Ampeg BA Series

These notes are for the OLD BA Series amps, originally produced by Ampeg in the SLM era. Ampeg's new owners, LOUD, are producing a NEW BA series, and I have yet to have any notes to report on those amps.

BA112

BA115

BA115HP

BA115HPT

BA210

BA300

BA600

There are so many of these Ampeg BA series combos!

By far the most common I see are the BA115 and the BA115HP.

PLEASE NOTE: This is not a manual or instructions for anyone else. These are my notes, open for public view. Proceed at your own risk! The reader should have a good understanding of electricity, electronics, safety, and the proper test equipment, including a light bulb limiter. I recommend sending your Ampeg amp to a qualified service technician if it requires disassembly.

DISASSEMBLY

The number 1 issue I have with these things is the cabinet design. Some of these will pull apart easily, and some will take a ridiculous amount of time to wiggle free.

I have found that placing the amplifier on its side, on my bench, 1/2 of it hanging off so I can access the screws, is the best way to disassemble.

The random factor is in the covering and whether or not is gets caught in the caged nuts as you wiggle the chassis free.

Key thing here, and with any Ampeg or Crate with this style cabinet/chassis is not force this. You can break off the little bits of the chassis, or just bend it badly. This is repairable, but adds a whole extra step and it's not that easy.

It may take awhile, but eventually you'll free the chassis. If it was relatively easy, then move onto the electronics. If it was difficult, then the covering is probably bunched up where it was caught. I cut away the excess material with a knife, just trying to avoid removing anything that will be visible once the amp is reassembled. Do this now before you forget! Assembly can be just as frustrating as disassembly with these amps.

You'll need to disconnect the speaker connection before you can fully remove the chassis.

MOSFETs

These are MOSFET power amps. Typically these will fail short and you can measure a low resistance between the terminals while in circuit. Go ahead and remove any suspect transistors, and then continue to check everything to the power amp's input (the input is usually the last op amp before the power transistors). You should make diode tests on all diodes (including zeners, just check for normal diode action) and BJTs (base-emitter, base-collector). Inspect all resistors with in-circuit resistance checks. Burned up resistors usually read high.

After replacing obviously bad parts and the fuse, keep the speaker disconnected, do not apply any signal, and power up with a light bulb limiter. If it passes the bulb test with no load, keep the bulb in and try it with a load. If it passes again, you can now remove the bulb limiter.

How do you connect a speaker without putting it back together? Alligator clips. Use your multimeter to determine which terminal is connected to ground. Put a red alligator clip on the other terminal. Put a green alligator clip on the chassis. Use the bulb first time you try this in case you have completely screwed up.

Note that the normal speaker "ground" pin, that is immediately next to the "hot" pin, travels "through" the headphone jack's sleeve contacts before hitting ground. That is why I would recommend alligator clipping straight the chassis first, but if you lose your signal when using the regular speaker "ground" pin, then you probably have headphone jack trouble.

Headphone Jack

The headphone jack can be a source of trouble. The speaker's negative connection goes from the PCB connector to a switched terminal on the headphone jack. The switch is normally closed and connects speaker negative to chassis ground. If headphones are inserted, the speaker gets disconnected (normal use). If the headphone jack gets oxidized over time or broken, then the speaker will lose this connection to ground even when headphones are not in use. Its good to double check this jack before reassembly.

If you want to replace this jack, you won't be able to source one from Ampeg anymore. The original part is a Cliff jack S4/BBB PC A. "PC A" refers to the "L" or offset pc pins. ("PC C," which is readily available, has straight PC pins.)

I've found Neutrik NMJ6HCD3 is close enough to work as a replacement. The thread pitch for the nut is finer than the original Cliff jack, so be sure to get all the hardware with the jack. The threaded barrel does not extend as far as the cliff jack, but I've found this does not prevent it from functioning just fine. I use 1 new thin washer that comes with the Neutrik, and then put the original thick white washer over that. It is still a bit short (distance from panel to the first threads), so I attach all the other components snugly before trying to attach the Neutrik jack.

BA115HP Hum issue

I have encountered several BA115HPs that seemed to have a fairly loud hum (60Hz/120Hz - which was it?). It is common enough that I stopped trying to troubleshoot it. It appears to be inherent in the design. I have replaced entire amp modules from Ampeg and the problem persisted. Don't waste time with this particular issue.

BA600 Issues

This amp only appeared briefly before being discontinued. There are a lot of negative testimonials out there on this amp. I have only seen one of these, but the customer's complaints matched the widely reported complaints online.

The two issues I am aware of are:

1) Screws come loose inside, possibly causing random problems

2) The protection circuit is overly complicated and overly sensitive. It also activates in an unusual way where the sounds cuts, but not completely, allowing a sound reminiscent of a fuzz pedal to come through (you would think it would cut out the sound completely). I was unwilling to take on either troubleshooting or modifying this circuit, since the wide reports of this problem indicated to me it was an engineering issue, and not something like a faulty component. Ampeg no longer supports this amp, and does not have replacement PCBs to correct this issue. I found that if I played the amp below the protection tripping point, it was fine. It seemed to function as a normal bass amp for straight bass playing. However, any type of aggressive use would trip the protection circuit, which means anyone trying to slap, pop, or overdrive the amp would be frustrated by this amp.

Bias Notes

BA112 - 2x TDA2050 output ICs. No bias control.

BA115 - 5mV across R61[0R1 3W]; no signal, no load.

BA115HP - 15-20mV across R72-R75 [0R33 10W]; cold start, no signal, no load.

BA115HPT - same as BA115HP

BA210 - same as BA115HP

BA300/600 has a class D module. No service notes are given in the schematic.