DOn't worry about the voltages.

STart with making sure power amp in jack si OK.  Patch a cord from preamp out to power amp in and see if it helps.

Put a signal into the power amp in jack.  Is that OK, or does it react the same way?

I'd be betting your output stage is not running on all cylinderrs.

Well, first things first.  How are the batteries?

Popping at turn-off is not generally considered a "problem," and often is not reasonably eliminated.  Sure one could engineer in some sort of speaker relay and get it to drop out before the power supply discharges.  But in reality, that pop cannot be any larger than the signal peaks - it is limited by the power supply voltage.  SO unless the speaker was underrated for the amp in the first place, it can;t hurt the speaker.

I'd have to wonder about that third LM1875 too.  Are you sure the third one doesn;t have only three legs and say 7805 on it?

You can copyright your schematic though.  Redraw it and you are OK.

All the TIPs will have the same pinout.

Did you leave out the mica insulator under any of them?  Go down the row of power transistors and see if the tab is shorted to the heat sink.

Really?

I looked in Mouser and all of the ones I saw listed were from NXP, and they were all non-stocked types.  Minimum order was 400 pieces.

Did you see something  different?

I have no idea what Crate sells them for.

www.bdent.com has them in Phillips brand for $16.

www.digikey.com lists them for $10 each, but shows none in stock.

You could also google the part number and I bet a number of suppliers would show up there.

And try your favorite parts sellers.

If DC offset is the problem, I'd bet my lunch money your power amp IC is bad.  After all, it is the entire powr amp.  is it a TDA1562?

None that I know of.

Did you buy it new?  If so, it is under warranty, and they will replace it.

If you did not buy it new, what is different about your amp compared to the FM212R.  As far as I knew, they were the same amp.  FM = Front Man.

The signal is so much larger on the bass.  100 watts is screaming loud for a guitar amp.  But 100 watts is not much for a bass.  When a player has a 1000 watt bass rig, it isn;t about how freaking loud it can be - he might not be any louder than a 200 watt rig.  But that 1000 watt capability allows a clear effortless sound to emerge from the speakers.

In a way it is like pulling a large travel trailer down the highway.  You can do it with a little Volkswagen bug or with a large powerful SUV.  One will struggle to do the job, the other won;t struggle at all.

SO when making a very low power bass amp, you must keep the limitations of the system in mind.

teemuk is certainly correct.

Not only that, but Marshall schematics are readily available.

A signal generator is fine and all, but you can use your guitar, or play a CD through the thing for a test signal.  Use your meter set on AC volts to check levels.

You tell us that Spice says there SHOULD be 600mv after the tone stack, but you don;t tell us what you actually have.  What do you have at the output pin of the first stage?

Clipping in the signal path is not chip failure.

You didn;t show us the actual schematic of your 2030 stage.  My first thought was your feedback circuit from the speaker out was not working.

What signal level is actually getting to the power amp in?

You can also use a voltage doubler/ tripler/multiplier circuit running off the same power transformer to generate a higher voltage for a tube.  Your common tube preamp stages only draw a milliamp or two.

If you use common tubes like 12AX7s in your preamp, each tube has two triodes in it.  The heater in each tube serves both triodes and can either be wired for 6V at .3A or 12v at .15 amp.  That is per tube.  How many tubes will you have?  If you have three tubes, then you can run their heaters in series wired for 12v each.  That makes 36v, and you could just power those heaters off your existing supply.  it wouldn;t be much of a strain to come up with a similar plan for two tubes or even just one.

In solid state systems, yes, the preamp is the main difference.  The power amps all tend to be full range and clean.  Bass and PA amps tend to be more powerful than guitar amps - who needs a stereo 800 watt per channel guitar amp?  IN a bass amp, 1000 watts is more about effortless clarity in sound than abiout sheer loudness.  But other than power levels, the power amps are all similar.

Preamps are the difference.  Guitar amps are meant to have a sound of their own, they are not PA systems which are meant to NOT have a sound of their own. All PA systems ought to sound the same.  They may not sound alike, but not because they weren't trying.  Guitar amps are part of the instrument.  They are fundamental producers of sound.  PA systems are designed to faithfully REproduce sound.