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.

Ahh yes nothing worse than intermittent crackles.
Yes a component dying or a cracked or cold solder joint,, my guess you have a cracked track hiding in the maze.
Get yourself a wooden skewer stick and while amp is on gently poke and move components on the PCB. you can get lucky and find the part that is causing the drama. The bottom side will not be easy to access so disconnect an pull the board out then with a magnifying glass in bright sunlight go looking for hair line cracks, especially around the solder pads.

I actually own a SS Laney amp, even earlier model that yours and it developed a crackle last year which drove me mad but finally found a hairline crack which was actually hidden by a solder lump so Even with magnification you could not see it.
Keep us posted, Phil.

Some helpful info here: https://ozvalveamps.org/varidamp.html
variable damping been known way back.

Front page; https://ozvalveamps.org/
Site setup by a wonderful Aussie Roly Roper, sadly no longer with us but His site is a credit to him as he lists most of all the Aussie Valve amps that were made in Au and NZ.
With tons of good advice from a highly experienced Teck.
Phil.

Re Current Feedback also called "Defined Impedance" of an amplifiers output.
Most SS HiFi DC Poweramps have a very high Damping factor. No over shoot of the Cone, The speaker follows the signal as perfectly as the laws of physics allow.

Ye old Valve Guitar amps tend to have low damping factor. The cone over shoots more. Partly due to the Output Transformer. Also remember every time the speaker cone moves it also generates it's own energy Called Back EMF. High damping reduces that effect.

CF is no magic bullet yes at high volume you may notice a little benefit not so much at low volume. If this Amp has an output TX then you already have the best setup and little benefit will be had by implementing CF. (and likely more complicated because of the OTx)

Rod Elliott has lots of mind bending info if you wish to know more
https://sound-au.com/articles/cfb-vs-vfb.htm

Home page here; https://sound-au.com/

Now Others here will know far more about this interesting subject and Some claim great benefit but for me having messed with this stuff years back there is only  marginal benefit and I doubt if you are loud and live on stage you would hear much difference.

There are tricks you can implement with CF that can alter the Frequency response, may be of benefit,  Peavey implement CF in a lot of their circuits as do many others.

Warning As With any TX coupled output make sure you have a load on the output before you crank up the volume.

If it was on my bench I'd be using a stand alone preamp or some pedals first as messing with the unit directly might reap little rewards. just  a thought?

Regards Bridging Amps,,, cricky just get a chip that delivers the Wattage you want and save all the messin around.
Member J,M,Fahey has designed and built likely 100s of SS guitar rigs and a lot of those use OTx. Good to see that he is still posting
He will have a lot to say about the advantage of using Tx coupled outputs on SS rigs.
Phil.

Yes you can use an attenuator on SS Power Amp but there is little benefit as most of them run much cleaner that a Valve Pwramp.
With SS rigs 90% of the OD effect is done in the preamp stages.
whereas with Valve rigs you can get extra sag and compression just from driving the Amp LOUD. Hence they use Attenuators on Valve amps to get the pwr section in sweet Spot But at reduced Volume.
The only time I use an Attenuator on SS amp is for bench testing a circuit under load, to make sure it can actually handle sustained full power.
You have a Master Volume on Rods Circuit and that will do the same as an Attenuator.

The diode Clipper before the Master Volume in Rods circuit is unlikely to give you full bore Screaming OD/Distortion.(If that is what you are chasing) You would need to drive it all with pedals if you want more Screaming OD.
Phil.

I lost my Wife last year And 20 years earlier she lost her son to suicide, she carried on but it left a mark on her outlook on life.
Yes the things that life throws at us can be soul destroying.
So I have some understanding of your great loss.
Phil.

I don't believe I mentioned Diodes as you don't need them,, maybe I'm having a brain fart? (head scratch)
 What you have is correct but Add a low value resistor before the Send otherwise the previous opamp output could get a dead short at some point.
Frankly there is no great benefit I can see to add the Loop, just plug in to the front of the amp,,,, but your choice.
Phil.