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Messages - phatt

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1
Yep Mr Jazz beat me to it,, Agree way over the top. Maybe keep those for a bigger build.

I salute you for you ability to deciyfur the maths I'm just painting by number most of the time as I'm way too old to go back to school now.  xP

Others here will correct me if I'm wrong but all that maths assumes the Transformer is an infinite current source and the tX is the size of a car battery.  :lmao:
Meantime back in the real world your 30vct Tx supply will dramatically sag long before the circuit can pull that much current.
i.e. Put your meter across the car battery while someone starts it. That 12~14 Volts will instantly drop back to 7~9volts depending on the health of the battery. Current goes through the roof and voltage sags,, same goes for Amplifiers. 8|
If you are well versed in sims you can probably setup full load conditions for a more realistic outcome.
For me understanding power supplies was likely the hardest thing to get to grips with. :duh :loco

Maybe look at what has already been built? Some cheap amps only use the case as heat sink.
Look at my Laney pic where I've shown the original back panel for comparison. Note the heat sink is only alloy angle bolted to the back panel.
That amp ran from +/-37Volt rails and rated at 40~50Watts. (more like 30ish watts clean to my ears)
Even with only that meager heat sinking it never got burn finger hot.
That design was very basic and yet performed well for 5~6 years before I rebuilt the power amp.
(just to get more clean headroom for My Synth)

Re Q on predrivers (Q5&6) on separate heat sinks.
That was just a safety backup while testing, they were getting hot until I found why.
Q4 (the Voltage stage) actually runs hotter as it's basically got the full supply across it.
In the final build Q4 has a heat tab added, hidden in pic.
Q3 (BD139) is the bias that needs to track the heat sink temperature and it can be seen in the pic.
The power devices are over speced but as I had these well why spend money.
Hope it helps, Phil.

2
Amplifier Discussion / Re: Peak dissipation in class B output stage
« on: September 27, 2020, 06:39:26 AM »
Oppsy forgot the Test board pic. :-[
Probably not the ideal test setup but when on a budget one has to be inventive.
Most of this is recycled El stuff I've collected over time.

As I've done a few times, I did again test to see if Current FB was worth the fuss but just did not hear any magic so gave it a mis. It's those two 5watt Resistors on the side, near the Grey output jack.
Not in picture but I used a light bulb limiter for every new tweak,, It saved me a couple of times during this experiment.  ;)
Phil.



3
Amplifier Discussion / Re: Peak dissipation in class B output stage
« on: September 26, 2020, 10:16:13 PM »
Hi Shasam,
                If you want the indepth analysis there are far greater minds here that can explain the finer points.
I was just a frustrated muso guy who got fed up with the crap that was being sold that never delivered so I bent my brain and read a whole lot of books and started building my own gear.
Most of my knowledge is just basic *Rule of thumb* thoughts I have gleaned from books and later  wonderful people (via the WWW) like Rod Elliot and many other great people who have shared there knowledge.

My rules of thumb;
1/ KISS (keep it simple stupid)
Don't get caught up with teck heads who want to analyze the black holes in amp design. i.e Current FB is it worth it? Well some claim a big difference others say nay,, only you can make that call  xP
My observation,,,It likely helps if the Amp is working at high volumes.
At bedroom levels I doubt if any ears could hear a dramatic difference. 
You have to use your nose (Intuition) to decide if something tecky is worth the extra effort (& often expense).

Yes all done in preamp,, I don't even use a guitar amp, lol.
It's an old SS Laney keyboard amp,, all the mojo tricks are in my pedal board so the Amp is just the power driver.
I have 3 Valve Amps but I use the SS Laney when playing live, it's just so versatile as I can play my El Acoustic, Strat or keyboards all through one Amp. 8|

I've lost count of all the circuits I've built in the last ~35 years and I've built a lot of disasters.  :-[ over that time I've designed about 4 success circuits that work really well (Some are on this site)

The Laney rebuild I'm posting for you to show the BBoard testing setup of a power amp
It's very basic and likely not an ideal bench test setup but well worth the effort.
As you can see in the pics the laney only had a small Tx and when using the Keyboards the supply could not keep up. I rebuilt the entire PSU and power amp section. The 3 Ch preamp section is basic but functional. (Hey it does have a Tank Reverb which is nice)
You can see in the pic the difference from the original back panel and the rebuild.

I've added the Schematic for you as that may help to understand the pictures.
This uses over rated devices as I've found with my basic rule of thumb knowledge. :P
As long as the transistors are way above what is needed then less chance of failure.
(i.e the 2SC5200 is a 250V-15A-150W device) They are the 3 main things to look out for.

As explained in Art of Electronics the whole idea of these DC coupled amp designs is to keep the DC Q point of *Output & the bases of Q1,Q2 at close to Zero as possible*,as I've noted on my Schem.
The Voltage and Temp measurements were taken from test circuit not a simulation.

Current limit is done with a trick little device on the output,a PTC (Poly Thermal Capacitor or just PolyFuse)
Hifi geeks might not like them but saves all the extra PCB space.
Short circuit testing was done with amp at full power on the test circuit pictured about 20 times for at least 10 Seconds.
The Poly Fuse will trip at ~110Watts, The colour will darken when they get close to trip.
Absolutely brilliant little device. :dbtu: :dbtu: :dbtu: :dbtu:

Pleae note;; This was 5 Years back I may have slightly changed something since then,, but it's all very close to how it was built.

Re bandwidth,
I used 250pF at C6 to help limit HiFreq. (Use 250V cap)
C4 sets the low Freq roll off,, smaller values can help roll off excess bass.
100uF there is likely of little use for guitar freq.
Have fun,, Phil.




4
Amplifier Discussion / Re: Peak dissipation in class B output stage
« on: September 25, 2020, 04:04:47 AM »
OK I now understand. :tu:
As you have already Found ESP site then dig deep into Rods fine web pages and you will find a lot of gems. :tu:

Be aware that Guitar Amps are so far away from perfect that they bare little resemblance to the Audio Fools magic smoke dealers.

The last thing you want for guitar sound is HiFi flat bandwidth so steer clear blameless amp stuff.
ODrive guitar through a dead flat EQ with 20/25khZ bandwidth is the fastest way to annoy the punters.
Some of the greatest guitar sounds ever heard were produced by what would now be considered a very bad design. :lmao:
As A friend of mine found out when he spent a fortune on a custom build by a hifi teck. He eventually came to me unhappy so I built him an amp from a pile of El crap for 1/10th of what he had wasted and he was blown away by the result.

As for design help if you can find a copy of "Art of Electronics" it's well written for those of us who are less gifted at complex maths. :duh

As noted in the ESP page you quoted, anything running below +/-35VDC is not likely to give much trouble and that covers a basic amp of 40~50Watts so 15 ~20 Watts is fairly safe.
Just wack a few ideas on a bread board and use a Light bulb limiter and see what you can come up with.
I'll try and dig up some old pictures of my testing circuits to give you ideas.
Phil.

5
Tubes and Hybrids / Re: VHT Classic 18 Preamp Modification
« on: September 25, 2020, 03:38:12 AM »


My only questions now are..
How do I properly add a volume pot to the new input?
And is a capacitor before the valve needed?

Also many thanks for your reply's!!! <3)

Hey Tom,
If you want volume control at input then Return to original input BUT insert a 100k ~250k pot in front of C52.
This will open up much better control over how the Amp responds to your rig.

If thee amplifiers overall system tone is bass heavy then you can reduce the value of C52. If on the other hand it lacks bottom end then remove C52.
Also R76 is 100k You can always make it 220k or higher,, that will give the input higher sensitivity.
Don't go crazy as it might squeal like a pig if the input is too sensitive. I'd try 220k.

Also worth note is you have all 3 plate resistors at 220k (being R3, R6 & R30) which to me is a bit too much. I'd be changing 2 back to 100k which might help civilize the outcome.
If you want to get more power stage magic OD then try changing R52 from 2k2 up to 10k or even 20k for some softer power stage compression.
A lot of the magic of the old famous valve circuits was in the power supply voltage drops,, far more useful than buying fancy pants  brand name Valves 8|
Phil.



6
Amplifier Discussion / Re: Peak dissipation in class B output stage
« on: September 22, 2020, 08:14:14 AM »
Well an LM1875 can easy produce 15~20Watts.
Just google it, there are tons of circuits out there using that chip, a few to be found here on SSguitar

The LM1875 delivers 20watts into a 4Ω or 8Ω load
from ±25Vsupplies.
Used within specs they are perfectly happy.

As long as using ONLY 8Ω load you can run them from ±30V for even more watts. ~30W would be the limit.

For such a small amp there would be little benefit from designing discrete stuff. Unless of course you just want the experience or learn how to blow stuff up. ;)
I sent many perfectly good transistors to magic smoke heaven learning how to build amps.  :-X
Phil.

7
Amplifier Discussion / Re: Peak dissipation in class B output stage
« on: September 18, 2020, 10:19:23 AM »
Likely over my head but I'll ask.
                              What is it you are trying to design?

Re the last part of the link to SOA by Rod Elliot, (Section5  Protection Schemes)
To my mind that is how you cover the extremes.
I doubt any circuit is totally bullet proof but if it's just a 100Watt guitar amp then considering how many millions of 100watt amps get flogged and survive for years ,,, maybe you are over thinking the design. :-\
Or have I completely missed the point? xP
Phil.

8
Amplifier Discussion / Re: Help with Crate CR112 Tonestack
« on: September 09, 2020, 08:12:50 AM »
Having found time,,,I just simulated the preamp up to C15 tone output,,
A couple of observations.
On Clean the bass response at the output of U1 starts with a hump at 2 hZ which is kinda silly.
But when drive is used the bass rolls off at a more sane 50~60hZ.
I can only assume this would create an obvious imbalance between clean and drive mode.
Maybe too much bass on clean and next to none on drive. In combination with the diodes D1,2&3 would likely be kinda harsh.

The mid control does little to create a mid cut at the classic 400hZ. so yeah proly not nice to play if you want a more fender tone. xP

I would first remove the diodes and use an OD pedal for dirt and just use the drive mode.
This will give the Amp a lot more clean headroom,, And leave the OD tricks for pedals which do a far better job.

If you need more bottom end up the value of C4.
As for the tone ,, you would have to rebuild it or I as mentioned easier to build an external tone pedal. 8|

Sorry  to say but if you try to turn your hamburger into a lobster you may only end up with a bad smell. ;)
Phil.

9
Amplifier Discussion / Re: Help with Crate CR112 Tonestack
« on: September 01, 2020, 08:01:38 AM »
Hi Jopy,
Having messed with circuits for many years now I've found it's often a lot easier and cheaper to work outside an amp than inside.
My suggestion is build an external tone pedal.

Why you ask,,
Most amps are PCB and later ones tend to use fine traces and solder pads. It only takes one stuffed trace or pad lifted only to find you just wasted all the time making land fill. :'( :'( :'(

As you are wanting the Fender tone stack may I suggest you would be far better served building a PhAbbTone.
This will be more fender than a,, hum what were they called?
oh yeah fender. ;)


         

Re altering tone stacks from Hiz To low Z.
Just divide the resistors x10 and multiply the Cap x10.
So a 250k becomes 25k and a 47nF becomes 470nF and it will work in similar manner but not always as it depends more on the circuit Z.
Has more to do with Circuit impedance (Z) and little to do with SS or Valves.
cheers, Phil

10
Amplifier Discussion / Re: Trying To Repair a Traynor TBM 25...
« on: August 20, 2020, 10:46:44 PM »
All seems to point to a speaker failure,,, maybe test using another speaker.
Phil.

11
Amplifier Discussion / Re: Trying To Repair a Traynor TBM 25...
« on: August 20, 2020, 08:46:13 AM »
Q does the speaker switch off when you insert the headphones?
Phil.

12
The Newcomer's Forum / Re: FX loop noise
« on: August 08, 2020, 10:28:41 PM »

With Zoom (or any other wacky do it all in one pedal unit) inserted in FX loop of any hi gain amp this is always going to be a potential squeal like a pig problem. :duh

Reason is because most hot rod guitar preamps already have a lot of gain  so the moment you use a zoom with hi gain patches then the oscillation will be hard to suppress.

Used in front of amp (as per normal) then it's usually ok.
or as already mentioned zoom into FX return (bypass the Amps preamp) ok.
But not wise to use both hot rod preamps. :loco
Phil.

13
OK sounds like there is a fault in the power stages.
Get a stick (Kebab skewers are ideal)  :tu:
Start tapping around the power supply and pwr amp section. Do this while passing a signal into the poweramp input. The idea is to try and isolate where the fault might be on the PCB.
You can also take the expert advice from *Enzo*,, just bang on the lid a few times and see if you can recreate the fault. :lmao:
Could be a cracked solder joint which can be a real pain to track down. Strong sunlight and a Magnifying  glass will help find what is often a tiny line on a track or solder joint. Larger parts are the most obvious places to look as they can vibrate and crack solder.
Those type of intermittent problems arise when big signals cause the crack to momentarily open causing a big crackle through the speaker.

I did once have an amp with a bad audio crackle on big signals,, turned out to be a failing contact in the power switch. Drove me nuts for a while. :duh

Still could be corrosion on the power amp input socket.
If it's the standard 6.6mm type then try a can of contact cleaner, spray into socket then insert the plug many times to clean the contact surfaces.
Phil.

14
Yes just measure the DCV on the terminals of the caps.
The mains voltage is unlikely to cause an issue as it's only ~10% difference.
The fan is likely a backup for cooling at high power so again it's not important unless it's part of an over load & temp circuit cut out system. (maybe check the schematic)
Fans are cheaper than alloy so some later amp designs do rely heavily on fans for cooling (cost cutting is the aim for big companies now which trumps intelligent quality designs) but I'm guessing this amp being older already has a substantial heat sink.
Phil.

15
Amplifier Discussion / Re: Crate G130C Amp
« on: July 24, 2020, 02:14:45 AM »

Quote;
* At some point I started having an issue with the Amp where the output sound would fade out.*
Then,,
Have you checked your FX loop? a common cause of intermittent signal drop out.
Phil.

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