Acoustic Control Corp Amp 260. Quasi-Complementary topology.
Q32 is a RCA 40409 NPN transistor. Some PBCs have the mod adding the 100ohm resistor to the base, some don’t. I have several units with and without, both function either way. What might be the purpose of adding in the 100 ohm base resistor to that location?
A different head with the R130 100-ohm mod.
Schematic: R130 upper right on base of Q32
It is a well known fact that BGTs are current driven devices. Putting a 100 Ohms resistor in series with the base will therefore reduce the current to the base of Q32 as a form of added protection and in case of stronger audio signals, preventing it from going into overdrive and/or (potentially) breaking into oscillation. One of the reasons also being because the amplification factor in BGT transistors (generally speaking) wildly differs from one piece to another. So, in my opinion yes is a form of protection. Also another advantage is that when you drive a transistor with a reasonable signal level, in turn the distortions will be lowered as well so is a “win-win” situation. If other people have other opinions/ideas well… I can’t wait to hear them please.
Side note: seeing the way the resistor is fitted as a “piggyback fit” it seems to be an after thought from the manufacturers part (if it is an original factory fit). It could be that this happens at the testing/final inspection stage (of the product) before the release to make sure that the final amplification levels/specs are met.
It’s defeinitly a later on design mod. From my understanding about the company, these ACC 260 amps were never really out of the design/engineering phase. Made popular through a promotional phase during the height of the The Doors touring. They are rare to find in working order and have a bit of a collector community.
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