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Messages - J M Fahey

#31
The Newcomer's Forum / Re: Marshall 5010 Needs Help
April 12, 2018, 01:48:36 PM
The YT Video you linked to does not show crossover distortion at all but something which scares unexperienced guys a lot:

* tube amps have a *smooth*  and continuous non stop variation from clean to distorted, imagine you are rolling a wheeled cart along a long ramp, you will go from a point to a somewhat higher or lower one with no hitch, going from, say, Volume 5 to 6 or 4 may mean going from 5% distortion to 6% or 4%, no big change (since you are never *really*  clean to begin with) , you can´t find a "frontier" since there´s not a defined one, and your ear accepts it.

* SS amps (*all*  of them by the way) are very very very clean before clipping (say 0.5% or less  :o ) to harsh distorted (10/20/30/50%) as soon as they clip, easy to see on a scope where you see a sharp edge at the clipping point.
Going from clean to dirty now is like hitting a staircase step: very definite, very sharp edge, "your cart bumps" on that edge, it is very annoying.
What the video guy does is play basically clean , rise volume until just the peaks distort, the resulting  mix is as harsh as putting mustard on your candy, you are jumping up down all the time between the top and the bottom of the vertical wall stair step.

* experienced SS amp Guitar players simply do NOT play over the step, they just roll their cart either in the clean area or in the fully distorted one.

Here´s one example of an experienced player testing the Marshall Lead 12, your exact preamp circuit, same gain, EQ, distortion mechanism (which does NOT use diodes of any kind but slams Op Amp against the rails, like a mini SS Powwr Amp driven to 11) .
In fact yours is the improved version, with higher power (same circuit but higher rail voltage, larger transformer and a 12" speaker) plus 2 Factory "Mods" :  they added a single transistor buffer to better drive the power amp and an adjustable presence control.
Of course yours may have some improperly changed parts values , but we have lots of doubts about the circuit.
I am certain circuit is the same (they used it on a ton of same era amps, why would they make a different one for this particular model?) BUT parts designation in the drawing and on the actual PCB might not match, so I ask you to print schematic on a large sheet (even better on 2 sheets and tape them together) and follow it part by part, following tracks where they go, then write actual (PCB) labels and actual part value on the paper one.
Then take a nice sharp picture of corrected schematic and post it here.
If in doubt with some part value write what´s printed on its body, don´t guess (as when you called pF what was actually uF).
That is a killer amp.
Power amp *might* be different, I´m puzzled at the "MosFet" label on front panel because those used TO3 metallic Darlingtons (MJ2501/3001).
They did also make a MosFet one, but those are VERY expensive transistors, justified only on the 100W one, definitely not in a way cheaper 30W one.

This guy ONLY plays amps full blast, with a high output humbucker equipped guitar and if not a combo, plugs them into a Marshall 4 x 12" cabinet.
https://www.youtube.com/watch?v=L-5IaYOvmjQ

To make it clear that even the cheapest cheesiest amps can sound GOOD when properly used (without any silly Mods) here´s:
https://www.youtube.com/watch?v=xwKZd-Vc3os
#32
The Newcomer's Forum / Re: Old 70's Randall Amp
April 12, 2018, 06:19:47 AM
Old Randalls came in two very different flavours.
Earlier ones , which you might have here, were all inspired by "Fender amps" which were king of the hill way back then, so: clean amps, bright (they were used by Tina Turner´s husband, meaning they were great for Funky guitar) , has Tremolo and Reverb , NO channel switching, Master Volume or any dedicated distortion circuit.
Second generation were "Marshall killers" so two channels: dirty and dirtier, both had Gain and Volume/Master controls, footswitchable, and a very aggressive buzzy sound (they were used by Pantera, that should mean something).
In any case, either squeaky clean or buzzy aggressive, none was "dull"  by any means.
So, which one do you have?
At least post front panel and gut pictures.
#33
+1 on the tank mounting.

Cabinet vibration shakes tank and causes feedback so manufacturers diminish that by "floating" it.
Minimum is using soft rubber grommets in mounting holes and NOT tightening screws, so tank has a little "play".

But rubber hardens, crystalizes and rots, plus some times eager owners tighten screws > instant feedback.
Fender even used a separate padded Tolex pouch to hold the tank, very loosely mounted.
#34
Agree.
Resistors overheat because *something else* is applying too much voltage to them or passing too much current through them.
First suspects are shorted transistors so check those suggested by G1.
#35
1) congratulations on what you are trying to achieve  :dbtu:

2) it *can*  be done  :)

3) that said, "the devil lies in the details".

As an example,let´s check the first gain stage, which tries to emulate the first gain stage in OR15:

* first triode has gain about 40 to 50X
Your first iteration also used 40X gain ... problem is that OR15 first triode is fed some 200/250V DC so it can put out some 70-80VAC without clipping , while an Op amp will clip above some 9V RMS.
So exact same guitar which is handled flawlessly by the Tube will horribly clip the Op Amp.

Of course, lowering gain (your second iteration was about 15X) will improve the outcome ... but as you see now we are getting away from original schematic operation so we´ll have to account for that somehow.

As said above, it can be done ... but not "literally", we´ll have to compensate, and that at every stage.

* clipping diodes clip, but:
a) they clip symmetrical (tubes do not)
b) they start clipping at 0.7V peak while Op Amp would only clip (on its own) at some 12V peak
This alone will also change your design even more.
Just as an idea: put a reverse Zener diode in series with each conventional one; say a 9V one on top and a 12V one on bottom.
Now signal will clip on +9.7V and -12.7V bottom.

Also use widely different resistors in series with each: low value on the 12V side so it sharply clips, a higher value one on the 9V side so it loses gain but does not actually *clip*  but sinewave gets rounded (like a tube reaching the saturation area).

Now you have a stage which mimics triode clipping way better.

Apply same kind of tweaking and correction to all tube stages and you´ll get quite closer to the original full tube one.

Just warning that power stages can get REAL complex ... and I mean it.

But congratulations again and keep experimenting  :tu:
#36
Amplifier Discussion / Re: Legendary Amplifier Insanity
October 22, 2017, 01:34:51 PM
Quote from: tarahall on October 22, 2017, 07:49:35 AM
What could it possibly have in it that would make it worth even 1/5th of that price is inconceivable.
Didn´t know about SRV , who I associate more with Fenders, but the Dumble was the *exclusive* amp both for Lary Carlton and Robben Ford (who actually paid around $4000 each for theirs).
Now if Howard Dumble can make , say, 6 amps a year (say 2 Months each, from order to getting parts to actual building to delivery) and he gets 80 orders a year, then, Offer and Demand rule the Market, nothing new here.

FWIW 100% pure Coke costs U$1000 a kilo in neighbouring Bolivia ... 20% pure in NY is $60/80 a gram so 60k to 80k a kilo of the adultered one , would mean >300k a kilo of the pure one.
Offer and Demand.

EDIT: Dumble would-be buyers are not kids, not Heavy, not Punk, but because of musical taste I presume middle aged , affluent males, maybe Lawyers, Doctors, Bankers, etc., who left the band days long ago to pursue other, economically rewarding goals (and they succeeded) and now want to "be" :
https://www.youtube.com/watch?v=9TaY_4wc9JA

since reaching this is possible but requires long years of hard practice, maybe they think they can *purchase*  the "magic dust"  instead.

In concept, not too different from the 13 y.o. kid who wants to become

and buys the starter ESP guitar and 10W amp, plus cord and lesson booklet for $199 .
#37
IF you have a bright LED, a 5mm photocell and some heat shrink tubing, do your own, that´s what I use successfully.
#38
Just saw this when going to sleep (4:20 AM here) so too foggy to answer, but will do tomorrow.
Take care  :)
#39
Cool.
You may "spread"  the adjustment range.
Say now you have to set the 100 ohm trimmer to somewhere around 62 ohms?
Replace it with a series combination of fixed 47 ohm resistor and 25 ohm trimmer, you physically widen the sweet spot 4X .

If you dare: use a fixed resistor 5 ohm below needed value , in series with a 10 ohm trimmer.
Now THAT will become a "fine adjustment".

Although the main problem remains: lack of thermal tracking.
#40
May be wrong but I think TDA2050 is the dropped out one and 2030 is still in the race.

Anyway, browsing through Mouser/Digikey/Farnell will clear the doubt.
#41
Fully agree.

If you go the surplus/leftover route, you can bolt 2 L or U aluminum profiles back to back to increase thickness, at least where the chipamp sits.
You can (should) add a little thermal grease between mating surfaces.
#42
Preamp consumption is nil, does not justify its own separate supply.

Get 9V with a reguator or simply a resistor-zener-capacitor net.
#43
You have 2 options:

1) get a nice thick aluminum extrusion, some which is available normally anywwhere for "normal"  uses by "normal"  people ... not us crazy experimenters  :duh
You can always find somebody who makes aluminum doors and windows who will have surplus odds and ends who he will sell cheap "by the kilo" or even free.
Some will make a nice, (if odd) heatsink.


2) add a small computer type fan blowing into the current heatsink.
Best is a standard 80x80mm PC supply fan, followed by two mini CPU fans in series.

Both will need a series resistor to drop from your supply (25/28V?) to 12V or 24V needed there.

Measure *actual*  fan motor current consumption using a 12V supply, then calculate needed resistor using Ohm´s Law.
Also calculate resistor dissipation, you might need a 2W to 5W resistor.
Bypass fans with a 1000uF capacitor, both to improve running speed and to minimize motor noise.

#44
Excellent choice.
You will have 20W instead of 30, excellent sond, for about 1/4 the size, weight and cost :) 
#45
Cool project, congratulations  :tu:

By the way, nice carpets, where are you from?

Always fill Country/City information in your user profile ... sometimes we are surprised :)