6G15

[tubeswell]

2 pics

Chassis is 16 Gauge aluminium sheetmetal (hand bent and riveted as usual) with laserflim on one side (which gets peeled off when the engraving is finished) 40cm (15.7") wide, 7.5cm (3") deep, 11.5cm (4.5") high - just a bit higher and deeper than a standard chassis for easier working in.

Cab is a mixture of timbers - Pine top and bottom, ply front and back and some oak drawer ends for the sides (found in a recycling centre for free - you can still see the handle holes in the middle). Clamped and glued together with cleats inside corners, dowels inserted for extra strength - whole thing will be sanded, routed and covered. 54cm (21.3") wide, 19cm (7.5") deep, 27cm (10.7") high

[txstrat]

Both pieces look very good. Expecially the hand bent aluminum chassis. Great work. What are you gonna cover the cab with?

[tubeswell]

Same stuff I used on my other builds. I've decided the faux-retro look of the little black & white gingham stripes'n'squares gives it a mesmerising euro wannabe '50s rock'n'roll suit look, (like something the Goons might've worn with their 'Twit Conway' impersonation of Conway Twitty).

[Sir Cuitous]

I have narrowed down my selection of isolation transformer to one of two or three Jensens, but I still don't know exactly what the output impedance is at the output jack of the 6G15. I believe that the Jensen JT-11 series is analogous to what is found in commercial products like the Nady HE-1. I have nearly every other part for this project. I will order the transformer tomorrow - probably the JT-11-FLCF.

[tubeswell]

Inching along on the woodwork

Ahhh... this is more fun than anything I know (how sad is that?)

Knocked off the corners/edges today with a 1/2" rounding bit. I'll get the engraver to work on the chassis next. I'll cover the whole thing in black'n'white retro fabric, so I'm not too worried about filling in the holes. Back panel is 3/8" ply (holes were made with a 2" diam hole saw and a gut-wrenching impact drill).

[tubeswell]

Air circulation qn

Well I have nearly finished the cab, bar covering it with fabric and putting the handle and feet on.

A question I thought about a few weeks back re-surfaced in my mind as I was looking at my handiwork - and no, its not "will it work?" - that qn is at the front of my mind ;-)

The qn is - will there be enough ventilation inside the unit?

It has 2" wide slits cut in the rear panel (see pic), each one is about 6 & 1/2" long (so that's about 24" sq of gap). Is that enough to ventilate 1 x 6V6 and 2 x pre-amp tubes and a PT? - Or should I make some more slots in the front panel? - I will be covering the front panel with dark-brown hessian (hence the dark brown 'camo' paint job on the ply). Hessian is quite a loose woven fabric and would let some air through, nevertheless, I could probably do quite a big cutaway in the ply panel (before I cover it that is) if I needed to. Or would that be overkill?

[Sir Cuitous]

"Heat is the enemy of electronics." - Morgan Jones

You could enlarge the ventilation on the back panel by creating two panels, one to cover the chassis, and a smaller lower panel at the bottom. Just an idea.

[tubeswell]

Ventilation Plus

Okay so with that quotation about heat in mind, I made an opening in the front panel before I covered it. It might make for an interesting effect viewed from the front when the lights are low. Anyhow that should be enough ventilation now methinks. The grille is detachable (with tee nuts). I also fitted the chassis to the cab. There is room inside for either a vertical mounting pan or a horizontal one.

[tubeswell]

Big iron or little iron

I put the iron on today and couldn't make up my mind which OT to run with.

The big one is a 25ishW SE Ot with 8k to 4R/8R/16R (so I'd have a couple of extra wires dangling around).

The little one is 10-12W 5k5 to 8R. The little one is a smidgeon lighter I reckon (but I'm not sure whether the big one will distort less in the bottom end).

On balance, I prefer the idea on not having extra wires hanging around (so maybe I'll keep the big one for another day)

I also cut out and mounted the eyelet board and a separate board for the rectifiers (from 1/8" electrical switchboard). With the thought of a bit of extra PIV buffer in mind, I made the 2nd board a bit longer so I can use two diodes in series on each side, with a 100nF 630V cap in parallel with each one.

[tubeswell]

Cab done

Dang I realised part-way through screwing the back panel on that I'd had it upside down. Ah well, I'll have to cover it with something

[tubeswell]

Eyelet Board etc

Finished the eyelet board and the rectifier board (with extra holes for zenerin' down the B+ if necessary later) - 4mm eyelets from Bunnings - they just tap in nice'n'snug.

Looking more complete with sockets and pots - only wiring up and pan installation to go now.

I need to score some chunky rubber feet with about 1/2" clearance.

(Damn that cock-up with the back panel)

[tubeswell]

Finished! (at 10:30pm on 31 Dec 2008)

I've still got to find some little feet so it can stand on top of my other homebuilds. Anyway here's some pics.

Key voltages:

Reservoir cap = 349VDC

B+ = 342VDC

Plate = 338VDC

Screen = 342 VDC

6V6 Cathode = 25VDC (990R resistor) = 25.25mA and 7.9W

CF Plate = 283VDC (I guess the cathode is about 1/2 that?) The heaters are elevated at the 6V6 cathode to make it a bit easier on the CF stage

I found some 3kV caps for the rectifier

I used shielded cable to the grids of the first two stages, and on the first leg to the grid of the CF stage), and split grounding, so time will tell how noisy it is - I haven't test driven it yet (waiting for tomorrow/later today when Mrs TW and the kids are awake).

I decided to build it stock and see how it runs. Maybe next time I'll add a parallel CF stage after the mixer to compare (or maybe I'll just wait for Sir Cuitous to build his one and see what it sounds like)

Happy New Year's BTW 8-)

[tubeswell]

Test-drive results Mk I

Varying results

With a real-short good-quality output (patch) cable - hardly any hum, but hummier with a longer cable (and I think this is maybe because of capacitance being induced by the longer cable?).

Also the hum is different into different amps. Into my 5G9 clone thingy, its a very quiet noise floor and hardly any hum at all with the short cable (- and to get sidetracked -the amp (5G9) sounds so gorgeous with the reverb unit connected that I'm thinking I might not mod that amp further after all). But into the 5E3 clone its quite a bit hummier. I am thinking the difference is that that 5G9 has a CLC filter in the power supply and is fixed bias, whereas the 5E3 has a CRC filter in the power supply and is cathode biased. (So - to get sidetracked again I was going to initially build the 5E3 clone with a switchable choke/resistor filter, and I am thinking I might now put that in and see if it makes a difference to the 5E3. The 5E3 clone sounds good and acceptably quiet by itself, but the reverb unit shows up some hidden flaws.)

The grounding system in the reverb unit is a split-ground type; with the CLC filter caps grounded together along with the output tube cathode resistor ground and output tube grid load resistor ground. All the other grounds, including the pot grounds (but not including the output transformer secondary ground, which is grounded at the reverb pan input socket) are connected via separate wires to a single wire going to the input socket ground. Also the output socket is bolted directly to the chassis with only a single wire from the mixer pot to the socket tip, and has no other ground wire attached. The heater CT is connected to the output tube cathode.

Anyway I wonder whether a little DI box with a ground-lift between the reverb unit and whichever amp would make any difference?

[mr.jetski]

nicely done TW

[txstrat]

Looks great. Congratulations TW