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Jumper K. Balls
Jumper K. Balls UberDork
1/18/17 10:48 p.m.

This build is going a bit past what I feel was proper for the guitar/bass gear thread so here is the proper build thread.

Some years back I found an old PA head at a auto parts swap meet. I bundled it with a turbocharger and got it for $5

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This lil' guy sports a pair of EL84 in the output section and a pair of 6EU7 as a preamp and phase inverter. A 6V4 rectifier provides the DC to all of them.

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So I scabbed in a random electrolytic cap to quell the hum from the bad filters and plugged a guitar and speaker in.

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It made noise. The transformers checked out and seemed perfectly suitable. I scoured through some circuits thinking of something like a Vox AC15 and settled on a Matchless Spitfire

Now this Power transformer doesn't have a 5V tap and after looking up the specs on the 6V4 I decided to bin it in favor of an EZ81 rectifier.

So now it was on to starting to build the amp. A piece of surplus stainless was bent into an 18X2X8 chassis

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And in an effort to be clever I re purposed a fiberglass safety sign and turned it into the eyelet board

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and mounted some components

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The chassis was cut for transformers and the ceramic tube sockets

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And the heaters and transformers were wired up. The output transformer is on a 3 way rotary switch to give 4-8-16 ohm outputs.

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At this point I needed to order the tubes. I set the project in a box, slid it under the dining room table and forgot about it. FOR 18 MONTHS! I am not sure if these speaks more to my abhorrent cleaning habits or total lack of use of the table.... and dining room.

RossD
RossD UltimaDork
1/18/17 11:01 p.m.

Nice work. 6eu7? Why not 12ax7 from the schematic?

Fitzauto
Fitzauto Dork
1/18/17 11:03 p.m.

This excites me.

Crackers
Crackers Reader
1/18/17 11:06 p.m.

I've been wanting to make a vertical 2x12 cab with a slot for a 2u rack mount bass amp.

I can't play well enough to justify it though. Lol

Jumper K. Balls
Jumper K. Balls UberDork
1/18/17 11:11 p.m.

OK. So I was doing my daily lunch break craigslist browsing and some dude was selling a new pair of JJ EL84 and a pair of JJ 12AX7. I handed over the $20 and decided to get back to the dusty box under the table.

It became painfully clear to me that the 1/8" eyelets were too small for all the component leads in certain locations. Also the Eyewash station sign seemed a bit cramped.

Enter a chunk of 1/8" Micarta sheet

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I sawed out a pair of 4X12 slabs and set them in the mill

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This was probably WAY too accurate for what I am doing

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A trip across the street to the Tandy leather store for a bag of 3/16" brass eyelets and setting tool

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The tool sucked, I think it must have been a 1/4" tool in the wrong bag. The arbor press and a dead center from the lathe did the trick

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These look GOOD! something about that canvas based phenolic and brass. Almost a little steampunk

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Started moving the components over

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I found the missus' jewlery pliers whilst upstairs and a spool of 12ga solid core so decided to get a little fancy on the ground buss.

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To mount the board to the chassis I turned some 0.5X1.125" standoffs on the lathe and tapped each end.

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and mounted them to the board

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And mounted the board to the chassis

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Then I changed my mind again and decided to go with a filter choke instead of a power resistor so I flipped the chassis over again and mounted a 5H 150mA Hammond unit.

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I also picked up some reproduction Harmony "cupcake" knobs

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And that is where I am at this point. I have a few packages in transit with USPS for the final bits to have it electrically complete. Then I will need to build the cabinet.

So next the next episode will be lead dress, voltage checks and powering up.

Stay tuned, or don't..... This is a car forum after all.

I should probably join a guitar forum

Jumper K. Balls
Jumper K. Balls UberDork
1/18/17 11:12 p.m.
RossD wrote: Nice work. 6eu7? Why not 12ax7 from the schematic?

The 6EU7 is what the PA came using. I am going with 12AX7

Ransom
Ransom PowerDork
1/18/17 11:37 p.m.

Keep it here. I don't want to join a guitar forum. That might have... consequences.

Keith Tanner
Keith Tanner MegaDork
1/18/17 11:55 p.m.

I'm with Ransom. This place is bad enough.

And Crackers, build the rig. Then you'll HAVE to get good enough to justify it. That's my reasoning, all my guitar stuff is far better than I am.

RossD
RossD UltimaDork
1/19/17 1:37 p.m.

Sweet. I wasn't aware of the 6EU7. I'll have to check my stash to see if I have any as the data sheet said they're meant for AF and that's all I do (guitar and hi-fi)

I'm sure you've seen Triad chokes but, if not, here's a link. They are pretty a reasonably priced alternative to Hammond. http://www.alliedelec.com/passive-components/inductors/?navigation=4294945837-4294965009

Crackers
Crackers Reader
1/19/17 3:32 p.m.
Keith Tanner wrote: I'm with Ransom. This place is bad enough. And Crackers, build the rig. Then you'll HAVE to get good enough to justify it. That's my reasoning, all my guitar stuff is far better than I am.

LOL I'm really just a stereotypical drummer who owns a bass. My skills are on par with football humping monkeys.

RossD
RossD UltimaDork
1/19/17 3:33 p.m.
Crackers wrote:
Keith Tanner wrote: I'm with Ransom. This place is bad enough. And Crackers, build the rig. Then you'll HAVE to get good enough to justify it. That's my reasoning, all my guitar stuff is far better than I am.

LOL I'm really just a stereotypical drummer who owns a bass. My skills are on par with football humping monkeys.

Great Band name! Whens you're first gig?

Ian F
Ian F MegaDork
1/19/17 4:37 p.m.

The Seymour Duncan forums aren't bad.

How much of the original amp are you using?

Jumper K. Balls
Jumper K. Balls UberDork
1/19/17 4:44 p.m.
Ian F wrote: How much of the original amp are you using?

Just the transformers at this point. They are the most expensive part after all

Crackers
Crackers Reader
1/19/17 6:47 p.m.

Jumper K. Balls
Jumper K. Balls UberDork
2/12/17 7:10 p.m.

OK, I have been hoping for a sick day or rainy day to finish this up but alas, I have been feeling just fine and the weather on weekends has been too nice to stay inside as well. Also my soldering station died so I ordered a new one. I don't recommend the Kendal 937D for this type of work. not enough wattage or something. Takes 4 times as long to heat these big joints.

In my attempts to keep the rapidly vanishing scavenged theme going I harvested some cloth covered wire from a 1953 Bristol wiring harness. This proved problematic due to the size. It was all 16ga and had a thick cloth wrapping. Actual current draw is so low that all I really needed was 22ga wire. There is also over 30 linear feet of wire inside this so I needed to save some space. I relented and ordered some genuine vintage fender style "pushback" wire.

Today I forced myself to sit down and finish it up.

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I now realize I made a mistake on the size of the new eyelet board. It is so big that while components have plenty of room the chassis is a bit crowded. I made it work.

All I need now are 2 100 ohm 3 watt resistors for the cathodes of the power tubes and a 180pf capacitor as the "bright" cap. I will grab those in the morning and get some Vietnamese food for lunch (need wooden chopsticks to push things around and find noises)

Once it works I can start on the cabinet. I am thinking of going with bandmaster/bassman style hardware to hold it onto the speaker cab.

Fitzauto
Fitzauto Dork
2/12/17 8:01 p.m.

Stop making me wanna build an amp.

Jumper K. Balls
Jumper K. Balls UberDork
2/16/17 11:46 a.m.

Figured I would start in on the cabinet.

Sometimes you want that E36 M3 square and your crappy table saw isn't good enough

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Can you spot the mistake?

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I should have drilled the longer pieces for the pocket screws. Getting the last piece on took forever with a 1/4" ratchet and the driver bit. But it is together, rigid and the corners are routed and rounded over

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And yesterday I finally installed the last few components, applied electrons and measured some voltages. I was concerned that with the volumes all the way up there was nothing coming out of the speakers. No hiss, no hum. Something was off and it wasn't working. Then I tapped on the input jack and heard the pop pop. I plugged a guitar in and got the single coil buzz and realized the amp just has zero background noise. After playing it I realize that it is way too quiet. Something is off. It is less than half the volume of my 5 watt stuff through this same cab.

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Tonight I will check the bias and see what current it is drawing. I am sure there is a simple mistake somewhere. It should be putting out 18 watts.

Ransom
Ransom PowerDork
2/16/17 12:02 p.m.

Looking forward to hearing it!

DocV
DocV New Reader
2/16/17 3:43 p.m.

Looks awesome. If I may ask, what is your background? I have always wanted to play with stuff like this (safely) but have not due to the cautionary warning about the voltages contained within tube amps and their capacitors ("you'll shoot your eye out!").

Is that a Squier '51 in the pics above?

SilverFleet
SilverFleet UberDork
2/16/17 4:24 p.m.

Pretty freaking cool.

Jumper K. Balls
Jumper K. Balls UberDork
2/16/17 5:01 p.m.
DocV wrote: Looks awesome. If I may ask, what is your background? I have always wanted to play with stuff like this (safely) but have not due to the cautionary warning about the voltages contained within tube amps and their capacitors ("you'll shoot your eye out!"). Is that a Squier '51 in the pics above?

My background? Industrial mechanic and auto restoration. As long as the filters are discharged you should be safe.

Yes. I played around with the Squier 51 for a few months and got bored with it so it was sold.

RossD
RossD UltimaDork
2/16/17 8:57 p.m.

If you need to take live voltage measurements, stick one hand in your back pocket, stand on a rubber mat, and use a bamboo skewer to poke around. Connect the dmm before turning it on if you can. Just be careful and use your head.

Jumper K. Balls
Jumper K. Balls UberDork
2/24/17 12:52 p.m.

OK. So after a lot of poking around the volume is still too low. Barely any clean headroom. Brittle, harsh sound. The power tubes are biased at 16.4 watts. I think something is amiss in the preamp circut. So while I fight that battle I am carrying on with the other parts.

Tolexing the cab

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And I started machining the piggyback hardware

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The plates are simple 1/8" 6061. The pass through fasteners for the speaker cab started life as 1/2-20X1" bolts that I turned the head smooth and drilled and tapped to 1/4-20.

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I need to zinc plate the thumb screws and pass through bolts before installation, but it is turning out alright

Jumper K. Balls
Jumper K. Balls UberDork
2/27/17 10:39 a.m.

Still slowly working through the electronic issues. Some dodgy tube sockets are giving me grief ande I can't find my spares to replace them

Got the cabinet a bit further

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Chrome corners and handle installed

Drilled the speaker cab

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And mounted the piggyback bits.

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I like the look of the zinc plated brass. After seeing the yellow knobs against the buckskin tolex I realize they have to go so I am gonna make my own knobs now too.

Ransom
Ransom PowerDork
2/27/17 10:50 a.m.
Jumper K. Balls wrote:  photo P_20170226_141236_zps8abw1jlf.jpg I like the look of the zinc plated brass. After seeing the yellow knobs against the buckskin tolex I realize they have to go so I am gonna make my own knobs now too.

But they match the wrap on the smaller transformer! Nearly.

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