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 Post subject: Guitar Amp for My Son
PostPosted: September 30th, 2015, 9:02 am 
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This has been a project I've been threatening to do for years! Almost complete, it finally works.

I had a month's long setback when I destroyed a perfectly good vintage KILLER power transformer with a dead short at the rectifier socket. A ground tab on the octal socket was touching one of the plates, out of sight, and the fuse (naturally) didn't blow.

Ordered a replacement (different planform :( ), so doesn't perfectly fit the chassis, but it works!

I ran DC for the 12AX7s and needed to add a small cap to ground to get rid of all hum! Imagine that, a vintage tube guitar amp without hum!

At the suggestions of musicians (who know stuff!), I built a replica of a Marshall Plexi series JTM45. Used a reproduction chassis and faceplate, built a case, and tried to stick with as much correct as possible. Obviously, the iron is not correct.

The larger output and power transformers mean that he can run any of the common output tubes: EL34, KT66, KT77, KT88, KT90, KT120, 6550, 6L6GC, etc. I'm supplying a printed list of bias set points for any of the tubes.

The bias check points that I added are across 1 ohm resistors, so checking and converting is easy: 50mV = 50mA.

For speakers, a kind friend donated an empty open back case, into which I added a pair of Celestion G12 Vintage drivers (on sale at PE).

Only stuff remaining is to cover the front plate with Tolex, add printed labels for the tubes, make the lad an owner's manual (few words, pretty pictures, (ADHD!), and get a couple of logos made for the front panel of the case and the speaker enclosure. I'm thinking "POLANSKY" in a script similar to the famous Marshall script.

Little bugger better like it and use it. I built my little brother a stereo: speakers, amplifier and turntable (made the plinth). He sold it for drugs!


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PostPosted: September 30th, 2015, 9:03 am 
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Pics


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PostPosted: September 30th, 2015, 9:04 am 
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Wiring with the old power transformer and without the cap to ground for the DC portion of the filaments.


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PostPosted: September 30th, 2015, 9:18 am 
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Good job, Stuart!

That is gonna be one freakin super loud amp.

My son is worried about blasting out the neighbors with 1 or 2 6V6 amps, up to a pr of 7591A...I told him," Let them call the cops. and if you blow up a speaker, I'll buy a new one." Dupont Circle needs more rockin out, to be sure.

"Let that boy boogie woogie," as John Lee Hooker advised.

Jojo is actually getting pretty good. Much better blues slanger than me, but my main instrument was blues harp back in the day.


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PostPosted: September 30th, 2015, 9:19 am 
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I just love the old tag board construction......

Stuart Polansky wrote:
Wiring with the old power transformer and without the cap to ground for the DC portion of the filaments.


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PostPosted: September 30th, 2015, 9:26 am 
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Great work. As J-Rob said, it's gonna be loud in the house... For home use, it's hard to beat the old Fender Vibro-Champ circuit. I'm working on one now that'll be basically a Vibro-Champ with the Princeton Reverb reverb circuit added...

Roscoe


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PostPosted: September 30th, 2015, 5:43 pm 
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Thanks to all for the kind words. Compared to DIY audio, this has been pretty straightforward, still rewarding though.

Tag board is wonderful to work with. So easy to swap out components without fear of lifting a trace, or having to undo tube socket terminals in the case of PP wiring.

The real reward will be hearing my son make music out of it! Unlike J-Rob, D. McGown, Charlie, Johnny D., and so many others in the group, my only hope is to recreate the music others make, I have no skills at making music, unless playing the accordion when I was a kid counts, AND IT DOESN'T!!!

As to loud, well the Celestions are neodymium magnets and 98dB/W/M, so considering I used a 60W hi-fi OPT (probably can do lots more, mid-band) and a big power transformer, I'm guessing well over 60W output. Since the speakers can each handle 60 watts, well, maybe 115dB if someone pushed it. I guess that's loud, even at Roscoe's horn loaded house!


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PostPosted: October 1st, 2015, 9:04 am 
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Joined: March 2nd, 2013, 2:43 pm
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Location: Potomac, MD
Stuart,

Looks like a wonderful gift for your son. Nice-looking build.

David


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PostPosted: October 1st, 2015, 10:20 am 
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Thanks again for the kind words. I need to take a picture of it now, with Tolex on the front. It turned out surprisingly nice. Just need some labels and an owner's manual.

Today is Alex's birthday. He came in with his guitar at my request. I presented the amp and speakers to him and unfairly asked him to play. He hasn't played for several years with any regularity. He sold his Hot Rod Deluxe and seven or so guitars when we had to move out of Plum Point Road. He was desperate for money.

I took a few short videos of him playing, and with a little practice, I think he'll be back to his former self. Next I need to get him a Strat or RG series Ibanez. Hopefully the link will work.

http://s894.photobucket.com/user/Sepola ... /Alexander


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PostPosted: October 1st, 2015, 10:57 am 
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I have a couple potential loaner strat type guitars...mostly homebrew by J-Rob Lutherie but also a few vintage jap models. Got a MIJ Fernandez that is a real nice vintage style player, to name one.

I also have a few import humbucker jobs that play great.

Building guitars is the next step, possibly even more infectious than building amps.

I have more guitars than most bands. Too bad I suck.


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