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?

 

 

[JapanAxe]

Well this is both annoying and encouraging.

 

Today I rebuilt the bias board with a voltage doubler, giving me a range of -56V to -36V. I still get a positive voltage at the control grids when I apply HT, and this time I got some sizzling and smoking from around V4. I wasn't able to determine exactly where, and there are no crispy components, but it's a start.

 

I then removed all 4 output valves and dropped the mains to 180V. With the amp in standby, I measured around -14V on the HT downstream of the standby switch, and it changes as I move the bias adjustment pots. There should be no way for DC to make its way from the HT to the bias circuit.

 

There's either a short or a leaky capacitor. I'm off to play sone music now to soothe my brain but I'll come back to sort this another day.

[JapanAxe]

It lives!

 

Last night I was trying to work out which HT node the excess curent was coming from. The various components up to the third node (C+) all tested ok but I was measuring 121k from C+ to ground where it should have been open circuit. Before I turned in for the night it occurred to me that I should look for leakage in the coupling caps. While I was falling asleep (in the hypnagogic state, no less) I kept thinking that there were resistors where there should be capacitors, but when I woke up I realised that was just daft.

 

This afternoon I checked over the circuit diagram and worked out there are just two 100nF capacitors which, if leaky, could allow positive voltage onto the control grids of the 6L6s, thus messing up their negative bias. I found them on the layout, but when I looked on my eyelet board, there was a 100k resistor fitted in each spot! I had written '100n' (for 100nF) on my layout but misread it as 100k (100kΩ). Consequently there was a straight resistive path from the anodes of the phase inverter to the control grids of the 6L6s, with no capacitors to block DC.

 

I took out the resistors, fitted the correct caps, and was able to bias the amp to a healthy place. I gave it a quick test through the guitar speaker cab that I keep in my music room, and all the controls seem to work as they should. Tomorrow I'll give it a try through my BF Super Twin while Mrs Axe is at work

Edited 23 hours ago by JapanAxe

[Richard R]

Woohoo!

[JapanAxe]

Well that was fun! It is in fact a working amp, with a huge range of tones on tap. As ever when I'm trying out my builds at volume, I get a lot of buzzes and rattles in the room that make it difficult to hear whether there is distortion from the amp, so a blast in a hall or something would be an advantage.

 

Background noise is acceptable with just a little 100Hz hum. I can hear no effect from the bright switch on the normal channel because I have the channel volume up a fair way, so I might turn that into a channel selector instead.

 

There's now way too much negative voltage available on the bias circuit so I'll experiment with component values and maybe add another filter cap in there - who knows, that might reduce that hum a little.

 

For now I think I'll work out the best place to fit the top handle and get it installed, although I think it may be a challenging one-handed lift!

 

[SpondonBassed]

Man that looks geeky.  In a good way, you understand.

 

It's beautifully uncomplicated in appearance.  I bet it looks good with all the valves warmed up and the lights down.

 

Why do you need a top handle?  It's asking for the whole unit to be mishandled if you aren't there.  Side handles would mean a two handed lift which is less likely to result in knocks or worse.  Just a feeling you understand, I'd feel safer.  Plus, you'd get to keep a nice clean top on it.

 

Great build thread.  Thanks for sharing.

 

 

 

 

 

 

Edited 2 hours ago by SpondonBassed