Thanks Jay (I assume) I appreciate the effort you have gone to, to explain it all.
I know it must be very frustrating for you.

I had a hair line crack hidden under a solder pad in an amp that drove me nuts for a very long time. It was by chance that I turned the amp on just before I turned on the bench light one evening and saw a tiny flash on the circuit board which led me to the answer to the intermittent bad hum.
The crack was one of the main filter caps solder joints. The slightly over filled solder was covering the Crack in the copper track.
Phil.

At RG100
From post11,
When you say;
  "It used to sound just like the tone in the Dokken 'Just Got Lucky' (guitar only) video."

Well there are a lot of things besides the amplifier that can alter the outcome.
so,,, the Q you need to ask yourself is,
What was your whole setup before it all went south?

Did you change speakers? Did you change PU's or electronics in guitar?
Many things besides an amplifier can easy rob you of sound quality.
Just changing speakers can alter tone in a DRAMATIC way. 4 G12's in a box will sound very different to a single G12 in an open back cab
Some speakers might flab more so that others.

At post 13;
You mentioned you have taken a speaker out of the Randell cab and mixed in another speaker. That will change the sound big time when compared to the original Quad setup.
How about going back to the setup you had BEFORE it went wrong,,,then start all over again.

Dirty or corroded connections can cause signal loss, guitar cords can slowly cause signal loss due to broken strands. It seems to me that you have made a false assumption,, ""as G1 has noted there is no crossover distortion.""
And besides biasing ANY amp hotter does not make for better tone.
A Bias pot is NOT a Friggin GAIN knob. It has nothing to do with the gain of the amp, it just sets the balance between the 2 output devices.

Yes there is a lot of hype regards to biasing Valve rigs hotter to get the Mojo sound. well they don't understand what they are talking about. I have shown a few players in real time the sonic result of hot verses cold bias.
Here is what happens, with hot bias the amp starts to break up on 4.
With cold bias it starts breakup on 7.
So hot or cold the amp sounds the same. only Q left to ask the owner is how often do you want to replace your power valves.

The SS amp design you have is not going to give you power amp Dirt or sag as they don't work the same way. Valve amps have a soft power supply while SS use a stiff rigid PSU but you can design them to sag a bit with some mods.

If the Amp is working,,,My advice, go back and Establish just where it all went wrong. Because it's a fair bet something else might be the culprit.
Phil.

Hi daydream,
Ok, look at the right hand side of Schematic, note how you have 5 sockets.
These are switching sockets, meaning when you insert a plug it also switches the signal path.
Know this; FX loops and like are a nightmare, just waiting to go intermittent.
I personally hate them       

Spray the hole with De-oxit then plug a lead in and out 10 times. If that does not help then the switching tabs inside might be dead.
Now rather than trying to replace them it's far simpler and cost effective to simply take a short lead and bridge the send/return which bypasses the switch.

I have done this simple fix many times with failing FX lopp setups.

If it's the internal switching in the head phone, well that is harder to fix.
You would need to work out a bypass or replace the socket.
Hope it helps.
Phil.

You need to get priorities in perspective.
forget the preamp the power amp has blown so please focus on that first. Fix one issue at a time otherwise you just make life hard for you as well as those here trying to help you.

If it's all too hard to follow instructions then it's a fair bet one or more Tr's have blown so you could just replace them from Q10 onwards, with equivalent or higher rated. Q8 & Q9 are likely ok as they don't have to pass big current.

Then with no load and light bulb limiter bring up the voltage and check DC voltages shown on schematic. The bases of Q8 & Q9 as well as the speaker out should be very close to zero volts. (i.e. under 100mVDC)

****Be aware this circuit has No Short Circuit Protection like later designs so even a momentary short at speaker out will likely blow transistors****
 No need to ask me how I found that out 
This is a very simple circuit so a good learning curve for you.

The simple fact that the trace burnt out tells you there has been a critical short causing big current to flow and melt the copper trace.
Take it one step at a time.
Hint listen to G1,, He has the experience to help you.
Phil.

Quote from: RG100ESROX on October 28, 2024, 10:17:41 PMWell, no sound. Other than the somewhat  diminished hum/buzz that I had previously.

I tested the thermistor and it worked very well when heat was applied. So, I don't think this is my issue.

I changed both TIP32C's and the one TIP31C. And both 1N5484's located near the bias trim pot.

Any thoughts??

What should I be checking next??

1N5484 is suspect,, No such diode can be found in my quick search.
The Schematic says 1n914 which are just small signal diodes.
Using the wrong part at that point will likely end in smoke.
don't frig anymore until you build the light bulb limiter.

This is a DC Amp which means, All the Tr's sit at different DC potentials so if something fails it can take out many parts in a flash of magic smoke.
Now I'm not the expert here but here is a brief run down.

Unlike a Valve amp there are no coupling caps between active Tr's, hence the term DC coupled (no Cap to decouple the DC of each section) The limiter will save $$ and lots of wasted time.

The input is similar to the PI input to Valve power amp, used differently.
The idea is to keep the bases of Q8 & Q9 And the Spk output at close to Zero Volts as possible. Q10 is the Voltage Amp. (I think these Tr's see the biggest voltage and I think they tend to run hotter than output at times)
Q11 & Q12 are predrivers for the Darlington pair outputs, formed by Q13 & Q15 and Q14 & Q16. The diode string and trim pot from Q10 is the bias circuit. the thermister usually mounted to heat sink tracks the heat and slightly alters the bias as the amp heats up.
Google differential SState power amp designs or something like that .. Phil.

Ed I just found this; https://www.eeeguide.com/bjt-power-amplifier-with-differential-input-stages/

You just need a 100k pot in front of "C", the input to the Power stage.
If you can only get to halfway before it distorts use a 50k pot or add some series resistance before the 100k pot. Your ears will tell you if it way off.
Phil.

If the instructions say 40/50mV across those resistors (sounds about right) and you can only get 25mV then there is a fault in the Amp "Not the instructions".

BTW,
Additional power is how you get tight bass that does not flab.
That is why bass amps are 4 or more times the wattage of guitar amps.
They have to have much bigger transformers to deliver the same "Clean" SPl at low Freq as a 40 watt guitar amp at higher freq.
Freq below ~200 ~ish hZ chew up heaps of energy.
Phil.

Adding any circuit is unlikely to improve a bad preamp design.
My advice is purchase/build a decent preamp circuit and bypass the whole front end of the Amp.
Phil.

I think you have made a false assumption that the power amp is at fault.
Larger caps won't do anything, you would only improve that with a bigger Transformer as that defines the power not the caps.
Far more likely the issue is in the preamp stages.
Also have you upgraded your Pickups to hotter ones?
Have you added Changes to any pedal board setups if used?
Any of the above would likely cause the problems you have.
A very common issue that can cause loss of power is corroded Efx loop sockets.
Channel switching circuits could be failing,, any contact points could be corroding causing signal loss.

As for biasing the power amp, I'll assume you do not have a scope?
To get very close turn off ALL distortion, you want a pure clean signal to hear this trick.
Volume up just enough to hear it with your ears close to speaker.
Now just pluck one string and let it ring out.
If bias is not correct you will hear a slight buzzing sound with the note. It sounds a lot like a torn speaker cone. Now gently adjust bias until the buzz just disappears.
Depending on the design you may not be able to make it perfect.

without more detail and a schematic it's hard to know what maybe causing your problem.
Phil.

Quote from: Miyagi_83 on October 09, 2024, 02:15:30 PM

Quote from: phatt on October 09, 2024, 07:37:47 AMI don't know what your aim is

I have nothing specific in mind, really. I'm just looking around for options. I sometimes happen to get my hands on a small, usually 8", practice amp and it seems that, usually, the weakest link in the signal chain there is the speaker, hence my interest in swapping.

Yes speakers play a big roll, BUT these cheap little budget amps are made to a price point.
Zero R&D is done to make sure it produces a quality sound/tone.

I have 2 small amplifiers here, A Vox Pathfinder 10 and a Peavey MicroBass which claims 20 Watts at 4 Ohms, Which would be less than 20 Watt into 8 Ohm. (both use the same poweramp chip so a fair comparison)

The Vox is definitley a cheap crap toy while the Peavey delivers a quite usable sound for El guitar. They obviously spent time matching the circuit to the speaker when designing the Amp which delivers a smooth well balanced tone even though it only has a 5.5 inch speaker.

I also have a very cheap Casino 12 watt junk amp, delivers a brittle harsh trashy tone. So I trashed the small cab but kept the small chassis to see if I could improve the sound. Just by adding a low pass filter and fixing the design flaw in the tone control part it actually works well through a 12 inch guitar speaker. Most small guitar amps are trashy and brittle because they don't spend time rolling off the excess hifreq produced by small speakers and small cabs.

There are a few Utube vids of guys running the LM386 smokey amps driving into a Quad box.
they only produce around 1/2 to 1 watt. The quad box rolls off the top end.
While a smokey into a 3 inch speaker will have extreme hi freq = trashy tone.
If using small speakers you need to rethink design of the driving circuit.
Phil. 

There would my 100's of obscure long forgotten brand names.
Forget the brand just get to know the difference between a Hifi woofer and a MI speaker.
If it has a foam rubber spider and it's easy to move the cone then don't bother.
If it has a rippled spider and the cone is stiff and harder to move the cone then it might be worth trying. In Australia Plessy/RCA and MSP speakers were used a lot in early hifi record players. but these are rare now and ones that do turn up the glue is way past the use by date and although they might work they will fail if driven hard by over enthusiastic bedroom thrashers with a 10 watt amp.
My rule of thumb for folks who are into speaker swaps "if the amplifier circuit is bright then use a darker speaker,,,If the amp tone is dark use a brighter speaker to balance the result.

I don't know what your aim is but there is a lot of talk on speakers making a huge difference and the market is full of fancy brand name speakers that claim to deliver tone heaven.
Yes true good speakers **CAN** make a big difference but it's not the whole story.
It's all about *Balance*
Each part of the amp circuit defines *the whole system tone* and it only takes one part to make or break that *Balance*.
You can destroy the *Tonal Balance* of a perfect Fender Twin Reverb Amp just by changing one 30Cent Capacitor.

If you are catching on the flip side of that is that sometimes just one Cap or Resistor swap in a circuit costing pennies can reap better results than expensive speaker swaps and a lot less work.
Of course this assumes you know what you are doing which requires some years of experience.
So for the novice swapping speakers is doable and the market gets flooded with fancy bright labels on the back of mass produced speakers that may or may not improve the sound. And so someone is making money from peoples lack of deeper understanding.
ps; I just realised that Mr Fahey has answered a lot of your Q's on another post,
 So yes take his advice.
I have personally learned a lot with His help.    
Phil.

Short answer, don't waste your time.
Hi Fi Drivers are high Compliance while MI Drivers are Low Compliance so they are worlds apart.
Hifi woofers rely on a defined space, a sealed box with or without a tuned port.
They will just wobble outside of a defined enclosure. They are mostly much lower on SPL levels. They give close to zero hi frequency while passing full range signal.

MI drivers can work in open or sealed boxes, I'm old enough to remember small pa speakers just mounted on a flat baffle board with no sides and a couple of angle brackets to hold them in a vertical position.

On the other hand keep your eye out for older hifi speaker boxes which used Low Compliance speakers as some will be able to reproduce a half decent sound for small guitar amps. don't expect them to handle hi power amps as they are very old and most will be low wattage.

Easy to see the difference as most hifi Woofers have a big rubber spider around the edge while low compliance driver will have the rippled spider often made of cloth or tarred paper.
Phil.

Thanks Juan,
            At least I have several recording of my wife singing that we recorded some years back. I'm so glad that I persuaded her as she was a trained singer with a unique tone and range. I was able to play one of her recording at her Funeral which got a huge response.
Now I just sing at open mic jams, the music keeps me sane.
Phil.

Thanks Joe,
Yeah, If I didn't already have a sound/tone pedal board that I'm quite happy with then I would certainly check this out.

Using Analog circuitry for cab sims and amp parts is likely part of the reason it sounds good. Full bore dirt is a bit weak in the low freq but hard to judge when it's recorded on a phone.
Reverb sounds ok.

Certainly far more usable than a lot of other pedals. (what happened to the thumbs up emoji??)  

The one sound a lot of these fancy pedals can't seem to crack is that magic sweet Santana lead tone, which delivers a huge cut through without the brittle irritating top end fizz. I'm yet to hear that done.
Phil.

Also with power on gently pressing on the board in different places
(with an insulated probe) can help track down an offending component.
Phil.