100W plexi...so um....are they supposed to be noisy?

[beasleybodyshop]

Hello everyone! I have just finished building a 1967-68 era 100W Marshall plexi clone. Amp is more or less the stock 1959 with a few twists of my own:

For the bass channel (820R/330uf cathode) I made the bypass cap switchable to a .68uf via a push pull pot. I find that when you jumper the channels for more gain this takes a lot of the low end mush out.

Used a 4700pf brite cap for the bright channel - Really like the trebely grind the bigger cap offers, plus i also made this switchable via a push pull pot so it wouldn't sound so shrill at lower volumes.

Added a parallel tube effects loop module - this little unit sounds great, uses a russian submini missile tube. it draws around 1 milliamp and has a trimmer send control. Sounds pretty good. It does have some coloration but its not as colored as other tube effects loops. Also has a PPIMV.

My question is...with the master cranked all the way up and the channel volume all the way down, it does hiss...is this normal? Is this circuit inherently noisy? Ive disconnected the tube loop and the PPIMV and it still hisses the same amount, so i think it's safe to say those aren't contributing to the problem. I also pulled the PI tube, hiss does drop dramatically. What do you fellows think?

[beasleybodyshop]

Oops. Had a duplicate in there:

[cbass]

Doesn't sound normal to me you shouldn't be getting much hiss at all with the preamp volume below 5 and the master cranked .

[beasleybodyshop]

Going to try to pull some preamp tubes and see if I can isolate it somewhere in the preamp.

[beasleybodyshop]

So, some more progress:

The hum is definitely 60/120Hz in origin. I pull v1 and v2, still hums. I pull the PI tube, hum goes away. i double check that my PI filter (two 50uf in parallel via can cap) is grounded and attached right after the PI dropping resistor. This is confirmed. Still hums with a PI tube. I disconnect the ground on the PI filter caps, to see if the hum gets worse. It doesnt, it just sounds the same. Ok, so maybe the PI cap is bad or leaky? I hook up my Eico 950B and test for leakage up to around 450v - Says it's not leaking. I dont have another filter cap handy or i would tack it in.

If the hum level doesn't change when i disconnect the PI filter cap from ground, it should be that particular cap right? On this type of plexi, the PI cap sits next to the power transformer and has its positive lead ran under the mains cap filter board all the way over to the dropping resistor where it connects to the circuit. Could the close proximity of the positive lead to the mains filters cause induced hum?

[Roe]

Try rotating the OT to mimimize hum. and experiment with bright cap to see if the hiss is affected

[mhartman]

Try swapping V2 (cathode driver tube) with a different brand. Some tubes are noisy in that position.

[Reeltarded]

Wait.. a question because it's killing me..

50u para for the PI? Huh?!

You have a minitube with a high current circuit running in a massive induction field. Not a good idea.

[Jana]

When all other avenues are exhausted, PI hum is almost always caused by a ground problem. The speaker jacks need to be isolated and the common from the OT secondary needs to be grounded at the same place as the PI--most commonly at the same place that the presence pot is grounded if using a 5K pot or the 4K7 resistor if using the other style presence. The OT secondary common is connected to the speaker jacks but a ground wire is run to the aforementioned location.

[Structo]

Most high gain amps do hiss.

But you don't really hear it unless you are not playing in a quiet place.

As mentioned, try different tubes to see if it changes.

As for the hum, 60Hz is usually related to the heater circuit and the 120Hz is usually a power supply problem.

[beasleybodyshop]

Ok, some updates:

I clipped in another PI filter cap. Grounded it where all the other filter cap grounds are, still hums. Ok, we can remove filter cap from the equation.

My OT secondaries are connected to isolated speaker out jacks. They are not grounded to anything else - i thought that connecting the OT common to ground would cause ground loop issues? Will try grounding PI filter cap to OT common - to ground. See what happens.

The noise is definitely a 60 hz or 120Hz buzz. I guess hum isnt the proper word. It is almost in tune with the B note on my guitar (2nd fret on the 5ths string)

Miles, isnt the typical marshall setup using a dual capacitor wired in parallel ...well....typical? lol. I was going for a mostly bone stock marshall type thing

[johnnyreece]

Here's what Aiken says:

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 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 currents from flowing in the chassis. Note that there still must be a 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 wherever the global feedback is returned). 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 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.

[beasleybodyshop]

Here's what Aiken says:

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 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 currents from flowing in the chassis. Note that there still must be a 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 wherever the global feedback is returned). 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 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.

This makes sense. I grounded the common to chassis. Noise went away. almost all away. I moved the ground for the PI and the OT common to the point where the presence control is grounded, noise pretty much completely disappears. Thanks for that tip Jana!

I dont know why, but i always thought that the OT secondary circuit should stay in an isolated loop from the rest of the circuit to minimize potential of noise. Guess I need to re-evaluate a few of my other builds and see if this eliminates some noise in them as well! Thanks everyone!!

[Jana]

The output jacks need to be of the isolated type so that they don't make the ground connection at the jack to the chassis at that point. They do, however, need to have a ground wire going from the common to the place I mentioned. Look at the schematic and think it through--the negative feedback network needs a complete circuit in order to function. The circuit is not complete until that ground wire is in place. Stare at it for a while, you will see it.

The secondary of the OT is the source of the signal. The 5K pot (or 4k7 resistor) is the load. The feedback resistor is connecting to one side of the load, what is connecting to the other side?

[Roe]

...

As for the hum, 60Hz is usually related to the heater circuit and the 120Hz is usually a power supply problem.

in these amps the hum is also related to the transformers. Specifically, the PT hums and the OT pickups up this hum even when the amp amp is on standby