A DIY 100 watt tube bass amp

[Passinwind]

I'll do a full build thread here in the next few days, but I only got this thing running properly yesterday and need to do a bunch of detailing before I do gut shots.



100 watts, 4 X EL-34, 1 x 12SL7, 2 x 12AU7

1/8th inch aluminum enclosure from here: https://landfallsystems.com/

Laser etched fascia plate from Marco Bass Guitars

Heyboer Hiwatt DR103 output transformer clone

Tube line driver based on the Aikido ones from Tubecad.com

Switchable input impedance (not yet implemented)

Power amp PCB and power transformer are NOS repair stock via friends from a now defunct amp company. Opamp input drives a stepup transformer which directly drives the EL-34s. Super low noise floor and an active bass or many pedals can drive the power amp board pretty well for practice purposes. I have a few rack and desktop preamps that work great into the Power Amp/Aux In jack too. Since there's a nice opamp power supply already on the power amp board I'm contemplating many SS add ons. The preamp build is highly modular and I left enough extra internal space to try many mods. Easy enough to drill more control holes and burn a new faceplate at any time as well.

 

[Passinwind]

Power amp board, DC filament regulator, filament voltage elevator board, preamp B+ supply voltage drop/filter board, DIY tube socket breakout boards w/SMD grid stopper resistors right at tube pins.



Preamp board originally from a rack preamp build in 2015, I would do many things differently by now. I think I see at least one labeling error there too.

 

[Asdrael]

Looks great! What are you using the DC for? The Novak and the OP amps?

 

[Passinwind]

Looks great! What are you using the DC for? The Novak and the OP amps?

The red board is a Glassware Audio 12V regulator for the preamp filaments, but that 12V rides on +68V derived by a splitter from the +280V preamp supply. This is done to keep cathode to heater voltages in bounds between the White Follower-ish (totem poled) line driver format and the typical common cathode gain stages.

The opamp runs off regulated +/-15V right on the power amp board (the gold heatsinks), off its own separate power supply tap. As you might imagine, ground management is hypercritical and there are a lot of ways to get it wrong. This is not exactly the most simple build, and it really helped to have the original power amp designer helping me out a little. He's doing bass amps at Mesa now, really great guy.

 

[Asdrael]

Yeah, grounding schemes between different regulated DC, elevated, B+ and audio can be hell. I usually go for buses that slowly branch out and are specific for each section and only meet at one point close to general ground. But never dealt with elevated…

 

[Passinwind]

Yeah, grounding schemes between different regulated DC, elevated, B+ and audio can be hell. I usually go for buses that slowly branch out and are specific for each section and only meet at one point close to general ground. But never dealt with elevated…

I used a little chassis ground isolator board a friend had shared on OSHpark and was careful about avoiding loopbacks, but the first time I used one of these power amps it totally gave me fits working out what went to dirty ground and what went to the clean side. I think I could still improve on that if and when I do a new preamp board, but with turret board wiring it could have been a lot easier! But I really dig doing PCB layout work, and hopefully getting smarter as I go.