Just for gits and shiggles I removed C6 10uf emitter bypass cap on Q1.

I know those emitter caps maximize the signal gain of the stage.

Well... squeal is gone tho the amp lost quite a bit of volume.
Also backing the Tone down from 10 even slightly, lowers the volume quite noticeably.
Tone at 0 and the amp is barely heard and quite dull sounding.

For fun I installed a 1uF cap instead of the 10uF.
Squeal came back at 9 on Vol and amp got a bit louder again.

So my initial uneducated (as yet), thoughts... randomly...

1. The squeal originates in Q1, it just has too much gain with the better PS.
2. Q1 is ok, but too much gain overdrives the driver circuit of Q3 and Q4.
3. There is a biasing issue. Somewhere...Everywhere...?
4. The new 2N2148 output transistors bias is wrong.
5. All of the above

Thinking about it... of course lowering the gain of Q1 reduces/eliminates the squeal because turning the Vol down basically attenuates the signal and gets rid of the squeal. "sorta" the same as reducing gain.

The difference is, with C6 installed and volume down the amp is very distorted at all non-squeal settings.
With C6 gone, the amp is mostly clean with just a hint of distortion, even at full volume.
Yes, I know there is a difference between lowering gain (voltage) and attenuation. They are *not* the same thing.

Hmmm, gonna sleep on it now. It's late here and I'm tired.

Ok, got a little work done.

1. Input jack shorting type
2. C16 now 1000uF
3. Added R13 1K
4. Added C9 1000uf

See schem.

Amp is "better" squeal now starts at @ 8.5 on Vol instead of @ 6.
I have been keeping the Tone on 10 as that makes it the worst.

Squeal happens even with input jack shorted to ground, so it's (obviously) the amp itself oscillating.

Hummm, I found a Data sheet for the 2N2148's...

It's a pdf but secured so I can't print it but I think I can link to it... 

http://alltransistors.com/pdfview.php?doc=2n2148.pdf&dire=_rca

Let's see if this works...

Quote from: Roly on August 29, 2014, 11:08:54 AM

Instability.  Your amp is acting like an oscillator.

Thanks for your explanation of this. 

Target For Tonight.... I will...

1. fit a shorting jack
2. increase? C16 to 1000uF or whatever I can fit there.
Is there any benefit to adding a small 0.1uF in parallel with C16?
3. add a 1K in the supply line to Q1 and decoupling cap nearby. Value? Large? Small? Both? I'll experiment...
4. ... and other things as I think of them... but one at a time! so as not to lose the rabbit.

Quote from: Roly on August 29, 2014, 11:08:54 AM
"Start"?  START!?!?  :trouble Your circuit is already at rev3.0 and counting!  You're on the third lap already.

Review this thread man, look around you, you are already in it knee-deep!  :duh  {And having a great time it seems to me}   :dbtu:

Ha Ha!  :lmao: Yeah, I should have said "where do I start from here"

Neck deep more like it...

Yes, having a great time!  Learning is FUN!  Esp. with such wonderful help here! 
Someday I hope to be able to nail jelly to a wall...  :lmao:

Magic Wand? Magic Wand?!!! In my trade (transmissions) I have a Magic Wrench,
I'm the guy they all bring the "tough" problems to...
I pretend to have a secret that keeps the others all in mystery and wondering "how did he fix that?"... (snicker)
If only they knew....
I'm just a little dense when it comes to this amp stuff... 

Ahhh yessss, bite sized...

Quote from: Roly on August 29, 2014, 11:08:54 AM
The power supply now seems satisfactory (if not actually magnificent).

Magnificent! Holy carp! High praise indeed. Thank you! 

Quote from: Roly on August 29, 2014, 11:08:54 AM
You were having thermal runaway (in the output stage?).  After all your supercharging we need to have a look at the output stage, its biasing, its thermal compensation (lack of), and its dissipation/heatsinking; its DC and thermal conditions.  I'm pretty sure that the bias tree R25/6/7/8 requires trimming, AND it has no thermal comp (so no wonder you are getting thermal runaway) so that also needs a mod.

Then we need to look at the AC conditions, is the driver doing the required job?

Then back another stage; stable?  Sufficient?

Finally the inputs, and a fully functional amp.

And from that stable foundation you can then tinker and mod with confidence.

HTH

Yes that helps immensely! But.. did you notice the output transistors?
I installed the T03 2N2148's (a few pages/posts back) they don't even get warm.
There is no more (doesn't seem to be) thermal runaway, and so far they seem to be doing ok even with the (probably) less than optimum biasing.
And Q3 and Q4 do not get warm either. Not even a little bit.

For all the rest of your above post...

Thanks! lots of good stuff there and food for thought.

I'm goin' back in swingin'... De-coupling, here I come! 

Quote from: phatt on August 29, 2014, 07:23:58 AM

You start by reading;

How about you go do some reading it will save you from all the frustration.

Thanks Phil, I will do that.

Thanks for the thumbs up Roly. 

So I got home and wired this up just like the schematic shows.

One exception... R7 at the base of Q1 is actually an 18K, not 10K.
Somehow that mistake crept in and I never noticed.
It's always been an 18K all along. (I'll fix the schem later)

Ok, so the amp is very loud and distorted like this. (BTW, the Trem still works  )
With Tone and Vol up full it squeals horribly.
Squeal (barely noticeable under the distortion) starts at about 6 on volume and gets worse as Vol increases.

At very low Vol it's "almost" clean. Any tone setting.

Can find a "sweet spot" balancing tone and vol to where it's a nice crunch but kinda muddy, lack of treble.
(But it will sustain for days...    :lmao: )

As an experiment I disconnected the buffer output after R4 10K, (so no signal on R5 at all) and jumped it to the junction of C7 and R11.

Still quite distorted (any vol except very low) and not a pleasant distortion.

My (random) thoughts/ideas/questions... no particular order... and I'm just guessing... 

1. Leaky germanium transistors can't take the good solid guitar signal from the buffer.
2. Simply too much gain somewhere for what the circuit can handle.
3. The tiny little phase transformer is saturating and not in a good way...
4. Replace all germs with silicon (one at a time and bias them appropriately? ), see what happens...
5. The above may not work because of the phase transformer?
6. Go back to 50K pots as those were probably bleeding off some signal to ground?
7. Figure out a way to eliminate the phase transformer?
8. The "power amp" section just can't handle a decent signal?

I've got small signal JFET's and BJT's and small MOSFET's etc, etc.
Even got some 2N404 germanium transistors.

Lots of questions and possibility's....

Where do I start?
Where would you start?
(besides gutting it and putting a real amp in it, still a possibility...) 

If I gut it, I still want Tremolo and Reverb to work.
I would need a preamp schem I can copy or...
Design my own? That may be over my head right now...

I do have this 60W to 100W compact power amp board kit from here...

http://www.lh-electric.net/projects/pa100ws.html

Thanks for any ideas you can throw my way.
I'm a little lost right now... Not discouraged, just not sure what to try next.

Edit; BTW I did physically remove Q2 and all associated R's and C's from the board.
Just in-case there was some parasitic signal bleed or whatever...

Ok, Had a nice break... back to it. 

And back to basics...

I came across this thing called SWTC, (Stupidly Wonderful Tone Control), and some modified versions of it.
In the schem I have used the SWTC 2.

Here I have stripped down the schem to "back to basics" to get the basic amp sounding good.
I didn't like the tone control and how it caused the amp to sound bad with tone on 10.
Also the volume control was not very "linear" so to that end we use an audio taper pot. 
Besides, the original pots were still a bit scratchy even after using spray lube/cleaner.

I've not yet made these changes to the amp but will do so tonight after work.

After I get the basic amp sounding good, I will add back in the reverb circuitry.

Note that compared to the last previously posted schem, I have eliminated Q2 and its associated parts.
Q2 was the original reverb driver which we don't have to use,
and now have the option of using it for an added gain stage or whatever.
The reverb can also be tapped into the signal chain at whatever point we want, but that's all for later...

I have also been considering trying different (silicon) transistors for Q1-3-4 but again... later...

This is becoming an almost complete re-engineering of a cheap amp that would have been easier to just gut the thing and put a "real" amp in it.

I don't care, I'm having fun and learning lots. 

Quote from: Roly on August 18, 2014, 09:47:26 AM
{I hate that, when the datasheet lies.  :trouble }

Ya, not sure if the data sheet is lying, or something is wrong with the one I got. 

That's why I ordered another one. 

Okay. 

With that cleared up...

I've been playing this amp for awhile now with the 2N2148's and it seems to be good.
With no effects in use the amp stays clean to max volume as long as the tone control is not above 6-7.
I have some ideas for that...

I've cobbled up a separate 5V supply for the reverb with the ground of that going to ground (+Ve) on the amp.
No more hum, but I did not yet put in an emitter follower.
I have C11 dialed completely out on Q2 and it's playable at low reverb settings.
Much past 4 and reverb gets over the top and all goes to mush.
At 10 its on the edge of oscillation/feedback and does oscillate if tone gets turned up.

All this stuff is interacting somewhat of course.
I expected that.

So 2 things... need to tame the tone control and get the reverb levels sorted.

I'll do the emitter follower first and see what happens,
but for now gonna take a break from wrassling with this thing.

I AM gonna wrassle this thing to the ground and make it beg for mercy.

Some might call it stubborn...

I call it dogged determination.  :duh 

BTW just a point of interest...
The reverb module specs say 100mA... but actual testing show it drawing @ 167mA.
With that in mind and to have something to compare, I have ordered the "newer" small reverb brick BTDR-2H labeled Accutronics rather than Belton.
Probably both made in the same factory somewhere in Asia anyway....

Quote from: J M Fahey on August 17, 2014, 08:59:35 AM
Maybe he already got sick of this endless thread.
Sorry.

I would not be surprised...

Am I annoying everyone with this?

Quote from: Roly on August 17, 2014, 06:01:14 AM
Doh!   :duh

1) for letting the blood rush to my head.  Sorry, just think, "add an emitter-follower".

Ummm ok, no apology needed. I can figure out an emitter follower.

Quote from: Roly on August 17, 2014, 06:01:14 AM
2) getting the wrong datasheet and thinking "silicon".

Again, ummm,
in reference to what? The emitter follower?

Quote from: Roly on August 17, 2014, 06:01:14 AM
That "heatsink" is a joke.

The stock original heatsink?... or the brackets I made for the 2N2148's ?
Because the 2N2148's run quite cool and don't even need heat sinking.
The brackets merely hold them.

Quote from: Roly on August 17, 2014, 06:01:14 AM
Grab a fan-blown CPU cooler and thermally couple it to that.

You could try increasing R25 which feeds the bias chain, say try 2k2 and see if that cools it down a bit, may also reduce output power a bit.

Try to avoid running it into thermal runaway - it isn't healthy.

The driver Q4 might need some thermal attention too.

Get some heatsinking in there!  As a germanium amp these really don't like heat.

{sorry again  }

Again, no apology needed but I am confused....
Are you suggesting I put the original 2SB324's back in with proper heatsinking?

For R25, I don't really want to do anything to reduce the output...
...unless you think it's really needed with the 2N2148's.

Thanks for the data sheet, that's the one I found too. 

Are you ok Roly?
The above was not your usual clear concise post.

Roly, in your circuit above did you mean to draw the transistors Q2 and the added one as PNP type?

I have some 2N3906.  Suitable?

Alright, I got the new transistors mounted.

The amp is sounding better than ever.
It's got some volume, tho not earth shaking.
Bass response has improved (which was another thing I hadn't mentioned yet, and wanted to fix).

The tremolo now has an interesting "pulse" or "throb" in behind the note that sounds kinda cool. 
I like it!

The tone circuit does need some attention because at 10 the sound has a harsh ice-pick distortion.
Tone at 4-5 and it sounds great!

Check out the pics and let me know what you think.

Now to get back to working on the reverb...

Quote from: geomiklas on August 16, 2014, 04:21:24 PM
The power supply fried on my Hog 20.  I found a second hand replacement which is rated 13.8vdc @ 1.7a ... So I tested the voltage and it registered at 17.8 volts.  I know that the 1.7a is sufficient as it is over 1500mA, but the higher voltage is my question.

Unloaded (not connected) power supplies will measure higher voltage compared to when the supply is loaded.
Manufacturers usually rate the supply as it is found in a circuit with a load.
To measure higher voltage out of circuit is normal.
I'd say go ahead and use it, but you may want to wait for more experienced members answer. 

HAH!
I found a pair of 2N2148 PNP Germanium T03 package in my junk. Way overkill I'm sure.... 

Quick and dirty wired them in with alligator leads (removed original 2SB324's of course)...
and it works!

Sound is a bit distorted and need the tone control at "0" else it squeals.
Maybe because of the long leads to the transistors?
Or biasing issue. Or both.

No heat sinks on the 2N2148's, they got barely warm after 5 minutes of playing and the amp did not fade out!
Yay!

Voltage stayed up around 24-25V. Didn't check the current.

I think I got some T03 sockets around here someplace....