Has anyone modded an EVH LBX II for less gain?

[calan]

Finally got hold of the schematic, and have an idea or two after posting on a couple other forums while waiting approval. I love the amp's tone and features, but the blue channel is just a noisy monster and WAY to much gain for my needs. I'd like to get a lot more resolution out of the gain pot, rather than everything past 9 being useless to me.

Here is the full schematic: EVH 5150 III LBX II Schematic Set.pdf

Was just wondering if anyone here has any experience with this amp. I'm a tube newb, but have messing around with electronics for a few decades.

Here is where I've landed so far, after some discussion over the last couple of days. I'm going to swap in a 12ay7 in V1, and then go from there. I may try the addition of R50 first, and then alter the gain pot circuit if neccessary.



Thanks for any advice!

[xtian]

First, replace V1 and V2 with lower gain tubes, like 12AT7.

In the schematic clip you posted, the Blue channel bypasses the bass shaping (C46) and the treble peaker (C45/R51/R69), and bumps the gain way up prior to V2a.

If you like the tone shaping in the Blue channel, but want to reduce gain, change R88 to a lower value (10K if you want to match the Green channel--then the only difference would be the EQ.

[calan]

First, replace V1 and V2 with lower gain tubes, like 12AT7.

In the schematic clip you posted, the Blue channel bypasses the bass shaping (C46) and the treble peaker (C45/R51/R69), and bumps the gain way up prior to V2a.

If you like the tone shaping in the Blue channel, but want to reduce gain, change R88 to a lower value (10K if you want to match the Green channel--then the only difference would be the EQ.

Thanks for the reply.

I'm replacing V1 with a 12ay7. I didn't like a 12at7 there.

R88 was my first thought, but after some discussion elsewhere the general consensus was that changing R88 could adversely affect the output impedance of V1-A, due to it being the only resistor in that path. (If the green channel is selected, r42 is in series with r48 to balance the impedance.) If I understood correctly...I'm still learning.

I don't know why they left R50 out of the circuit, unless they decided later that the blue channel needed more gain and pulled it. Doing the math shows that adding it would barely affect the green channel's signal level, but would drop the blue channel significantly.

I took off the bottom cover, and it looks pretty straight forward to add it and a switch. I moved the standby switch to the front already, so I could add a switch there to bypass R50.

[calan]

SO...

I only had a 100k and a 68k on the bench, so I series'd them up and tacked them onto the R50 pads, and (drum roll please...)

Huge improvement!

The gain knob now has much more resolution, and the amp just sounds cleaner. Still plenty of gain on tap, but not like a can of pissed off bees. I can actually max out the blue gain and have a tone that is usable, for that really saturated singing lead stuff. Back it off to 12 o'clock, and it's pure 80's Marshall. Taking it down to 9 gives a nice heavy breakup/crunch tone, and it almost gets to clean if I back off the guitar volume. (I couldn't get it past 10 or so before without creating a big fizzy mess, and nothing close to a usable crunch). The actual tone doesn't seem to have changed at all and there is no added noise, and it seems much more responsive to the guitar volume knob now.

Now I'm curious to try some other resistors, and A/B the values. I may see if I can find a 150k or 200k trim pot to play with and safely wire it outside the chassis, probably through that big ole' switch hole in the back.

[Bmorjamn]

Calan, it seems by my searching for mods you have worked hard to figure out a solution. I wonder now if you left the mod in and still have the amp or if you moved on ?
Thanks , James

[xtian]

Just got one of these amps, and, like others, found the gain on blue channel to be absurdly high (I also own an EVH 5150III 50w to compare). I added a 180K resistor at R50, and behavior is MUCH better.