View Single Post
Old 2nd Oct 2004, 2:01 pm   #2
paulsherwin
Moderator
 
paulsherwin's Avatar
 
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,787
Default Re: Bush PB53 Output stage problems

Hi David.

The DC resistance of the O/P transformer isn't too important. What matters is the impedence. You need to use a transfomer which has more or less the correct impedence for the valve anode load. In practice class A O/P valves are pretty tolerant of this so long as you check the current being drawn is correct and modify the cathode resistor if required. The worst that's likely to happen is you'll lose some volume.

The fact that the speech coil is slightly wrong is also not significant. It's the impedence that matters, not the DC resistance. These DC resistances both sound plausible for a nominal 3 ohm impedence speaker.

The hum and field coil values do seem wrong, so the speaker has probably been replaced at some time, but I wouldn't worry too much at this stage so long as it works. I can't see anything here that's going to cause any damage.

I can't suggest another O/P valve as sets of this period aren't my speciality, but subbing another valve should be OK if the characteristics are similar - again, you may need to adjust the cathode resistor.

HTH, Paul
paulsherwin is online now