Vintage Blue

I got a modest start on the power supply tonight.  I installed the newly potted power transformer, as well as the new multi-cap can and 3 new switches.  I got the 5v and 6.3v heater wires, and the power transformer secondary wires connected to the tube sockets.  I temporarily mounted the new turret board, in order to get the transformer leads run in an orderly manor.

Heater and secondary leads connected

My next step will be to wire the primary side of the transformer.  I will hold off wiring-in the fuse holder until I have everything wired to the cap can, since that’s easier to do with the fuse holder out of the way.

I got my potting compound yesterday, so I got the new power transformer potted.  It took more compound than I anticipated so I held off on potting the output transformer.  Once I have more compound I will pot it too.  I am using Hammond transformers: a P-T274BX power transformer and a P-T1650G output transformer.

Transformers w/temporary base plates

I decided to try something new while potting these transformers.  I wanted to be sure that the transformer leads were bundled and properly positioned during the potting process so that they would line up with the holes in the amp chassis.  To do this I took two pieces of 1/16”  aluminum and drilled holes in them to match the holes in the chassis.  This allows me to pull the leads through the large hole and attach the aluminum plate to the transformer can, thereby ensuring that the leads are properly positioned while the compound sets up.  It seems to have worked fine on the power transformer so I’m sure it will work okay on the output transformer as well.

I’m looking forward to getting the power transformer installed so I can start working on wiring the power supply.

This amp came with just the output transformer; the power transformer was missing.  Due to the overall condition of the amp, I was not comfortable trying to reuse the existing output transformer, plus the wires looked to be in pretty bad shape.  That meant I would need to remove the OT from its can so that I could repot a new one in it.  Fortunately, I also have a spare can here that I will be able to use for a new PT.

I had un-potted a power transformer about a year ago.  While it was somewhat messy, not to mention smelly, it wasn’t too bad of a process.  Removing this OT from its can proved to be more difficult, however.  Since the OT is smaller than the PT, the transformer had small blocks of wood wedged on 2 sides to keep it centered in the can. 

Melting potting compound in oven

This made the transformer much more difficult to remove, since it did not just drop out like I expected.  Once I discovered the blocks of wood, I was able to pull them out with a pair of pliers.  This allowed the transformer to drop out as I had been expecting it to, but not before I had created a bit of a mess with the liquefied (and extremely hot!) potting compound.

I cleaned up the 2 empty cans and decided that this would be a good time to give them a fresh coat of paint.  I had originally planned on using a black, crinkle finish paint but then I saw something different at Home Depot.  It was a black, textured-finish paint from Rust-Oleum (7220 Black),  so I decided to give it a try.  I was very pleased with the results!  To my eye the texture matches the original, subtle texture of the cans quite well.  I’d like to be able to use it on the tube cage too, but I’m afraid it wouldn’t hold up well with the heat from the tubes.

Everything is now ready for re-potting the transformers; I’m just waiting on new potting compound, which should be arriving today.

I got a modest start on the wiring by wiring the heater circuit and hum control pot.  After seeing photos of other people’s work I had pretty high aesthetic aspirations for my wiring.  I used solid, continuous buss wire to daisy-chain the sockets together, slipping Teflon tubing over the wire between sockets for insulation.  I found the combination of stiff wire and cramped space to be tougher to work with than I thought.  The end result looks okay but I was really hoping for better.  I did start improving toward the end, so I’m hoping to get better as I gain experience.

Heater & hum control wiring

Rebuilding the power supply seems like the next logical step, so I’ll need to get my power transformer ready.

Gutting the chassis was fairly straight forward.  Everything that I planned on reusing was placed in plastic bags and labeled.  This included items such as the control packets, hum control pot, 1/4″ jacks, etc.  The switches looked pretty rough so I’ll be replacing them with the correct models from Carling.  Since this was originally a B15NF, it utilized a printed circuit board.  As such, the tube socket tabs were bent over and soldered to the board.  Several of the tube socket tabs had become brittle, which caused them to break while desoldering them from the board.  Although I hadn’t anticipated this, it turned out to be a good thing.  This forced me to replace all of the sockets with matching NOS sockets.  In addition to installing new tube sockets I also installed a socket so that the amp would be able to use a removable power cord. 

Power cord socket

Since this amp was so messed up to begin with I’m not concerned about making such alterations.  I’m also considering making the speaker cable removable.

Outside of amp (tube cage not shown)

I purchased a B15N amp head on eBay back in August.  The head was pretty cheap, since it was being sold for parts.  I purchased the amp with the intention of using it as a tool to learn more about tube amplifiers and, more specifically, the B15N.  My goal is to gut the interior, rebuild it, and learn something in the process.

Damaged PC board

When I received the amp it looked pretty much like what I expected – the power transformer was missing and it looked like something had burned out pretty good in the power supply area.  Although the tray-mounted schematic that came with the amp was for a B15NC, the amp itself was actually a B15NF model.  Since I wanted to build this into a B15NC, I knew I would have to build a new circuit board to replace the damaged B15NF printed circuit board (PCB).  Before starting any work I photographically documented the amp’s current condition, then started gathering the various parts and supplies I was going to need.