Marshall AVT150H Gain mod

[TolerBear01]

Hey. I have a Marshall AVT150H and was wondering if there's any way to get more gain out of it without using an external pedal? I thought about doing the Tube Screamer or EQ in front of the amp for a boost in gain but I was wondering if there was a mod that I could do to the amp so that it's plug and play. Thanks!

Sent from my SAMSUNG-SM-N900A using Tapatalk

[J M Fahey]

We should see the schematic for any accurate answer, but I guess that all gain stages are already maxed out, doubt there's much left to tweak, if any at all.

And for really noticeable effect, gain boost should be *at least* 10 dB (~3X) and preferrably 20 dB (~20X) , while any tweak would be at best a couple dB, not worth it.

Since you are ready to solder anyway build yourself a clean boost pedal, which in its simplest version would not even have any pot or switch  :duh  , besides the footswitch that is.

I do that myself, I added an "extra distortion" setting in my Guitar heads which simply slams a fixed 100X gain stage    :duh at the input jack , go figure.

Works like a charm and nobody complains, all understand it once I explain: "the regular distortion pot goes from 0>10 , after you push the "Distortion II" button it then goes from 10 to 20 " .

For a fully developed and documented project, search for the pedal guys (GGG? - ROG?) version of MXR clean boost, they even post the ready to transfer PCB or you can build it on perfboard.

The only slightly hard to find part is the "C taper" reverse Log gain pot, I think it's 500k , but there's 2 McGyverish solutions to that problem, so no big deal.

[TolerBear]

I'm just trying to figure out how I could get more gain to hit the distortion circuits of the amp. I don't know if it can be done but maybe someone who knows how to read the schematic better than me could give me some pointers. I'm attaching the schematic to my reply.  8|

[J M Fahey]

Easiest is replacing OD2 gain pot from A50k to A250k (5X more gain) or A1M (20X more gain) and you affect just that channel.

If you only want to shred and clean is unneeded, replace R5 with 1k and C4 with 1uF, polarity is not important, you get 5X gain which is *multiplied*  by the boost in OD2 channel , but the guitar will sound crunchy even if "clean" because of the gain excess in Ic1a which is the front end for all of the amp.

*Really*  you shoukd add an extra audio gate connected to the footswitch so the last boost happens only when Od2 is engaged.

That said, it's much easier to build an up to 100X gain clean boost, you don't mess with the amp, etc.

[TolerBear]

Cool. I noticed the OD1 circuit has an A200k pot (VR1) and it has less gain than OD2. How does that work when I replace the A50k pot with a A250K pot  (VR3)? What value would I replace VR1 with to get it hotter? I'm trying to understand how that affects the gain structure. Ultimately I'd like to end up with an OD1 channel I can more easily get better pinch and natural harmonics and an OD2 make it my lead channel where the harmonics are almost effortless. I don't want to introduce allot of noise into the amp though. I looked and couldn't find A1M.

[J M Fahey]

The pot is but *one*  gain setting element, there's others in the circuit which complement it.

I found OD2 has more possible extra gain on tap with leasier modding (just 1 pot)  and less impact on original amp "voice".

If you can't get A1M use A500k but *really* A1M *is*  available, search a little more .

FWIW I can easily get them in Argentina  , worst case mine are special ordered from some manufacturer or larger distributor or I even have them custom made, although in that case minimum order is 100 units, with 300 having the best price.

[TolerBear]

Oh. The A1M is a variable resistor? A 1 Meg? Perhaps for the OD1 circuit? VR1? I'm sorry. I'm trying. Lol.  :dbtu:

[J M Fahey]

Oh. The A1M is a variable resistor? A 1 Meg? Perhaps for the OD1 circuit? VR1? I'm sorry. I'm trying. Lol.  :dbtu:

Easiest is replacing OD2 gain pot from A50k to A250k (5X more gain) or A1M (20X more gain) and you affect just that channel.

Mouser has as many Audio taper 1M 16 mm pots as you want, or 390 of them in stock for immediate delivery, for U$ 1.79 each  http://www.mouser.com/ProductDetail/Alpha-Taiwan/RV16AF-20-15K-A1M-3/?qs=SLyRFcDoYzEft%252b7mGJOdgg%3d%3d

[TolerBear]

Nice. Is there anything I can do with the OD1 channel to get more gain out of it without changing the amp voicing?

[TolerBear]

Anybody? 

[J M Fahey]

Gain 2 channel is far easier to mod than Gain 1 one to achieve what you asked.

You may pull and replace Channel 1 capacitors, resistors and pots to turn it into a Channel 2 clone; I fail to see the advantage of working more to get the exact same result you can get by simply replacing a single pot which to boot is of reasonable value ; to do a similar thing on Ch 1 you'd need a 3M or 10M pot *or*  replace all caps and resistors ... and then you'd get what Ch 2 does 

[TolerBear]

I was wondering because I use channel 1 for my rhythm channel and channel 2 I use for my lead channel. I like allot of gain because I use allot of harmonics in my playing and the extra gain helps to make it easier to do with better sustain with more gain. I don't want it muddy though. My style is similar to Dimebag Darrell 's, George Lynch, CC Deville, Reb Beach, and Bill Leverty in that I use long natural harmonics.

[phatt]

Just use a 250k pot,, or go right over the top and try 1 meg.
Bear in mind what JMF just noted. I'd be trying a few pedals before messing inside perfectly working amplifiers.

The reason I say that is simply because as you go up to 1 meg it will alter the tone range
50/50 bet it will be a problem.
Phil

[TolerBear]

Thanks.