This is a cool little solid state bass amp, I'm guessing dating to the 60s. No idea if Silvertone manufactured this, or if it was a rebrand, as there is very little information about this model online.
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| Solid State Bass 35 |
No output signal. Only 5 transistors, so surely this would be easy? Instead of getting through this quickly I had it for several months, only looking at it when I had a few minutes to spare. If I had more space to work with this could have been much faster and easier. This has been a trend with physically large gear, I should really think about learning a lesson and getting a wide bench and keep it clean somehow.
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| Single sided phenolic PCB |
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| Full of cracks and mild corrosion |
This amp has been worked on in the past, and has some of the electrolytics replaced. It was also dropped on it's faceplate at some point in the past, and the phenolic PCB has cracked. The ground pour is broken in a few places and it has been bridged with solder.
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| Original output transistors. |
Most of the transistors appear to be branded with house-markings, as I can't find any references to their part numbers. I initially though output transistors were bad. They are marked "761" and "CG121", no info on these. One seemed to have a very low Vbe when measured with a DMM diode mode, which looked like a shorted junction. The TO-3 pinout is fairly standard for transistors, so I was assuming all cases were collectors. Collectors voltages were +35V and 0V, so I made guesses as to which output transistors were NPN and PNP types and then replaced with new 2N3055 and MJ2955 in TO-3 packages, new mica washers and new thermal grease.
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| Dead bass control/driver transistor |
After tracing the signal through the amp with a scope I found that nothing was getting to the output transistor bases. Nothing was passing the third transistor that drives the output transistors and implements the bass control. It was marked "5981 274 S6721", no idea. I desoldered this part and found that it was dead, completely shorted leads. I replaced this with a BD139. Based on a guess of the polarity of the original and the size of that package I thought this should handle the current requirements. With the BD139 I got some life, there was an output signal sometimes. Turning power on and off would sometimes get it working, sometimes it would stop. I found that there was a high-frequency oscillation at the output stage, if it drifted in and out of oscillation then things would work. Capacitors from collectors to base pins fixed this, values were just picked experimentally.
It still didn't sound right. There was an odd overtone riding on top of the signal that couldn't really be dialed out, though it sounded better at higher volumes. Another parasitic oscillation? Bias issue?
Around this time I bought a Peak Atlas DCA 55 Semiconductor tester. This is a very useful tool for testing transistors and diodes, and will also attempt to identify parts based on I/V characteristics. I still had the original TO3 power transistors, so I tried them out. One read a Vbe of 0.19V but was identified as PNP germanium, and working. I had assumed Vbe this low was a problem. After putting the old transistors back into the amp with new grease it sounded great, not very clean but not totally distorted either. The replacement silicon part wasn't biased correctly, the odd sound was probably some crossover distortion.
There is a diode from the base of CG121 which is probably used for biasing, I would guess that this is also germanium and modifying the amp to use new silicon transistors would involve replacing this as well.
Here are measurements of the originals, they are surprisingly mismatched. This may not be a push-pull design at all.
7671
NPN silicon
hfe 108 @ Ic 2.5 ma
Vbe 0.643V
cg121
PNP germanium
hfe 59 @ Ic 2.5 mA
Vbe 0.190V
Ib 4.781 mA
Hopefully this is good for another 40 years.
