building a Fender "Pro-Amp" from scratch

[ELS]

Here's a modified layout.
You can wire the filament winding to the pilot lite Then to the output tubes, a bit less wire that way but it adds another connection that can fail.

Keep the output transformer plate wires as close to the chassis as possible, route it very straight and don't have excess wire in the chassis. And route the grid wires for the output tubes as far away from the plate wires as possible (don't get too close to the screen wires either)... 6L6GC's are very prone to oscillating so the wiring layout in this area is critical. If you still have problems you can put 1.5k resistors in series with the control grid, Place the resistor right next to the tube socket, even 1/4" excess wire can cause a problem at the control grid. So mount some terminal strips to connect the other end of those resistors.
Don't twist the OPT plate wires, I've found this to be more detrimental, probably because in doing so you lift the wire slightly further away from the chassis which in this case seems to actually increase the RF radiation from them. I try to run them pressed up against the chassis, and right next to each other if possible, I sometimes tape them down with aluminum air duct tape for a tiny bit of extra shielding (and mainly to hold them down).
If you opt for the control grid resistors it's far less picky but adding them slightly removes the "singyness" of the amp, it's just a bit but I prefer to not use them if possible
ALso note how the output tube which has the longer grid wire (since it's further away from the circuit) is connected to specifically the 200V rated cap side of the phase inverter, this is intentional; The impedance there for this type of phase inverter is much lower than the other side, resulting in less chance for it to pick up the plate signal and oscillate.
If you need to swap the phase, swap the OPT plate wires or speaker output wires instead.

For the earth-ground terminal, use a split washer not those many-tooth ones, they are the most resistant to getting loose under vibration.

 

[ELS]

Also, things like this are a no-no:

That terminal has ~500V on it since it's the first B+ node, and that screw is grounded. They're too close to each other. Whoever made this should've offset the terminal to the other side, farther away from the screw.
This can cause carbon tracking over time, where the PCB becomes conductive, can blow parts or start a fire.
This is what happens when the PCB carbon tracks:
That's red hot from all the current that's flowing trough the PCB and heating it up. Here the reason for the carbon tracking was: Connetions too close together and after years of soaking up moisture, dust, it arced over, and the arc converts the PCB material into carbon which then keeps conducting
This is the reason modern PCB's have cut-out slits where high voltage is, if there's no PCB there it cannot carbon track

 

[Rotorcraft230]

Thanks ELS, that sure is a lot to take in and I already practice some/most of that, but let’s remember it’s not a 10,000 $$ hifi amp, it’s a guitar amp. And like I had said in the past he doesn’t mind hum, as long as it’s not excessive it’s not a problem.

I’m an amateur builder and I’m teaching my son how to build and maintain his amps and we are having the time of our lives with it and it will always be something he can look back on far after I’m gone and realize how much fun we had.

Cheers

 

[ELS]

Thanks ELS, that sure is a lot to take in and I already practice some/most of that, but let’s remember it’s not a 10,000 $$ hifi amp, it’s a guitar amp. And like I had said in the past he doesn’t mind hum, as long as it’s not excessive it’s not a problem.

I’m an amateur builder and I’m teaching my son how to build and maintain his amps and we are having the time of our lives with it and it will always be something he can look back on far after I’m gone and realize how much fun we had.

Cheers

Well it doesn't cost much to have a thought-over layout. And when you get used to it, it just becomes automatic.
For what it's worth I still haven't found quiet enough preamp tubes to hear hum from my vibroverb-style amp and I used filter caps from the 60s in that amp, still no hum and has been working for a year now

 

[Rotorcraft230]

Fender sure does know how to make a quiet amp. I’m hoping that when I incorporate the grounding scheme with a few exceptions that the amp would have minimal or even no hum.

Vintage fender amps are my favorite amps to work on.

 

[Dblgun]

When in good repair Fenders ground method worked well and provided minimal noise. There is no reason to try and re-invent the wheel. I have no doubt your research, planning and attention to detail will result in very acceptable results. Build on!

 

[Rotorcraft230]

Kinda slow going but started to wire in the PT. And made the bias supply board. Found a tubular rivet setting tool for my hand squeezer to set the eyelets and I also have a heavy duty 1/8” hole punch for punching the holes in the eyelet board.

Learned something on old transformers, the wires are very stiff and don’t want to play very well. I usually like more tidy wires but because they are old I didn’t want to move them too much.

 

[Marcomel79]

Kinda slow going but started to wire in the PT. And made the bias supply board. Found a tubular rivet setting tool for my hand squeezer to set the eyelets and I also have a heavy duty 1/8” hole punch for punching the holes in the eyelet board.View attachment 133916View attachment 133917View attachment 133918View attachment 133919View attachment 133920View attachment 133921View attachment 133922

Learned something on old transformers, the wires are very stiff and don’t want to play very well. I usually like more tidy wires but because they are old I didn’t want to move them too much.

Its a Fender, its gotta be messy

 

[Rotorcraft230]

Yep, it will be in typical fender style. Anyway, it’s slow going, had to put it aside, got a 1962 Brownface Pro to repair.

 

[Rotorcraft230]

Finished up on the pro, needed all new electrolytic caps and a new set of power tubes. They were too far apart and mismatched by brand. One tube was 11.2w and the other one was 15.6w, not very good

Anyway,I didn’t like the double pull switches, too big and the bias board wouldn’t fit past them so I put in some Carling switches. Much better now. Wired up the PT and the secondarys on the OT.

 

[Gene Ballzz]

Kinda slow going but started to wire in the PT. And made the bias supply board. Found a tubular rivet setting tool for my hand squeezer to set the eyelets and I also have a heavy duty 1/8” hole punch for punching the holes in the eyelet board.View attachment 133916View attachment 133917View attachment 133918View attachment 133919View attachment 133920View attachment 133921View attachment 133922

Learned something on old transformers, the wires are very stiff and don’t want to play very well. I usually like more tidy wires but because they are old I didn’t want to move them too much.

WOW! Although you hide it well, I think I've discovered that you are a card carrying "TOOL JUNKIE" and it takes one to know one! I often have a chuckle when I think of the confusion when after my demise, someone is wondering: "What the hell is this for?" when looking at some of my most unique and prized gadgets! The sad part is that they will likely have no idea of the exorbitantly high price it should command! I guess the joke will be on them!
Nice Work, As Always!
Gene

 

[ELS]

Finished up on the pro, needed all new electrolytic caps and a new set of power tubes. They were too far apart and mismatched by brand. One tube was 11.2w and the other one was 15.6w, not very good

Anyway,I didn’t like the double pull switches, too big and the bias board wouldn’t fit past them so I put in some Carling switches. Much better now. Wired up the PT and the secondarys on the OT.View attachment 134384View attachment 134385View attachment 134386View attachment 134387View attachment 134388

Yeah... the most planned projects seem to take the longest
I wouldn't sweat the power tube mismatch, But the weakest tube on the side which is fed from the 100k plate resistor side of the phase inverter, the 82k side has a bit less signal.
My janky tube selection and biasing method is; take a couple tubes, if one is much hotter to the touch than the other, swap the coolest tube with a different one until I run out of tubes. Then I turn up the bias, wait ~30seconds, turn it up more, until I see the tubes start to red plate, then I turn it down a bit. make sure it doesnt start red plating after playing too.

 

[Rotorcraft230]

WOW! Although you hide it well, I think I've discovered that you are a card carrying "TOOL JUNKIE" and it takes one to know one! I often have a chuckle when I think of the confusion when after my demise, someone is wondering: "What the hell is this for?" when looking at some of my most unique and prized gadgets! The sad part is that they will likely have no idea of the exorbitantly high price it should command! I guess the joke will be on them!
Nice Work, As Always!
Gene

Yes, you are correct with everything you just said. I’m a mechanic and frequent pawn shops and can’t resist a good tool, I repair aircraft structures so the sheet metal tools are plentiful, and you don’t even want to know how much Impaid for some of them. I am teaching my son how to use them and will all get past along at some time.
I love special tools!!!!!!

 

[Rotorcraft230]

Yeah... the most planned projects seem to take the longest
I wouldn't sweat the power tube mismatch, But the weakest tube on the side which is fed from the 100k plate resistor side of the phase inverter, the 82k side has a bit less signal.
My janky tube selection and biasing method is; take a couple tubes, if one is much hotter to the touch than the other, swap the coolest tube with a different one until I run out of tubes. Then I turn up the bias, wait ~30seconds, turn it up more, until I see the tubes start to red plate, then I turn it down a bit. make sure it doesnt start red plating after playing too.

Thanks for the 100k/82k tip I’ll try that in the future. The owner opted for new tubes.

Anyway, worked on the amp this weekend.
Incorporated. Some of the grounding scheme, but didn’t stay true to it. Got some nylon hardware coming to use for the turret board close to the HV cap and V1 plate resistors. Turret board mounting will surely be thought out better on the next build.

 

[ELS]

Thanks for the 100k/82k tip I’ll try that in the future. The owner opted for new tubes.

Anyway, worked on the amp this weekend.
Incorporated. Some of the grounding scheme, but didn’t stay true to it. Got some nylon hardware coming to use for the turret board close to the HV cap and V1 plate resistors. Turret board mounting will surely be thought out better on the next build.View attachment 134579View attachment 134580View attachment 134581

Tie Pins 1 and 8 together on the output tubes, allows you to use EL34's with rebiasing ofc.
6L6's and alikes still work as those have pin 1 not connected to anything. while EL34's have it connected to the suppressor, which for normal operation should be tied to the cathode.

 

[Rotorcraft230]

Tie Pins 1 and 8 together on the output tubes, allows you to use EL34's with rebiasing ofc.
6L6's and alikes still work as those have pin 1 not connected to anything. while EL34's have it connected to the suppressor, which for normal operation should be tied to the cathode.

Won’t be using 34s. I have the RCA 6L6GCs for it dated 1961

 

[Rotorcraft230]

 

[Rotorcraft230]

Little more work, started on the inputs and I also got threaded nylon standoffs and nylon screws. The board mounting wasn’t fully thought out and had gotten a little too close to the HV cap

 

[Guitar-Rocker]

I really enjoy looking at your work and can appreciate the time it takes to achieve that look. Congrats on another great looking build.

 

[Rotorcraft230]

I really enjoy looking at your work and can appreciate the time it takes to achieve that look. Congrats on another great looking build.

Thanks, worked in it last night but didn’t get any pics taken, but I will. Looking like I ready for the turret board to go in and start connecting to sockets and pots.