That´s why you must post pictures even from burnt stuff.
Model ends in "1N" and I seem to recognize the Signetics "S" .
That plus "16 pins" and "compressor"  makes me think of Signetics NE571N (now NXP)
Check that datasheet pin function matches that in your amp:http://music-electronics-forum.com/attachments/33954d1431700495-ne571-compander.pdf

Quote from: vidmachine on March 30, 2017, 12:16:43 AM

Quote from: g1 on March 29, 2017, 10:53:11 PM
Measure with DC millivolt range.  One probe at each end of resistor.
Post your results for each and specify whether .6 or .27 resistor.

I put the controls all at noon, no signal plugged into the input, reverb off with footswitch, and both channels on with fs-5 footswitch

Put all controls to 0 , we don´t want any signal at the output, not even self generated hiss or buzz.

QuoteLooking at the picture I posted starting with the top left .6 Ohm
.6 = 3.3 mV then runs to zero
.27 = 106 mV holds steady

Weird, according to schematic you should have 30 to 50mV across the 0.6 ohm ones, and *zero* across the 0.27 ohm ones.
Maybe somebody pulled them and replaced the wrong way ? 

Quotenext bank next to the molded connectors
.6 = 6 mV then runs to zero
.27 = 83 mV holds steady.

Same here, we expect 40/50mV across the 0.6 ohm ones and zero across the 0.27 ohm ones.
In any case we do NOT want more than 50mV across *any* of them.

QuoteI have adjusted the trim anywhere from .550 to .650 Volts.

550/650mV *where*?

QuoteAnd there is a sweet spot that sounds pretty decent, but I can feel the back heating up a bit more than yesterday.

You must NOT repeat NOT adjust bias "by ear", not even by bias voltage, only by measuring transistor current which is voltage drop/emitter resistor ; in this case 50mV across 0.27 or 0.6 ohm resistors means around 185 or 80 mA through each transistor, whih is already a lot and should never be surpassed.
In fact I would be happy with half that.

QuoteMy initial reason for the investigation started when I noticed the Bass was sounding loose and flabby.

Maybe true but that is absolutely unrelated to bias, so do not mess with it.
And in any case you are supplying values opposite to what is expected.
please reread as suggested above.
EDIT: measure bias with NO speaker or load connected.
Now that you are there, also measure DC offset, from Spk out to ground.
We expect less than 100mV DC there.
And recheck that the 0.6 ohm and the 0.27 ohm resistors are in their proper places.
Even if parts are not labelled on the PCB:
* R55 0.6 ohms must be connected to emitter of Q13 which is the one which has its base conected to Q11 emitter and 100 ohm R53 . Follow wiring to confirm this.
* R57 0.27 ohm is conected to Q15 emitter whose base is connected to R55 and Q13 emitter. Confirm.
* R56 0.6 ohms must be connected to emitter of Q14 which is the one which has its base conected to Q12 collector and 100 ohm R54 .
* R60 0.27 ohm is conected to Q16 emitter whose base is connected to R56 and Q14 emitter.

QuoteI changed the filter caps to 6800 uF - 50V cans.  Some of the ever-present hum cleaned up, but the bass was sounding off.

If hum lowered then new caps work way better than old ones.
If anything, Bass should improve.
If you still don´t like it, at least now you are certain that it´s not a supply problem any more.

QuoteIt was suggested on the thread https://www.ssguitar.com/index.php?topic=4201.msg33508;topicseen#msg33508 to change a few capacitors that I haven't had a chance to swap.

*Maybe*, but your sound as shown in your mp3 is killer as is.
If anything, it might be more of a "taste"  problem than a "tech problem"  one,and it´s difficult to argue on taste, I won´t even try.

The old schematic you found belongs to the "real" Sun Coliseum 300 from the 70´s ... and it never had compression, nobody did way back then.

IF it does, then it´s something else, a "reissue" or something a new brand owner made to cash on the name.

IF made by Fender, they bought the Sunn carcass but continued only with the PA equipment, *might*  have made some Bass amps, then they should have a suitable schematic, ask them i any case.

We are in complete darkness here, at least post some pictures, including any "Made by / Made in" labels, some gut pictures and some closeups showing any labels on the boards themselves, both silkscreen and copper side.

That said, "compression"  and "16 pins"  in the same phrase, make me think of LM13600, a very popular gain controlled OTA:
http://www.b-kainka.de/Daten/OPV/LM13600.pdf

For starters check whether datasheet pinout matches what you have on PCB or not.

That said, a cracked IC makes me suspect supply overvoltage or some similar gross problem, check you have proper +/-15V rails or whatever that preamp uses.

FWIW that amp might be a generic Korean OEM amplifier with a Sunn label glued to it.

Of course those sound good !!!
Excellent cleans and for dirt there´s 1000 pedals to choose from.
But tubeheads were angry when these appeared and bashed them beyond belief.

Agreed, they were not Twin Reverbs ... but then which SS amp is?
Besides Lab L5 of course

Yes, gut pictures will help a lot.
maybe you´d like to put them side by side in an extra picture to compare space needs or availability; but is suspect the long narrow guitar preamp PCB is longer than TOA chassis width.
You *might*  also build a "fat Twin" sized combo: 2 x 12" side by side, a slot cut on top for the Silvertone preamp and the TOA amp solidly bolted in the bottom, you may even "unify" mains connections and use only a single mins cable and turning everything ON with a switch mounted in the Silvertone (I guess you´d have to upgrade it though)
You use just the Silvertone preamp and disconnect its power amp, you may even pull its chipamp, no longer needed.

Polytone amps were conceptually the same, a bottom mounted powr amp and a top preamp, with speaker(s) filling the available space in a very compact package.

of course, mount the Silvertone preamp up front like it is now, forget the all-you-can-stuff crazy fiberglass filling, and your bottom mounted TOA will be in its own enclosure, while this one is a bare chassis, no big deal since it´s a sealed enclosure, but you´ll probably want an open back or semi open back cabinet.

But although not very common today (it was all the rage in the 40´s) , the idea is practical and thousands of Polytones keep working today, some over 40 years old.

*Most*  footswitches , specially old style such as those, work by grounding the tip and sometimes the ring connector in a plug, in this case one or both "ears"  to ground.
This picture I found shows a 3.5mm "headphone"  one but principle is the same with a 6.35mm guitar one which I bet your Yamaha uses.
So get a 1/4" (6.35mm) stereo guitar plug, pull thye plastic cover, insert it in thejack you are testing and alternatively short either lug to ground with a piece of wire, and check for yourself what happens.

Then add wire as needed and a footswth at the other end, so yo again ground active lugas at will, just with your foot.

Then you mount it on a suitable, pedal type enclosure, such as the cast aluminumHammond ones or anything you can build.

No big deal as far as Electrinics, but you´ll have a *packaging*  problem.

Not sure the Silvrtone front panel and presumably full width PCB will fit inside the TOA cabinet and viceversa with the TOA power amp, power supply, heavy transformer and heatsinks.

personally I would build a powered guitar cabinet since ypu will need some cabinet anyway: build a 2 x 12" cabinet, maybe a Twin Reverb like side by side which will be some 65/70 cm wide, or one on top of the other, this time the same width of the Silvertone which I estimate in 45 cm wide, or thereabouts.
Same width (or slightly more) is needed so the Silvertone sits nicely as a "tall head"  on top of the 212 cabinet.
Then in the bottom of the cabinet backside you mount the TOA amp, as is, hardwired to the speakers, and add an input jack which goes straight bto the power amp input or some suitable Aux one.

That way for home practice or easytravel toa friend´s home you just use thye light and handy Silvertone, and when rehearsing or playing LOUD you also bring the powered cabinet and join both with a Guitar cable. Of course each will need its own Mains connection, no big deal.

I suggest this because for Homebuilders the main problem is not the Electronics side but the "mechanicals" .

c=a

Killer sound as is   :dbtu:

Did it sound "better"  sometime in the past?
Who knows?
Very much doubt so.

*Tubes*  age, every second they are ON some atoms boil off the cathode coating , eventually said chemical coating gets depleted and tube is too weak to function properly.

In the long road between fresh out of the box and the corner dumpster, sound is changing all the time, while Transistors either work or die.

So power transistors are the last in a LONG chain of suspects, including strings, pickup settings, cables, ambient humidity and temperature, other instruments playing along, wall voltage and 1000 more, not forgetting signal generator (Musician) or Meter Microphone and processor (Musician´s ears and brains).

Agree and add:

Quote
Someone suggested the output transistors?

WHO suggested such a thing?
Certainly nobody here.

Sounds like the typical average Forum answer, by somebody who has absolutely NO clue, but has itchy fingers and MUST write "something" , no matter what.

A variation of the classic: "it must be some thingie in there" 

Quote from: vidmachine on March 16, 2017, 11:36:30 PM
what are the symptoms of a bad transistor?

It doesn´t work that way.
You tell us what is the actual problem with your amp or describe its symptoms, then we proceed.

Not sure about what you mean by "stompbox" 

All over the Civilized World it usually means an Effects Pedal (Distortion/Chorus/whatever) but in wild Australia  :lmao: it often means some kind of percussion unit, a (literally) "box which you stomp/kick" with your feet, with usually a Piezo microphone glued to it, although the even wilder version uses a standard small speaker (say, 2" to 6") as a pickup, coupled to a reverse wired power transformer (say 9/12V to 220V) .... so which one are you referring to?  :cheesy:

One real world example, about "fabled tone" .
One Music Electronics Forum member, John Frondelli, runs a mythical New York Service Shop, where *all*and I mean *ALL* stars, nopt only Rock but everybody else (including Disney Studios, all NY Studios, Oscar organizers .... up to and including the Navy band and CIA  ) send their stuff to be repaired, every month a truck stops at their door and blocks the street with stuff from, say, Megadeath or Prince or whomever is going on World Tour and wants everything checked for safety, nobody wants to rush find a Tech 8000 miles away from home.

One day he said U2 had left their truckload of equipment, and everybody asked him about a fabled Fender Princeton which allegedly had been used to record all famous band Hits.
Intrigued he checked it, since in his eyes it was "just a normal Princeton, bought at a regular store and never modded" yet it was labelled with Sharpie on a piece of paper strip: "repair but do not modify".

He checked it part by part, schematic in hand, ticking one by one what he found "normal", all he could find was that it was exactly the same as all others (at least nominally) with the single exception that a .022 coupling cap had been replaced by a .01 one.

Not a Mod, it was there from Day 1 , put straight at the Factory.

Net effect was that it cut a little Bass, so when overdriven, distortion was slightly cleaner than all others; in any case a subtle effect, not day and night.

In another Tech Forum, a member used to be a Fender VP for some time, and was asked about it, "what were they experimenting or trying to achieve and why weren´t all other Princetons built the same way, given it "worked better"

  His answer? : "don´t read too much into it, probably that day they were out of .022 caps, they *had*  to ship out some of them, and all they had on hand was .01 caps" 
Since apparent effect was very small and still acceptable, so be it.

This matching what Enzo said about amp to amp variations, in this case because of an actual value change, not just tolerance spread.

Clearly The Edge tried a couple Princetons at some shop and bough the one he liked most

Just to check the *awesome*  customer list who relie on that particular shop:
http://www.dbmproaudio.com/customers.html

Short answer and backing what was said above: do not touch it.

I thought it was a little 1x8" 15W job or something equivalent, but it´s a mid sized single 12" 40W amp.

be certain that Fender Engineers whoare no fools, already milked that cow to the last drop, neither you or any of us won´t casually "improve" it.
If a hole in the back could improve sound they would have done it themselves, after all it costs nothing.

You want to *really* improve sound, make it sound louder/better, with more projection*

Replace the cheesy (but adequate) Chinese speaker it has now with a Celestion Vintage 30 and stand back.

Or add an extension jack and whenever possible plug it into a Marshall 4x12" cabinet or equivalent ... and stand back.
You´ll get something like *this*:
https://www.youtube.com/watch?v=eNHdJmsfdMM

Mmmmmhhhh, that´s a very old amplifier, your internalpicture shows almost nothing, not even a PCB.

Pull that chassis out in the Sunlight or equivalent and send us a couple sharp and defined pictures (the back one is somewhat unfocused, while the front one is perfect) , *maybe*  we can "detect" where preamp ends and power amp starts, so you can add a couple (or single switching) jack there.

That said, you won´t get more power than what´s available there, you can test different preamp sounds though.

We´ll wait for our friend Galaxiex to chime in, he´s a fan and expert on old weird SS amps like that one
Just warn you that even he had to hand draw the schematics for the ones he bought