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Messages - Dino Boreanaz

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In the top-right corner of the schematic it specifies that IC1 is an MC1458.  This is a dual op amp that provides the two gain stages shown in the schematic as IC1a and IC1b.  Pin 8 connects to the positive rail as shown and Pin 4 (not labelled on the op amp) connects to the negative rail as shown below R7 in the schematic.  This is the same op amp application used in many solid state Marshall amps of this era.

As suggested, this circuit can use a variety of dual op amps.  I've read about people trying many different dual op amps in these applications.

I've been considering building this amp too, and would be interested to follow your progress.

Amplifier Discussion / Re: HELP Marshall 5203 Master Reverb 30
« on: March 07, 2019, 05:51:23 PM »
I don't own a 5203 and I'm not an electronics expert, but I have several small, solid-state Marshalls from the same time period.  Can you read the model number from the top of the transformer?  What input voltage are you using?  The following may be useful for comparison, but keep in mind that mine are 120V input.

My 3005 (Lead 12) uses a 4942 transformer and its schematic specifies +/-14.3V output with 120V input.  I've measured +/-14.8V (29.6V between terminals) output with 122V input.

My 3505 (Micro Bass) uses a 4937 transformer.  I've measured +/-17.3V (34.6V between terminals) output with 122V input.

The only photo I could find of the 5203 showed a 5010 transformer which is the SEMKO model number shown on the schematic of the 5002 (Lead 20) which uses the 4937 in USA for 120V.  So based on this, I wonder if the required secondary voltage of your 5203 is the same as that of my 3505?

Hopefully someone with more direct experience can offer some feedback.

Amplifier Discussion / Re: OK to run Marshall Lead 12 into 4 ohm load?
« on: December 03, 2018, 11:46:34 AM »
I like the tones I'm getting playing my bass through this amp, so I'd like to make it as reliable as I can with the 4 ohm load I'm running it into.  I'm also quite new to tinkering with electronics, so I'm really enjoying the learning that goes along with any modification process.  This site has been a fantastic resource for knowledge as has the very highly recommended "Solid State Guitar Amplifiers" by Teemu Kyttala which I've been slowly making my way through.

Amplifier Discussion / Re: OK to run Marshall Lead 12 into 4 ohm load?
« on: December 03, 2018, 08:15:35 AM »
Just wondering now ... would it be better to replace the Lead 12 transformer with a higher VA rated unit,  but the same secondary voltage (so as to avoid changes to the rest of the power stage circuit) along with improving the heat sinking of the output transistors?

Amplifier Discussion / Re: OK to run Marshall Lead 12 into 4 ohm load?
« on: December 02, 2018, 09:39:09 PM »
That's what I suspected and that's why I've been looking into using the transformer from the 3505 MicroBass as it's intended to produce 30W with a 4 ohm load.  It's also the same transformer shown on the schematics of several other Marshall solid state amps in the 20W to 30W range some of which drive 8 ohm loads and others that drive 4 ohm loads.  Do you think my intention of trying to replicate this power amp for use with my 4 ohm load makes sense in this case?

Amplifier Discussion / Re: OK to run Marshall Lead 12 into 4 ohm load?
« on: December 02, 2018, 12:37:36 PM »
I've done some more investigation that I'd like to get your feedback on.  I'm thinking about making changes to the power stage of the Lead 12 so that it matches the power stage of the MicroBass since the MicroBass is intended to be used with a 4 ohm total load.  On the attached schematic of the power stage I've shown the component differences between the Lead 12 and several 20W and 30W Marshall solid state amps.  Some things came to my attention that led to some questions:

- In some cases (C8, R12, C14, & C15) the two Bass amps use one component value while the two Lead amps use another component value.  Is it reasonable to think that these differences are related to the "tone" rather than the power output?

- I measured and compared the secondary voltage of my Lead 12 and MicroBass amps.  The MicroBass is about 17% higher.  Is it correct to assume then, that the MicroBass power stage operates at rail voltage that is about 17% higher than the +/- 19V shown on the Lead 12 schematic (about 22.2V)?

- The schematics of the 20W and 30W amps all show the same T4937 transformer while the Lead 12 uses the smaller T4942 transformer.  Is it the higher rail voltage that results in the greater power output?  Considering these four amps as "known" quantities, the lower rail voltage of the Lead 12 into an 8 ohm load produces 12W, the higher voltage of the Lead 20 and Bass 20 into an 8 ohm load produces 20W, and finally the higher voltage of the MicroBass into a 4 ohm load produces 30W.

- Where the Lead 12 uses 1K resistors from the +/- 19V rails the others all use 2K7 resistors.  This difference seems to correspond very closely with the different voltage drop if I assume the power stages operate at 22.2V and drop to the same 16V preamp rail voltage as the Lead 12.

- R18 has me a little puzzled.  Here the two Bass amps use the same component value, but the two Lead amps do not.  Furthermore, the Lead 12 uses a lower resistance than the Bass amps, while the Lead 20 uses a higher resistance than the Bass amps.  So I'm not sure how to determine an appropriate value here.

- There are two component differences between what is shown on the MicroBass schematic and my MicroBass amp.  While the schematic shows the use of a 500mA fuse (same as the schematics for all of these amps) the board of my MicroBass states that it should be a 1A fuse.  And the schematic shows a W005 rectifier while my amp has a 2W02 rectifier.  So I'll probably go with the higher voltage 2W02 rectifier and the 1A fuse.

- Lead 12 schematic shows that C17 & C18 are 25V electrolytic capacitors.  Is this too close to the higher 22.2V rail voltage with the 30W transformer?  Should I replace these with higher voltage caps if I swap the transformer to produce the higher rail voltage in trying to mimic the power stage of the MicroBass?

Thanks as always for everyone's input.  I'm really enjoying the learning process with your guidance and advice.

Amplifier Discussion / Re: OK to run Marshall Lead 12 into 4 ohm load?
« on: November 29, 2018, 07:01:37 AM »
That is the case here.  Above the speaker jacks it says 8 ohms.
Below the jacks, it says "Do not use loudspeakers of less than 8 ohms.  IE. - 1 x 8 ohm or 2 x 16 ohm."
This is correct for the Lead 12 that I'd like to use.

The rear panel of the 3505 head says ""Do not use loudspeakers of less than 4 ohms.  IE. - 1 x 4 ohm or 2 x 8 ohm."  So I'm just wondering what I can do to modify the Lead 12 head to work with the output impedance that the 3505 is designed for.  The heat sinking is the obvious difference and I'm certainly planning on improving that area.

Thanks also for the caution about swapping the transformer and the suggestions to check the rectifier and fuse as well.

I'll compare the two schematics again to identify any differences, but from what I recall the Lead 12 and 3505 have extremely similar power section circuits and components.

Amplifier Discussion / Re: OK to run Marshall Lead 12 into 4 ohm load?
« on: November 26, 2018, 08:54:00 PM »
Wow, thank you all for the responses!  I didn't expect to get this much input on a rather oddball question.

This is intended only for home use, it's not like I'm going to be using this amp to play with a band.

I have a Marshall 3505 MicroBass (that I was never really happy with) that is rated at 30W into a total load of 4 ohms and uses the same output transistors, but with different heat sinking.  I've attached a photo for reference, but it doesn't seem radically different to that of the Lead 12.

Wondering if you think there would be a benefit to trying to swapping the heat sinks for the ones in the 3505?  Due to the slightly different orientation of the transistors on the board, the heat sinks might require slotting the mounting holes.

Another potential option would be to fabricate my own custom heat sinks.  That would allow me to keep the hole locations to suit the existing component orientation, but add more fins and make them much taller as there's plenty of room in the cabinet.

Should I be concerned about the transformer?  The 3505 transformer is larger than that of the Lead 12.  Would it be worthwhile swapping it?

Thanks again.

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?

Yeah, I wasn't sure what to make of the Vac readings either.  I'm using a digital multimeter, nothing fancy, but it has always given reliable measurements.  I can try re-measuring some of these values.

Based on the difference in the resistor readings, I'm suspecting bad caps.  I've probably gone as far as I can without taking some components out of the circuit, so this may have to wait a little while before I can devote some more time to it.

I measured the DC voltage before the 2.7k resistors and both + and - were nearly identical at about 22.7V to ground, 45.3V between them.

I could not measure a stable AC voltage before or after the 2.7k resistors.  The readings continuously ranged from a few hundred volts down to less than 0.1V.

The DC voltage after the 2.7k resistors measured the same 13.9V & 10.2V as I had previously measured, but this is in-circuit so I don't know whether it's due to the zener "dumping" too much voltage to ground or the resistor dropping too much voltage before it gets to the zener.

So I also measured the voltage drop across the 2.7k resistors and this accounts for the difference.  The voltage drop across one is 8.8V and the other is 12.5V.  These add up as you'd expect: 13.9V + 8.8V = 22.7V and 10.2V + 12.5V = 22.7V.

Then I measured the resistance of the two 2.7k resistors again and found that the resistance of the one on the side with 13.9V started significantly lower than 2.7k, but slowly rose to over 2.6k.  The resistance of the resistor on the side with 10.2V read a very stable 2.63k immediately and its reading did not change.  Does this indicate that the capacitor on the 10.2V side is bad?  Since these measurements are all made in-circuit, I would assume that the measured resistance of the resistor should change as the meter charges any connected capacitors.  Does this sound right?

I am planning on putting the 5002 preamp into the 3505 chassis, but it will require a new board since the circuit is quite different.  So I can't put the new components into the existing 3505 board and there will be some additional length of wire to connect between boards and from the new preamp board to the pots.  I'll have to try moving things around to see whether the squeal changes as I move the long jumpers that I currently have running between the breadboard and the existing 3505 board.

As for the op amp supply voltages, the zeners are in place.  I've tested the resistors in-circuit with a multimeter and the 2.7k resistors seem to measure right around 2.7k.  With the power on, the two 15V zeners measure 13.9V and 10.2V and the other 9.1V zener measures 8.6V.  It does seem like one of the 15V zeners is faulty; what effect would this have on the sound of the amp?

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?

Hey Phil, thanks so much for your feedback.

Just to clarify though, I'm not planning on running both preamps or trying to make some ultra-versatile, do-it-all amp with two complete sets of controls.  If I like the sound of the prototyped 5002 preamp, I'll make a version of the preamp that I can permanently replace the existing 3505 preamp with.  So the front panel would remain the same as it is now with its two inputs, gain, volume, and tone controls.

Any thoughts on the type of resistors and capacitors I need for the 5002 preamp?

What about the huge difference in the volume pot values?  Do you think this is simply due to the difference in the impedance of these two different preamp circuits and the net effect is that each feeds the power amp with approximately the same signal level?

Amplifier Discussion / Re: SWR Workingman’s 160 Bass Amp Crackle
« on: January 20, 2018, 10:07:33 AM »
I had an amp with terribly scratchy controls that I was able to clean up with DeoxIT and now it works great.  If the crackling isn't too bad, you may be able to get rid of it simply by rotating the knobs back and forth a bunch of times as they tend to get scratchy when they're not used regularly.  If it's worse try spraying some DeoxIT into each pot and then rotating the knob back and forth a few times.  You don't need to spray very much into each pot and be sure to clean up and dry off any residue or overspray.  If that still doesn't fix it, you may need to replace the pots.

As for the orange spots, it's tough to say without seeing them, but it sounds like it could just be some rosin flux residue from the solder.  If that's the case, I would not consider it to be corrosion.

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