MOD 5F1 kit - voltage issues

[martinlb]

I'm trying to get a MOD 5F1 amp kit working but since I haven't yet found an electronics course, I'm left more or less stuck.

The power light comes on and there's power to the tubes, but there's no sound and my readings at the voltage test points are off.

I'll list them here in hopes someone will have some advice:

Eyelet 19 sb (should be) 293VDC but reads 374
Eyelet 18 sb 250VDC but reads 321
Eyelet 17 sb 18VDC but reads 0
Eyelet 12 sb 1.1-1.4VDC but reads 1.79
Eyelet 15 sb 155-170VDC but reads 192
Eyelet 14 sb 155-170VDC but reads 207
Eyelet 13 sb 1.1-1.4VDC but reads 1.68

These are listed in the order given in the manual for testing.  I'll upload an image of the board with the numbered lugs.

[tubeswell]

I'm trying to get a MOD 5F1 amp kit working but since I haven't yet found an electronics course, I'm left more or less stuck.

The power light comes on and there's power to the tubes, but there's no sound and my readings at the voltage test points are off.

I'll list them here in hopes someone will have some advice:

Eyelet 19 sb (should be) 293VDC but reads 374
Eyelet 18 sb 250VDC but reads 321
Eyelet 17 sb 18VDC but reads 0
Eyelet 12 sb 1.1-1.4VDC but reads 1.79
Eyelet 15 sb 155-170VDC but reads 192
Eyelet 14 sb 155-170VDC but reads 207
Eyelet 13 sb 1.1-1.4VDC but reads 1.68

These are listed in the order given in the manual for testing.  I'll upload an image of the board with the numbered lugs.

Without seeing what other components those ‘lugs’ attach to, the readings are a little bit meaningless. However, I’m guessing ‘Lug 17’ goes to a 6V6 cathode and that this thing is cathode biased. If it’s reading 0V, then the 6V6 (or whatever tube you have there) won’t have any cathode current. Start there.

[acheld]

Photos of your build will really help us help you.   

I was going to (gently) rag on you for the lack of a schematic, but when I looked at the documentation for this kit quickly realized that they did not supply one for you!

Here is one from Doug's huge library:   https://el34world.com/charts/Schematics/files/Fender/Fender_champ_5f1_schem.pdf  I'm not sure how closely your kit adheres to the old schematic, but I bet it is very close.

Since there is no voltage at lug 17 (which should be connected to pin 8 of your 6V6), that points to (possibly) no current flowing in your 6V6 output tube.   So, check your wiring of that tube socket against your manual closely.  The other voltage discrepancies are likely related to this.

[martinlb]

Thanks for the help.  I'll get to checking th wiring on the 6V6 socket.  In the meantime here's a shot of my build:

I'll also get a shot of the wiring to the sockets.

[martinlb]

Here's the sockets.

[acheld]

Do you have heater wires soldered to pins 1 and 7 of the output tube socket?  It's a little hard for me to tell for sure.    Heater wires go to pin 2 and 7 for a 6V6.

[Calboy]

It looks like  the 470ohm cathode resistor is connected to pin1 instead of pin8 on 6v6 tube.

[tubeswell]

It looks like  the 470ohm cathode resistor is connected to pin1 instead of pin8 on 6v6 tube.

Yes if the brown wire coming from the OT primary is going to Pin 3 of the 6V6 socket, then the heaters next to that would be on Pin 2, and so on...


(Edit: @martinlb - cathode on a 6V6 is Pin 8, plate is Pin 3, screen is Pin 4, control grid is Pin 5,. With the amp plugged in and switched on, I assume you can see the little orange glow of the heaters coming from the middle of the bottle?)

[martinlb]

I did have the heater wires connected correctly, but YES, I did have the 470 ohm resistor connected to pin 1 instead of pin 8!

Testing the operation of the amp to happen soon, after more verifying pin connections.

Thanks!!!  I'll report back once I've finished testing.

[martinlb]

You guys nailed it!  With the resister connected properly, all the test voltage fell into place and...

The amp works!  🫡😆😂

Thanks SO much!!!

[dogburn]

From your pictures, it looks like you still have the neutral line from the power cord connected to the on/off switch, as was pointed out in your other thread. That really needs to be changed. The black (hot) wire should go to both fuse and switch, then to transformer. White wire should go directly to transformer.

[scstill]

On that neutral line switch safety issue,  I reached out to MOD about this design flaw in many (all?) of their kits (instructions show neutral switching) but have received crickets. Most MOD builders are following MOD instructions and are generally inexperienced. This flaw could come back to haunt MOD. And it opposes National Electrical Code.

[mresistor]

They don't provide a schematic either... not a good look for them.

[dogburn]

On that neutral line switch safety issue,  I reached out to MOD about this design flaw in many (all?) of their kits (instructions show neutral switching) but have received crickets. Most MOD builders are following MOD instructions and are generally inexperienced. This flaw could come back to haunt MOD. And it opposes National Electrical Code.

That is really bad that they have not responded to you on that. You caught a significant flaw in their instructions, and they should be willing to fix that right away. This attitude make many of their products suspect.

[martinlb]

From your pictures, it looks like you still have the neutral line from the power cord connected to the on/off switch, as was pointed out in your other thread. That really needs to be changed. The black (hot) wire should go to both fuse and switch, then to transformer. White wire should go directly to transformer.

I'm reluctant to mess with things now that it's working but maybe I need more info.  What's the problem with doing things the way the instructions say to do it?

[dogburn]

I'm reluctant to mess with things now that it's working but maybe I need more info.  What's the problem with doing things the way the instructions say to do it?
[/quote]

The instructions have you running the neutral line through the switch and the hot line directly to the power transformer via the fuse. This way you have the 120V power from the wall directly connected to the power transformer - this is the line that should be running through the switch. I can't say what dangers are now lurking within your amp circuits, but there's a reason that it's against electrical code (as scstill notes). You are much better off fixing it now than finding out later.

[dogburn]

It's a simple fix: Remove the black power transformer wire from the side lug of the fuse and remove the power cord's white neutral line from lug on the power switch. Splice those two together and cover with heat shrink tubing. Then run a wire of appropriate gauge (use black if you can so it matches the rest of the hot wire) between the side lug of the fuse and the empty lug on the power switch.

[martinlb]

Is there a diagram somewhere for this

[scstill]

see the link in reply #2

[mresistor]

This should help to visualize the correct wiring   difference being your on/off switch is on the vol pt not separate.


sorry for the crude spelling my mouse ain't the greatest..   

[Backwoods Joe]

Glad you have it working now! What brand of transformers come with the MOD amp kits?

[Platefire]

My first amp build was 5F1 back in 2000. As I recall, it worked on the first fire up but it's took me 25 years worth of tweaks to get it working to my satisfaction. I just had a hard time understanding signal flow, different amp stages and power supply and how all worked together. I was learning to read schematics but at first it was Greek to me. So I drew this graphic layout that seemed to help me understand all the connections a little better.
 
Of course this layout is a modified version of the 5F1.Now the schematic is the first thing I go to for understanding the makeup of an amp. Attached also is the Schematic of my Champ.

Platefire
 

[martinlb]

It's a simple fix: Remove the black power transformer wire from the side lug of the fuse and remove the power cord's white neutral line from lug on the power switch. Splice those two together and cover with heat shrink tubing. Then run a wire of appropriate gauge (use black if you can so it matches the rest of the hot wire) between the side lug of the fuse and the empty lug on the power switch.

Wouldn't that leave it without the fuse working?

[martinlb]

Thanks for those images mresistor!

[scstill]

Wouldn't that leave it without the fuse working?

just follow the diagram in reply #19 and defined in the schematic in reply #21

[scstill]

BTW - Amplified Parts sells these MOD amp kits.
I reached out to AP so we'll see what they have to say since MOD has still not replied (over a week)

The transformers seem to be generic.

[mresistor]

Amplified Parts = Antique Electronic Supply = CE Distribution

[scstill]

email from MOD...
with their explanation and resolution.
Not sure if I agree with rationale. In those days designers were not aware of safety issues.
I wonder if MOD would design in a death cap to honor vintage design....
Plus if MOD is providing vintage design why do they include a grounded AC cord.

[tubeswell]

I wonder if MOD would design in a death cap to honor vintage design...

LoL

[dogburn]

I looked at their 5E3 kit, and it is wired according to current safety standards, despite the original layout having one wire fused and the other switched. So the vintage design argument is just an excuse to avoid admitting their mistake on the 5F1 kit.

[scstill]

The MOD 5E3 doc here
https://www.modelectronics.com/sites/modelectronics.com/files/media_files/K-5E3%20Instructions.pdf
page 33 shows a DPST switch on both Line and Neutral.

But they have apparently added a switch to lift the HT center tap.
Haven't seen this before but better than the death cap design I suppose.
Does this approach work?

BTW with the exemption of missing schematic, the instructions seem very thorough and well illustrated and should be great for the novice to follow...

[DummyLoad]

Technically, it's best to switch both mains wires. Best common practice as well as mandatory requirements for most safety certification agencies, as well as fusing for every secondary winding, if you really want to split hairs over these things.

If the power switch is attached to a tone control or a volume control, then routing and switching the neutral wire reduces hum. On thoses builds it's best to relocate the power switch to come into compliance.


--Pete