Jose Plexi mods

[Spencerleehorton]

Yes it seems there are several ways to do gnds that’s why I made the layout diagram to show how they are, not ideal but it does work.
Just wondered if there was a better way?
Also remove the hi/low switch and screen the PPIMV cables?
How’s the power wiring looking in the diagram?

 

[Spencerleehorton]

Have just drawn out my B+ nodes and pretty sure I’ve done my b+ wrong, will follow the slo B+ and change then clean up the rest of the lead dress.
Take out the hi/low switch and put in screened leads for the PPIMV, this should then bring any noise down to minimum, it’s not too bad at the moment but I’m sure I can get it better.

 

[Alva Goldbook]

I'm not a big fan of PPIMV's. Just simplify things and attach the MV after the tone stack. Simple. And works like a charm.

 

[Alva Goldbook]

Here's a schematic of a pretty standard B+ power supply as found on a Marshall style build. Note, no standby switch is included in this diagram, for clarity purposes.

The transformer B+ winding: The approximate rectified B+ voltage can be calculated by taking one half of the B+ winding and multiplying by 1.3. The total current of the winding, as listed by the transformer manufacturer, should slightly exceed the total current drawn. The total current consumed by the tubes can be roughly calculated by choosing 1mA per 12ax7 triode, and looking at the data sheet for the power tubes, and choose the limiting or max figures for the plates and screens.

The rectifier: Here we have a full bridge rectifier, but it can also be a full wave rectifier, a voltage doubler, a tube rectifier, or what have you.

Node A: Node A should only supply the power tube plates, through the output transformer center tap. Nothing else should be connected to Node A. In most Marshall style designs, each filter cap is 50uF, but in most modern amp designs Node A is larger since Node A draws the vast majority of all the current for the B+ circuit. Values as high as 220uF is not uncommon. Take note that Node A's filter cap is commonly referred to as a the reservoir capacitor.

Choke: A choke is often used to filter B+ more, but it doesn't have to be included. Most modern designs exclude chokes and just use more filter caps instead since it's cheaper. The choke will drop a few volts. Usually only 1-2 volts total.

Node B: Node B is typically used to supply the power tube screen grids. Typically, Node B has dropping resistors attached directly to the screen grids. These should be metal oxide resistors, and the common values are 470 ohms for beam tetrodes such as 6L6's, and 1k for pentodes such as EL34's. Pentodes use higher values because their screen grids are more fragile than on beam tetrodes.

Dropping Resistor: These should be Metal Oxides, 2 watt typically in Marshall style designs. 10k is a fairly universal value here. Since B+ is not 100% silent, the value should create an RC filter below the human hearing range of 20Hz. 10k and 22uF will create an RC frequency of 0.7Hz and a 47uF cap will create an RC frequency of 0.3Hz. How much voltage the resistor drops will depend on how much current is flowing through it. If we drop 10 volts through the 10k resistor, then 1mA is flowing through the resistor. Drop 20 volts, 2mA is flowing through it. Drop 30 volts, 3mA is flowing through it. In this way, we can test for current just by 1) always using 10k B+ dropping resistors and 2) just measuring how much voltage is dropped.

Node C: This typically powers the phase inverter.

Node D: This typically powers the tone stack.

Node E: This typically powers the first gain stage

220k: This is a safety filter cap drain resistor. When the amp is on, it effectively disappears from the circuit. When the amp is off, it will slowly and gently drain off the filter cap voltages. These can be 1 watt metal film resistors. Do not use a low value like 100 ohms. High values like 220k are preferred.

 

[Spencerleehorton]

where should the 6.3v CT, Speaker Jack GND, HV winding CT goto?
on the SLO it looks like there is a star gnd over by the power side?

 

[Alva Goldbook]

You should disregard this layout and not use it. Your entire B+ circuit should be linear and not have any parallel offshoots like we have here.

When you build a house you don't assemble the roof before you assemble the foundation. You don't install doors before you assemble the walls.

This is why you should not use a PPIMV, instead you should use a standard master volume. You should not use an FX loop. No FX loop should be in the amp until the base circuit works flawlessly. Build your foundation. Test it thoroughly. Only then add your bells and whistles.

The Input jack should be connected to the chassis. This is your earth ground. Meaning it's physically connected to the earth. This in turn should go to some kind of ground plane. A bus bar is common. Each node's ground is connected to this bar. From the rectifier ground to the input ground. The HV ground is the rectifier ground. This should not be connected to the power transformer mounting nut. Why? Power transformers get warm and this heating and cooling can loosen the mounting hardware over time, cause the ground to disconnect.

If your HV winding has no center tap, then you use a bridge rectifier, like in the diagram I made above. If your HV winding does have a center tap, then you use a full wave rectifier instead and the CT is grounded just as it would be with a bridge rectifier.

Fender amps had a steel chassis that you could solder to, and they would solder the rectifier ground directly to the chassis. On a Marshall style build (an SLO is a Marshall style amp) the rectifier ground gets connected to the bus bar, as does each node.

The Presence control is the one exception. You can put the Presence ground on the bus bar, but sometimes this will cause noise. when that happens, just mount the Presence ground directly the the chassis, via screw, nut, and terminal tag. The reason why this noise happens on just the presence control is a bit complex, and it's not really important.

How you ground the speaker jacks will depend on what jacks you're using. Almost all designs just have the ground on the sleeve of the jack. In Marshall designs the tip of the speaker jack goes to the ohmage wire, whatever that may be. Often these will be like yellow, green, orange, purple wires all having the different output impedance. And then you have a common wire, that's usually black. The common is the ground. The word common refers to "all the other things that are connected to it". For instance a DPDT switch will have a center common lug, but that does not mean that lug is ground.

Almost all amps did this, but early Fender tweed amps often soldered the common wire to the output transformer body. Once mounted to the amp, that wire would then be connected to the amp chassis, and the metal Alpha open frame speaker jacks then connected the jack sleeve to the chassis. This is not really a good way of doing it. Fender stopped this and started wiring up their speaker jacks so that they were grounded when not plugged in. Marshall never did this.

6.3v heaters can be arranged in several ways. The most common method is to use a heater CT if the winding has one, or an artificial CT. Many amp techs much prefer the artificial CT and will tape off the CT wire and use an artificial one instead. This is because the resistors will act as faux fuses should too much heater current gets pulled by one of the tubes for some reason. This can save a power transformer from getting destroyed. And it's a lot cheaper to replace a couple resistors than a power transformer. Another method is to use a humdinger, but I would advise against it until your lead dress improves.

https://www.valvewizard.co.uk/heater.html

A star ground is not a ground that is connected to a chassis. A star ground is a ground that has everything else connected to it, so that the ground wires going to it resembles a star. The grounds on the right of that diagram are chassis grounds.

A chassis ground and an earth ground are both different and kinda the same. In principle, an amp should only have 2 chassis connections. At the wall inlet and at the input jack. And each should be at opposite corners of the amp. But there's a "where the rubber hits the road" reality where it's just not practical to put a cap can ground on a long wire to feed it back to the input jack. So instead you just ground the cap can to the chassis close to where the cap can is mounted. This was a "screw it, it's not technically right, but it works" solution. These days, modern amp designs should avoid cap cans, so the issue should be moot. Cap cans will not be made much longer, and they don't perform nearly as well as modern radial caps that are a fraction of the size, weight and cost.

But if your amp uses them, do what Marshall did. Ground the cap cans to the chassis right next to where they're mounted. Make sure it's on a screw, locknut, with a terminal. Put a drop of nail polish on the screw thread so it don't come loose.

If it's not any trouble, it's more ideal to mount the cap can ground to the bus bar. I've done this with my own designs, and it works very well, assuming the layout of the amp is better thought out than how Marshall and Fender did it back in the 60s

 

[Spencerleehorton]

Perfect, most of what you have said im aware of but always good to go over it again.
as per the cap cans gnd close to them, thats exactly the explaination i was look for.
Star gnds i see what you mean now, just a terminal which isnt actually down to gnd but gnds share.
The heater CT i will change to the 2 x 100R as this is a no brainer, much rather save my OT!
Speaker gnd i will change to a jack which connects sleeve to chassis.
i'll take out the FX loop for the moment, one less thing to confused the issue but when i put back in i'll put the gnd to speaker gnd.
I'll also remove the PPIMV and the hi/low switch as both not needed now.
The HV ct / Rectifier gnd i will attach to bus bar
as per the presence pot, i will try it connected to bus bar and then to its own gnd point close by and see if any less noise

thanks again for your time and patience.

 

[Alva Goldbook]

I usually use two 220R resistors for the heater artifical CT. This protects the PT, not the OT.

Most speaker jacks have a floating ground, as in not really grounded at all. The common goes on the sleeve. With a Marshall style CLIFF jack (plastic jack), there is no connection to the chassis.

A solid state FX loop should have a bipolar power supply. Typically it’s powered from the heaters, or a SS winding from the power transformer. It gets rectified and well regulated to get +16v and -16v. The -16v acts as a ground, but it is not the same as 0 volts earth ground. These two things should not be connected.

 

[13engrsapper]

Thanks for that resource!! That’s great, I’ve found it hard finding stuff through the other forums and tbh I like this forum and wanted to try and build similar info here but have it more schematic based and get rid of the myths!!
I did noticed on both the Jose schematics that I’ve got now that there is no grid resistor on the extra gain stage, and it keeps the 68k into the next stage.
So have read that you are meant to have a grid resistor of at least 10k but the Jose way doesn’t seem to agree with that?

Perhaps we need to request a guitar amps, and an effects Forum in the project specific forums, or both in one.

 

[Spencerleehorton]

You should disregard this layout and not use it. Your entire B+ circuit should be linear and not have any parallel offshoots like we have here.

what do you mean by this?

that layout is the SLO-50 B+ ???

 

[Alva Goldbook]

You should disregard this layout and not use it. Your entire B+ circuit should be linear and not have any parallel offshoots like we have here.

what do you mean by this?

that layout is the SLO-50 B+ ???

Well, think of it like this. A pure linear supply is like a straight line. Node A goes to Node B goes to Node C, etc.

The SLO you showed me has some parallel offshoots. As in Node A doesn't go to Node B so much as Node A goes to Node B in one direction and Node C in another direction. So instead of the circuit going in a straight line, it goes in a Y direction.

While there's nothing wrong with this in principle, it's a far more advanced way to do things, and you're not an advanced builder yet. So keep it simple. Keep your B+ circuit perfectly linear like how I drew out above. This is not just for the sake of simplicity, but also for the sake of trouble shooting.

Merlin discusses this here, but his book has a more thorough explanation, and I highly recommend reading it.

https://www.valvewizard.co.uk/smoothing.html

This diagram explains how useful a linear B+ supply is.

Because everything is linear, it means all current draw can be precisely calculated between each node. How much is the voltage being dropped over the resistor? Super easy Ohm's Law calculation. That's why I say to always use 10k dropping resistors. It makes the calculation of current draw that much easier. If you use parallel offshoots, then you lose this advantage, and trouble shooting becomes more difficult.

Everything this layout does, is doing the same job that an SLO circuit is doing. This is just easier for a beginner to trouble shoot. The SLO has all the bits that a Marshall has. Node A to output transformer. Node B to screens. Node C to phase inverter. Node D to tone stack. Node E to preamp stuff before the tone stack.

 

[Alva Goldbook]

Do you have a layout for the entire amp? When building Step 1 is drawing up a schematic. This is assuming you're going from an original design and not building a clone, but the steps are the same.

Step 1, draw up a schematic. Step 2, draw up a layout. Step 3 assemble the amp. Step 4, trouble shoot any errors or problems.

Each step should be done in that order. Before you build, you should have both the full and complete schematic and the complete layout in front of you. Do you have these things? I ask because there could be any number of things wrong with the amp that I can't ascertain because I'm not looking at them and you don't have enough trouble shooting experience to spot.

 

[john12ax7]

Most speaker jacks have a floating ground, as in not really grounded at all. The common goes on the sleeve. With a Marshall style CLIFF jack (plastic jack), there is no connection to the chassis.

I don't think this is correct. The plastic jack might not connect directly to the chassis but it still connects to ground and chassis somewhere else. Most amps do not leave the secondary completely floating.

 

[Alva Goldbook]

I don't think this is correct. The plastic jack might not connect directly to the chassis but it still connects to ground and chassis somewhere else. Most amps do not leave the secondary completely floating.

Aiken Amps covers this topic in one of his white papers, unfortunately, he seems to contradict himself a bit. Perhaps Merlin can help clarify this.

https://www.aikenamps.com/index.php/grounding

"What about the output jacks?

"The current in the secondary winding of the output transformer can be very large. For example, in a 100W amp, the secondary current into a 16 ohm load is 2.5A. It is even higher into a 4 ohm load, at 5A. This means that you need to pay special attention to the grounding of the output jacks and the output transformer. It is important not to use the chassis for this return path.

"The output transformer secondary has a common wire and one or more speaker taps, usually at 4, 8, and 16 ohms. The speaker taps usually go to an impedance selector switch, and then a single wire goes to the output jack tip connection. The common wire should never be connected to the chassis right at the output transformer. It should be run all the way to the output jack and connected to the sleeve connection of the jack. This accomplishes two things. First, it maintains the continuity of the connection in the event the output jack becomes loose. Second, it keeps the heavy secondary ground return currents from flowing in the chassis.

"Note that there still must be a ground return path to the rest of the circuit if the amp uses global negative feedback. This should be in the form of a wire from the sleeve connection of the output jack to the preamp ground point where the phase inverter common connections are grounded, or to the ground of the portion of the circuit where the global feedback from the tip connection of the output jack is connected. Note that there will be no heavy currents in this wire. The speaker output jacks can be either isolated or non-isolated if you follow this plan, but it is usually best to isolate them to maintain control of the return current path for the global negative feedback, to insure it doesn't flow through a part of the chassis that may contain power supply ground currents.

"Sometimes it helps to ground the common (sleeve) side of the output jack to the chassis even when no global negative feedback is used. Occasionally, an amplifier will have a high-pitched oscillation noise, or other type noise that will go away if you ground the output transformer common wire at the speaker jack sleeve terminal. In addition, there may be a potential for a small AC current to flow between the floating sleeve and chassis if the secondary is not grounded. This current is due to capacitive coupling in the output transformer, and may cause a mild shock if the speaker plug sleeve and chassis (or guitar strings) are touched while running a signal through the amplifier. Even though the potential for dangerous currents is low due to the galvanic isolation of the output transformer, the shock can still be annoying. For this reason, it is best to always ground the common side of the secondary even when no global feedback is used."

 

[Spencerleehorton]

I understand what your saying now after the first 10k in my layout it Y's off to other 10k's (SLO way) rather than having them in series as per the linear way.
now slightly confused about the speaker jack gnd? i can see from what you have put that its preferred be an isolated jack and the common from the OT secondary goes to it, but where within the GND placement should it go? as its potentially lots of current shouldnt it go down with the filter caps gnd?
Also im assuming!! that the 1R resistors connected to pins 1+8 on EL34 should goto a gnd close by on chassis?

 

[Alva Goldbook]

I understand what your saying now after the first 10k in my layout it Y's off to other 10k's (SLO way) rather than having them in series as per the linear way.
now slightly confused about the speaker jack gnd? i can see from what you have put that its preferred be an isolated jack and the common from the OT secondary goes to it, but where within the GND placement should it go? as its potentially lots of current shouldnt it go down with the filter caps gnd?
Also im assuming!! that the 1R resistors connected to pins 1+8 on EL34 should goto a gnd close by on chassis?

I’ve never grounded my speaker jacks. I just put the common on the sleeve and the ohmage wire on the tip. Done. I usually don’t use NFB.

The 1 ohm resistors get grounded right by the tube socket. They’re only needed to measure bias.

 

[Alva Goldbook]

This is from my last build. Common to sleeve, ohmage wire to tip.

 

[Alva Goldbook]

I asked a few amp building friends, and they seem to be in agreement that the speaker jacks get grounded. I have never done this. I found this bit from Merlin.

https://www.valvewizard.co.uk/Grounding.pdf



I also took a look at this layout, and it suggests local grounding of speaker jacks. I don’t recall off hand ever seeing this however in any Marshall amp.

 

[Spencerleehorton]

That’s put the cat among the pigeons!!!

 

[Spencerleehorton]

From what I’ve read it’s a good safety option so I’ll take that!!
It does then bring me to think where is the best noise option for gnd for the speaker jack?
as its high current potential shouldnt the best linear position be at the filter cap?