DIY Tweed Deluxe 5e3 question

[Duelling Banjo]

Hi all,

My first post to this forum, hope it is the correct one for my question.

I'm currently building a 5e3 clone from a kit supplied by Modulus Amplification in the UK. They provide a wiring diagram though no instructions. Not being an electronics expert I need instructions to follow so I am following the instructions for the equivalent StewMac kit and cross checking between the Modulus and StewMac diagrams in case of any differences. I've nearly completed the build, however, I have noticed a discrepancy between the circuits which I don't think I can resolve without the help of someone with expertise in this area. In the attached StewMac jpeg, soldered to the lamp terminals are two 100 ohm resisters which are connected to a green wire, the other end of which is grounded to the chassis. In the Modulus pdf this part is missing from the diagram. However, the two resisters and the grounding terminal are included in the Modulus kit which makes me suspect they should be connected up as per the StewMac diagram. My only reservations of connecting are; 1) If the Modulus diagram is wrong you would have thought someone would have reported it and they would have updated it by now. 2) There are legitimate differences between the two circuits in terms of the Modulus having a standby switch and a different approach to grounding.

If anyone can clarify the purpose of this part of the circuit and whether or not it is required in my case I would be very pleased and I can move on to complete the build.

Many thanks in advance.

 

[Phrygian77]

You need two 100 ohm resistors on the filament supply as a ground reference, if you don't have a center tap on the filament supply. The heaters have to be referenced to ground or you'll get horrible hum. Does your power transformer have a 6.3V center tap?

 

[Scottcurry13]

@Phrygian77 beat me to it. Did the power transformer come with a spec drawing? It will show whether the 6.3v supply has a center tap to ground.

 

[Duelling Banjo]

Thanks for your prompt reply @Phrygian77 !

Excuse my ignorance but I don't understand your technical terms. I'm not aware of the PT having a centre tap, certainly it doesn't have a wire connecting to ground if this is what you mean. Are you able to explain in simple terminology please?

 

[Duelling Banjo]

This is the transformer, sadly no wiring diagram

Deluxe Power Transformer

DELUXE STYLE POWER TRANSFORMERThis is a Deluxe style power transformer made to be a direct fit on our Tweed Deluxe chassis and Princeton Reverb

modulusamplification.com

 

[Duelling Banjo]

Here is my power transformer wired up in the chassis, hope this is clear enough for you to see whether it has a centre tap.

 

[D'tar]

It would appear you will need an artificial heater center tap. Welcome to the madness!

Have a gander here....

5E3 Mods

5E3 Deluxe Amplifier Modifications

robrobinette.com

 

[Duelling Banjo]

Thanks @D'tar, so in simple terms that means I need to connect the two 100 ohm resistors to the lamp terminals, solder a wire to them, then ground the other end of the wire to the chassis as per the StewMac diagram, right?

 

[sds1]

If the Modulus diagram is wrong you would have thought someone would have reported it and they would have updated it by now.

Indeed. Hold up!

Typically the filament center tap is color coded green with yellow stripe.

You have that wire, it's connected to the standby switch at the same terminal the power supply ground bus is connected to.

The standby switch seems to be a ground lift type but I don't understand that the black PT wire on the other side of the switch.

We should understand what that black wire is before proceeding here as it seems like you indeed have a filament CT already. Black has to be the HT center tap yeah?

And so then where is the circuit's chassis reference?

 

[Duelling Banjo]

@D'tar I just followed the link, in this diagram the 100 ohm resistors are connected directly to one of the power tubes.

My initial question was do I need to attached the two resistors at the top in the diagram below (StewMac wiring), then connect the green wire and ground them to the chassis?

Now I'm really confused because I don't know if I need those resistors at all, and if I do, I now have two options as to where I connect them...

I thought this would be a really simple question, how naive of me

 

[Duelling Banjo]

Thanks @sds1 , any suggestions for how we figure out what these wires are? Is there any additional info I can give you?

These pics from an earlier stage in the build might help;

 

[D'tar]

@D'tar I just followed the link, in this diagram the 100 ohm resistors are connected directly to one of the power tubes.

View attachment 1086182

My initial question was do I need to attached the two resistors at the top in the diagram below (StewMac wiring), then connect the green wire and ground them to the chassis?

View attachment 1086181

Now I'm really confused because I don't know if I need those resistors at all, and if I do, I now have two options as to where I connect them...

I thought this would be a really simple question, how naive of me

Disregard my comment on you needing the artificial center tap. As @sds1 noticed you may be covered. Appologies for any confusion. You may like to do some confirmation with your meter to ensure that wire is a heater center tap.

 

[sds1]

Thanks @sds1 , any suggestions for how we figure out what these wires are? Is there any additional info I can give you?

Yeah like @D'tar mentions above you can put your ohmmeter on the windings to get some add'l info. Make sure no tubes or pilot lamps are installed when you do this. Measure resistance between the following PT wires:

Red -> Red
Black -> each Red
Green -> Green
Green/Yellow -> each Green
Black -> The chassis (I assume you must be getting chassis reference via the PT housing??)

See what you come up with there.

Appologies for any confusion.

Not your fault, this Modulus layout is hard to look at! Waiting for someone to come along and tell me I'm wrong.

 

[D'tar]

Not your fault, this Modulus layout is hard to look at! Waiting for someone to come along and tell me I'm wrong.

Right, Im on my phone to boot!

 

[chas.wahl]

Unfortunately, @Duelling Banjo, you've wandered into a vortex of "issues" that have their source in the electrical practices of amp antiquity, and the way they've evolved since then. Safety is one reason for that evolution.

The original 5e3 Deluxe was powered by only two mains conductors (hot and neutral, without a ground wire) and because the amp's power cord might have a plug which did not enforce polarity, and also because the socket it might be plugged into could be wired with polarity reversed, the Deluxe had a "Ground" switch to reverse the polarity of the connected mains hot and neutral (after one had been shocked). It also had a "death cap", I think provided as a safety device; with its own dubious history -- nobody but an idiot installs one of those.

Nowadays a separate ground is required as part of the incoming mains power (3 wires, hot, neutral, ground, the latter fastened according to requirements of the applicable electrical code, which I believe in the UK means mechanical fastening, separate from any other electrical connection -- that's shown at the bottom left of your Modulus diagram. If you read Merlin Blencowe's (fellow UK resident, aka "Valve Wizard") article on Grounding, he'll explain that for you.

The second has to do with how heaters for the tubes (green wires from power transformer, and in this case green wire with yellow stripe) were wired by Fender back in the day (using the chassis itself as one "conductor", by soldering one of the green wires to the chassis), and how they're generally done now (two green conductors to every tube, isolated from the chassis to reduce hum and noise). Blencowe also has an article about Heaters that will explain how it should be done now.

The third issue has to do with standby switches. Merlin Blencowe also has an article about those, as you might suspect. In the 5E3 case, many manufacturers of kits and chassis for this amp have simply converted the Ground switch (that being patently a Bad Idea, and in fact probably contrary to electrical code) to a standby switch. Opinions vary about whether a standby switch is a Good Idea or a Bad Idea (Blencowe is in the latter camp, and I'm inclined to agree) -- it has advantages and disadvantages, and the hole is there on most chassis; not in a great location for repurposing in a way that has anything to do with the preamp portion of the amp circuit. You could just leave it as a dummy.

As it turns out, it appears that the Modulus approach, which is to use the former Ground as a Standby. It's not absolutely clear to me how that is done (is the switch a 3-position one or two?) but it appears to connect or disconnect the heater ground and ground bus (black wire from circuit board upper left corner to bottom of standby switch) from what I can only guess is the power transformer's B+ (high tension/high voltage secondary) center tap (again, black wire in the Modulus diagram). I am sure, and also hope, that someone else here will provide a more conclusive description of how this works. I think that the net effect is that the heater circuit always has its ground to chassis (connection to the switch is inconsequential, but just a convenient place to connect it, though it could be connected directly to the ground bus too -- thing of the ground bus as beginning, at the power section, at the bottom of the standby switch); the real operation of the switch is to disconnect the B+ center tap from the ground bus.

Your power switch (to the right of the standby switch) is connecting/disconnecting both the hot and neutral legs of the mains power (neutral apparently fused), as Merlin Blencowe notes is required in the UK.

You have at least two choices here:
1) wire it up as shown on the Modulus diagram
2) Don't connect anything to the standby switch. Leave out the black wire from circuit board to standby switch and connect the black wire from power transformer to the end of the ground bus at upper left of circuit board. Connect the green-yellow wire from power transformer to the same place, or else follow Merlin Blencowe's advice and connect it to the cathode of the nearest 6V6 tube (pin 8) to elevate that heater ground on a modest DC source (about 19 V) to reduce hum (see Heater Elevation in his Heaters article).
3) Have you tried to call or email Modulus about this? I strongly suggest that you do this, before taking any advice from a random internet guy (me), to confirm that I haven't gotten something wrong -- especially to confirm what is the correct function of all the wires (particularly that black one, which is conventionally red with a yellow stripe if it's B+ center tap) from the power transformer.

If you're really a newbie to all this, then Blencowe's articles may take you further into the weeds much more quickly than you'll be comfortable with. Unfortunately, that's how it is with amp electronics -- Blencowe is the example that I can think of where basic things are explained with the greatest clarity, though. Stick with it and you'll learn a lot.

For those who are wondering where the ground bus is connected to the chassis, I believe it's at the lower right input jack (looking at layout) -- take a look at the entire Modulus layout PDF downloaded from their website, attached.

 

[Duelling Banjo]

Thank you @chas.wahl for so much information, I really appreciate that

 

[sds1]

For those who are wondering where the ground bus is connected to the chassis, I believe it's at the lower right input jack (looking at layout)

Sure enough, the ground busss runs underneath the board. Very sneaky, could barely see those lines.

 

[YellowBoots]

Who votes @Duelling Banjo as the best first time poster? Do you work in customer support?

 

[chas.wahl]

Thank you @chas.wahl for so much information, I really appreciate that

You're welcome, and I hope it helps.

Just to be a bit clearer about the heater: Blencowe's article has a few diagrams beginning at "Transformer Centre Tap". Your PT has (or appears to have) a center tap wire (green-yellow), like the topmost of those diagrams. While convenient and simplest, the problem with this is that the voltage on either side of that center tap (nominally 3.15 V each side, green wires) isn't always balanced, really, which can cause noise. But you don't need an artificial center tap, as is shown on the next diagram down (the Stew-Mac layout presumes an artificial center tap, hence the pair of resistors -- some PTs don't provide one). Blencowe's "humdinger" (next diagram down from that) is a way of making an artificial center tap that can be adjusted to achieve the best balance between the voltages. That's a nice touch, but requires a pot, another hole in chassis, and abandonment of the green-yellow wire, which is then superfluous.

The elevation of the heater on a DC voltage at a power tube cathode (2nd diagram at "Heater Elevation") is shown using an artificial center tap, but could also be implemented with a humdinger, which is just a deluxe version of the artificial center tap.

I don't intend that you should follow any alternatives to simple use of the heater center tap wire, just wanted to make it clearer different ways to do it -- in case you wish to "improve" (or "muck with") things when you're unhappy with the noise floor of your amp. That's a very common problem, which people have a highly variable tolerance for, and there are many aspects to it, heater circuiting and wiring being only one of them.

 

[King Fan]

Welcome, @Duelling Banjo . You're now hearing from some of the smartest folks on the forum, which means lots of good info coming at you. D'you feel good going forward? Let us know if there's any part you don't understand (but want to -- as noted, a lot of amp theory is pretty esoteric).

For anyone who didn't click on it, here's a pic of the Modulus layout you attached in the first post; just for grins, I drew in a dotted line to show the ground wire under the board, which I too had missed on first inspection.

Oh, also, re the Mojo instructions, they do several things differently from Modulus (as you've noticed). Some are big, like power entry and ground. I haven't gone item by item, but where they differ I'd generally go with Modulus. Mojo is a good little shop, but some of their wiring choices can be idiosyncratic.