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April 16, 2021, 03:16:01 AM

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91
Amplifier Discussion / Re: Acoustic 370 Output Level Issue
« Last post by Loudthud on March 15, 2021, 02:08:55 AM »
Normally the diodes CR301-304 don't conduct. If they do, the input signal is too big. When the signal at the output of the power amp gets high enough, the JFET turns on shunting the input which in turn reduces the output.

A too big signal on the input makes me think the gain of the power amp is too low somehow.

The gain of the power amp is set by R314, R313, R315 and C306. R312 helps set the DC gain (operating point) of the output. The pot R304 is used to adjust the DC (45V) on the output. The low frequency corner is about 30Hz, set by R313, R315 and C306.
92
Amplifier Discussion / Re: Acoustic 370 Output Level Issue
« Last post by gbono on March 15, 2021, 01:28:34 AM »
The waveform shown is on the line input side of R301 if that makes sense. The waveform on the other side of R301 is almost a square wave (if I remember). How does the limiter work on this amp?
93
Amplifier Discussion / Re: Acoustic 370 Output Level Issue
« Last post by Loudthud on March 15, 2021, 01:14:59 AM »
Which side of R301 is the waveform from ? One side is essentially the same as the Line outputs, the other might be limited by the JFET compressor circuit.
94
Amplifier Discussion / Acoustic 370 Output Level Issue
« Last post by gbono on March 14, 2021, 11:34:03 PM »
I'm stumped why the output of this amp is limiting or at least I think that's the case.
DC tests:
The bases of Q301,302 are both at 12v and Q304, the bias transistor, has VC=48.3 and VE=46.1. The limiter circuit has the collector of Q316 at 8.55V and the voltage at node R333/4 is 8.48V? The base of Q316 is .259V
I replaced Q315,Q316 and Q303 and the tantalums.
CR301-304 are neither open or short.
Attached waveforms are voltage at R301 (sinewave) and the voltage waveform at the base of Q310

What am I missing?
95
Amplifier Discussion / Re: Fender Frontman 212R with distorted sound
« Last post by g1 on March 14, 2021, 04:30:50 PM »
When I listened to the mpg file my first thought was "that's a bad speaker".
But then I saw that you verified them as good.
Exact same thing here.
Quite relieved to see it was actually the speakers and our instincts are still good.   :)
96
Amplifier Discussion / Re: Fender Frontman 212R with distorted sound
« Last post by Jazz P Bass on March 14, 2021, 01:17:25 PM »
When I listened to the mpg file my first thought was "that's a bad speaker".
But then I saw that you verified them as good.
Glad that you got it sorted out.
97
Amplifier Discussion / Re: 1983 Fender Showman Channel Switching Issue
« Last post by phatt on March 14, 2021, 07:58:18 AM »
Hi DJ,
Ok thanks, now I see where you are coming from an that's cool.
Just be aware this stuff can become very addictive, my wife thinks I'm quite disturbed,  :lmao:.

Most young guitar players will tend to fall for the hype that if you have the latest model Amp/Gear/Guitar you will be in mojo heaven and will play better and you will be BROKE $$$.
Reality is with some basic gear and some understanding of electronics and Sound you can escape the hype bubble easy.

Read Bobs comment for just how cheap you can run a big rig setup.
https://www.ssguitar.com/index.php?topic=5003.msg39230;topicseen#new
Note the Amp used is one of the cheapest model amps you can buy.
Very Clever for a chap who has little electronics knowledge  8|

Re modding amps;
Yes you can alter some PCB's but depends on how much work is involved. Some circuits leave no room for much alteration and it's just not worth the trouble. (your amp is likely one I would not bother to try and alter).

For me, Having tried all sorts of ideas I've found from experience that a lot of Dedicated guitar circuits are way to extreme and many owners of multi channel amps tell me they like one channel and end up back at relying on there pedals for all the other sounds.

So as you wish to learn my advise is buy a Bread board and start building circuits to see what suits your playing style. You are welcome to my circuits if you so wish ,, most of them are on this site anyway but there have been a few alterations in the last couple of years.
-----------
To understand whatzhappining; :duh
 Bandwidth.
By shear design the power stage of old Valve amps limited the bandwidth of the signal.
WHY?
Most valve amp circuits have a lot of Decoupling between each stage including the power amp. They also have a Transformer coupled output to speaker.
Most guitar OTx's are often low speced and can't transfer really hi frequencies so straight up limited bandwidth by design. Add things like miller effect and you have an amp that sounds sweet even when distorting.

NOW Enter SS where most power stage circuits are DC (Direct Coupled) so no caps to wipe off a little bass,,, and no OTx to wipe off excess hi freq content. The result is often HARSH TONE. :'(

But don't panic,, there are many ways to recreate the sweet stuff most of it can be done in the preamp stages.
It's not about the Mil Spec transistors or the Gold plated print on Valves it's about understanding what is happening inside a circuit.

It's the RC coupling and boring maths stuff that makes the mojo not the cost of fancy mil spec Chips/Valves actives.
-------------

90% of the freq of guitar is under 1khZ,, add for some harmonics gets up to around 3khZ. Past 5khZ is just adding crap that will destroy the CLARITY and Definition of the notes you play.
Of course we all tend to fall for the MORE is always better than LESS,, humm.
Not so once you understand what is really going on. To cut through you only need to focus gain at say 1khZ to maybe 3khz.
This is where the whole system has to come together to create that TONE FOCUS. So when you turn up the treble knob you don't want to turn up frequencies way past what is needed,, otherwise your tone will be way too harsh/brittle. Add a lot of distortion and now you just annoy people.
My cab sim which is permanently on for that very reason to help tame that top end fizz.

Regards to OD/Distortion.
I have 3 OD pedals,  ALL 3 the gain is never above 2. I've found trying to get the OD out of one stage or circuit never works.
The idea is you build that OD each pedal adding a little more Grit/Edge.
With all 3 on, I'm into ZZ Top kinda distortion.
No reason you could not build some of the these into a rack case but you will still need a control board.
My pedal board can go direct into FX return of the Laney but I do like my Spring reverb and that extra Volume control on the Amp Cab is handy. :tu:
I've added a pic of my home made pedal board. 4 are obviously brandname the others are my own designs.
The ugly black box has a Compressor and Cabsim.
The white GeOdrive is my design and my favorite the PhAbbtone.
Enough for now I have work to do. bummer.
regards,, Phil
98
Amplifier Discussion / Re: Fender Frontman 212R with distorted sound
« Last post by Superpunky on March 14, 2021, 05:46:52 AM »
SOLVED: I borrowed another working 212R to make some cross tests and it turns out that the distortion depends on bad speakers (both of them). I had them tested in first place, but I didn't realize they are bad (lack of experience). I feel so stupid  :-\
Thanks for your precious support  <3)

PS: I ordered a low cost function generator and an oscilloscope because I would like to keep up with this hobby
99
Amplifier Discussion / Re: Fender Frontman 212R with distorted sound
« Last post by Jazz P Bass on March 14, 2021, 12:42:42 AM »
The dc voltages don't look bad at all.
You don't have access to a scope do you?
100
Amplifier Discussion / Re: Fender Frontman 212R with distorted sound
« Last post by Superpunky on March 13, 2021, 04:31:26 PM »
U6 OA voltages:
Pin 1: 5,1 mV DC
Pin 7: -235 mV DC

Drivers base voltages:
Q18: 1,225 V DC
Q19: -1,17 V DC

Output transistors base voltages:
Q20: -0,62 V DC
Q21: 0,56 V DC

Is it possible that the distorted output is the effect of this little asymmetry in polarization?
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