Big-A$$ Bass Head Build

[rwillett]

I think this is why I don't work with valves and large voltages. I've read this 2-3 times. It looks like English, it sounds English but I think it's actually in Klingon.

 

Hope you sleep well and I look forward to whatever you do being done. 

[JapanAxe]

Well that's interesting. I've discovered that, when HT is applied, my nice negative bias voltage all but disappears, leaving the output valves to go full-on Fukushima. 

 

Which bit of this circuit is not a tried-and-tested design, but my own concoction? Yep, the bias supply. I consulted the writings of good old Merlin Blencowe, and discovered that my choice of component values may have left the output valves with too high a grid leak resistance. Time to re-design the circuit!

[JapanAxe]

Well that's annoying. I rebuilt the bias board with the correct component values but the result is the same - the bias voltage disappears when HT is applied. With 450V of HT there should be about -37V on the control grids accordoing to the 6L6GC spec sheet.

 

I'm pretty certain that the tapping I am using for the bias is a completely separate secondary as I checked continuity when I first got the transformers out of the Bugera. It has a centre tap (unused) but the HT secondary doesn't as it went to a bridge rectifier (which is what I'm using on this build). I'll check that again, and then consider the following options:

1. Voltage doubler on the 41V AC secondary. It may be that it just can't produce a high enough negative bias without this.
2. Creating negative bias by dropping the HT with a potential divider.

Merlin Blencowe goes into all this.

 

Sadly it's looking less. likely that I'll have the amp ready for the SW Bass Bash

[SpondonBassed]

28 minutes ago, JapanAxe said:

Sadly it's looking less. likely that I'll have the amp ready for the SW Bass Bash

 

We're looking at The 2026 Midlands Bass Bash being held at Bentley Heath.

 

Relax.  Take a deep breath, hold it  then slowly exhale while letting go of all those imaginary gremlins scampering around your mind.  Now you can do your troubleshooting at leisure.  See you there?