Sorry for the delay but here's what I got from my guy at TI. He's not an actual employee as far as I know, more of a paid consultant. The TI engineers are talking about those designs where you use two chips in parallel or two chips in a bridge drive configuration. I think Marshall has a design that uses four chips for amp. Hard to keep working blowing up chips all the time.

Yea LM3886 is back in stock. I've printed a few PCB batches
for a guitar and Bass power amps to build with them but after
talking with TI engineers they steered me away from them to
use Class D. I've yet to build any amps with the LM3886.  They
pretty much implied it was designed for single module application
or dual in stereo and not recommended for multi modules designs
to double the power. They said rail voltages needed to achieve the
result I expected would exceed the maximum continuous voltage
and cause them to fail. So I get they exaggerated the true power
output and adding additional chips don't increase total power
output in expected quantities. They also exaggerate about the
Class D chips. In reality the advertised output is at 2 ohms and
maximum rail voltages before it burns!! So if you want 200 watts
in the real world you must get 700watts to come close.

Quote from: J M Fahey on December 30, 2024, 07:31:41 PMI love LM3886 but they are out of production for years now.

Only non original ones freely available.

I don't think this is true. There was some availability issues around the time of Covid, and there was a switchover from National to TI when TI bought National, but I think TI is still cranking them out. I will check with a guy I know who works at TI, may take a day or two.

That's not to say there aren't fakes out there. Like always, buy from a known good source like Mouser or Digi-Key.

I was thinking that a Cliff type jack might briefly short the cable as the Ground contact of the jack slides over the insulation between the tip and sleeve of the plug.

A few words about the need for short circuit protection. In addition to the bad speaker cable as mentioned by J M Fahey above, there is the very real possibility of a momentary short occurring when using Switchcraft brand jacks (or similar) on the speaker cabinet. Cliff type jacks don't have this problem.

Most Guitar amps use Phone plugs to connect speakers. When a cable with Phone plugs is inserted into a Switchcraft jack, a momentary short occurs across the cable. If an amp is Powered On and you first plug the cable into the amp, no problem yet, until you plug the cable into the speaker cabinet. The amp sees the momentary short and could blow up. Best to leave the amp off while you connect the speakers or always plug into the cabinet first, then the amp.

Those 60s solid state Thomas-Vox amps didn't have short circuit protection. How did they get away with that ? First, they used XLR type connectors for speaker connections. Those won't short the signal. Second, they used an Off - Standby - Play power switch that didn't connect the amp to the speaker until you were in the Play mode. Pretty clever

The transistors on the layout have Q numbers, but not on the schematic.

The driver transistors can be spun around swapping Collector and Emitter because of the single inline footprint on the PCB. That makes a lot more sense.

It would be nice to know the C+ and C- Voltages.

The circuit looks strange, I suspect it may contain mistakes. I can see where one transformer winding powers the 12.6V tube heater and goes through a Voltage Doubler and could produce roughly +/- 15V. What is strange is the other winding has no center tap shown but drives two 4700uF 50V caps in series with the center grounded.

Another strange thing. See the output transistors. Looks pretty normal, NPNs on top, PNPs on bottom. Now look at the transistors just to the left. See how the NPNs are driven by an NPN transistor, but it's emitter is connected to the C+ rail. Same with the driver for the PNPs, a PNP transistor with it's emitter connected to the C- rail. Is current supposed to flow backwards through those transistors ?

There are surface mount fuses that look like a small dowel slightly smaller than the body of a 1N4007 diode with the leads cut off. Easy to remove if you have two soldering irons. Next, clean off the pads with solder wick. Add a small lump of solder to one pad. You'll need tweezers to hold the part, melt the solder on one end and position the part in place. Solder the other end, add solder to the first end if needed and you are done. I recommend you clean up any flux with flux-cleaner or isopropyl alcohol.

Quote from: Jazz P Bass on November 21, 2024, 06:11:07 PMTo try and prove whether or not the power switch is the issue, turn the power switch to On & then plug the power cord in to the outlet.

Did you do this ? What were the results ?

Quote from: RG100ESROX on November 26, 2024, 02:22:31 AMQ: Should I be concerned with the millivolts across the .62R resistors while biasing the amp?? Does this matter?? What's considered to be too high of a voltage (if there is?)

This is a long thread and I don't want to contradict anything from members who might have more experience with this amp. I have none. This amp is a little unusual in that one pair of output transistors have the 0.62R emitter resistors and the other pair has 0.27R resistors. The first pair act like driver transistors to the second pair that act more like a Class B output stage.

Second point, going by the schematic in post #7, there is no info regarding biasing from the original designer. We don't know where the bias was set when the amp left the factory. If we did, I would go with that. Without guidance from Randall, I would start with 100 millivolts across the 0.62R to see how that looks with a dummy load. A crossover notch might not be exactly at the zero crossing but at a slightly higher Voltage where the second pair of output transistors turn on at around +/- 3V.

Quote from: RG100ESROX on November 26, 2024, 02:22:31 AMOr am I only concerned with the notch in the sine wave?? Which there is very little of at min bias adjustment.

The problem with bipolar transistors is that if the bias Voltage on the Base it a fixed Voltage, the transistor will draw more and more current as the transistor heats up. In a power amp, the output transistors are going to heat up, they can get so hot that even with a large heat sink, they will get too hot and fail. This is called thermal runaway. Special circuits have been devised to keep the current under control as the transistors heat up. These circuits include the thermister and the diodes near the bias adjustment.  Keeping the bias as low as possible while still eliminating the crossover notch is the goal.

Quote from: RG100ESROX on November 26, 2024, 02:22:31 AMAlso, I used to get -.6v on the bottom lug of the trim pot which I don't have anymore. It's now a positive voltage. Starting at .876V at the minimum bias setting. It only increases with the adjustment of the bias pot.

Without knowing exactly how the amp is wired, I can't comment on that and I'm not looking back on a 13 page thread to find that info.

Quote from: RG100ESROX on November 26, 2024, 02:22:31 AMI also have a bad hum that gets worse as the bias is increased.

Where is the hum coming from ? Do any of the Volume or Gain controls affect the amount of hum ? If you ground the signal at the Effects Return jack, does the hum go away ? If it does, suspect C45. If it doesn't, suspect C38 and C39 or possibly C31. If that doesn't fix it, suspect any mechanical ground connection. Tighten any screws or nuts where things are grounded to the chassis. Cables that go between PCBs might need to be jiggled or give them a shot of contact cleaner.

Bias current is usually adjusted with no signal, then look at the output with a scope and a small signal to confirm there is little to no notch where the sine wave crosses zero Volts. Sometimes a dummy load will disturb the DC readings across the emitter resistors in the power amp if there is DC Voltage on the output. In that case, disconnect the dummy load before adjusting the bias. Then reconnect the load to check for crossover notch. In some cases you might adjust the bias with a small signal to observe the crossover notch and set the bias to the lowest value that eliminates the notch. If you are really diligent, you might check the bias when the amp is hot after being played for some time to see if the temperature compensation is working as it should. You should see no notch, but bias current may have drifted up or down with the change in temperature. That's OK, don't adjust the bias unless you see a gross notch.

Clipping is when the input signal is too big for the power amp to reproduce without distortion on the peaks. The peaks of the sine wave are "clipped" off so the wave starts to look more like a square wave. This insures that the transistors in the power amp can withstand the Voltages that they might see in normal use, not just the Voltages with no signal.

The next test is with a dummy load. This test makes sure the transistors can withstand the current required for the amp to produce full power. If the power amp can pass this test, it us usually good to go.

Myself, I would look at the output with a scope and drive it to clipping with no load. If the amp survives, try it with a dummy load resistor. If that's OK, hook up a speaker and Rock ON !

Those Voltages in the preamp are close enough for Rock and Roll. About as good as you can get with JFETs and fixed (not pots) resistors.

The one with the smaller cap is providing a treble boost. The one with the larger cap sets the low frequency response.

You can't really expect anyone to reply without knowing exactly what model amp you have. A quick ooogle search shows at-least a dozen models. With only a 12V supply, my guess is it's probably a Class D design. If Fishman won't fix it, trash it and buy another.

Edit Oooge shows no "Loudmini". There is a Loudbox mini, but it has an IEC connector for a line cord. No input for a 12V supply

Edit 2 Ok, found it. Its a Loudbox-Mini-Charge. Uses a 12V 5A power supply. Same advise applies. If Fishman won't fix it, trash it and buy another.

What a poor design. No gain from Q1. Q1 is a switching FET not intended for Audio. High Impedance tone stack more appropriate for a tube amp. Low input impedance inverting opamp stage loads the tone stack too much. Power amp is OK but 100K input is higher than recommended for that chip.

Looks like it was hacked together by some internet hack without a clue.