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Topics - Dino Boreanaz

I've had a Danelectro Nifty Fifty for over 20 years now and recently I hadn't been playing it as much as I used to.  I quite enjoy the tone, but only in the first quarter of the gain (dirty/sweet) control's sweep; above that it becomes too fuzzy and fizzy for my taste.  I also found the volume (level) control to be VERY sensitive to the slightest movement near its minimum setting which is where I use it most as I am an at-home-only player.  So, I looked into making some modifications to make this a more useable and enjoyable amp to play.

The quick summary is I changed the volume pot from linear to logarithmic taper, reduced the maximum gain by changing the value of the gain pot, and changed from symmetrical clipping LEDs to asymmetrical silicon diodes resulting in a more useable amp for my at-home use.

Warning, the following is a MUCH more long-winded explanation of my tinkering:

I replaced the stock 100k linear volume pot with a 100k log pot and this alone did wonders for making the amp more enjoyable at home.  I cannot understand why the stock volume pot is a linear taper rather than a logarithmic (audio taper), but I would recommend this change to anyone wanting to use this amp at home.  I can now smoothly control the volume from off to a reasonable in-home level without fear that the slightest touch of the volume knob will cause it to become either silent or unacceptably loud.

I then went about making the gain control more useable.  I started by creating a schematic of the circuit (attached below) up to the volume control from the board (because I could not find one online) and then modelled the circuit in Micro-Cap to get a better understanding of how the components contribute to the gain and tone.  I found that the gain is primarily controlled by the value of the gain pot (VR1) and the values of R3 and R4.  I also found that if the sum of VR1 and R3 is kept constant, the overall "tone" (shape of the frequency response plots) and minimum gain do not change while the maximum gain is determined by the value of VR1.  This seemed like the right place to start since I only wanted to lower the maximum amount of gain available in order to make more of the knob's sweep useful to me.  The stock values are 100k for VR1, and 10k for R3, so I tried using a 10k VR1 pot and a 100k R3.  This did keep the overall tone the same as well as the same fully clean character at minimum gain while drastically lowering the breakup at maximum gain.  This works fine for me since I only use this amp for clean to cleanish tones.

Another area that I experimented with was the clipping LEDs.  The stock circuit uses two anti-parallel red LEDs (LED1 and LED2) hard-clipping to ground.  I tried replacing these with just about every imaginable combination of different colour LED as well as plain silicon diodes as well as removing the clipping diodes altogether.  I found the stock red-red arrangement did not produce very much breakup and most other LED colours did not change this much, except blue-blue which resulted in very little noticeable breakup even at maximum gain.  I then used the plain silicon diodes and this produced noticeably more breakup.  I tried a variety of asymmetrical arrangements as well and found that my favourite breakup was produced by two silicon diodes in series clipping one half of the wave and no clipping on the other half.  The two silicon diodes in series result in a DMM-measured forward voltage of around 1.1 V (about halfway between that of a single silicon diode and a red LED).

I did also experiment with different values for R3 and R4.  I found that changing R3 raised or lowered the amount of gain (and breakup) without a noticeable change to the overall tone, but I was satisfied with the previously-mentioned 100k in this position.  The gain of the second gain stage is controlled by R4 and here I tried a variety of values from 100k to 2Meg.  The lower values in this position reduced the overall gain and made the tone brighter, while higher values increased the gain and made the tone darker.  In the end, I found the stock value of 680k to be the best compromise for me.

I also tried swapping the 10k VR1 gain pot for a 50k pot in order to get a broader range of gain control and experimented with the value of R3 to again try to find an acceptable compromise between the inherent tone and the overall minimum and maximum gain.  In the end I found myself not using the upper end of the control with the 50k pot, so I went back to the 10k pot.

This involved a lot of back-to-back testing and repetitive playing (apologies to my family), but I've now got something that is much more useful to me.  If anyone is interested, I'd be happy to share the Micro-Cap file if it's of any use or interest to anyone.
I've got a Marshall 3520 Integrated Bass System 200W head that I really like, but I'm finding that it is too loud for home use.  The volume controls (separate treble and bass volumes) are barely above zero and the slightest movement takes it from silent to way-too-loud.

Can I reduce the output power in half by eliminating two of the four output MOSFETs in order to make the power amp like that of the 100W 3510 model?  I've attached power amp schematics of these two models and encircled the differences between the 3510 and the 3520.

If so, would this be as simple as desoldering one leg of R17 and R22 while leaving everything else in place and undisturbed?
I've just purchased a Lead 12 head (3005) to use as a bass amp into two 8 ohm cabinets for a total load of 4 ohms.  I know it works and there is no immediate damage because I used it for about 30 minutes today.  It wasn't too loud as I was just playing around to see what tones I could get and I was a little unsure of whether I would hurt anything, but it sounded just as I had hoped it would.  Everything I've read on this site about this amp leads me to believe that the power section of this amp can handle the lower load, so is it OK to use this head (rated for 8 ohms) into a 4 ohm load for extended periods of time at louder levels?  If so, are there any things I can (or should) do in order to improve the long-term durability while running into lower load?
On the advice and encouragement of members here, in an attempt to get a more guitar-like tone from my bass amp, I have prototyped a Marshall 5002 (Lead 20) preamp on a breadboard and run this into the power amp of my Marshall 3505 (Micro Bass).  The result is everything I hoped it would be ... brighter tone, clearer mids, and a really nice overdriven distortion at higher gain settings.  Due to parts availability, I used slightly different pot values from those shown on the schematics.  I used 25k instead of 22k pots for the gain (VR1) and mid (VR4), and a 250k instead of a 220k pot for the treble (VR3).  I've attached the schematics for reference - the 5002 is the top portion of the hand-written schematic showing multiple amps.

There is just one issue that I need to fix before I make this a permanent modification.  There is a high-pitch squeal when I have the gain, treble, and mid all at maximum (10).  The squeal changes slightly in pitch and loudness as these three controls are slightly lowered, and is eliminated at gain settings below about 8.  By changing the capacitor in the second-stage feedback loop (C2) from 220pF to a much higher value like 0.01uF (10,000pF), the squeal is also eliminated, but all the treble is gone and the amp sounds very muddy.  I've tried adding capacitors in parallel to gradually increase this value in hopes of finding a compromise between eliminating the squeal and retaining some brightness, but it seems that any values that are high enough to eliminate the squeal also take too much treble out of the tone.

I'm assuming a stock Marshall 5002 does not squeal at any combination of gain, treble, and mid settings.  Is there some interaction between the 5002 preamp and the 3505 power amp that is different than the stock 5002 which would cause this preamp to behave differently in front of the 3505 power amp as compared to its intended combination.

Are there circuit changes (different value components, additional components, etc.) that I could make to eliminate the squeal while retaining the current clear, bright sound at the highest gain settings?

One final question, possibly unrelated to the squeal issue.  I measured the supply voltage to the op amp and found that it measures -13.9V and +10.2V.  These values seem quite different than the +/- 15V specified on the 3505 schematic.  Should I be concerned about this even if it is not related to the squeal?
On the advice of several members here I am going to try converting the preamp in my Marshall 3505 to the preamp of the Marshall 5002 in order to get a more guitar-like tone from this bass amp.  I've attached the schematics of both amps for reference (the 5002 is at the top of the multi-amp drawing).  This will be the first time I've tried anything like this so I have a few questions and concerns:

Can I desolder one leg of the input R1 and R2 on the 3505 board and jumper these to a prototype board so that I can build the 5002 preamp on a prototype board?

Can I desolder one leg of C3 on the 3505 board and jumper this to the wiper of VR2 (volume pot) for the output back to the power amp?

Can the +ve, ground, and -ve be jumpered to a prototype board from any convenient connection point on the 3505 board or should these come directly from the diode bridge and transformer?

Should I be concerned that the volume pot of the 3505 preamp is 4.7k while the volume pot of the 5002 preamp is 1M?  This seems like it would result in a very different input level to the power amp.

Will 1/4 watt, carbon film resistors be OK for all resistors in the preamp?

What type of capacitors are required for the various preamp capacitors?  Disc, electrolytic, mylar, silver mica, orange drop?

What voltage rating do these preamp capacitors require?

Do any (or all) of the 3505 preamp components need to be removed from the 3505 board in order to test the prototyped 5002 preamp?  In other words, can I simply connect the prototype 5002 preamp in place and effectively bypass the original preamp?

Just out of curiosity, on the 3505 schematic what do the arrows on one side of ZD2 & C13 mean?  Does this indicate a connection to ground?  These two components are at the top, centre of the 3505 schematic, but I've seen arrows like this in other schematics and wondered.

Sorry for the long post, and thanks in advance for any feedback and input you can offer.
After initially thinking there was something wrong with this amp, it does seem to be working just fine - it just doesn't have any breakup at reasonable home volumes.  So I'm looking for your thoughts on whether it is possible to get a more over-driven sound (distortion, fuzz, dirt, overdrive, gain) from this amp.  If so, what specific changes would you recommend this electronics novice start with?
I had been lurking on this site for a while in order to learn more before I bought my Marshall Lead 12 3005 Micro Stack a few months ago.  I've been playing and loving that little guitar amp ever since.  So much so that I just bought a 1987 Marshall 3505 Micro Bass stack to go along with the 3005.

The 3505 is similar in appearance and controls to the 3005, and it has the same controls and layout as the 5502 Bass 20 combo, but it is a 30W bass head running into a 4 ohm load.  It was part of Marshall's Integrated Bass System (IBS) line and came with two 10" cabs loaded with 8 ohm Celestion G10L-35 speakers.

I've been having fun playing it, but I've found that is has essentially no top end or any overdrive breakup at all.  Although I only use it for bass, I'd love to know whether anyone has any suggestions on how to increase the treble and add some gritty overdrive to the tone of this amp?

In addition to these two Marshalls, I also own Danelectro Nifty Fifty and Nifty Seventy combos.  I don't know how much input I'll be able to offer this forum since I'm certainly not an electronics expert, but I will continue to enjoy reading, learning, and contributing as much as I can.