5E8-A build: grounded secondary center taps, speaker out

[electrical]

My first amp build is a 5E8-A build. I finished all of the wiring last night and started going through the amp with a multimeter gave me concern.

I have connected the neutral from the power cable to its own power transformer bolt and the circuit ground to another transformer bolt, these show continuity to each other and chassis. I implemented a star grounding scheme with separate ground leads for input jacks, the output speaker, all the ground spots on the layout, and most importantly the secondary center tap for 355v (red/yellow) plus heater center tap (green/yellow).

I measured and marked each part on a layout when populating the board, and I have double checked each lead from the board to its outlet. All parts are laid out correctly, and all leads are going to the correct location.

I have not plugged the amp in yet and powered on to test voltages, nor have I installed any tubes yet. I want to first make sure that my questions below are not indicative of a bigger issue that might fry my build or me. I also feel a healthy bit of respect for high voltage in my reluctance to plug in and power up for the first time.

Schematic here: https://www.mojotone.com/Amp_Kits/Tweed/Twin_LP_5E8A_SCH.pdf?_gl=1*7s6trw*_gcl_au*Njk1NzAzNjgyLjE3MDMwODcxNDk.*_ga*MTM3MjIxNTAwMy4xNzAzMDg3MTQ5*_ga_82F088JBDD*MTcwMzA4NzE0OS4xLjEuMTcwMzA4NzE1NS41NC4wLjA

Layout here: https://www.mojotone.com/Amp_Kits/Tweed/Twin_LP_5E8A_WD.pdf?_gl=1*7s6trw*_gcl_au*Njk1NzAzNjgyLjE3MDMwODcxNDk.*_ga*MTM3MjIxNTAwMy4xNzAzMDg3MTQ5*_ga_82F088JBDD*MTcwMzA4NzE0OS4xLjEuMTcwMzA4NzE1NS41NC4wLjA

Photos of my build here: https://imgur.com/a/YEX4ZY8 and I am happy to try uploading higher resolution or close up photos should they be of assistance.

The only modifications I made from the layout are that I wired the heater center tap to ground rather than using 2x 100 ohm artificial taps off the pilot light; I omitted the death cap on the ground switch; and my power transformer is a custom order from Heyboer at the recommendation of a colleague to handle the increased current draw of the parallel rectifier tubes. The spec of the power transformer is as follows:
black-black 120 primary
red-355, red/blue-55, red/yellow-0, red-355 vac @ 200mA
yellow-yellow 5 vac @ 6A
green 3.15, green/yellow 0, green 3.15 vac @ 7.5A

After making some measurements, I am uncertain about these:

• After hooking up the filament heater leads from the power transformer, I'm now getting continuity across all heater current lines in all tube sockets. Is this correct, since they are connected to each other through the secondary?
• I am also getting continuity from heater filaments to chassis/ground, is this correct because current is flowing through the center tap? One thought I had was to disconnect the green/yellow heater center tap and heat shrink it off, then connecting two 100 ohm (one reads at 103.3 ohms, the other at 102.6 ohms) to the pilot light leads as an artificial center tap - which puts resistance between the heater lines and ground. Would this be a good idea?
• I am getting an open lead reading from tip of each instrument input to chassis (tested by plugging in a guitar cable). I am getting continuity from the speaker jack tip to chassis and circuit ground, is this correct because current flows through the OT secondary which then connects to chassis ground? Or is it because the speaker jack is connected to the chassis? I am using a J12A shorting jack that came with my kit.
• The negative feedback resistor is reading on my multimeter as 1.5k, despite clearly being a 56k resistor (close up pic here: https://imgur.com/ET2RQEo ), and it read at 56k when I installed the resistor into the board. Do I need to replace this resistor?
• I am getting continuity from the red/blue lead connected to the 3.3k resistor all the way at the left end of the board and ground, and I'm additionally getting continuity between that point and the power/standby switches. Is this correct because current is flowing through to the 0v center tap of the 355v secondary?

Overall, my questions can probably be summarized as uncertainty as to whether secondary transformer taps should be reading ground to chassis through their respective center taps, and whether the speaker jack tip should be connected to chassis through the output transformer secondary coil. Do my measurements suggest something wrong with my wiring, or can I safely start powering up the amp to check for voltage on my heater filaments and secondary taps?

Thank you in advance for your patience and help - I've learned a lot from browsing this place, and I hope to learn more over time.

[shooter]

post the schematic in the thread so we don't have to play ping pong links n get cross-eyed.
 IF.. your filament CT is at ground then readings along the string will have a path to ground
The OT secondary is one wire to speaker + (tip), one to speaker - (ring) (which should be grounded at chassis near the speaker jack.)
IF so...then measuring tip to chassis will get ohms through the secondary

[sluckey]

The negative feedback resistor is reading on my multimeter as 1.5k, despite clearly being a 56k resistor (close up pic here: https://imgur.com/ET2RQEo ), and it read at 56k when I installed the resistor into the board. Do I need to replace this resistor?

No. It's all fine. If you examine the schematic you will see a 1.5K (look at the presence pot). That 1.5K is parallel to your 56K NFB resistor via the OT secondary which is only a couple ohms.

[scstill]

might consider this for your startup
https://robrobinette.com/Tube_Amp_Startup.htm

[electrical]

Hey everyone, thanks for the insight. I contacted Mojotone, they confirmed that my readings are correct signs of a healthy build and those readings connect to each other/ground through the center tap. I followed Rob's steps for tube amp startups and measured correct voltages across the board - it's a healthy and successful build! I tapped a guitar cable tip to make sure the speaker hummed, but given my neighbors' proximity to my practice room I'll wait until tomorrow to enjoy the build.

post the schematic in the thread so we don't have to play ping pong links n get cross-eyed.
 IF.. your filament CT is at ground then readings along the string will have a path to ground
The OT secondary is one wire to speaker + (tip), one to speaker - (ring) (which should be grounded at chassis near the speaker jack.)
IF so...then measuring tip to chassis will get ohms through the secondary

Good to know! This confirms my intuition on grounding center taps. Next time I need assistance with a build I'll post the layout and schematic in the thread to avoid link jumping, thanks for that tip.

The negative feedback resistor is reading on my multimeter as 1.5k, despite clearly being a 56k resistor (close up pic here: https://imgur.com/ET2RQEo ), and it read at 56k when I installed the resistor into the board. Do I need to replace this resistor?

No. It's all fine. If you examine the schematic you will see a 1.5K (look at the presence pot). That 1.5K is parallel to your 56K NFB resistor via the OT secondary which is only a couple ohms.

That would explain it perfectly, thanks!

[Esquirefreak]

I don't think power cable neutral is meant to be grounded. Connect it directly to the transformer with some heat shrink wrapped around or via a mains switch.

/Max

[pdf64]

 …
I have connected the neutral from the power cable to its own power transformer bolt …

The photos indicate the words got mixed up, not the wires 
I dunno why DIYers think that’s the most appropriate fastener for the safety earth connection to the chassis. Whereas it’s actually rather unsuitable, as they will tend to work loose over time, given the mechanical and magnestriction forces acting on them.
Electrical safety good practice requires a dedicated fastener for the chassis safety earth connection, with a star washer between the chassis and earth lug, and locking nut. The wire lengths should be such that if the cable retention fails, the earth wire is the last to be pulled off.

[tubeswell]

I have connected the neutral from the power cable to its own power transformer bolt...

What on earth did you do that for? The neutral mains wire goes directly to the neutral return side of the PT Primary. Only the mains earth goes to the amp chassis (and it’s best to have that fixed by clamping a ring terminal on the unsoldered stranded end of the mains earth wire, and bolting that securely to the chassis with a locking nut)

[tdvt]

As pdf64 suggests, I think it's mixed up terminology; neutral vs. ground or earth.


Though it does look like a fused hot & a switched neutral from the pic I looked at.

[electrical]

Indeed, mea culpa its a terminology mix up. I wired hot to the fuse, neutral to the on switch, and ground to a transformer lug.

 …
I have connected the neutral from the power cable to its own power transformer bolt …

The photos indicate the words got mixed up, not the wires 
I dunno why DIYers think that’s the most appropriate fastener for the safety earth connection to the chassis. Whereas it’s actually rather unsuitable, as they will tend to work loose over time, given the mechanical and magnestriction forces acting on them.
Electrical safety good practice requires a dedicated fastener for the chassis safety earth connection, with a star washer between the chassis and earth lug, and locking nut. The wire lengths should be such that if the cable retention fails, the earth wire is the last to be pulled off.

Noted on this, I do have a dedicated #8 fastener for the ground lead and it is longer than hot and neutral. I’ll grab a #8 star washer from my local hardware shop, and place it between the fastener and the chassis.

[sluckey]

As pdf64 suggests, I think it's mixed up terminology; neutral vs. ground or earth.


Though it does look like a fused hot & a switched neutral from the pic I looked at.

That's correct. He was just following Mojo's schematic/layout. And Mojo was just following Fender's schematic. Works just fine and allows a convenient place to connect the power cord, although I prefer to connect the neutral directly to the PT primary.

@pdf64... I agree about using a dedicated earth ground bolt. However, I have never witnessed a loose kep nut on a transformer bolt.

[pdf64]

… He was just following Mojo's schematic/layout. And Mojo was just following Fender's schematic.

Fender’s design comes from an era of unearthed, unpolarised mains connectors. It’s incorrect to consider one side as live, the other as neutral, as it was completely pot luck which was which. So it wasn’t possible for Fender choose to fuse the live and switch the neutral, or visa versa.
Hence the ground switch; if Fender could know which wire would be neutral they would have saved the cost of the switch and hardwired the death cap to neutral.
 Mains powered electrical equipment being built now should comply with current standards.
 

I have never witnessed a loose kep nut on a transformer bolt.

When I was servicing amps it was fairly common to find the mains transformer of a TR etc with loose mounting bolts, on a couple of amps it was hanging on to the chassis by its ‘fingertips’. Invariably it was with through hole type transformers, mounting bolts through the lamination stack, hence the mention of magnestriction.
So, that’s why I bang on about it.
There’s only one safety critical connection in an amp, it’s hardly that much of a burden to do it properly.

[scstill]

I wired hot to the fuse, neutral to the on switch, and ground to a transformer lug.

On/Off switch should NOT be on neutral,
It should be on hot side between fuse and PT
otherwise you don't switch the line (hot) off.

[electrical]

It's entirely possible I have my terminology wrong. Here's how I wired the leads of my power cord:

-green is the longest lead and connected to its own transformer bolt. I have confirmed continuity between the round end of the male plug and the chassis through this lead
-black is connected to the fuse. The other lead of the fuse is connected to the first PT primary lead.
-white is connected to one of the on/off switch leads. The other lead of this switch is connected to the second PT primary lead.

I am now having some issues with the sound quality of the build, I have measured voltages and some of them around V1 seem suspect. I am not 100% sure what forum etiquette dictates, should I start a new thread for those debugs or keep posting in this thread? edit: I have started double-checking all leads and components/parts, per the instructions sticky'd to the top of this board I'll re-measure and revisit each component and lead, checking for continuity where I expect to find it and marking things off on my layout before I ask of more assistance from the community. Thanks so far to everyone for all of your help and the high level advice, it's deeply appreciated.

[tdvt]

From what Sluckey said earlier (without seeming your paperwork), that is how it is drawn in the vintage schematic, & while that works, the others are saying that is not really the safest or best practice (or even code, these days) to wire the incoming power that way.


The green ground or earth sounds fine, though I believe it is meant to have it's own dedicated connection, not shared with anything including fastening the transformer.

The best practice is to have the fuse AND the switch on the line voltage HOT lead (black in the US) coming from the wall. Then it continues on to one of the primary PT leads.

The neutral lead (white in the US) is connected directly to the other PT primary lead, either on a terminal or an in-line splice with heat-shrink tubing.


The issue with the way you have it wired is that the hot-side of the line voltage from the wall is not being disconnected from the amp, even when the switch is "off", which poses a shock hazard.

You want to break the current path of the line voltage coming from the wall before it reaches the amp circuitry; either intentionally by the switch or by the fuse, in the case of a fault.

Sort of surprising that Mojo doesn't have an addendum to that old design, as it's akin to a two-wire power cord & the death cap.   

[PRR]

> hot-side of the line voltage from the wall is not being disconnected

In UL/NEC terms, this is a Plug Connected Appliance. You WILL un-plug from the wall before you stick your hand inside. Right?? If not, you have only yourself to blame.

[shooter]

I have my terminology wrong

when I landed here from the digital world, terms took a minute
Vcc is now B+
Vdd is now ground
there are no collectors or bases, they're plates n grids
99% of the schematics have no grid call outs, R37 at loc A3 is now, see the resistor on V2 screen


and no, Google won't help 

[Esquirefreak]

I always use a dual pole mains switch, to switch on/off both the LINE and NEUTRAL. EU safety code and doesn't cost much more to do.

/Max

[JPK]

I don't like using the transformer mounting studs for any ground connections. Those are for mounting the transformer. It's just lazy practice but I see it in layouts all the time. I prefer separate chassis grounds.

[electrical]

I can definitely create a dedicated chassis ground in the future, I have plans to add a bias pot in the next few months and when I do that I’ll drill a hole specifically to mount the power cable earth on its own screw, star washer, and keps nut for its current fastener.

After probing voltages, I found a cold joint on a jumper going from the preamp mixers to the tone stack - that fixed my amp right up, and I’m proud to have completed my first build.

Regarding the power cable, I would like some clarification for what is dangerous about my wiring scheme. Is it that the primary is always at voltage when the amp is plugged in, even when the circuit is not complete from turning the on/off switch? I see that it is recommended I wire one primary lead to neutral, would it be wise to wire the hot lead to the on/off switch, and connect the other end of the on/off switch to the fuse and on to the remaining primary lead?

[Esquirefreak]

Or just fuse the HOT and use a dual pole switch and all will be well

/Max

[sluckey]

Regarding the power cable, I would like some clarification for what is dangerous about my wiring scheme. Is it that the primary is always at voltage when the amp is plugged in, even when the circuit is not complete from turning the on/off switch? I see that it is recommended I wire one primary lead to neutral, would it be wise to wire the hot lead to the on/off switch, and connect the other end of the on/off switch to the fuse and on to the remaining primary lead?

There is no danger to a user. A potential danger exist to someone servicing the amp. A non-savvy tech may think that when the switch is off the 120VAC is dead, but this ain't so with your scheme. 120VAC is still present up to and through the PT primary. So a possible shock hazard still exists. IMO. a non-savvy tech has no business being inside an amp chassis. A smart tech would probably unplug the mains cord before poking a finger or soldering iron to the mains circuit.

As far as I know there is no "rule" that states the neutral shall connect directly to the PT primary and only the line shall be fused and switched. However, this has become standard practice and I agree with it. Anything I build will be wired like this..., line to fuse, fuse to switch, switch to PT. Neutral to other side of PT. Ground/Green to chassis. I also do not assume that every amp will be wired like this, and that's OK. I know how to be safe even if the neutral is switched.

This notion probably comes from a NEC (National Electrical Code) rule that states that the neutral SHALL be continuous from the distribution panel (breaker box) to every circuit in the building. No fuses, switches, etc., are allowed on the neutral. But the NEC has no rules for plug-in appliances such as your amp. Still a good idea to do so and I wish everyone would adopt this practice for standardization. I also don't think old amps that are wired as your mojo schematic need to be "corrected".

If you're still not clear about this, just unplug the amp anytime you need to be sure there is no shock hazard inside. Discharging filter caps is a separate issue. Usually only take one mishap to figure out what to do!   

Here's my preferred mains wiring.