5E5A build from a kit

[Hope]

Hi there, I'm trying to complete an amp kit for a Fender 5E5A (https://www.mojotone.com/Mojotone-Tw...mplifier-Kit_2). It's my first try at building an amp.

I chose this amp model because I had a Blues Deluxe reissue that burned out. Possibly because I tried to use it with a power attenuator. So I thought I would try to remove the old circuitry and replace it with this kit, keeping the cabinet and the speaker (Celestion Gold 12").

I got pretty much everything connected and then got lost trying to check everything.

A few problems came up that I couldn't figure out.

1. The output transformer has three wires: yellow, black and green. I assume that the green is for use if you want to output to 4 ohm speakers, so I just connected the yellow and black and wrapped the green in electrician's tape. Is this correct?

2. I tried to check the grounding of the amp using a guitar cable in the speaker jack. I found that the casing has continuity to the sleeve. However, if I disconnect the yellow wire from the output transformer off of the speaker jack, it also has continuity to the sleeve of the guitar cable in the output jack. That's not right, is it?

3. I think the layout diagram and the schematic don't say the same thing around the choke transformer. Maybe I am reading one or both of them incorrectly. the layout shows pin 4 of the second power tube connecting to the right-hand side of where the choke connects to the 475 v capacitors. But shouldn't the two pin 4's really connect to the left-hand side, so that the signal has to go through the choke to get to the red wire of the output transformer?

Any help on this project would truly be appreciated.

[The Dude]

1) Usually yellow is the 4 ohm tap and green is the 8. Read the part number on the transformer and look up Mojo's datasheet to be sure. They have datasheets available for all of their parts.
2) In your test, reading the yellow wire to sleeve (or ground) you are reading through the OT secondary winding, which will be very low resistance- almost a short read with a DVM, so this is normal.
3) I'm not sure I understand your question. Signal doesn't go through the choke. It's part of the power supply. Plate supply (or connection to the OT primary CT) is on the rectifier side of the choke, and screen supply is on the other side of the choke. I hope that answers your question.

[Enzo]

The output of your transformer - the speaker connection has a VERY low resistance. SO it may seem like the tip has connection to ground, but I suspect it is OK. Yes, just tape off the green wire if you do not use it.

The choke is in the power supply, no signal runs through it.

I can live with "left and right" referring to the schematic, but on the layout, it leaves me cold. The choke has no polarity. One end of the choke is wired to the red wire and the standby switch. The other end to the screens (pins 4) and the rest of the circuits. It appears to me the schematic left and right ends of the choke are reversed on the layout drawing. However as far as I can see, the schematic and layout drawings agree with each other. Follow the drawings.

[The Dude]

I think that's what he did. I see a taped off green wire in the first picture. I'm not sure the right tap is being used, though. Usually green= 8 and yellow= 4. The datasheet would say for sure. We also don't know what the speaker's impedance is, so maybe the 4 ohm tap is what is needed.

[loudthud]

The 5E5 {nothing} is like a 5E3 (except the extra input triodes) , it has no choke and just 3 knobs. The 5E5 A is one of those amps where everything goes through the choke. The layout drawings show this style choke connection. The schematic posted in post 1 shows the more conventional choke connection where only the screens and every thing down stream goes through the choke. Did that schematic come with the amp ?

[Enzo]

LT, he mentioned it was a Mojotone kit, on their web site the schematic for the kit is the one he posted in the photos

[g1]

The 5E5 {nothing} is like a 5E3 (except the extra input triodes) , it has no choke and just 3 knobs. The 5E5 A is one of those amps where everything goes through the choke. The layout drawings show this style choke connection. The schematic posted in post 1 shows the more conventional choke connection where only the screens and every thing down stream goes through the choke. Did that schematic come with the amp ?

That is the schematic they have under documentation for this model on the mojotone website. Agree that it is an error for 5E5A (see correct Fender version attached).
It's a 100mA choke that they supply (MOJO777).
Their website photo of completed chassis seems to agree with their layout. (but red & white wire connecting to pin4 of left 6L6 rather than right)

[g1]

And the Fender version of the 5E5A layout attached. Note to Hope , the red OT wire can connect to pin4 of either 6L6, it doesn't matter which.

[loudthud]

If the choke is only rated at 100mA, it's not suitable for the original 5E5A connection. I would suggest the more conventional connection and 470 Ohm screen resistors.

Edit: I remember a thread about the E series choke from some years ago. I ask the original poster to post a picture of the choke from an original Fender amp and it was HUGE.

[Hope]

thanks for the helpful comments everyone
thanks for the original Fender schematic, G1, I only had the one from Mojotone.

I checked the schematic drawings for the output transformer and the choke:

https://www.mojotone.com/transformer...ER-MOJO769.pdf
https://www.mojotone.com/transformer...77%20Model.pdf

My speaker impedance is 8 ohms, so, based on the drawing, I connect with the yellow and black, not the green, I think.

The choke is rated at 100 mA on the drawing.

So following the layout drawing, the power from the standby switch goes through the choke, then on to the pin 4's of the power tubes. The amp seems to work set up this way. What would be the optimal kind of choke for this set up, if not the one in the kit?

Is the recommendation for screen resistors of 470k 3w, connected between pin 4 and the power from the standby switch (through the choke)?
https://www.evatco.com.au/3-watt-metal-oxide-resistors

[Helmholtz]

What you describe is the original Fender wiring.

As loudthud said above you can't use the 100mA Mojo choke with the original Fender 5E5A wiring.
Though you won't notice a problem at idle or low output, the choke will saturate and overheat at large output.
For the original circuit you need a choke that can handle 200mA to 250mA.
Check the Hammond website for 5E5A replacement choke.

You might use the Mojo choke if you change the wiring as shown in the Mojo schematic.
Means that the CT of the OT primary should go the first power supply node, i.e. the first filter cap.
(The original schematic doesn't show a standby switch, while the layout does.)

[Enzo]

The Mojo schematic shows the switch. Other than the confusion at first, is there any reason not to use the circuit as drawn from Mojo with the Mojo kit? The choke was chosen for their circuit, not the Fender circuit.

[g1]

Is the recommendation for screen resistors of 470k 3w, connected between pin 4 and the power from the standby switch (through the choke)?

470R, not 470K.
As the others noted, either change the OT center tap connection to match the mojo schematic, or get a bigger choke if you want to use the Fender schematic.
The mojo layout drawing is incorrect for the choke they are supplying.

[Helmholtz]

This choke should work fine with the original wiring:
https://www.hammfg.com/files/parts/pdf/194G.pdf

I'm sure there are more options.
More inductance improves ripple filtering but either requires a larger core size or a higher DCR.
More DCR increases sag and lowers available output.