Monthly Archives: February 2014

Potentiometer and Input Jack Wiring

The next step was to mount the pots. I went with sealed the P.E.C. brand that I purchased from Valvestorm. The trouble with these pots is that they are stainless steel and very difficult to solder a bus bar across the backs. Doing some research I found that a buss bar could be run just connecting the lugs that needed to be grounded.

*** NOTE – I later abandoned the buss bar and switched to the “Larry” grounding scheme due to a hum in the amp. ***

I next wired up the input jacks using shielded cable. I originally had the 68K resistor mounted on the board but after several suggestions I moved it directly to pin 7 on the V1 preamp tube. This is to help reduce noise.

*** NOTE – I later replaced this very skinny cable with Lava ELC which has a braided shield for better noise rejection. It is also a thicker gauge so breakage on the solder points is minimized***

Switch and Back Panel Wiring

I installed the power and standby switches. I am not putting the front panel on yet as this only needs to be installed when the pilot light is mounted.

The left lug of the power switch is wired to one conductor of the mains fuse. The right lug of the power switch is wired to the black 120V primary wire from the power transformer.

I installed the Neutrik powerCON connector. I wired the ground to it’s own ground lug, the neutral to the to the power transformer primary common, and the live to the other terminal on the mains fuse.

The left lug of the standby switch is wired to the left filter cap. The right lug is wired one conductor on the HT fuse.

The other lug of the HT fuse gets the output transformer center tap and one wire from the choke.

I grounded the 6.3 volt center tap and high voltage center tap to a transformer bolt.
***Note – I later moved these to a cap can clamp bolt***

I soldered the 1K 7watt Welwyn resistors to pins 4 and 6 on tube sockets V4 and V5 as well as a length of wire to pin 6 on V4.

Next I wired the bias test points. These are female banana connectors. I installed 3 across the back panel. Two red with a black in the center. Between each red and the center black connector I soldered a 1ohm 2 watt resistor. I joined pins 1 and 8 from the EL34 tube sockets V4 and V5 and then soldered each one to the nearest red test point. The black test point is soldered to a ground lug on the V4 tube socket bolt.

 

Switches and back panel wiring.

Switch and back panel wiring.

 

 

Output Transformer and Choke Wiring

The next step was to wire in the output transformer. Using a diagram provided by Chris Merren, I wired the white primary wire to pin 3 on the V5 tube socket and the red primary to pin 3 on the V4 tube socket. The back amp plate must be installed at this time. I fed the secondary wires from the output transformer through the impedance switch hole to solder them to the switch before bolting it in. I soldered Yellow (4 ohm) to pin 1, Green to pin 2, Gray to pin 3, the orange wire is left hanging for later connection to the speaker jack. I found this method easier than trying solder the switch mounted. I used #4 stainless machine screws and Nyloc nuts to bolt the switch in place.

One of the choke wires is twisted with the OT center tap (Brown) and will go to the HT fuse holder. The other is left hanging at this time .

Small cable ties keep everything neat.

***You will notice that the extra cap is missing. This step is slightly out of order as I changed the position of the cap from the chassis wall to right next to the choke as described in the previous step. The cap clamp on the wall of this picture was removed.***

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Capacitor Mounting

I am using vintage reformed ARS multi-section capacitor cans that I picked up from Valvestorm. I was originally going to mount the third can on the wall of the amp as in my previous builds, but instead drilled an additional hole in the chassis using a monster step bit,  so there is more room for wiring. It is also provides a more traditional look. Since the ARS capacitors are slightly smaller than a standard 1-3/8″ can clamp, I had to slightly modify the clamps. I used #6 Stainless bolts to bolt the clamps to the chassis and #4 stainless bolts in the hole that pinches the cap. I am using 20 gauge PTFE wire for the leads.

Time to Build The Amp

I have most of the parts collected and it is time to start putting this beat together. I ordered stainless steel hardware from Bolt Depot. I go as heavy as I can on the hardware and every nut and bolt is stainless. I also use Nyloc nuts where ever possible so the amp stays together when vibrating during transport or at loud volume. To quote Spinal Tap, “Heavy Duty – Brings out the duty in my soul.”

I inserted the 1/2″ and 3/8″ rubber grommets in the chassis that prevent the output transformer and choke wires from rubbing on metal. I got these and many parts from Valvestorm. They have a very cool 50 watt checklist of parts. This list does need to be slightly modified for a 2204 circuit.

I bolted in a Merren 1202-118 power transformer with #8-32 x 2-1/2″ bolts and Nyloc Nuts. I put three grounding lugs on the bottom right bolt.
***Correction*** I later moved the ground lugs to the bottom left bolt and used only two.

I bolted in a Merren 784-139 output transformer with #12-24 x 1/2″ bolts and Nyloc nuts.

I bolted in a Marstran 1999 choke with #12-24 x 1/2″ bolts Nyloc nuts. I used a tapered reamer to widen the holes just a bit.

I bolted in Amphenol Bakelite power tube sockets. Chris Merren (transformer supplier) insisted that I used Bakelite sockets as he swears they improve the tone. Who am I to argue? He sent an eBay link and I purchased them right away. I did have to use a Dremel with a wire brush to clean up the terminals. They are pretty tough to get centered in the tube socket holes just right as they use a spring clip. I used #6-32 x 3/8″ stainless bolts and Nyloc nuts.

I bolted in Cinch Bakelite 9-pin tube sockets for the remaining tubes. I used #4-40 x 3/8″ stainless bolts and Nyloc nuts.

I used a drill to twist a black and red pair of 18 gauge PTFE wire for wiring the heaters. These went from the 6.3V heater terminals on the power transformer to pins 2 and 7 on the power tube sockets and pins 4/5 and 9 on the preamp tube sockets. This takes a long time to get neat. I cursed many times getting them perfect.

I bolted in 3/4″ stainless standoffs with #6-32 x 3/8″ machine screws to support the circuit board. I had to custom drill these for my unique circuit board. I put a ground lug on the standoff closest to the input jacks for grounding.
***Correction*** I changed these from 3/8″ to 3/4″ in height to provide more wire clearance under the board. I also moved them over by one hole (3/8″) closer to the power transformer for more accurate circuit board mounting.