While I hate to see butchered amps, there are some modifications that are worth performing. Of course, the work has to be done with care. Jess Oliver, the designer, used to service these amps and performed a number of upgrades on them. Things that he thought should be done.
ONE IMPORTANT SUGGESTION. CHANGE ONE THING AT A TIME AND TEST THE AMP AFTERWARDS.
Changing to a three-conductor power cord is a good idea. When doing so, test or remove the so called death cap. The black cap in the image above.
When this amp was designed, the Ampeg tubes sold at the time were inexpensive and pretty much plug-and-play. Resetting the bias was not usually necessary. Setting the bias required changing a resistor which meant soldering on the PCB. Today, tubes are expensive and less consistent. In order to optimize performance setting the bias when changing the power tubes is often required. Having a pot adjustable bias is a good feature. Another upgrade Jess performed and it's in all the modern Ampeg B-15 revisions.
There are known issues with these amps. The original Mallory cap can does not last 60 years, the rubber seal dries and cracks with the internal pressure. When amps that have sat unused for long periodsare put back into service, it can hasten the failure of this component.
The high wattage resistor in the bias circuit runs hot and tends to drift out of spec, both in the cathode and fixed bias revisions. The diode in the fixed bias revision is prone to failure.
The braid in shielded wires in the preamp is prone to rusting. The rusted areas tend to crack, this affects the shielding and leads to noise. Apexjr.com sells a very good one conductor shielded wire that I use.
A common issue is oxidation behind the input jack. This is the bus ground point and can make the amp hum. This point needs to be inspected and cleaned. Often the locking washer is so badly corroded that the plating is gone and it needs to be changed.
Baseed on my experience, the PEC tone control modules can have issues and should be inspected. They are an encased network of ceramic capacitors and carbon composition resistors. The tolerance of the resistors were +/- 20%, 1/5 Watt. The capacitor tolerance was +50% and -20% in the 1000mmF range, and +/- 20% in the 100mmF range. The working voltage of the capacitors was 450 VDC, 800 VDC flash.
Issues with these modules that I've seen include broken wires, cracks in the body, out of spec components. With two channels, ideally you want both models close to each other, they can be far apart. When a channel doesn't sound righ to my ears, it can sound muddy or what some call cloaked, changing this part can make the channel sound quieter and clearer. I don't condone changing them as a matter of course but sometimes it is necessary, even if the components read within spec
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Some of these amps do not have grid stopper resistors on the first stage input of one of the channels (channel 2, intended for accordion). It can help with RFI pickup to install one.
Check the isolation washers on the jack on the back. The EXT SPKR jack shoud have an isolation washer set to avoid a ground loop. Amps with speaker cords are isolated, if this has been modified to install a jack, I like to isolate the jack rather than ground the return on the chassis. I find that the speaker return wire is quietest at phase inverter ground point on the bus.
The beauity of these amps is that if they are healthy, they can be very quiet. The quietest that I've heard. The tone is amazing.
