Electronics, programming etc.

About this section:
I made this section to share some electronics projects. I’m primarily a hobbyist in this field, although I often find use for these skills in my line of work as well.

I have no relevant education, and have learned electronics mostly by trial and error. These mods and projects have worked well for me, but copying my work is at your own risk.
Power supplies, amplifiers and other electrical equipment can kill you, even when not plugged in. Be careful.

Peavey 5150 / 6505 series mods

About the different models:
The Peavey 5150 should need no introduction.

The 5150II is a 5150 with an improved clean channel, and a modified distortion channel. It also has a different power supply and a few changes in the power section. You can't make a 5150 sound like a 5150II or vice versa without severe modification, but you can get them a bit closer by some simple mods.

Later, the 5150 was relaunched as the 6505, and the 5150II as the 6505+. They made no changes to the circuit at this point. If a 5150 sounds different from a 6505 it's due to different tubes, or the age of the components, not a change in design.

The 5150 2x12 combo is different from both the 5150 and the 5150II. Tone-wise it's said to be somewhere between the two, but I don't know this circuit.

The 6505+ 112 combo has a preamp nearly identical to the 5150II, but a new fx loop and power amp.

Make your 5150II sound more like the original 5105

This mod will make changes to the preamp section to make the ultra channel of your 5150II match the specs of the original 5150. This without affecting the clean/crunch channels. Your amp will be be growlier, more organic, more gainy and get a chunkier attack, but it will also become less tight.

It will not sound 100% like the 5150, as there are some differences in the fx loop / power amp stage as well, but it will take it in the right direction.

5150 Pre

This mod is also applicable to the 6505+ 112 combo, but the component names differ.

5150II head:
Replace C17 (470pF) with a 470K and 0.0022uF capacitor in parallel
Replace C2 (0.001uF) with a 0.022uF capacitor

6505+ 112 combo:
Replace C5 (470pF) with a 470K and 0.0022uF capacitor in parallel
Replace C26 (0.001uF) with a 0.022uF capacitor
Replace R12 (37.4k) with 39k (R12 probably won't make much difference, the value is already quite close)

6505+ 112 PRE

Mod the 5150/6505 lead channel to match the specs of the 5150II/6505+ You will get a more twangy top end, more fizz, not as much of a raw growl to it, and a more processed feel/sound overall. If this is what you want, do the opposite of what's stated above.


Bias Mods:

The 5150 is biased very cold. Cold bias gives higher output power and adds crossover distortion.Crossover distortion sounds scratchy/buzzy. There's too much, it will sound as if something is broken. Also, crossover distortion is most noticeable at low volumes. Hence, if you play loud your bias current can be set low to extend tube life.

Some like the crossover distortion in the 5150, but a lot of people think it sounds better with a cleaner power section. Higher bias current will also reduce the max output power, thereby making it possible to attain power tube saturation/distortion/compression at a slightly lower volume level, which may be something you want.This mod will make the bias voltage adjustable:
R68: Replace this with a 5k to 7k resistor and a 10k pot in series. Start out by setting the combined resistance to 15k (stock value)

5150 II:
This amp already has the potmeter built in, but the resistor value is too large. Even with the bias set to max current,  the  bias is quite low. Replace R68 with a 5k to 7k resistor

5150 bias mod

6505+ 112:
it is possible. I recently got an email from a man called Brian Mangan, saying that you can adjust the bias of this amplifier by altering the value of R14. I have not taken the time to check the schematic or try it myself yet.

EQ Mods:

The EQ on the 5150 isn't great. The main problem is that the mid control doesn't work in the right frequency range. In my opinion, its' focus point should be at a lower frequency.

The tone stack on a guitar amplifier is usually a very simplistic design. Leaving every knob in the middle position rarely yields a "neutral" frequency response, and changing the value of one component will usually alter the response of more than one frequency band. Here are 2 ways to improve the 5150 Eq; It's probably best to do only one of these 2 mods:
OPTION1: Shift the mid frequency downward
This is a very simple mod which will move the focus point of the MID band to a lower frequency. 5150/5150II:
Replace the 50k mid pot with a 25k or 22.5k pot.Or, if you're lazy, solder a 47k In parallel with R95. This does pretty much the same thing.

6505+ 112:
Exactly the same, except R95 is called R48

OPTION2: Make the mid frequency sweepable
This mod will make your EQ sweepable, however, moving the mid freq position will also affect the response of the bass and possibly the treble bands.

Replace R28 with a 10k resistor and a 250k pot in series.

6505+ 112:
R28 is called R47

5150 tone stack

Other preamp mods:
I found these mods in the Rig-Talk forum, and they were originally posted by Mark Cameron. They are designed for the original 5150, and may or may not work well with the 5150II or combo versions:

V5B(middle preamp tube): add 10pf silver mica cap to pins 7 and 8. This will remove hiss, make the gain less fizzy, make the high mids clearer and improve clarity in chords. between pins 6 and 7 it's even more efficient, but will also darken the amp a lot.

R9: piggyback a resistor on top of the 1 meg resistor. 120k is a good starting point. You can try out 68k, 100k, 120k, 150k, etc. Lower value means tighter amp, but if you go too low, the amp will be choked out.

C26: cut one leg for less bottom end on the rhythm channel

C15: bypass cap for better "feel"

R22: bypass for better dynamics and clearer picking response

I tried tinkering with R15 and using Marshall values, but it took too much away from the original sound. The 39k is the SLO value as well.

Power amp mods: replace screen resistors (with 1k 5W) add choke somewhere. I have to study these further. To be continued.

Credits: I didn't figure all of this out by myself. I've gathered some of this information from forum threads, and here are some of the sources:
http://www.mrbobo.com/music_5150_mods.htm (5150 bias mod and tone stack mod1) http://www.ultimatemetal.com/forum/backline/596150-5150-mod-thread.html (EQ mod 2 + etc) combo has a preamp nearly identical to the 5150II, but a different fx loop and power amp.


Motherboard Repair

Like most consumer electronics, computers tend to break down due to low quality capacitors that fail before their time.  In this example, my brother's computer switched itself off after a period of instability, and would not turn on again.

Visual inspection revealed a bulging capacitor close to the south bridge. (Look at the pictures below, the top plate with the vents is bulging slightly) I de-soldered the part:

As you can tell from the pictures, the part is:
Manufactured by TK, max voltage 6.3V, capacitance 820uF, max operating temperature is 105 degrees Celsius, and it's from the ATWY series. 646A is a date code.

In most electronics repair situations you'd simply replace the broken capacitor with a new one of equal capacitance and the same or higher voltage rating. This part measured an ESR of less than 1.1 ohms, which for most circuits would indicate a healthy component. In modern computers however, specs are much tighter, and parts that would work perfectly in most circuits can make a computer fail.

A google search for "TK ATWY" led me to the data sheet, which revealed that this 6.3V, 820uF capacitor has an Impedance (other word for ESR) of 0.036ohms, and a max ripple current of 1140mA.

We're looking for a part with the following specs:
Capacitance: 820uF (this value must be identical to original part)
Voltage: 6.3V (this value can be equal or higher)
Temperature: 105C (look at the placement of the part; how hot will it get in that position? In this case, very close to the South Bridge; 105C or more is recommended)
Impedance/ESR: 0.036 ohms (this value must be equal or lower, but not too much lower)
Max Ripple current:  1140mA (this value can be equal or higher)
Physical dimmensions: Diameter: 8mm (just make sure the part you order will fit on the board)

Then simply make sure you get the polarity right, and the board should work again. If not, this means the failing capacitor probably blew other components as it went out of spec. In most cases, replacing the faulty capacitor(s) solves the problem.

After a repair like this it's probably a good idea to clear CMOS+RTC, just in case something got corrupted when the computer became unstable.


12AX7 comparison

I decided to buy a selection of 12AX7 tubes to find out how they differ. Remember to switch to 480p for better sound quality:
I tested the following tubes:
Shuguang 12AX7-B
old used chinese 12AX7A
Sovtek B
Sovtek LPS
Mullard Reissue
Svetlana Reissue
Tung-Sol Reissue

I tested the tubes by placing it in the V1 position, which is where 75% of the tone is made. the other 3 tubes (the last 25%) are Shugang 12AX7-B.

Settings on the rock master is ultra channel, gain pulled, max gain, EQ 3, -3, 3. The cabinet is close miced using an E906.

To get the same performance on all tracks I prerecorded a clip of dry guitar, and fed that to the rock master using a reamp box.

In my opinion, the differences between the tubes are negliable. It was however possible to reveal some difference by comparing two out of phase samples.

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Tube mic build

1. Got a cheap mic and disassembled it

2. Ripped out all the electronics

3. built a proper head amp

4. Put the new electronics into the mic body

Here's how to turn a $35 chinese large diaphragm condenser microphone into a high quality tube microphone.
The result may not sound like a $10.000 U47, but it should be comparable to products in the $1.000 range.

This mod is usually referred to as the "Royer Mod". It was designed by Dave Royer, and the original article can be found here: http://recordinghacks.com/articles/one-tube-microphone-from-berlin/ I based the mod on an MXL 2001 mic, but it can be applied to a lot of cheap microphones with good results.

This mic uses a capsule (diaphragm) that is copied from the Neumann KK67/87 capsule, which was used in the Neumann U-67, U-87, SM-69 and M-269. And it is a pretty decent copy, it sounds very good!

Head amplifier
The head amplifier is where the problems start in this mic. The original circuit is not horribly bad, but it could be better. The circuit I built is similar to what is found in the Neumann U47, but modified to match the impedance of the capsule. As the tube in the U47 is very rare, it has been replaced by a more common 5840, which is electrically similar to the AC701K tube used in the Neumann M-49. I ended up using a 6205, which is a very similar tube. It's said to have less noise and higher shock tolerance, and it has a slightly different pinout compared to the 5840.

The output transformer in the MXL 2001 is horrible, replacing this attributes to probably 50% of the improvement in sound quality. Replaced it with a Jensen DB/E.

Power supply

The tube requires about 100V DC to operate, and the 48V phantom power is far from sufficient. I built the power supply into the case of an old AT computer power supply. Keeping the power supply away from the head amp circuit is not only important due to size limitations within the mic, it is also crucial for eliminating noise.

Head basket
Like most cheap LDC's, this mic has too many layers of mesh obstructing the capsule. Manufacturers do this to protect the capsule, at the expense of sound quality. I got rid of one layer as a start, but should really get rid of one more.

5. Replaced the output transformer

6. finished the wiring

7. Took the head basket off

8. removed one layer of mesh

9. built a power supply

10. PSU done

11. All done!

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