HAPPY to see Mouser now has them in stock, in large amounts.

It was not so for *years* , where they did not straight say "forget it" but gave ridiculous lead times for restocking.

Oh well, all is well that ends well.

In this case, it becomes again an option for a Guitar amp.

Personally much prefer the metal tab version

A slice of mica or a laminated silpad or even Mylar insulator will *always* be thinner (so better heat conducting) that some Epoxy or any other plastic that has to be *injected* into a die, and flow to fill it.

The OP can use that KT something board or any other tried and proven one.

There is a guy at DIY Audio called "Brian GT" who has been supplying high quality LM3886 boards for years now, hundreds have been built by Forum Members with success.

I love LM3886 but they are out of production for years now.

Only non original ones freely available.

Most are straight junk; *maybe* a honest chip factory (in Asia of course) cloned one or makes something which reasonably works, who knows?, but that is still betting on unknown horses, how would you know who is who?

For "just one" not worth the risk.

The real ones are made by TI; they are not "officially" discontinued but typically on heavy backorder because they have WAY more profitable products to make, think car, solar energy, electric vehicles, etc. stuff.

A hobby type (commercial factories have switched to Class D) market is not even a drop in a bucket.

There is a specific tread on LM3886 unavailability on DIY Audio.
Currently 29 pages long, maybe that means something

https://www.diyaudio.com/community/threads/whats-going-on-with-lm3886-availability.386577/

Now and then a few appear at Mouser or Digikey or Farnell, they are swept out within hours.

*Maybe* you get lucky, I commercially make amplifiers and can not work with that uncertainty.
 

Quote from: Tassieviking on December 28, 2024, 04:12:32 AMI think that PCB is not well designed, you will need a good heatsink on the power transistors but the transistors are right in the middle of the PCB, right behind the transistors where you should mount the heatsink bracket are 2 small footprints for 7 watt resistors.
Have I been asleep for centuries and 7 watt resistors have now shrunk right down in size ?
I would throw that PCB in the bin, I would never try to build that amp since it will undoubtedly lead to heartache.
You can buy a 100 watt class D amp for less then the cost of the components you need to complete it, and the transformer will be expensive as well. There are so many better and cheaper options out there.

Fully agree.

Checked KM Tech site out of curiosity, thinking it might be a Chinese site, it reeks of it.

To my surprise, it IS UK based, but otherwise the description fits .

There is a Business Model based on ordering stuff in China , wholesale, and reselling it.
PCBs can be ordered from fabricators such as PCB Way and others for peanuts, think 10 boards for $5 or so.
https://www.pcbway.com/
https://www.pcbway.com/orderonline.aspx?x=100&y=100&num=10&Layersquote=2&Thicknessquote=1.6

KM seems to be a hardworking PCB design office, churning out tons of PCBs.
Their catalog shows over 200 different designs, they have sold over 20000 empty boards, so the Business model is working.

Most are for the Retro Gaming market, think Amiga, Playstation, Atari, Amstrad, etc. but they also offer some Audio boards.

Most look reasonable, but when being so broad banded sometimes a lemon sneaks in.

Their LM3886 power amp looks fine, the Chipamp is "heat sink mountable" along an edge, they show a populated picture, proof it is "buildable":





Now the "150W Darlington Amp" looks like it was hastily thrown together.
It may meet the "electrical" connections but layout is ludicrous, the thing is unbuildable; so much so that they show a *simulation* where to boot Power Transistors are NOT inserted 
Because they can´t.



Bonus points:
* as noted above, real 7W resistors will NOT fit there, no way.
* speaker ground return track is impossibly thin, it will explode on the first drum roll or something.

Funny notice: LM 3886 board has the "Use heat sink" warning printed on the silkscreen side ; "150W" one does not even mention them. 

That said, IF you already bought it and have no option, it *may* be built, sort of (assuming design is fine, of which I am not sure) by mounting TIPs on a heatsink proper, and running wires to PCB holes.
Same with 7W resistors.
But why bother to correct a botched board design which to boot is also poor electrically?

If you want the hands on build challenge, fine with me, just get the Velleman board, either as a kit or on its own, use your own heatsink and transformer.

If you find another kit that looks suitable, feel free to post/link it here so we can have a look.

You are lucky: exactly that same basic amp but properly made (not much more added, yet ...)is offered in kit form with an *excellent* 20 page assembly manual.

Enter the Velleman k8060.

Proper biasing, short protection, very flat assembly (board,  supply,  power transistors at the ends) make it easy to mount inside a Guitar amp chassis or a powered speaker back aluminum plate.

Circuit:  (eerily similar to the crude one):



Full  manual :  https://cdn.velleman.eu/downloads/0/illustrated/illustrated_assembly_manual_k8060.pdf

from:  https://www.velleman.eu/products/view/?id=360242

They even offer a heatsink, custom drilled or not, and a suitable power transformer.

Of course you can buy only the PCB and parts kit and mount it on your own heatsink or aluminum chassis and get a cheaper and maybe closer to home ANTEK toroid.

Anything from 26+26 to 30+30 VAC, 2.5 or 3A will do fine.

Amp marketing is "optimistic" as usual, forget the meaningless "200W peak power", it has honest 70-80W RMS into 8 ohms, which is perfect for live use.

Use it as is, crude but does its job.

Later you can build a better one but it´s fine for experimenting.

It´s somewhat on the edge, use +/-40V rails or so (30+30VAC PT, 2,5A), 8 ohm load, beware it has NO short protection.

No big deal in a Combo or powered cabinet, somewhat dangerous on heads because someday you WILL meet a bad cable.

Power output is similar to a Bandit, VS8080, Crate or Laney 60, etc. , enough for playing around with a good speaker, say an Eminence or Celestion or modern Jensen

Not HiFi, Car, Home Theater class speakers.

Use good heatsinks.

Maybe.

Way back then FETs were a newfangled premium component and deserved expensive metallic cases.

Today?

Apparently not worth even a humble plastic TO92 case because these are being discontinued, everything is shifting towards SMT cases, SOT23 and similar.

In any case, what matters is pinout; if you need to do some lead bending, so be it.

Different pinch-off voltage?
Sure, but in any case switching line applies 14-15V to gates, way above any pinch-off voltage, so they are forced to work.

I suggested testing the Distortion FET switcher, basically to *measure* voltages around it and take that as a reference.

All others must work about the same.
Best FET in the World will not switch if proper voltages are not applied, that why I insist you measure them.

J111/12/13 are modern stuff, straight designed fr switching, almost nobody makes FET preamps any more.
Doubt they were around in the late 70s

Most probably it has this inside, the datasheet example:

Quote from: edvard on December 08, 2024, 01:38:34 AM

Quote from: Tassieviking on December 05, 2024, 11:06:33 PM...
I have read somewhere that those small amps have an input impedance of 50k ohms so just a capacitor on the input might be enough.
...

Yep, the manual says so: https://www.farnell.com/datasheets/1712767.pdf

That is the *preamp* datasheet.

The power amp itself does not state any input impedance but it suggests using a 10k input volume pot.

IF it has a TDA2030 or similar inside (most probable) they often have 22k input impedance but it *might* be 50k.

I would NOT use the 10k volume pot IF straight fed Guitar , lowers impedance way too much, just use guitar own controls, and add a Bass limiting cap in series with input.

Try 4700pF or .01 uF, this would approach "VOX EQ" as in simplest AC15/30 where they cut everything below 600-700Hz or so.
Crazy as it sounds, it works fine in the Guitar World 

You can use almost  any pedal (except, say, a FUZZ one or an old Wah) you currently have as a Preamp, even if set to minimum or no effect, it will work as a Buffer.

Back in the day, it was common to use a simple pedal such as an MXR Distortion+ as a Preamp, go figure.

Volume set to 10, Distortion turned to Minimum or almost, just enough to provide some boost.
Still needing to cut Bass at your power amp input, back in the day it went straight into any Fender or Marshall amp.

Not applicable to true bypass pedals, which route Guitar straight through when turned OFF

PS: you may use 2 input jacks in your powered cabinet: a straight/flat one to listen to Music; the other through the Bass cutting cap for Guitar use only.

You need a steak and fries Boomer soldering iron, not an organic almond milk Latte Gen Z one 

Quote from: Paul Marossy on December 19, 2024, 01:25:26 PMYeah that was my thinking but it does not work at all with 2N5457, J201, or the J113s that are in it now! It works sort of with 2N5952s but it's still not right.

That was my thinking, any FET ought to work but that has not been my experience in this case.


J113s are what is in it now and it does not work!

When I started on this project I had no reason to suspect the FETs but thru the process of elimination I have come to suspect them. I'm getting all the right voltages. Already have looked for broken tracks, etc. but maybe I'll do that for a fourth time just for the heck of it. :-D

Attached is the data sheet I have for PN4093. 2N5952 seems closest but still not quite right. I have not found anything that is a direct replacement for a 2N4093.

Just thinking aloud: then maybe you do not have a FET problem but something else.

These FETs do not amplify or create any effect or anything, they just let Audio through or not.

Here is one which is easy to test functionally, Q2.
Switch Distortion  ON/OFF.
Does that work?
IF so, Q2 is working.

Even more, I would inject 100mV 1kHz at input and check for signal level/presence/absence on nodes C21/R19 and on U5 pin 6




If not: does voltage change on nodes R20/D10 or R20/R21?
I would expect 0V (Dist ON) or -15V (Dist OFF)

What dmeek already suggested.

Notice FET is switched ON by merely applying 0V to gate and OFF by applying -15V to it, which is WAY higher than needed to turn ANY FET OFF

I reluctantly suggested shotgunning at first but clearly we are getting back into actual troubleshooting.

Maybe because it works? 

Don´t worry.

2N4093 is a very common plain vanilla NCH FET used only as a switch, easy peasy, even the humblest FET can do that.

They used it because it was common way back then, but many modern cheap ones can do its job.

Metallic case original ones are still available by the thousands, that´s how common they were, but they charge "museum" prices, Mouser asks $6.61 for the ones they have in stock, no need to pay that much.
https://www.mouser.com/c/?q=2n4093&sort=pricing

I used them a lot, but the plastic cased ones, called PN4093 ("part number"4093 go figure) and still have a few stashed somewhere.

Mouser has them for all of 50 cents each.
Currently non stocked but that means they are still very much in production, so "others" will have them (Digikey, Farnell, etc.)
https://www.mouser.com/c/semiconductors/discrete-semiconductors/transistors/?series=PN4093&srsltid=AfmBOorb2Ku6gas7wqsBRPXCaV81SfgDvib-XI8UhUFmXkH9SZziRHj7

Just as an example: Digikey *Spain* has them for 0.3 Euro, about U$ 40 cents.
https://www.digikey.es/es/products/detail/rochester-electronics-llc/PN4093/11528016?srsltid=AfmBOooSZvOLeFKtGcpW6k2pBfgfvbsziL3x0ZvG1MZVAJaBudBRKtMj
Guess Digikey US also has them.

But not even that, most *any* N channel plastic FET can do that audio switching job (they are usually advertised as "choppers"); I would use any of these, only paying attention to pinout.
Write D G S labels on PCB with fine point Sharpie but in any case, I would leave them as last ones, it is very unlikely they failed.

This is the modern widely available FET I suggest:
https://www.mouser.com/ProductDetail/onsemi-Fairchild/J112?qs=ljbEvF4DwONoDl3CPnuhug%3D%3D

Stock:
    39,608 Can Ship Immediately 

ONSEMI © Semiconductor Components Industries, LLC, 1997
March, 2023 − Rev. 5
1 Publication Order Number:
MMBFJ113/D
N-Channel Switch
J111, J112, J113,
MMBFJ111, MMBFJ112,
MMBFJ113
Features
• This Device is Designed for Low Level Analog Switching, Sample
and Hold Circuits and Chopper Stabilized Amplifiers
• Sourced from Process 51
• Source & Drain are Interchangeable
• These are Pb−Free Devices
As usual, doublecheck pinout matching.

I would start with ICs first, check connections, replace what´s *missing*, cheek cracked solder pads and tracks, etc. , before replacing perfectly good parts.

Again: pamper the PCB, use good solder wick to fully clean pads before removing parts, etc.

PS: Using N Channel FETs as switches was much the rage in the 70s 80s (then people started using analog switching ICs such as 4016 4093 etc.) , this substitution I suggest also works on old Acoustic amps and many more.

EDIT:s ad to know you bought the expensive OEM ones.

I do NOT suspect them, zero stress applied to them, but instead they may not be getting the proper control voltages, or there are cracked/broken tracks or pads in the Audio path.
Any connectors too.

WEIRD amp.

Not sure what they were trying to achieve?

Just (wild) guessing: maybe they wee accused of being way too conservative? Which is not bad at all in the Jazz Guitar community they are so popular in.

So they hired an Engineer who decided to add some special effects to flavour the sauce?

Only he went wild and chose 2 barely usable effects nobody else does, either as a pedal not even less "fixed" inside an amp.

Octave divider is a "use once for 1 song and forget it" effect type, can´t wrap my head about the "harmonic divider" or something.
If they wanted to be "unique", they certainly achieved that.

Is there a YT demo showing them?

Maybe somebody disliked them so much that he lobotomized them by cutting IC pins OFF?

Usually against blind shotgunning, much prefer to test circuit functionality but here I have no clue as to what they are even supposed to do, so guess best course of action is just to **carefully** remove suspect or visibly mutilated parts in that area, PAMPER THE IRREPLACEABLE PCB , replace them with fresh ones.
Then cross your fingers, pray, and hope for the best.

Lighting some black candles or stroking a rabbit´s foot might help

Worst case, you still have a loud and otherwise functional Polytone amp.

Used to the ultra compact cubes, can´t imagine a 4 x 10" one, can you post some pictures?

Quote from: Kaz Kylheku on November 24, 2024, 08:11:43 PM

Quote from: g1 on November 24, 2024, 07:59:24 PMI believe the plain M-80 is the HM series amp.  Schematic attached.

I'm not looking at them side by side or anything but the power section looks identical to me based on the details I remember from studying the chorus model's power section yesterday. Of course the problem might not be the power amp; you never know.

Yes, the power amps are identical.
Only difference is that the mono one has 4 power transistors for 100W into 4 ohms, while the stereo one has 2 50W amps with a single transistor pair each.

But operation and troubleshooting are the same.

Most probably.

Set meter to 200V scale (just in case), Black probe to COM, measure all others with red probe, you will get positive and negative values, attach little stickers to identify them.

In no particular order and agreeing with my own "Law"  :

1) "do not mess with PCBs unless FORCED to", derived from the Truth: "you can buy *any* component for an Amp ... except the PCB", I suggest, "leave as is" for now.

2) marginal, but "if they work, they work", so ....

3) FULLY agree with G1: TO92 transistors are FINE for MosFet outputs, because MosFets draw NO drive current.

To be more precise, power MosFets have significant Gate capacitance, this is not 100% accurate but for analysis imagine they have, say, 2200 to 4700 pF in parallel to gates, which have to be charged/discharged on AC signals, but current involved is around 1 mA, easy peasy.

4) while those *bipolar* TIP31/32 will take Base current, big time.

Let´s do some "back of an envelope" Math:

47V rail/peak voltage into possible 5.5 speaker minimum impedance (DCR) amounts to worst case but possible 8.5A peaks.
Say 8A discounting transistor voltage drop but not much less than that.

So 4 A peak per device.

BUT TIP31/32 are THREE Ampere transistors 
Two of them would handle 6A peaks ... still short by a wide margin.

They also dissipate 40W each (under ideal conditions) so both "could" dissipate 80W

Now a classic similar power stage would use a pair of 2N3055 transistors, or modern equivalents.
Which are 15 Ampere transistors rated 115W dissipation and would be "just enough" there.
Now you know why this particular amp design sounds crazy to any Tech.

5) also: from datasheet, TIP31/32 current gain can drop as low as 10x at 3A (4A is not even mentioned, it is outside the curves) so drive current could be as high as 8A/10=0.8A which is stressing (bordering on impossible) for 2N5401/5551 which was our original concern.

But now we find more and more wild choices.

Oh well, if it works, works, leave as is.

IF you have any trouble, come back and we will help you solve it.

6) just as a side comment: both schematics posted by G1 look WAY more reasonable, not World´s clearest layout or drawing but no glaring errors either.
Tube cathodes are grounded, or better said, go to a connector pin which *might* be Ground or even -15V or so (have seen it in other amps) to increase tube working voltages.
Might even go to -69V ; not too easy to navigate around that schematic.

7) that schematic and the kind of errors it shows *looks* like those made by non-Techs trying to reverse Engineer (i.e. "copy") a Schematic by looking at a populated PCB.

Often 90/95% accurate, but always some errors creep in.

A Tech looks at them and says "no, this can´t be possible" (unconnected cathodes, wrong polarity and orientation drivers, impossible NFB connection, no PT center tap, etc.) and rechecks; a non Tech just draws what he thinks he saw.

Main transformer winding IS center tapped (in G1´s schematics).