Tuesday, May 29, 2007

Success is Thick and Juicy!

So it's been a while since I last blogged. Here's why!!

I've been working to complete my most challenging project yet.

A single ended KT88 tube amp. This amp is a variation of the amp schematic from Angela Instruments (www.angela.com) from their how to section - the El34 schematic.

I've changed some parts out as they were called out in the schematic/write-up but it is still essentially the same.

So here's the details of how this project deviates from the schematic:

1. I changed the choke to handle higher current model. Hammond 193M
2. I changed the power transformer to the Universal transformer (also higher current capacity). Hammond 372JX
3. I used a 12ga solid copper ground bus
4. I bypassed the output stage capacitors with a Dayton 1uf polypropylene capacitor.
5. I wired it with Dayton microphone cable on the signal side. (using the shielding as a drain)
6. I used flat speaker wire for the output side.
7. I used Kimber hookup wire (I think it was the 19 ga stuff).
8. I used KT88's from Tube Depot (matched). Currently the Sletvana SED's.
9. I used the Hammond 1627ESE Output Transformer
10. I changed the output cathode resistor to 500 Ohm (with 28 V at the cathode).

Construction Notes:

Since this amp was created from just the write up off of Angela's web site I had to do my own wiring/chassis layout etc. If you're interested in building one - let me know and I can be cajoled into sending you the plans.

Hammond Box - I used a 10x17x4 grey chassis - if you're going to veneer the box - use this one as the black one is textured. If you aren't going to fiddle with veneer then go with the black box. The transformers I used are so heavy the box top caves in (a very little). I choose not to worry about it - if I didn't say anything you wouldn't have noticed.

If you are interested in this project, please email me at (hudsonmiller ampersat gmail period com) and I'll send you the layout/drilling plans for the lid. I didn't do one for the ends so you are on your own there.

Drilling the chassis - use a drill press and a 1 1/8" uni-bit for the tube sockets. Deeply punch the starter holes or the drill bit will walk off center. (it may still). CLAMP your work.

I used a bottom plate and constructed feet out of cleko's - apparently these are used in the airplane industry to clamp sheet metal together while riveting. I love them - no screws - just a $6 clamp tool and the cost of the feet. HIGHLY recommend these.

I used the parts express speaker posts and RCA jacks. Other IEC sockets, switch, and fuse holder come from either PE or Rat Shack. The speaker posts are to long to work in the chassis without a spacer block. Drill holes into it and put the speaker posts through then make your connections to build up the back a little.

Wiring is a 2 evening affair. Especially if you get me to send you the wiring diagram.

The tough part of a project like this is the box work - cause if it doesn't look good it may sound great but will still be garage junk to the wife.

I choose to veneer the box in burled maple on the sides and red mahogany on the top. I would have done the whole thing in maple but I couldn't find a sheet of burl long/wide enough here to bother. The maple is sanded, tack clothed, 1 coat of clear lacquer, tung oil, then 4 more coats of lacquer. I did this order to control the color the tung oil brought out. I found on my sample piece that if I put the oil down first it got a bit dark for my taste.

For the veneer I used the oil based contact cement. As before with the LP#9's I used the scraper method to apply pressure to the veneer. I did get one bubble - which I subsequently pierced and then ironed flat. This has worked so far.

Copper detail is kitchen door hardware - I had to drill out the volume knob to fit the volume pot shaft. - there's also a set screw in the bezel of the knob.

For wiring - I just started with the transformer wiring and built it up from there. The ground bus I'm very pleased with. Initial power on for this amp had NO HUM. at any volume level. Literally - no tweaks since I turned it on for the first time. I'm thrilled. And I am convinced it's because of the ground bus.

All point to point wiring. Where resistors/caps hit more than one pin on the tube socket I left the leads long and ran the lead through both pins and soldered both down to minimize connections.

The output cathode capacitors are twisted together, then a piece of stranded wire connects the cathode resistor to the pair of output capacitors. I ran the strands into the groves of the first twist, soldered the joint - and then trimmed the leads so one capacitor's lead came out one end and the other cap's lead the other. Then hot glued them all together.

As you can see in the wiring - power for the heaters is on one side - signal on the other. Where the heater wiring is close to signal (in or out) I've tried to cross at 90 degree angles and give some vertical separation tool.

It sounds - well - "Thick and Juicy". The bass is fantastic and the mid's and treble are just there - pleasantly present - not in your face. I'm absolutely thrilled with it and it has matched perfectly with my LP#9's. I've stopped using the Champ for now - it's waiting for a pair of low efficiency speakers next winter...

Lessons Learned
1. DO use Angela. Their service is exceptional.
2. Do use your community for questions. Paul Joppa, David Walters, Luther Ward, and Ray P. have been an immense help. See the Bottlehead forum
3. Do get the tube amp bug by buying a Bottlehead kit and get your feet wet.
4. Do call Steve at Angela and ask him a technical question. After/When he shuts you down "I'm not sure you're qualified to do this project" - you will be pissed and vow never to do business with him again. Then, when you realize he's in business to send you parts and DIY means just that DIY - you'll turn your well deserved insult into a new mantra to learn your stuff instead of trying to take the easy way out and ask him. Angela's service is exceptional - be happy with it.
5. Don't try and cut your power switch with a dremel - I've not found a good way to do this without fixing it with bondo.
6. Do use handles on the front/back of the amp - this thing is heavy.
7. Do buy the GZ37 rectifier tube before they are all gone.
8. Do put considerable time into figuring out what your first album will be - it will be a great experience and should be well planned.... ;-) Enjoy!!!