To boot, I bet the 2 x12" 16 ohm cab is WAY better than the usually cheesy 8" speaker in a shoebox typically fitted with these amps.

So even if it worked, most of the power would go to the worst speaker.

Not good or advisable.

Notice the linking jack is on the cabinet but not on the Joyo head.

Quote from: RG100ESROX on November 25, 2024, 11:44:15 PM

Quote from: Loudthud on November 25, 2024, 11:06:11 PMMyself, 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 !

When you say drive it to clipping. You mean adjust the bias up to the point where it just starts to clip?

How much clipping are we talking about here??

We're not going to be measuring any voltages to bias the amp??

No PLEASE!!!
Completely unrelated and you will overheat/blow your amp.

Do not mess with bias which clearly you do not understand.
Sorry. 

Be safe.

Quote from: Davidiot on November 18, 2024, 01:11:21 PMPurchased this Amp in '97. I want to separate the circuit board which is absurdly mounted above the tube. How hard would this be?

Do yourself a favour and chalk it up as "impossible" so you do not waste time on than "non problem solving" idea.

. If that is correct, it means that (1) there is louder bass and (2) the low-damping effect around speaker resonance should be nearly absent.

There is some of that happening but not drastic at all.

I wouldn´t worry about it.

In no particular order:

* that amp is very repairable, but is a weird design, which *can* bring lots of problems if not addressed.

* design is not *bad* but clumsy and uses weird transistors nobody else does.
Even worse, they do not match Industry Standard packages and pinouts which makes replacement harder, normally involving some surgery.
As a bonus, they are out of production for many years now and only remaining "suppliers" are VERY suspect.
IF at least they were available at Mouser, Digikey or Farnell, but no, only "mystery suppliers".

* above mentioned 2n6488 2n6491 are different case (TO220 instead of TO127) but much worse, opposite pinout, which will make amp explode if just replaced without important modifications (cutting and rerouting tracks, etc.)

* I can suggest modding and improving it by using Industry Standard TO218/TO247 case TIP142/147 transistors (what EVERYBODY uses: Fender Crate Marshall Laney Peavey Ampeg etc.) which can also replace drive transistors, BUT it requires mounting them "on their own" on heatsink and connecting them to different points on PCB using flying wires.
Most probably also rebiasing.


* For now do not start wholesale "improving" PCB, you will add more unknowns, just replace 1 or 2 obviously blown/leaking/dead supply capacitors, do not replace perfectly good diodes, what for?


AFTER amp is working you can get into details, not yet.

WHERE is "local"?

Post pictures showing board and heatsink closeups, to suggest new power transistor mounting.

Full size pictures please, not small unreadable thumbnails as on post 1

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.

Both are "basically" same, only the stereo version has *one* pair TIP142/147 per channel, each driving some 60W per single 8 ohm speaker.

The non chorus one is Mono, drives (roughly) same 60W into 8 ohms but almost 100W into 4 ohms, so it needs *two* pairs to meet current needs.

Troubleshooting is same for both. 

Quote from: argenta on November 24, 2024, 09:49:33 PMOk. So DC offset was 0.003mV, which is negligible. I notice it will buzz when switch ed on, even with volume turned down, and pop loudly on switchoff. The buzz is a quick sound. When I plugged in to the headphone input it was quiet. Not sure I can eliminate this problem, as there isn't any way I can put a snubber circuit across the mains. Unless you have thought of something I haven't?

Different problems:

Turn on *thump* comes from empty electrolytic caps charging up, so for a fraction of a second amp "wakes up stupid" (just like me ), it´s almost inevitable.
Buzz comes from same.

BIG amps (PA, rack type. (powered mixers) suffer the same but hide it by using a time delay relay.
Too expensive for small amps.
Some incorporate some kind of mute.

You *can* add a snubber.

If double power switch (hot + neutral) add one snubber per switch section .

Or a single snubber across PT primary terminals.

It will work, with a 20V supply you´ll get some 20W into 8 ohms, not bad at all.

Remember output is balanced, so no output terminal is grounded, use a plastic jack.

It´s a common beginner´s mistake to use a metallic guitar speaker jack, screwed on chassis.

Coolidea, thnks for the desigs.

You can always have some local print shop to Laser print them in glossy adhesive paper, same as they use to print labels.

You can even ask (paying extra) to have these labels/stickers laminated with a thin transparent sheet over (similar to your packing tape idea but covering the full printer size sheet and some also offer spraying a transparent protective varnish over them.

But of course, the DIY at home idea has its merit too
Congratulations.

Johan Segeborn tests this all the time.
Here is one of his videos.

Sadly, it involves a cheap amp and a (relatively) expensive speaker.

Even so .....

Result is *incredible*

Miyagi asked:

QuoteHello, señor Fahey.
I've read your post on low-pass filters in cheap guitar amps where you stated that you manufacture your own speakers (I've read quite a few of your other posts here and there as well) and it got me interested. Because I sometimes modify small combos myself, I started wondering if you have ever tried any off-the-shelf midrange or woofer drivers in those. Such speakers can often be bought for peanuts (at least compared to speakers meant specifically for guitar) so they look pretty tempting. Do you have any experience with that?
Thanks in advance,
Maciej

Hi Maciej, nice to meet you.
Those speakers *can* be used at home, or for experiments, no damage done, but when playing live, even during a garage rehearsal, they "disappear" the moment a drummer enters the room.

Guitar speakers are at least 2-4 times louder than an equivalent HiFi soeaker, and I am talking an expensive one.
Cheaper ones?
Even worse.

Guitar amps were developed in the 40s

Way back then, power was at a premium, very hard to get. a typical Home "Combination Radio-Grammophone" was about 5W RMS, Guitar practice amps (Champ) the same, cheap beginner amps, think the countless "widow maker" amps such as Silvertone and many others, sporting 35W4 50C5 12AV6 tubes (table radio tubes) about 1 to 2 W RMS

So speakers HAD to be loud, efficient and ear biting.

But then power became cheap and abbundant, HiFi demanded flatter and less distorted speakers, real Bass, so they changed A LOT.

40s speakers whose "recipe" is followed today "because it works" had paper thin lightweight cones, often a lot of "cone cry", small light voice coils, fast sensitive response, a large peak around 3 kHz which is tinny annoying for Music but gives guitars "bite", etc.

Compare "exact same speaker", meaning same frame, magnet, diameter , price, but built either for Hi Fi or Guitar.
Even so this one is PA type, not true Hi Fi, which would be 6 dB or worse less efficient.

Eminence Legend 1258 (guitar):


Eminence Beta 12 (PA):


Now $13 Parts Express woofer:
https://www.parts-express.com/GRS-12PF-8-12-Paper-Cone-Foam-Surround-Woofer-292-412?quantity=1



notice very flat response, NO bite peak, average sensitivity around 88dB (vs Guitar/PA speakers 98dB)

Again: alone at home this one will "work"; but playing with others? ... it disappears.

In the old days PCBs were fully hand drawn.

Problem being than these designs are very interactive: you first design a draft board, then improve improve improve, push this component there, bring that one closer, separate sensitive tracks, reroute grounds for lowwer hum or interference, the works, as many times as needed.

When boards were hand drawn, you improved them a little and leave it there, because it´s a lot of work.

With software design, it is easy peasy to reroute and redraw everything on screen.

Here board will not be smaller (maybe somewhat narrower) because pots and jacks need to fit on front panel, but you can improve layout and grounding big time.

And order many for low cost.

Quote from: Kaz Kylheku on September 24, 2024, 03:08:28 AMA while ago this thread prompted me to go look for some videos demoing this amp.

There is even a "will it chug" one, by Ola Englund. (Spoiler: it did chug.)

In all the videos where they close mic this amp, it sounds nothing like an 8" speaker box. There is very little high end.

I spotted a comment by an owner who said that it sounds way too dark to be used with a clean Fender Strat.

Traditional combo amps with 8" speakers are not like that; they are bright and usable with your Teles and Strats for twangy music.

So, yeah, this is low-pass filtered to heck.

I don't understand why they wouldn't instead experiment with a different speaker. I mean, there are 8" woofer drivers that can make bass. It seems inefficient to be driving a speaker like this with a frequency range that is not right for it.

I wonder what something like this sound like: https://www.solen.ca/en/products/ws20e

Even if it needed EQ, I bet it wouldn't be as drastic.

There are 5 reasons for that.

In no particular order:
* cost
* cost
* cost
* cost
and last but not least:
* cost 

Serious, they must buy these by the containerload (not exaggerating) and use them *everywhere*

I estimate cost between $2 and $5, in large quantities. 

I commercially manufacture Guitar speakers, should NOT be able to comloete with Chinese suppliers (Marshall counts as one  ) except that their infinite greed leaves a little niche market for improvement.

I sell LOTS of 8" guitar speakers to Musicians who want to improve their 15W practice combos.

Mine is on left, Marshall on right.

I use 2-3 times as heavy magnetic system , 25mm voice coil (instead of puny 19mm), and to boot chose a non-ribbed and somewhat thicker cone for better mids and low mids ("warmer sound") , also somewhat less highs (less shrill-buzzy tone)





But it *still* is a Guitar speaker, not a Woofer or Hi Fi one.

You may have a point there.

Marshall uses a certain 8" speaker on small cheap models, say the ubiquitous "15W practice amp" , a market section covered by *everybody* because there are millions of users out there, which has a very small 19 mm voice coil (Jensen C 6-8-10-12 R uses 25 mm) which is also used in some Tweeters.
Couple that to a small magnet ("no Bass") and paper thin cone and you "almost" have a Tweeter there, or at least an extended range speaker, with a great potential to sound reedy/buzzy.



That would justify a treble rolling filter.

That speaker is Marshall _brand_ , Asian made, definitely not a Celestion.

1) that is a STRONG low pass filter and is NOT common or needed, at all, in normal SS guitar amps.

2) you do not show where they lead to, but a few amps or preamps or pedalboards, digital or analog, include similar 24dB/octave or stronger low pass filters as "cabinet simulators".
Normal load for a guitar amp is a guitar speaker which has a built-in "mechanical" low pass, usually above 4kHz or so.
If signal is fed into a recording or PA mixer or into a powered cabinet which have way wider response, tweeters or drivers, etc. , sound is *unbearably* buzzy, so such a filter is added to mimic guitar speaker rolloff.
Your circuit is incomplete but I guess it either feeds headphones/line out or somebody complained about buzzy sound and designer included it for good measure.
Again, not "common" by any means, you do not *need* it.

As a real world example, Hughes & Kettner Red Box: a  signal attenuator -direct box - cabinet simulator, including such a filter:
https://elektrotanya.com/hughes-kettner_red_box_classic_sch.pdf/download.html

I´m impressed by the designs and the looks, congratulations.

This is a DREAM opportunity for somebody wanting to build up a small Factory, either a Tech who wants to take the next step or an investor, who wants to multiply his money and hires a Tech to do the soldering and assembly, he takes care of sales (and collecting money ).

The amps can be assembled and sold as-is in those futuristic head-shells or rehoused in more conventional ones, his choice.

The see-through illuminated front panels look breathtaking on any stage (or Album cover ), of course conventional punched and painted (or polished) Aluminum can also be used.