April 11th, 2012, 12:36 AM
As I was afraid of, I finished the amp build in just a few days. Didn't bother doing any voltage testing or anything, just plugged it in and turned it on and it worked perfectly!
Two boxes delivered Friday evening by the man in brown.
Dweezil was convinced there was something in the box for him.
Very considerate little guy, had to check off the packing list for me to make sure all the parts were there. Nicely packed and labeled, each component in a well-labeled plastic bag.
Very nicely built cabinet. All I did to it was a couple coats of Verithane and black paint on the bare wood inside.
There were a couple mistakes on the diagram so it was very well worth taking careful note of the schematic and also checking Weber's diagram for the 5F2A. One discrepancy on a cap and another with a couple resistors were cleared up with the Weber diagram.
For my first build, it looks pretty neat to me!
Best part: It Works!!
Too bad I have to wait until tomorrow when the upstairs neighbor is at work to crank it up.
April 11th, 2012, 12:48 AM
Congrats on your first build. It looks great!
When I built my first amp, a 5E3 I twisted the wires from the tranny to the pilot lamp, then twisted again on to the tube heaters. Also the 2 wires to the main power switch. Not sure if it is necessary, someone must have suggested it to me at some point. I believe it is supposed to prevent hum. But hey, if yours is quiet, then no worries. Make sure you check back and let us know how it sounds!
To answer your "Now what?"..If you find yourself craving more, build another tweed! Pick another model, there are so many to love!
Here is my twisted wire work:
April 11th, 2012, 12:55 AM
it does look good... regardless of it being a first build. it's a shame though that with all that real estate you have such a small speaker. is that a 6 or 8 inch?
oh, also if you wanted to have a "champ" you could replace your tone pot with one weber sells that is a 1m p/p dpst. lift the ground on your tone pot and take it out of the mix. for good measure you could also lift the one of the other connections. it does make a difference. an amp i'm messing with right now has a princeton setup in it. i used a 1m with an on/off switch on the back and lifted the ground. both settings are nice to have.
April 11th, 2012, 03:28 AM
I did not twist the power wires to the pilot light, but I did twist the wires to the heaters. The twisted heater wires seem to be totally hidden in this picture. it is an 8 inch speaker, which is the stock speaker size from Mojo.
April 11th, 2012, 09:17 AM
Did Dweezil initial a QC? ;)
April 11th, 2012, 01:34 PM
nice looking job for sure but the baffle does dwarf that little 8"..
April 11th, 2012, 08:23 PM
that looks awesome! i am planning on doing a 5f2a sometime in the near future myself.
question regarding the mojo cab - how do they attach the baffle? the traditional 4 screw way fender did, they use more screws, glue? just curious.
April 11th, 2012, 09:12 PM
Well, the amp has a very nice tone to it but it does have a bit of a hum going as well.
I will get back in there at some point and try twisting the pairs of wires coming off the transformer as recommend. Not a huge hurry to fix the hum as it is not very audible at condo practice volumes.
Might the amp benefit from Aluminum or copper tape on the inside of the back cover to shield the chassis?
Also, there was a .047 600v capacitor shown on the Mojo wiring diagram from one side of the main fuse to ground, which was missing from the kit. This cap was not shown on other versions of the 5F2A that I found, such as Weber's diagram. Since it was not there and not shown on other 5F2A schematics/diagrams, I didn't worry too much about it but I wonder what its purpose may be.
April 11th, 2012, 09:14 PM
The baffle is attached with four nuts/bolts.
April 11th, 2012, 11:44 PM
After you twist some wires, If you do things safely, with one hand behind your back and respect live power, you can poke around with a chopstick. Poke around some wires and see if the hum changes or goes away. This will help you track down the hum. If possible try and place any wires crossing the tube heater wires (also pilot lamp wires) at a 90 deg angle. I found plastic zip strips helped keep wires where I wanted them. Do not do this unless you understand and respect the power in your amp. It could kill you.
I believe some amps that had a 2 prong cord had a "death cap" off the main ac power supply. This was a capacitor that held power somehow i think to prevent a shock or something. Once the capacitor dried up and died, then you may also die when you got a shock. This may be shown on some Layouts but not others as it was removed at a certain point during production. I don't believe it serves any purpose if you are running a 3 prong plug.
April 12th, 2012, 12:15 AM
Yeah, have to go find a suitable resistor for cap draining - on my task errand list for tomorrow. Will do that before I take the back panel off again. Re-routing the heater wires is on the short list for the first round of tweaks.
April 12th, 2012, 03:02 AM
thanks itsforrest, thats good to know. & i appreciate the reply!
April 12th, 2012, 05:17 PM
If you separate the preamp and power amp grounds the hum will probably be gone. The power supply grounds can go to one of the PT mounting bolts, the preamp grounds can go to the input jacks.
April 12th, 2012, 05:39 PM
Get a simple wire that has rubber-insulated alligator clips on each end (Radio Shack).
Use the 100 ohm preamp plate resistor(s) that are already built into your amp circuit to drain your filter capacitors.
Put one clip to ground.
Put the other clip on one of the 100 ohm resistors leads.
Located between your Orange Drop Coupling Caps, these resistors are in a V shape. Put the clip on one of the tails of the V. Not the pointy end of the V, but the open end.
Now that resistor is draining your caps slowly without sparks or burns.
See, those resistors are fed by your third filter cap. All the caps will be drained in a few minutes. Remember the wire you ran under the board from your filter cap to those resistors? That supplies the voltage to your both sides of your double triode preamp tube.
April 12th, 2012, 11:54 PM
Thank you TNO!!!
Separating the grounds got rid of the hum.
The preamp grounds to input jack, filter cap/power amp grounds go to PT lugs, OT grounds to output jack.
While I was poking around I found one of the ac input wires pinched under the chassis mount nut, too.
Mojo also confirmed that the cap on the main fuse is not needed with three-wire power supply. So of you live in an old house with ungrounded outlets, that cap might be a good idea.
April 13th, 2012, 12:02 AM
Great news. I'm glad you got everything sorted out. I'd love to hear a clip of your 5F2A sometime.
April 13th, 2012, 12:09 AM
I was looking at those and wondered if the cab/baffle would accomodate a 10" speaker. It looks like there's plenty of room.
April 13th, 2012, 12:41 AM
I am currently in the middle of a 5F2A project I am building. I had a cab built and cut for 10" speaker. I am waiting for the chassis to come in, but the speaker is mounted in mine and there seems to be plenty of room.