New PCB Express build - seeking advice

[mattvon]

Hi all, after a bit of a delay, I'm back on track to finish up my PCB build...I have a couple of questions regarding the PT wiring...

I'm using the Toneslut 11410 PT, so I have to connect the following color wires...here's how I'm planning on doing it:

Black wires (120v, pair): one each to the power switch lugs
Yellow wires (300v, pair): one each to the diode stack solder points on the PCB
Yellow/Red wire (0v): to standoff which serves as ground for the 80uf filter cap
Brown/White wire (center tap): to standoff which serves as ground for the 80uf filter cap
Brown wire (6.3v, pair): already wired in to V5 heaters

Does this look correct to you all? I know the build guide says to ground the Yellow/Red and Brown/White to the ground lug under the power cord, but I read earlier in this thread NOT to do that--instead, ground to the first filter cap ground point instead.

Thoughts? It's nice to be back after this, so thanks for all your help!

[Colossal]

I know the build guide says to ground the Yellow/Red and Brown/White to the ground lug under the power cord, but I read earlier in this thread NOT to do that--instead, ground to the first filter cap ground point instead.

Thoughts? It's nice to be back after this, so thanks for all your help!

Yes, that is correct and considered by many to be a better and more modern approach which results in quieter amps. The mains ground wire (or IEC socket) should ALWAYS have it's own dedicated chassis ground point close to the mains inlet (whether IEC socket or traditional strain relieved 3-wire power cord). No other wire should share that Mains ground. The PT secondary (300-0-300) center tap should return to ground at the first filter cap. This forms a closed circle if you think about it visually. You want each "node" in the power supply to form a closed loop where each section of the amp served by each B+ is returning to ground at the cap supplying it. This minimizes the risk of ground loops forming. Grounding can be a black art so there is much debate about what constitutes a good grounding scheme. Many classic amps "get away" with things that aren't considered (by modern standards) to be good technique.

Several guys here built quiet amps with the PCB approach as a proof of concept for RJ's prototype rev0 board.

[mattvon]

Ok, thanks! I was hoping I'd read that correctly...

Just to double-check (before I cut them short)...the pair of yellow 300v secondary wires connect to the two solder points feeding the diode stack, correct?

Here's where I'm intending to connect them...

[Colossal]

Ok, thanks! I was hoping I'd read that correctly...

Just to double-check (before I cut them short)...the pair of yellow 300v secondary wires connect to the two solder points feeding the diode stack, correct?

Here's where I'm intending to connect them...

Yep, you got it. Connect your PT's centertap to the ground side of the 80uF cap (but NOT after that 220k bleed resistor to ground) and you're good to go.

[Koop]

Hey Collossal - I noticed on your build the wires from V4 and V5 to the OT run underneath the board. On some of the other PCB builds they run over the board and down to the sockets. Is there any advantage going underneath? Over the top would make it easier to leave the wires long until I'm sure they're on the right tubes, then I can trim them and resolder without removing the PCB. Thanks in adavnce.

[Colossal]

Hey Collossal - I noticed on your build the wires from V4 and V5 to the OT run underneath the board. On some of the other PCB builds they run over the board and down to the sockets. Is there any advantage going underneath? Over the top would make it easier to leave the wires long until I'm sure they're on the right tubes, then I can trim them and resolder without removing the PCB. Thanks in adavnce.

For the PCB build, there is definitely an advantage to flying the leads to V4 and V5 plates in that you have access to them if you get them reversed (causes squealing). You get a 50/50 shot to get that right when you power up the amp for the first time. If you get it wrong and you had your OT wires underneath, you'd have to lift your board, desolder and flip the leads and put it back. That would cause a lot of swearing and would be very invasive.

But my plate leads are actually above board. If you look at the photo, I have two switches on the back: the one labeled PRIMARY IMPEDANCE SELECT allows the user to switch primaries (5k2 or 6k6) if desired. The OT wires (there are four of them, two for each set of primaries) go to that switch from under the board, but the commons from the switch then go to V4 and 5's plate connections on the tube socket. They are flying above the board. The two leads coming from the switch are black and form a lazy L.

Let me know if you can see this now and if not, I'll mark up the photo to show you what I mean.

[Koop]

Yes, I see it now - thanks. Your build is awesome, but I have to admit it's a bit intimidating. Many mods and lots of black wire to second guess myself on!

[Colossal]

Yes, I see it now - thanks. Your build is awesome, but I have to admit it's a bit intimidating. Many mods and lots of black wire to second guess myself on!

Thanks for the kind words Koop. Don't be intimidated. Yes, I was short on my usual inventory of colored wire so I just went all black. It really is more of a 2D/planar build so it's not too hard to follow the circuit despite just the one color of wire. This was a proof-of-concept for RJ's benefit to test the viability of PCBoarding a Trainwreck traditional layout so I wasn't so much worried about observing the color coded wiring system that I normally use. Yes, I threw some mods in there mostly to satisfy my own curiosity about what might or might not be complimentary to the TW format. It is not intended to be a purist build. I have the goodies for pure Express and Liverpool builds coming up when I get done moving and some amps off my bench. I'm actually thinking of selling that PCB amp.

If you have other questions, I'll be happy to mark up the photo to clarify (making some sense out of all that black wire). Good luck with your build.

[mattvon]

This is very timely, I was just measuring out those leads as well and was planning on routing them under the board. I'll go over the top now.

If you'd be so willing, I'd love a diagram showing the wiring--especially the standby switch, presence pot, and the 25w metal resistor most guys mount to the chassis.

Thanks!

[rock_mumbles]

Here's how the standby switch is used to hook up the OT center tap and 1k 25 watt dropping resistor
[img938]http://rh-tech.org/public/Trainwreck/Ot ... tandby.jpg[/img]

[Koop]

I've stalled on this project due to a death in the family, but I'm trying to get back into it.

The picture above has me confused. At the top of the photo there are 2 resistors labeled 220k 3W. Most of the schematics I have show R23 and R24 as 100k 3W. The parts I received from RJ also show R23 and R24 as 100k. Did I miss something?

[Colossal]

The picture above has me confused. At the top of the photo there are 2 resistors labeled 220k 3W. Most of the schematics I have show R23 and R24 as 100k 3W. The parts I received from RJ also show R23 and R24 as 100k. Did I miss something?

I would go by the photo. Use 220k 2 or 3W (higher is fine if you are short on 220k). On the original Express, two 100k resistors were placed in series to ground. This is the bleeder resistor for the B+ rail.

[Koop]

Thanks Colossal. If they are bleeders then they really exist only to "bleed" the capacitors at shut down, right?

[rock_mumbles]

If they are bleeders then they really exist only to "bleed" the capacitors at shut down, right?

Correct

A 'real' Trainwreck uses the two 100k resistors in series which would be in the left-hand corner bleeder position on the pcb ... you could install your resistors there and then when the amp is turned off, flip the standby swith to play position to bleed the rest of the power supply circuit ...

[Colossal]

... you could install your resistors there and then when the amp is turned off, flip the standby swith to play position to bleed the rest of the power supply circuit ...

RJ thought this one through with the PCB design. By having a bleeder on both sides of the standby (if you're sticking to the Express schematic for the placement of the standby), the entire rail drains after shutdown with no further action required by the player. That's great design and an improvement in safety.