hello
I think my diy 300B SET amp is causing the woofers to pump on both sets of my speakers. I am playing cd's, so it is not turntable noise. One set of speakers is very efficient but the other isn't, and neither has ever shown this problem before, so I think it is the amp. This seems to be a recent development. The amp was in storage for about 3 years and I just took it out to use it again. I never noticed it before I put it in storage.
I am wondering what some of the causes for this might be so I know where to start looking for the problem.
All help appreciated.
Thanks
Vinne2
 
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Sound like low frequency oscillation / instability. I'd probably check the values of the components in the feedback circuit, if it has any, as well as the wire routing to make sure none of the output wires are too close to the input wires. Might also be worth making sure your amp either has a DC blocking capacitor on the input jacks, or that your CD player or preamp isn't putting any DC voltage into the 300b amp.
 
Sometimes this could be referred to as "Motorboating". Since the amplifier has been sitting for a while, could it be the filter capacitors in the power supply have "gone soft" meaning lost value, high ESR etc.?
 
Yes, they are the same amps.

It looks like you are on to something Scott. I took the right hand output tube out and when I turned it on there was no pumping in either channel. Turn it off and put the tube back in and turned it back on and still no pumping. After I turned it on and off two more times its started pumping again. When I took the output tube out again both channels stopped pumping again.

What is going on here Scott??
 
Scott
Forgot to mention one thing.....I used a 47 mfd cap in place of the 15 mfd cap shown on the schematic. Not sure why I did that as it was some time ago, but i am wondering if that could be part of the problem?
Vinnie
 
I think you have a power problem.Do you have the filter caps on the amp chassis?If not,I think adding the 50mfd to the B++ line on the amp chassis will solve your problem.
I don't think increasing the filter after the rectifier would cause your problem,it may shorten the rectifier life though.I think there is a voltage drop between the amp and power supply.There is also a possibility of a ground problem between them also.
 
Another thing I forgot to mention is that I put a 47 mfd 'electrolytic cap instead of the 30 mfd in the plans. Do you still think I should add 50 mfd?
 
Also should tell you that this is a two piece amp. Power supply on one chassis and the front end on another.
 
Sorry, you already know it is two piece. I missed that. The filter caps are on the PS chassis, including the B++ cap. I will double check the ground for good connections. The harness is home made using an insulated terminal block between the two chassis.
 
You need to have some of the power supply caps on the amp chassis , long B+ runs with no local bypassing can cause the type of problems you are having .

Mark
 
Update

Put a solen 24 mfd in parallel with the 47 lytic on the b++ and the problem went away. Thanks for all the help!
 
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