Aguilar DB359 schematic

I recently got an impressive used Aguilar DB359 all-tube bass guitar amplifier for a decent price. About 200 watts, [email protected] 12AX7, [email protected] 12AU7, [email protected] 6550 outputs. Toroidal power transformer and toroidal output transformer. DI uses Jensen transformer for balanced out; effects loop, fan between the main chassis and output tube cage. Nice piece of equipment for the most part, if rather heavy as could be expected for a 200 watt tube amp with a steel chassis. Front 2-unit rack panel is definitely not up to actually supporting the amp for shipping, and eBay or Reverb often have listings selling bent-up used units. There are no rear rail brackets. It has no front vents and is neither front-to-back rack flow standard more back-to-front standard.

Unfortunately Aguilar doesn't release their schematics except to what they consider "commercial" service shops!

I find this really irritating! The more familiar I become with tube amp schematics the more I expect to review one before I discuss an amp, much less buy it.

The only issues mine suffers from seem related to a silly airflow pattern for the fan cooling, which has a big "short circuit" hole opposite the fan which can allow hot air from the output tube cage exhaust to recirculate back into the main chassis and fan input. It's more like stirring up the air than forcing a 100% exchange. The output tubes themselves are horizontal (which I'm not particularly fond of) with the tubes and tube cage sticking out the back. The intake is from both sides and exhaust from the output tube cage is to the back and top. With it getting pretty hot, people are reluctant to rackmount anything above it and sometimes mount a blank panel.
That can make things worse by allowing hot air to exit the top and get sucked back into the sides. I can't imagine what they were thinking, unless perhaps they wanted to make it live a bit longer if some idiot blocked the back against a wall by laying it naked atop a speaker bottom. Of course, agitating and circulating the air is important, and maybe they wanted the temps more consistent by blowing around hot air instead of cold? I often wish fan-cooled amps were also designed to allow direct infra-red radiation out of the cabinet to the outside room.

The company responded to my request by claiming it was to protect customers from getting electrocuted and to limit their liability, which does not explain the non-disclosure agreement they have shops sign before sending the schematics. The danger is real, but the risk and liability are easily managed with appropriate warnings and by supplying the schematics only in a service manual with appropriate warnings. They claim they are afraid that publishing their schematics will be seen as encouraging users to service their own amps, causing them to be liable for injury. It's for my own good because I'm not a "professional"... There are good "professionals" and bad. I can see them judging who they will reimburse for warranty work, but some touring acts looking for out of town service won't want to wait for three mail trips for a non-disclosure form, return with signature, then sending the schematic...they want a service manual in-hand.

I'm inclined to trace it out, and publish the schematic as an educational study. That way I'm taking the action, supplying an attached warning to cover my liability, and solving their problem by relieving them from liability. Won't they be grateful. Grumble grumble.
Last edited:
I really would like to know whether they are ultralinear, and a little more about the feedback loop. Those are things that affect the sound, especially when overdriven.

I hear that toroidal output transformers are sensitive to matching tubes carefully. At least there are external bias probe jacks and trimmer pots, one for the push pair and one for the pull pair of 6550.