Most amplifiers have a 20 to 100 pF cap from collector to base on that transistor. The TIP30 sort of has that built-in. The TIP30 will have more distortion because that capacitance is nonlinear, but you don’t have to add the capacitor. Its not really good enough for hi-fi, but the bar isn’t that high here.
The amp is also missing the zobel network - a 10 ohm with 0.1 uf in series, hung on the output. Sometimes needed for stability with no load.
The amp is also missing the zobel network - a 10 ohm with 0.1 uf in series, hung on the output. Sometimes needed for stability with no load.
> for stability with no load.
Or for inductive loads like loudspeakers. G-Amp speakers, even more than Hi-Fi speakers, have impedance rising toward infinity above the audio band. "8 Ohms" is often >50r @ 20kHz.
Or for inductive loads like loudspeakers. G-Amp speakers, even more than Hi-Fi speakers, have impedance rising toward infinity above the audio band. "8 Ohms" is often >50r @ 20kHz.
It´s a very poor design, barely adequate, and didn´t oscillate fresh out of the factory because poorish transistors didn´t have enough "muscle" and sort of self compensated.
Anything you improve may easily send it over the precipice.
But 100pF across 2N4402 BC plus a Zobel at the output should tame it, it´s what everybody uses today.
Transistor dissipation IS over max suggested, PRR's Math is right, but a beer can aluminum heat sink (I would add a dab of thermal grease betaween them) will certainly help.
Never saw original amp guts, but I bet they used a small clip-on heatsink there.
And you do not need to *strictly* follow original schematic, IF slight modern improvements are possible, apply them.
This is not the sacred amplifier used by Jimi Hendrix or Keith Richards in 1969 to record some ground breaking song, by any means, it was just an entry level amp, not even designed or made by Gibson themselves but by some subcontractor.
Personally I would use a TO126 or similar case transistor, which even no heatsink just standing on the PCB will easily supass the > 1W rating.
And the TIP30 transistor you tried even more.
Once you get it working properly, "stop fixing it" he he and start enjoying 😛
Sure enough, 47pF across Q3 BC straightens it right out. I had tried similar from collector to the high rail, attempting an output low-pass, without success. My intuitions about these things still aren't great.
The heatsink on Q3 (I did use some white thermal grease) gets hot but it no longer smells like the part might fail, and I think this sounds a bit cleaner than the TIP30 (though maybe the same as the TIP42).
Sounds actually nice with this 6x9 mounted in a tiny cabinet. Not a versatile amp by any means, but there's something nice about it for certain kinds of mostly-clean sounds. I'll post a video this weekend.
The heatsink on Q3 (I did use some white thermal grease) gets hot but it no longer smells like the part might fail, and I think this sounds a bit cleaner than the TIP30 (though maybe the same as the TIP42).
Sounds actually nice with this 6x9 mounted in a tiny cabinet. Not a versatile amp by any means, but there's something nice about it for certain kinds of mostly-clean sounds. I'll post a video this weekend.
Took me longer to get back to this than I expected, but I finally made a quick video: GA-5W demo - YouTube
The speaker is a Jensen 6x9 — smaller than the original, but the sound is similar. I tried to show off some of the mostly-clean but slightly gritty, rattly sounds that I like from this amp. Sorry for the poor video quality / sloppy playing. Thanks again for your help!
The speaker is a Jensen 6x9 — smaller than the original, but the sound is similar. I tried to show off some of the mostly-clean but slightly gritty, rattly sounds that I like from this amp. Sorry for the poor video quality / sloppy playing. Thanks again for your help!
Oh, I should explain the second input: it's an insert after Q1, so that I can experiment with FET and tube preamps and different EQ possibilities. I wound up moving the volume and tone controls to after Q1 (between C3 and a new capacitor going to the base of Q2). The insert is after the tone control but before the volume.