Woodo wrote:
I struggled to get to 5v max was 4.8 adjusting the trim pots. So i thought that’s the best i could do. I took some measurements and i’m getting a little over 6v after the bridge rectifier where i would expect more ie at least 8v.
This is rather unusual for a 1.2A load on a 2.5A winding. Further, the LT1085 is a 3A LDO regulator. At 1.2A load, the voltage drop should be just about 1V. A 4.8v output voltage infers an input voltage of ≈5.8v at the filter input. So if you're getting a little over 6V at the filter output you should be able to get the required 5V at the filament with the trimmer. Something is not right here.
The buzzing you're getting could be due to inadequate filtering in the filter. Or the regulators may be struggling to properly start up into the load and going into an oscillatory state. Here a look at your waveforms would be very helpful. Can you scope the input waveform at the regulator input? I also suggest you make sure that the bridge rectifiers are properly connected and the filter caps are all per the specified design.
I have never had this type of issue before with the LT1083/4/5 regulators. Look carefully at the circuit to make sure that the rectifier bridges are wired correctly.
Also, look at the post here on the filament circuit design:
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=2699&start=127 Note that I said:
I chose the Hammond 266L12B (or the 266L12). This is a dual secondary 6.3v 2A (2.5 for the 266L12) filament transformer. The 300B heater current load of 1.2A with derating requires at least a 2A secondary.It could be that the 2A secondary is marginal and a 2.5A secondary is really required to prevent startup problems. Hammond transformers tend to have higher secondary resistance than most other power transformers and this could be contributing to your issues.