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Messages - phatt

#1
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. 8|

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  :duh
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. :tu:
Phil.
#2
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/
#3
Amplifier Discussion / Re: Power amp in
October 21, 2024, 07:11:50 AM
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. 8)
Phil.
#4
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. 8|
Phil.
#5
Amplifier Discussion / Re: Power amp in
October 20, 2024, 08:01:04 PM
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.
#6
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.8|
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.
#7
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. 
#8
There would my 100's of obscure long forgotten brand names. xP
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. 8|
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. :dbtu:  :dbtu:  :dbtu:
Phil.
#9
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.
#10
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.
#11
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.
#12
The Newcomer's Forum / Re: Laney GC50 Problem
September 05, 2024, 12:45:49 AM
Also with power on gently pressing on the board in different places
(with an insulated probe) can help track down an offending component.
Phil.
#13
The Newcomer's Forum / Re: Laney GC50 Problem
September 03, 2024, 08:41:45 PM
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.
#14
Schematics and Layouts / Re: Realistic mpa-20
August 31, 2024, 01:07:03 AM
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.
#15
Schematics and Layouts / Re: Realistic mpa-20
August 30, 2024, 09:51:51 PM
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.