Firefly build

[xtian]

I'm gutting the Packard Bell chassis and turning it into a Matchless Lightning clone. The chassis is compact (11 x 7 x 1.5"), so I'm keeping the 4-section capacitor can on top to maximize space inside for a custom turret board. I'm not going to put this inside a cabinet--I'm going to build it with the tubes exposed and treat it like a piece of studio artwork. Furthermore, I'm hoping to keep costs under $200, hopefully by selling the existing transformers.

First question: Lightning schematics call for 33/33/22/22uF, 450V filter capacitors. What will be the effect if I change those two 33s to 40s? I ask because tubedepot.com has a 40/40/20/20 can. Otherwise, I'll have to rebuild myself, or order a custom can.

Thanks, guys (and gals?)!

[M Fowler]

Yes you can use that can cap just fine.

I never thought about making my own can cap. What do you do open the old can cap place in two 33 and two 22 radial caps soldered to the pins and close up?

Seems like an octal tube socket with a cylindrical container glue over the top could also be used for cap can.

Just thinking out loud which could get me into trouble, Aye

[dynaman]

I've known folks to make their own multi-section caps by gutting the old metal ones. Used to be a site somewhere depicting this but I can't find it.

Ooh, I like the idea of using a tube socket. Many old input transformers employed octal sockets and I want to say some caps did too. Hmm....

[M Fowler]

I was thinking if you wanted to save space, the octal socket with some radial caps could be the ticket and then scrounge the garage for a canister of some kind. Apply some retro decals and presto.

I need to get back to work

[xtian]

I'm gutting the Packard Bell chassis and turning it into a Matchless Lightning clone.

About face! I'm building a Firefly! All the pretty stuff is finished…just have to do the hard part (the circuit!). Maybe I should stop now to avoid possible disappointment if it doesn't work when finished? Naw!

My build includes a rotary switch for 4/8/16 ohm speaker out, a bias switch for 12AU7 or ECC99 output tube, and no standby switch. Gonna make my own turret board. I ordered a walnut and maple wooden base to bolt it to.

I bought a unibit for drilling the chassis. Best money I've spent so far! I love that thing!

[Structo]

Interesting chassis there.
Looks like a good one for prototyping.

Although I wouldn't advise putting the fuse holder on the side.

[xtian]

Although I wouldn't advise putting the fuse holder on the side.

Why dat?


So I have a Hammond 269EX. Paul Ruby's startup guide (http://paulrubyamps.com/info.html#FirstPowerUp) says that with the PT not attached to anything, "verify that the connection points for the PT secondaries are open circuits. With no tubes and no pilot lamp, the 6.3VAC and 5VAC secondaries should be open circuits, no continuity."

Not true on my PT. The 5vac secondaries (green) read 0.5ohms, and the HV secondaries (red) read 276ohms.

What's the deal?

[xtian]

Really hoping for an answer about my PT before I apply power!

[Structo]

The fuse holder is in an inaccessible spot if the amp is in a cabinet, how would you change the fuse without pulling the chassis?
Typically you find the fuse next to the power cord or IEC connector.

Also it appears you have wires going through chassis holes without grommets. The wires could chafe and short out to the chassis.

I think he means the resistance between the various taps should be infinite.

I have read that start up tutorial and while it is a very safe way for the amp, in inexperienced hands you have wires hanging out that potentially have lethal voltage on them.

I find building a light bulb current limiter works pretty darn well to protect expensive components.

I power up the first time with a 25 watt bulb and if that shows no shorts I put a 75 watt bulb in and power up again.
If all is good I install the tubes and try the limiter one last time before plugging into the wall.

[txbluesboy]

You must be misreading his suggestions because the none of the windings are going to be open circuits when reading from one end of the winding to the other.When you say you are reading the secondaries(red) I assume you are reading from one red wire to the other red wire, in this case the reading you noted would be correct. Across the green wires you should have almost no resistance which you have. I think you are good to go. I always start my amps for the first time through a light bulb current limiter just to be safe.

[xtian]

Structo: This chassis is not going inside a cabinet. Good eye, though.

Yes, Paul Ruby's wording is very confusing. I found a couple other mentions on the web that put my mind at ease, and thanks for confirming.

[selloutrr]

its a quick easy build and sounds pretty neat exp considering it's using only preamp tubes. i modified my daughters amp, adding a tone control and line out w/ dummy load so it can be used as a distortion box. depending on the tubes and speakers you can get a very smooth breakup and nice studio amp out of this build.

[rfgordon]

On your filters, remember that the Lightning runs the plates at about 350 VDC and has an extra dropping resistor of 22k to a 33 mF cap in order to drop the screens to around 250 VDC.

[xtian]

On your filters, remember that the Lightning runs the plates at about 350 VDC and has an extra dropping resistor of 22k to a 33 mF cap in order to drop the screens to around 250 VDC.

I can't reconcile your comment with the schematic. See attached.

[xtian]

See photos. I'm following Paul Ruby's startup guide, but now that I have hooked the HV secondary pair from the PT to the filter caps, things are going badly. I began to smell something burning (a bit like laquer, not like PCB or electronics) and the PT got very hot. I measured about 225 vac between B+1 and ground before I noticed the smell and shut down.

The HV pair goes to the two diodes (correctly oriented with the bars toward B+). The LV pair goes to the heaters (no tubes are plugged in, so this is a dead end. I did check to see that there are no shorts).

Please review my grounding choices: The green ground wire from mains goes to the chassis. Everything else is grounded to a star ground point on the negative side of the filter caps, and there is a short lead from that star ground point to the chassis point at the same place as the mains ground. The PT center tap is also connected to the star ground point. You'll also see a big cement resistor to help drain the caps. This resistor is not connected when in test mode, only after shutting off the power.

I don't think I burnt anything up yet…but I probably got close!