300b S/E problems.

[pre65]

Hi-i have had my 300b amp for a while now,and although it didn't suit the KLS3 it was a bit happier with the Adires and a bit bit more with the Fostex.

Now i bought this cheapish because of a declared hum problem (still not sorted) but today i took the valves out to measure some basic voltages.this is mainly because it was built in germany and presumably set up for 220v.

Mains 239v 6CG7 heaters 7.3v 5R4 heater 5.3v 300b heaters 7.5v !!!!!!!!

but according to my spec sheets 300b heaters should be only 5v.

what effect would i notice running it as i am ?


Philip

[NickG]

If you have taken the valves out, then the supplys will be running off load. The voltages will be less with the valves in place. Its only then you can see what the valves will be running at.

[Paul Barker]

dead 300b's in a matter of weeks. Oh sorry you measured them off load. If it's ac heated the'll still be well over voltage 'cause a transformer with 50% regulation is not likely 20% yes very common in cheapskate stuff, but if it's dc then you have no idea until you plug in the load.

[pre65]

Hi-newby error !

with the valves in place i get

6CG7 = 6.2v 300b = 4.9v 5R4 = 4.9v so that looks ok.

there are two separate dual 6.3v transformer feeding the 6CG7/300b for each channel with the 300b having a couple of wire wound resistors between the hum pot and valve to drop the extra 1.3v

i feel happier now knowing that at least this part of the amp is ok.

i might rewire the 6CG7 supply with a pseudo CT mod using a couple of 100R resistors,it cant do any harm and i might loose my hum.

Philip

[Paul Barker]

yes, might even be the problem.