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Topics - Dino Boreanaz

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Amplifier Discussion / OK to run Marshall Lead 12 into 4 ohm load?
« on: November 25, 2018, 07:24:54 PM »
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?

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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?

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The Newcomer's Forum / Prototyping a Different Preamp in an Existing Amp
« on: January 15, 2018, 02:10:59 PM »
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.

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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?

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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.

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