Let's build an amp! Greg's journey to electrocution

[Minerman]

Are you gonna put the little bias jacks on the back of this thing that you plug your meter into??? If I overlooked this, pay no attention dude...

If you are, I was gonna suggest putting the bias pot on the back panel, but I guess it really doesn't matter because you'd have to pull the chassis out of the headshell to swap tubes anyway, right???

You do some pretty neat work Greg, so far this thing looks really good...

[Greg_L]

Are you gonna put the little bias jacks on the back of this thing that you plug your meter into??? If I overlooked this, pay no attention dude...

If you are, I was gonna suggest putting the bias pot on the back panel, but I guess it really doesn't matter because you'd have to pull the chassis out of the headshell to swap tubes anyway, right???

You do some pretty neat work Greg, so far this thing looks really good...

I am planing to add bias test points to the rear of the chassis.

The bias pot is gonna stay where it is though. I don't want that thing exposed, getting bumped, getting touched, etc. Also, putting it on the back would require long wire runs and I don't want that.

The head box is configured with a back panel that can be removed so I can swap tubes and adjust bias without removing the chassis. The bias pot sticks through the chassis and is adjusted externally.

[rayc]

That's a lot of not much today.

[Lt. Bob]

Beautiful work Greg.

I knew it would be.

[WhiskeyJack]

Dude. Thats sick.

So awesome. I honestly think that when this is done and dusted it really deserves a follow up video man. I know you do not do videos but i really think it will need a video. Even if just a walk through.

[Greg_L]

Beautiful work Greg.

I knew it would be.

Thanks Boob. It's going well....so far.

Dude. Thats sick.

So awesome. I honestly think that when this is done and dusted it really deserves a follow up video man. I know you do not do videos but i really think it will need a video. Even if just a walk through.

Well, I don't know. I don't know how to do video beyond just filming with my phone, and that aint no good. If I ever do a video it'll have to be good quality, and that aint happening.

[Bubba]

Still enjoying this immensely.

[liv_rong]

Just put your phone on a tripod holder thing and film it. Then just upload it. It doesn’t gotta be fancy, the material is what’s important. And people love this nerdy shit!

[Greg_L]

Just put your phone on a tripod holder thing and film it. Then just upload it. It doesn’t gotta be fancy, the material is what’s important. And people love this nerdy shit!

What would yall wanna see? A large portion of this so far has been me just sitting there measuring wires out and going back and forth to the schematic and test fitting things and boring shit like that. For every one solder joint there is like 30 minutes of prep. Lol. I'm having great fun but I can't imagine someone watching video of this.


But here's today's progress in pic form....

I feel like I've spent a lot of time today for not much stuff. But it's been important stuff.

We left off at that little bias board. Well here it is all wired up.



And here it is installed in it's little corner.



And here is the bias pot 2/3 wired up. The middle lug will have a wire that goes to the turret board.



Here's where I had to get shade-tree mechanic on one of the pieces. That big ol impedance selector switch that I've been undecided on...well I decided. It was too big for the chassis, and I really wanna use it, so I fired up the air compressor and dug out the old pneumatic cutoff wheel. I had to lop off the bottom part of the switch so it would fit in the chassis...and bend the pins in.




And.....it fits!!!! And most importantly, it still works.


And finally the rear panel is almost complete.



Now we gotta wire it to the output transformer secondaries and speaker jacks. This is "Marshall style"...so naturally I love it. The yellow (4 ohm), Green (8 ohm), and Brown (16 ohm) wires come from the tranny, go through the switch, and on to the speaker jacks through the blue wire. The hard to see black wire hidden in all that is the OT ground and it grounds at the speaker jacks. I'm not 100% on grounding it like that yet, but it will work. I'm just not sure about ground loop potential.



So the power supply is pretty much all done. This is that area of the chassis. I also added the 470 ohm screen resistors on the power tubes. Each tube's pin 6 is not soldered yet because there will be another wire added to that pin later.


Impedance selector switch area.



And the whole shebang as it sits right now. One week ago it was an empty chassis.



So, all that's left for the rear panel right now are the RCA jacks for the reverb/tremolo transformer and footswitch. Maybe tonight I'll work on that. Or maybe that's enough for today. But we are getting very near to populating the turret board. That will be nerve-racking.

[rayc]

The black'n'red wires neatly twisted & reflected by the sides look like plaiting. Impressive work overall.
I have no real understanding of such hing s but, "...470 ohm screen resistors on the power tubes..., was that your decision? If Y, why?

[Greg_L]

The black'n'red wires neatly twisted & reflected by the sides look like plaiting. Impressive work overall.
I have no real understanding of such hing s but, "...470 ohm screen resistors on the power tubes..., was that your decision? If Y, why?

Thanks Ray.

No they're part of the schematic. The screen grid resistors help control oscillation and help keep the screen grid positively charged but keep current in check to aid the plate in sucking up electrons from the cathode. 6V6's don't draw massive screen current, but the grid control resistors need to be there. I'm using 1 watt, but I may bump them up to like 3-5 watts...because this amp will be abused.

[Greg_L]

These....

screen grid resistors.jpg

[rayc]

Thanks Greg.
"...this amp will be abused." is a positive statement.

[Greg_L]

Today might be mad scientist day. I'm gonna have to think about this one for a bit.....

I have this auxiliary mains outlet that is common to most vintage Fender amps. I had no plans to connect it. It would just be there to plug the hole. But....

What if I can re-purpose it for something useful? This idea popped into my head last night. I have little multimeter test jacks on order to make external bias testing points. The idea is to check and set bias without removing the chassis from the head shell. But to check and set bias, you need to know the plate voltage, and that requires removing the chassis, thereby totally defeating the purpose of the external test points. So I'd also need an external plate voltage test point. Plate voltage is dangerous though.

So my idea is to use that auxiliary outlet as a plate voltage or bias test point. It could be safe because you have to probe way in there to get a reading.

Option #1
Use the plug for the two bias test points and ground. Use one of the little sockets I've ordered for the plate voltage.


Option #2. This is probably safer. Use the wall plug for the plate voltage test point. Use two tip jacks for the bias points.


I think #2 would be better.

[Lt. Bob]

pretty cool idea

[Greg_L]

pretty cool idea

Thanks, now I'm thinking it's probably not all that necessary. I think I'll put a pin in that idea and maybe revisit it later. The bias test point is necessary, but I probably don't have to read plate voltage all the time.

[Minerman]

That is a cool idea Greg, it could be dangerous to someone who didn't know wtf it was for, but it'll be your amp, so that's probably not gonna be an issue...

[WhiskeyJack]

Today might be mad scientist day. I'm gonna have to think about this one for a bit.....

I have this auxiliary mains outlet that is common to most vintage Fender amps. I had no plans to connect it. It would just be there to plug the hole. But....

What if I can re-purpose it for something useful? This idea popped into my head last night. I have little multimeter test jacks on order to make external bias testing points. The idea is to check and set bias without removing the chassis from the head shell. But to check and set bias, you need to know the plate voltage, and that requires removing the chassis, thereby totally defeating the purpose of the external test points. So I'd also need an external plate voltage test point. Plate voltage is dangerous though.

So my idea is to use that auxiliary outlet as a plate voltage or bias test point. It could be safe because you have to probe way in there to get a reading.

Option #1
Use the plug for the two bias test points and ground. Use one of the little sockets I've ordered for the plate voltage.


Option #2. This is probably safer. Use the wall plug for the plate voltage test point. Use two tip jacks for the bias points.


I think #2 would be better.

This is me. right now.

[Greg_L]

Didn't do much today. I'm still having a debate with myself about which way to ground the RCA jacks for the reverb/tremolo footswitch. I've read too much. I've de-convinced myself. I'm gonna just go with my gut.

In the meantime, just to get something done today, I started the planning for my main buss ground. I don't want to ground everything at the back of the pots, like most amps, so I'm going to do a "floating buss rod" thing....made out of old wire coat hanger.


More on that later. But before we even get to that, I need to get the input jacks wired up because they are virtually impossible to do in the chassis, and super-impossible with a bunch of other shit in the way. So like doing guitar pots, I made an external template board and wired the jacks together on the board to be dropped into the chassis as an assembly..

Normal Channel jacks.


Normal channel jacks installed. See? Not much room for anything.



Same deal with the Vibrato Channel jacks. Start on the template....



Vibrato channel jacks installed with a dummy plug to make sure nothing is touching that aint supposed to be touching.



Next I will finalize the rear panel. Get the RCA jacks done and tidy up a few final touches and then we'll go to the grounding rod thing.

[JD01]

This is great, Greg. When do you think you'll be able to plug it in?