Let's build an amp! Greg's journey to electrocution

[einstein magoo]

Reckon what this thing would sound like through a single 15 in a nice tight cab? I think some of the old surf guys used fender combos with 15s?
Probably never know cause you are a Marshall cab guy Greg. Just a thought.

[Greg_L]

Reckon what this thing would sound like through a single 15 in a nice tight cab? I think some of the old surf guys used fender combos with 15s?
Probably never know cause you are a Marshall cab guy Greg. Just a thought.

Well, for me, I am not a fan of 15" speakers. Fender had single 8, single 10, single 12, two 12s, four 10s, single 15...lots of cab/speaker options came with this "blackface" amp circuit. None of those cabs were "tight" though. Most of them being ported or fully open backed doesn't equate to "tight".

I'm not so much of a Fender amp purist that I have to have a Fender type cab and speaker. I'm not chicken-pickin or playing Texas blues. The intent from day one was to have it work with my Marshall cabs.

[einstein magoo]

Understood! It was just a fleeting thought. I really have no experience with single 15" in a guitar cab, just curious if you ever had experimented with them.

Thanks for the reply!

[Lt. Bob]

actually, a good 15 has about as much highs as a 12 ..... they just have more lows.

For a full-tange sound a good 15 is great.
That's why pedal steel players often use them.
But rock guitar isn't really a full-range sound .... too hi-fi sounding.

[Greg_L]

Understood! It was just a fleeting thought. I really have no experience with single 15" in a guitar cab, just curious if you ever had experimented with them.

Thanks for the reply!

I don't like 15s for anything. Not even bass. 10s and 12s are IMO perfect guitar/bass speakers.

In my mind, and ear, a British voiced speaker like a Celestion in a closed Marshall cab lends itself nicely to a mid-scooped-by-design circuit like the Black/Silverface Fender amps. It's not the traditional way to do Fender tones, but I'm not using traditional Fender tones. The lack of mids in the amp is sort of helped out by the midrange hump of the speaker. I don't know why more people don't use them this way.

[WhiskeyJack]

When i think of 15inch cabs i think of this.



It is from the movie back to the future if you do not get the reference.

[BBvideo=560,315]https://www.youtube.com/watch?v=a-njcAia6nY[/BBvideo]

[Greg_L]

I remember back in the late 80s/early 90s all the hood rats had 15s and 18s in their cars. It was bass mania. I don't hear that trend so much anymore, thank goodness.

[WhiskeyJack]

I remember back in the late 80s/early 90s all the hood rats had 15s and 18s in their cars. It was bass mania. I don't hear that trend so much anymore, thank goodness.

Oh god. me too.I grew up in a poor rural area so it was just Walmart car stereo accessories. I'll let your do the math on how awesome that all sounded.

[Greg_L]

Oh god. me too.I grew up in a poor rural area so it was just Walmart car stereo accessories. I'll let your do the math on how awesome that all sounded.

I had a buddy in high school that had like a 78 or 79 Datsun station wagon. Nothing special....except he filled the back with 18" woofers and had some kind of 6x9s in the doors and dash. Totally stupid. I don't know how many watts it was, but it was loud as fuck and he didn't even listen to rap. He blasted shit like Metallica and Anthrax through his ghetto setup. Lol. Totally crazy. He managed to find the bass in Metallica's Justice For All.

[JD01]

Haha, someone has actually done a version of that album on YouTube called ...and Justice for Jason. I don't know if they've retracked the whole bassline over the top or what, but you can hear it!

[Minerman]

I had a buddy in high school that had like a 78 or 79 Datsun station wagon. Nothing special....except he filled the back with 18" woofers and had some kind of 6x9s in the doors and dash. Totally stupid. I don't know how many watts it was, but it was loud as fuck and he didn't even listen to rap. He blasted shit like Metallica and Anthrax through his ghetto setup. Lol. Totally crazy. He managed to find the bass in Metallica's Justice For All.

I was "that guy" for a long time dude...The last stereo I bought/installed was in a 2003 GMC Sierra 4-door 4wd truck...All Kenwood, reciever/cd player, 2 power amps, 2 - cabs behind under back seat (15" & 8" in each), plus the factory speakers were replaced with Kenwood, IIRC, a 6" in each front door, 2 tweeters in the dash, & 4"x6" in the back doors...I had one amp for the extra cabs, & one for the stock location speakers...I bought the installation kit from Crutchfield, which had instructions how to replace the factory speakers without destroying in interior & it looked great...

That stereo fucking rocked, but it should've because it was quite expensive...Those were the good old days of Alice In Chains on the cd player with my daily dose of about 300 mg of Oxycontin raging through my bloodstream

The guys at work could hear what I was listening to on the way to work long before I got there which was quite a feat because it's noisy as fuck even outside of a mine with the fan & belts running...

[Greg_L]

Update:

The amp still works awesomely. I'm a genius!

[rayc]

Update:

The amp still works awesomely. I'm a genius!

Undoubtedly - though the question remains...are you a stable one?

[Greg_L]

Undoubtedly - though the question remains...are you a stable one?

I think the amp is definitely more stable than I am.

[rayc]

PERFECT response.

[Greg_L]

Update:

As this amp approaches one year old, it's still going strong. Not even a hiccup....not since all the initial hiccups. And it's been ridden pretty hard in it's usage up to this point. I think that if I was gonna have any catastrophic failures they would have happened by now.

I've come to realize what I knew would happen all along - I don't use the "Normal" channel. The Vibrato channel has better tone, slightly more gain, and the reverb/tremolo, which I love, only exists on the Vibrato channel. The Normal channel is bare bones and just sits there.

Sooooo...I think I'm gonna mod the Normal channel. I've been looking at the schematic and the Normal channel is totally independent of the Vibrato channel until they come together at the phase inverter. This means that I can do whatever to the Normal channel and it won't have any effect on the Vibrato channel - which I definitely don't want to mess with. So what to do? I'm gonna turn the Normal channel into a Plexi!

It looks like there are only four components that I'd need to change. Here's a pic. I boxed the Normal channel preamp in red, and the changes needed circled in green.

normal to lead mod 2.jpg

1) First stage cathode/bypass changed to .68uf/2.7k - plexi bright channel spec
2) Plate load resistor changed to 220k. Marshall specs a 100k and that's what this amp already has, but the 220k should bump the gain a bit. Might just leave this alone.
3) Coupling cap changed to .0047uf. This will shave off some unnecessary low end.
4) Not sure yet on this one. Each channel has it's own preamp tube. The second half of each tube for each channel share a cathode bias and bypass. I don't think I'll have to split them, but I might. The stock Fender spec is the same as Marshall spec.
I don't know yet if I'll need to mess with the tone stack. I might just leave it alone.

These changes will boost the gain and brightness of the Fender Normal channel to Plexi bright channel preamp specs. After all that, the signal will pass on to the phase inverter and everything else will be unchanged. So if it all goes to plan, the Normal channel will be like a bright channel Plexi preamp going into a 22w 6V6 power section. It just might work!

[WhiskeyJack]

I would never in a million years look at an amp and be like" hmm, i never use this hole what can i do with it"

I am looking forward to seeing how that all comes together for you. I don't imagine it will be a very costly venture either? I stand to be corrected.

[JD01]

Greg, would it be possible to make this amp a switchable two channel amp? So you could switch between plexi crunch and fender cleans

[Greg_L]

I would never in a million years look at an amp and be like" hmm, i never use this hole what can i do with it"

Lol. I'm just the opposite. I see an unused feature and wonder what I can do to make it useful. The only thing that separates a Fender preamp from a Marshall preamp is a small handful of capacitors and resistors.

I am looking forward to seeing how that all comes together for you. I don't imagine it will be a very costly venture either? I stand to be corrected.

It might cost like $2.00 in parts and about an hour of time, and I probably have the parts already.

[Greg_L]

Greg, would it be possible to make this amp a switchable two channel amp? So you could switch between plexi crunch and fender cleans

It already would be switchable, as in, you reach over and switch the guitar cable to the other channel.

If you mean footswitchable or all going through one input....that's possible but above my level of understanding right now. That would involve relays and maybe even ICs and I don't wanna go that route.

Edit...actually [mention]JD01[/mention] , yeah, I could put it through a DPDT switch and have it all go through one input.

Guitar > input jack > switch up > Plexi channel - or - switch down > Vibrato channel. That could maybe work.

I could scrap the Normal channel "low input" and put the switch there. It'd still be manual switching, but the whole amp could be accessed through one input jack.