May be wrong but what I saw is a single channel 30W Class A amplifier, 2 required for Stereo, plus larger power transformer and *huge* heatsink, maybe a fan too.
Not too guitar friendly. 

I used to have that schematic some 3 computers ago, it´s probably on some old hard disk.
If anybody has it, please post it here, otherwise "R4,13,19,20,22,24,26,27,28." and others mean nothing to us.

In any case start building a lamp bulb limiter, you´ll need it , search this very same Forum for it.

Quote from: g1 on September 29, 2017, 01:37:22 PM
Yes, heat.
In this case, the heat is a result of the power going to the speaker.  Voltage squared, divided by the resistance of the voice coil.
Why does the power from DC burn the coil, when AC voltage usually does not?
DC is constant polarity, so the coil sits in place.  The usual heatsinking of the magnet and polepiece when the coil is moving from AC does not take place.
Often the DC is even big enough to push part of the coil out of the gap, and that part will burn first.

Fresh voice coil on right, burnt on left.
Notice the back area looks almost normal (it stayed inside the gap), but forward one which was pushed out (speaker cone pushed forward by positive DC) is toasted:



same but higher power amplifier involved:


notice burned glue and insulation and wire on its own travelled even forward.

Thanks Sajy  :dbtu:

Now we need some pictures of the completed build and the cherry on the cake would be some tasty guitar playing demo  :dbtu:

I bet the schematic is fine, oscillation problems come from poor layout or shielding, poor grounding, etc.

Yes, between 4n7 and ground.
You will have a huge volume difference.
You might use a "big Muff"  type control, which gives you huge range with a single knob, check the TSC tone stack simulator for suggestions and adjustments.

A guitar pickup can only drive the crudest tone control: a capacitor to ground killing highs, so for anything more sophisticated, you need some active electronics.
Meaning it should be *after*  the first gain stage.
As of making it switchable, be my guest

QuoteJM can you please clarify this. When data sheets (and I quote the LM3886 here) says things like:

"68W cont-avg-output power into 4 ohms" or "38W cont-avg-output power into 8 ohms"

what does the term "cont-avg-output power" mean?
is that their way of saying 68w RMS or 38 RMS?

They are speaking with the Dictionary in hand, which is fine with me.

Quotecont-avg-output power

is what average people on the street (and thousands of brohures, magazine articles, user manuals and probably even some books) calls "RMS power".

Strictly speaking there is not such a thing as RMS power, but RMS voltage or current.

That said, what everyday talking "RMS power" actually means (or should) is "power calculated using RMS voltage, sustained during a reasonable time"

personally if if I check an amp puts out 20V RMS, when fed some classic test tone, usually 1kHz or 400/440Hz , into a 4 ohms resistive load, without visible clipping and for at least 10 minutes, would prefer at least 1 hour, then *I* won´t argue at all if you say it is a "100W RMS amplifier".

There´s been tons of arguments lately arguing gramatics while the real problem lies in the monstrous fantasy claims posted everywhere using undefined, pure fantasy numbers.

Arguing against a time tested, **easily repeatable and consistent** system which has been used for decades and does not require hard to find equipment , difficult Math and on which most agree (or have agreed since forever until the latest Revolution)  sounds stupid and dangerous to me, because after dissing and putting out of the way a stable and repeatable test procedure, anything else goes.

FWIW the FTC itself has dropped **officially** any pretense of measuring or norms enforcement.  :grr

Dear ILYAA: don´t know where are you from.

If USA, and within warranty period, send it back, period, and get a real JTM45 or build one out of a Ceriatone kit, a suitable prebuilt cabinet and your choice of speakers , no need to make everything from zero up.

If in  Country with no *real*  access to Factory resources and where getting a full refund is a problem (such as here in Argentina), then it may be worthwhile trying to reform it, but it won´t be a standard repair job , it will be reengineering , and definitely expensive. (not in parts but in applied knowledge).

Don´t understand why Fender Authorized Service could not repair it or worst case get a replacement unit from Factory.

The 8085 circuits schematic makes not sense and the half covered one is unavailable, Roly too
Just look at the impedance correction networks in any practice TDA2030/2050 Guitar amp (Peavey/Fender/Marshall/Crate/Laney/etc.) and use the same with the LM3886 , they will work properly

All these chipamps are "powerful Op Amps" , just varying in supply and power output, same design rules apply to all.

Google "power scaling"  and read what you finf, there´s various schematics posted online.

Usual (and the real thing)  varies the whole +V voltage, tube plates and screens , plus to compensate also varies fixed bias.
Of course MosFet dissipates a lot of power, gets very hot, etc. really needs a good heatsink and a fan, tube amp guts are very hot .

Cheesy versions *just*  vary screen voltage, (plus tracking bias), way cheaper, less dissipation, in my view they miss the boat.

So first read about the full one, then the screen controller, and compare that to what you have.

Worst case you will be able to bypass it, feed plates straight to +V (I think you already have that) , screens also to +V but a secondary one,you probably will need to add an extra RC section (1k 5 or 10W plus a filter cap) for them.

Can´t get into much detail lacking the proper schematic but that´s the basic idea.

Although I´d personally send that back to Fender and ask for a refund, since they can´t make it work. Period.

Nominal 8 ohm input tanks have DC resistance about 1 ohm, just check Accutronics/Belton site.
Typically transformer driven by a 12AT7 1W power amp in old Blackface/Silverface Fender, in fact you could test reverb drive by hooking a speaker there, but I remember Roland used same "speaker impedance"  tanks driven by a Car Stereo type chip amplifier, just look at the schematic or inside your chassis, you will find it.
Modern SS amps use mid impedance tanks (some 200 to 600 ohms impedance, DC resistantce 10X smaller) which can be driven straight from a humble (cheap) TL072 .

Seller claims are wild unbelievable, from 120W out (out of a max. +24Vdc supply) :loco  to being able to be powered from USB (lower voltage than minimum acceptable) to being powered from AC (I´d LOVE to see that).
That said, it *should* work like a plain TDA2050 single supply datasheet example, so "just trust the datashet, not the seller".

Post here the preamp schematic you want to use with it.

If powered from 9V, you can easily get them from your +18V supply, all it takes is a resistor, a 9V Zener and a 100uF x 25V cap.

Nice project, now you must hunt for a suitable speaker.

Link to some Aussie site selling speakers to help you choose one.

Basically light, thin paper cone, like those cheesy 6 x 9 speakers old Falcons used to carry as factory installed ones in the 70s; you do NOT want "woofer" anything which will be dull and inefficient with guitar.

Or hunt pawnshops or Aunt Edna´s attic for old "home console" speakers, killer sound with guitar.

Quote from: zadmight on September 11, 2017, 03:52:03 AM
I did it but the result wasn't what I expected  ... the sound is still pretty "cold"

I roll back and then start building a Guitar Tube Pedal... Hope to get some more warm sound this way

Ouch !!!   
Thanks for experimenting and taking your time to share  :dbtu:
Problem is (IF that´s a problem) that Randalls are designed for that Pantera like brutal sound, quite the opposite of "warm",  so modding just one section will help somewhat but not fully.

Our friend Phatt has experimented a lot on getting good warm and punchy sound out of SS amps, his main point being a clever tweaking of bandwidth and frequency response to keep warmth and lose harshness, so take your time to search and read his posts, and listen to some samples uploaded here, it is very much worth it.

KMG has also uploaded killer JCM800 emulations, also search and listen at them.

Maybe you should try some of those projects, and keep your Randall as a power amp and speakers platform to amplify them, bypassing its own preamp.

Dunno, you tell us  :tu: