Big-A$$ Bass Head Build

[JapanAxe]

I’ve just snagged a 120W valve guitar amp head from eBay and all being well I’ll pick it up tomorrow morning. My plan is to use the iron and valves (and possibly some of the hardware) for my build, harvest any other usable parts, and move on the headshell. I have a slightly more compact headshell that I obtained in a trade.

 

The new amp should deliver 100W+ from four 6L6GC valves but I haven’t yet decided what to base it on.

Edited April 21 by JapanAxe

[SpondonBassed]

1 hour ago, JapanAxe said:

I’ve just snagged a 120W valve guitar amp head from eBay and all being well I’ll pick it up tomorrow morning. My plan is to use the iron and valves (and possibly some of the hardware) for my build, harvest any other usable parts, and move on the headshell. I have a slightly more compact headshell that I obtained in a trade.

 

The new amp should deliver 100W+ from four 6L6GC valves but I haven’t yet decided what to base it on.

 

Will my bum look big in it though?

 

 

 

 

 

 

[JapanAxe]

54 minutes ago, SpondonBassed said:

 

Will my bum look big in it though?

 

 

 

 

 

 

I hope never to find out...

[JapanAxe]

Below - the donor amp, minus front panel. The seller was proudly demonstrating its clean, crunch and filth sounds when I admitted it was going to be reincarnated as a bass amp. I think that made him a tiny bit sad...

 

All the 6L6GCs test good and are perfectly matched so need to drop a ton on buying a matched quad. Two of the 12AX7s are good, two test as 'worn'.

 

Result.

 

Edited April 22 by JapanAxe

[JapanAxe]

I've decided to base my build on the Fender Bassman 100, with a circuit more or less like the one below. In the past I've reduced such circuits to one channel with a single input but I like the idea of being able to use this as a guitar head too. Rather than jumper the inputs (which would work, as the channels are in phase), I'm thinking of having a single socket that connects to Input 1 (high gain) of both channels. Then I'd have a 3-way switch to select channels (Bass-Mix-Guitar) and use the channel volumes to mix their relative levels.

[Lo-E]

This looks like fun. Are you going to scrap the PCBs and make tag boards, print new PCBs or wire it up point-to-point?

[JapanAxe]

3 hours ago, Lo-E said:

This looks like fun. Are you going to scrap the PCBs and make tag boards, print new PCBs or wire it up point-to-point?

The Bugera PCBs won't be involved in my build. I'll be making my own eyelet board - the layout will use some aspects of the original, but modified to accommodate the smoothing capacitors which Fender used to put in a separate 'dog house'.

[JapanAxe]

Well that's interesting.

 

I have been studying the guts of the Bugera with the schematic to hand. I identified all the connections to the output transformer (OT) so that's good. There are three secondaries on the power transformer (PT), which you can see where they connect to the main PCB in the photo below:

1. grey/grey - I measured 373V AC, bridge rectified (there's no centre tap), with a B+ of about 500V DC
2. orange/purple/orange - 20.5V-0V-20.5V AC, rectified to give a split rail supply of +/-24V DC. This is further reduced to create a DC supply for the heater filaments in the three preamp valves and +/-15V to run the digital reverb board. 20.5V AC is also run through a couple of stages of voltage doubling (C73-C75, D20-D22) to create -76V DC, then dropped to -56V DC for the bias supply (all measured).
3. yellow/yellow - 6V AC (measured - within 10% of 6.3V so it's fine) for the heaters of the phase inverter and the four output valves.

The Bugera can run EL34s in place of 6L6GCs. EL34s need 1.5A heater current each but 6L6GCs only 0.9A so there is sufficient capacity to run all the valves in my build without messing about with DC supplies. Also I can tie off the purple centre tap and rectify the orange/orange 41V AC to give me a nice -57V DC for my bias supply.

 

We have a plan!

 

 

[Lo-E]

Well, that worked out rather nicely, didn’t it?

[JapanAxe]

This is the new head shell. It was originally made to re-house a 'D'-style amp that was in a combo, hence the Voxy fabric on the lower panel. I don't love that look, and fortunately that front panel is easy to remove - I'll likely replace it with some black steel mesh to allow ventilation, as this is pretty much forced to be a 'valves-hanging-down' layout.

 

I think I can live with the natural wood finish; it would feel wrong to cover it with Tolex or Tuffcote!

 

[itu]

Just oil the wood, and it will faintly resemble Mesa/Boogie, although cooler.

[JapanAxe]

40 minutes ago, itu said:

Just oil the wood, and it will faintly resemble Mesa/Boogie, although cooler.

The outside has already been varnished so no need to do any more with that.

[JapanAxe]

I rounded up the parts I already have, and placed an order for the rest with Modulus Amplification.