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Tubelab SE 300B low level hum, help

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abacus

Member
Joined 2007
Hello

My Tubelab SE 300B amp have a low level hum, its barely to hear with my ears close to the speaker(5cm). It stays consistent regardless to the volume level. Volume pot case is grounded (blue Alps 100k), disconnecting the input wiring from the PC board no change. OPT James JS-6123HS, choke installed 6H, 200mA, changing PT orientation, no change. Tubes, Shuguang 300B-98B mash grid, rectifier Sovtek 5AR4.Any help about this problem?
And what grounding scheme will be the best to use, i used the first scheme from the Simple SE page.

Thanks.
 
Tubelab_com

Tubelab_com

Member
Joined 2005
tubelab.com
Power and output transformer too close together? Orientation?
Click to expand...

It sounds like you have tried repositioning the transformers, but Is the hum audible immediately on power up, before the tubes have had time to warm up, or without the rectifier tube? If so transformer coupling is a strong possibility.

Try the amp with the 5842's removed. If the hum is gone, it is getting into the input stage. If the hum remains it is getting into the output stage.

There are 3 possible paths for this to occur. The B+ supply could couple hum into the output supply, but it sounds like you have tried several of the usual cures on the B+ which had no effect, so I must assume that this is not the path.

The next possibility is the bias supply. Ordinarilly the low pass filter formed by R6 and C7 assures clean DC for the bias supply. Any hum that is present is further reduced by the voltage division formed by the 20K mosfet source resistors and the low impedance presented by the mosfet. But 50 Hz power coupled with a less than perfect C7 could allow some low level hum. Since no capacitor is perfect, try adding another capacitor in parallel with C7 to see it it makes any difference. If there is no change this is probably not the hum entry path. If there is improvement the usual hum prevention methods can be applied (more, or better caps, and even a small choke in place of R5).

The last possible entry point is the filament supply. The regulator IC normally provides about 60db of hum reduction on the filament supply to the output tubes. This value diminishes if the regulator does not have enough voltage across it. There should be at least 5.5 volts at the input of the regulator, the more the better. This voltage can be measures across C1. Try adding another capacitor across C1. This should raise the voltage slightly and reduce the ripple. Both will make life easier on the regulator. I have seen the filament supply cause hum with some Sovtek 2A3 tubes that drew slightly more than 2.5 amps each drawing about 5.2 amps from the 5 amp regulator. It took a large external capacitor across C1 to eliminate it.

In any case if external capacitors are added to the board they should be connected directly across the existing capacitor using the same connection points. The PC board was carefully laid out to insure that ground currents do not flow through signal grounds. Connecting a capacitor to a different ground point could increase hum.
 
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