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Author Topic: Marshal Amp 7815/7915 swap to 7818/7918  (Read 1604 times)

engelheimer

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Marshal Amp 7815/7915 swap to 7818/7918
« on: December 18, 2015, 08:41:32 PM »
I posted this on the Marshall Forum last week and they gave me the boot, here is my post:

Here's one have been doing on most of the solid state Marshalls since they came out. From the Valvestate, AVT, MG, Mosfet, 5210, DFX, DSL and MB series. Are you with me now people?

Swap the 7815/7915 TO220's with 7818/7918 TO220's, with the +3 and -3v supply boost you get headroom and crunch out the wazoo.

Should clean the white silicone grease (if any) on the TDA chip, regrease w/clear and tighten it up. Do a continuity czech on the tab to chassis. If you see a short replace the mica insullator and/or plastic washer, grease both sides. czech it again.

On amps w/o a fan, an external heatsink (vertical fins) should be used. Use your noodle to figure this out, be sure to clean and regrease, for maximum heat xfer.

Enjoy!

This was my first post on 3 discussion groups last nite, and got an e-mail asking to introduce myself, and post on the workbench so here goes:

My opinion on Marshall, is they are all superior amps right out of the box.

My fave is the DSL5C
.
Compulsive/obsessive with a bit of ADD thrown in - the crux of the biscuit is who we all are.

My objective here is to entertain, annoy, confuse and send others off on tangents.

At least I know ya’ll are keeping out of trouble and behaving yourselves.
This was my first official post on any forum, and do consider myself a luddite. Every once in a while I visit this one and other sites to get a laugh from the plethora of “google techs”. Many (including myself) can’t leave well enough alone and are just born to tweak.

Do have plenty more updates/revisions for these and many others. Can’t use the word “mods” cuz my definition of it is: (https://en.wikipedia.org/wiki/Mod_(subculture) Just talk’in bout my g g generation. Since everyone’s personal taste/opinion is different, your mileage may vary.

Have been doing this stuff long before the www. My first cohort in crime was Kenny Fischer, then Gerald (El Presidente) Weber. My hearing is gone, have to use an oscope and someone else who is a tone monger to judge improvements.

About my first post: Op-amp data sheets have a maximum voltage to keep the specs in tolerance, noise/gain etc… you can go higher. If one fails, (have yet for one to fail) it means the chip was weak to start with. Will give you an opportunity to update key op-amps in the signal path, (after  8 pin sockets are soldered in). This makes it easier to find one of the many types available, bipolar/jfet/lo noise/high slew etc, as a suitable replacement. 

I always use a 100W light bulb limiter to bring an amp up, but for the anal just use a variac and the limiter. If it holds, burn it in for a while before you plug it in the wall. A shorted op-amp is easy to find by measuring resistance between the +/- pins with a good meter (Fluke 115) and good test leads. The one with the lowest resistance is the culprit.

My visit on this forum may not be long, and might hang around a while to share some experiences and observations. Am busy with many other HD audio/RF endeavors and this was just a temporary diversion. Will use this thread to post, and would like everyone to first review:

 https://en.wikipedia.org/wiki/Hypersonic_effect

R.G.

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Re: Marshal Amp 7815/7915 swap to 7818/7918
« Reply #1 on: December 19, 2015, 05:58:20 PM »
I posted this on the Marshall Forum last week and they gave me the boot,
That's a pretty ominous start.

Quote
Swap the 7815/7915 TO220's with 7818/7918 TO220's, with the +3 and -3v supply boost you get headroom and crunch out the wazoo.
That doesn't make any technical sense. The difference between ±15 and ±18V doesn't make that much difference to the signal an opamp can put out. Even if it were clipping at 15V and not at 18, the difference in signal level is only about 1.6db.

This might be significant if there was something following the opamp that clipped sharply at, say ±15.5V, but that is antithetical to the idea of "increased headroom", and increased headroom is also antithetical to "crunch out the wazoo".

Doesn't make sense as stated. Of course, you go on to say:
Quote
My objective here is to entertain, annoy, confuse and send others off on tangents.
So maybe you're being absolutely straight with us - you're here to cause havoc. Most forums do not take kindly to that. There's quite enough of that with everyone trying to play nice.

You may want to rethink your approach to forums; otherwise many forums may give you the boot. I've seen it happen before, to others with what turned out to be similar agendas.


phatt

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Re: Marshal Amp 7815/7915 swap to 7818/7918
« Reply #2 on: December 19, 2015, 07:02:58 PM »

Arrh huh,, thanks RG, I was rather confused. You are Obviously a man with Knowledge, Wisdom and Common sense. A rare commodity in this crazy world. :tu:
Phil.

Roly

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Re: Marshal Amp 7815/7915 swap to 7818/7918
« Reply #3 on: December 20, 2015, 05:09:03 PM »
Well, at the risk of taking this seriously, I have to agree with R.G., I just can't see the point, and I certainly don't like going off the datasheet - the manufacturers really do know best.

One question must be, what are you trying to do, more headroom for clean, or more crunch for grunge?  These are contradictory.

If you look at circuits for s.s. amps you will notice that they typically have clipper stages that depend on diodes and don't allow the surrounding op-amps to clip, and there is a very good reason for that - they tend to sound nasty.

Mods have their place but they need to be thought through, and I'm much more inclined to first decide what I want and build from scratch.
If you say theory and practice don't agree you haven't applied enough theory.

g1

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Re: Marshal Amp 7815/7915 swap to 7818/7918
« Reply #4 on: December 20, 2015, 08:59:43 PM »
And further to the good points RG and Roly have presented, you can't make extra power out of thin air.
Those regulators need an input voltage maintained at 3V more than than their output, so circuit adjustments may be required. 
This is assuming the power transformer has the capability to provide the extra voltage/current.

 

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