Safe to put V1a coupling capacitor on a switch for variety?

[Chuck H]

...but the high gain input squeels with my p90's with the head open like this at anything above maybe 2 on the gain knob.

That could still change when the amp is properly shielded in the chassis. Otherwise, it's either a proximity amp/guitar issue (not really the circuits fault) or your P90's need potting? If it's microphonic pickup squeal I wouldn't change the amp as a correction.

Sounds like the resonance function is deleting too much NFB at higher frequencies.?.

WRT the "extra gain stage hole" (I like it) better too much gain than too little as long as the amp is stable you can always trim it down (or make it adjustable).

Nice it's going well so far

[Mr. Bill]

I think I'm going to make the slope resistor switchable too since I have free hole right next to it. Any danger of added noise with such a low impedance run right after the cathode follower? I've gotta go about 2 inches worth of wire to get to it.

Also the higher gain channel seems to make the guitar cable micro-phonic. It makes a crystal ting sound when you tap on the cable itself. I've had this happen before in some high gain amps, but always wondered what causes it.

Had a couple guitar players stop by to check it out and they both loved it. It has such a wide pallet. Also was much more quiet with a LP with 490t's, as was expected.

[Chuck H]

I think your safe to run a short lead off the CF. But why not make it more of an instant gratification switch and change something else in the tone stack as well as the slope resistor. Like bump the treble cap value to 1000p AND change the slop resistor. Whether changing the value up or down the bump in treble cap value will add upper mids like a boost. Great for soloing.

[Mr. Bill]

Random question.... When using a transistor as a diode, should you short the collector to the base or just clip it off?

[Mr. Bill]

At this point the amp is sounding really good but I'm chasing down noise trying to get it as quiet as possible on the highest gain channel. At the volume in which cabinet excursion starts (what I consider recording level) there is about 75 dBA at 1 inch from the speaker with nothing plugged in, and ~86 dBA with a guitar plugged in. It's a combination of 120 Hz hum from the filaments and broadband hissy static from about 1k to 6k where the speaker rolls off.

Elevating the haters 40 volts didn't change anything at all (if I did it correctly), but a 6 volt battery powering the preamp filaments brought the 120 down where the signal at speaker is 70 dBA unplugged and 85 dBA plugged. The actual 120 Hz hum is reduced by about 8 dB, but the overall signal is still overpowered by the hiss.

I do have a large power transformer on my property that I believe dumps a lot of noise into guitar pickups, so there's that.

Also tried 6 different outlets on different circuits through my house, no change. Also cut power to the entire house and all lighting leaving only one circuit open, no change. Also tried an entirely different set of tubes, both preamp and power, negligible change.

All of this noise is negligible for the 3 stage hole, but the 4 stage is where it begins to get a little out of hand.

For reference, full signal guitar playing on that channel is ~130 dBA at 1 inch from the speaker. So about 45 dB signal to noise at speaker.

[eschertron]

Some random thoughts:
45dB SNR is not bad for an electric guitar. JM2C
I wonder what the noise floor is with the speaker cab unplugged (everything else in the room the same).
I wonder what the noise floor is with V1 pulled (speaker cab returned to service).

[Mr. Bill]

Not sure how to check noise floor with no cab plugged in. Noise with V1 pulled is 65 dBA at 1 inch. Of course most of the hiss goes away at that point and it's pretty much pure 120 Hz.

Edit: Noise floor with standard AC grounded to 0 volts.

[Chuck H]

Filament hum would be delivered right from the PT secondary without rectification and should be 60Hz. 120Hz would be rectified power. So, ripple in the power supply that is not filtered or grounded well enough. Ground scheme can have a large impact on this. There are a couple of good threads going here right now where R.G. takes some time to explain grounding. Definitely worth reading.

I wouldn't trouble to test the noise level with a guitar plugged in anymore. For the purposes of designing a quieter amp it only confuses the matter because noise entering the amp through the guitar pickup is a different matter that adjustments to amplifier circuitry can't fix.

I also wouldn't trouble to test the amp without the cabinet plugged in unless you have a dummy load. It's not good to run a tube amp without a load (though I wouldn't expect much risk if you're not conducting signal). If you think your PT may be humming mechanically you should be able to tell by separating the amp and speaker as best you can and just listen at the PT.

Hiss is a consequence of thermal, or Johnson noise. The higher a resistance through a circuit for a relative amount of gain, the more hiss you will have. It's a remarkably small amount until you exponentially elevate gain well into clipping levels. Then it sounds like someone is letting the air out of your cabinet Some designs use more series resistance than others, but all high gain amps hiss. There are notoriously bad hissers (some that players love anyway) and the rest that have just a normally huge amount at higher gain settings. All you can do to combat hiss is reduce series resistance. Especially through the early stages of the preamp. Doing this so that the same gain structure is maintained will usually result in lower impedances through some circuits. This can change the character of the tone in a couple of different ways for better or worse. But I bring it up because some designers still do use high-ish resistances for certain circuits because they make the guitar sound "right" for that design. extra hiss be damned. One place you can reduce series resistance without significant change in tone would be the first triode input resistor/s 68k being typical is much larger than necessary to block RF. A 22k will do the job, sound pretty much the same and you will probably notice the reduction in hiss. Another thing that can cause hiss is shot noise. Little bitty arcs of electricity coursing though microscopic voids in a resistive element. This shouldn't apply to your amp significantly unless you've used carbon composition resistor for the preamp plate loads or the cathode follower cathode resistor. I use metal film in the typical 1/2W type and it's said that 1W rating is better.

[Mr. Bill]

Thanks Chuck.

I am not scoping that 120 Hz by the way. That's a test mic in front of the cab sent to a spectrum analyzer on the computer. It could be possible it's a harmonic generated by the amp/cab since it can't really reproduce much at 60 Hz. However, running the filaments off of a 6 volt battery reduced this by 8dB.

Also, V1 (or V0 as I've been calling it) has a 33k grid stop instead of the 68k to reduce hiss already. V2a has the stock 68 though.

Another thing I've been meaning to ask is that I have an unused triode in V0 but I am heating both sides. Going by the data sheets for both EL34 (1.5A heater current) and 12AX7 (300 mA heater current), I am exceeding my PT specs on the heater winding of 7A by 200mA. It may be a good idea to disconnect the heater going to the unused triode, which will reduce my excess current draw to 50 mA. I don't think it would be a noise source, but I should have paid more attention to the data sheets when ordering the transformer.

[Chuck H]

I wouldn't worry about 200mA extra on the filament circuit. It should be able to take it with no problems. That 7A rating actually means that the winding delivers 6.3V at that current level. It's not a current limit, per se. Obviously the manufacturer wouldn't design a unit to run at it's max spec full time, right? So you're cool there.

Ok... Since there's at least 8dB to be gained (lost?) by attacking the issue at the filament circuit I'll cover a couple of options, but you won't like it

One thing would be to fully bypass the first triodes cathode circuit. That would be something like 22uf rather than the .68uf you have there now. This is considered good practice for reducing induced filament hum in any design. The obvious trouble with this is a change in tone. BUT...

Though I have taken to designing with fully bypassed first gain stages as a matter of good practice, I didn't always. Choosing instead to use the first stage cathode as another EQ shelving circuit for tone adjustment. And I can tell you that if your ground scheme is good that the difference in hum between partial bypass and full bypass on hum is minimal if it's even noticeable. But your ground scheme has to be idealized to avoid the problem.

[eschertron]

I didn't mean to horn in on Chuck's analysis (sorry, Chuck!), but there's something to be said for getting context around the numbers you've reported. That's why I asked about noise that might be generated in the room not through the speaker. A dummy load or a shorting jack on the speaker output would be the proper solution. Essential for a 15-second noise reading with no signal to the amp? That's the owner's call, I admit.

8 dB reduction in overall SPL from DC heaters is significant. Since all noise sources are uncorrelated, they add via a square-root-sum, like solving for a right triangle. If the total noise was reduced by 8dB, the heater noise is a greater number. A narrow-band analysis of SPL around 120Hz (60Hz) might be revealing. Regardless, Fixing first-stage heater hum is one issue, ripple hum is another, and the hiss is completely different. You'll have to attack each on their own.

[Mr. Bill]

No need to apologize! The more the merrier! I need all the help I can get. I'm sure once I finish my engineering degree I'll actually start to understand this stuff.

DC heaters seem awfully tempting on that front, but I'm not sure I have the current for it.

[Chuck H]

Thinking about it... No, the Marshall ground scheme isn't ideal, but it never was and it would be a PITA to change the ground scheme this far in.. There may be a trick you can try that I never have, but I thought it was nifty when I saw it. Mesa, being a pioneer in higher gain amps for a long while, did use typical AC heaters for some very high gain builds they designed before DC filaments were all the rage. Something they did was to hang a lead off the filament circuit at one of the preamp tubes. This lead wasn't terminated anywhere, it was just clipped flat on the end. It was positioned near a signal lead (probably the third gain stage input grid lead) to induce filament hum in a stage that was out of phase with the input triode. Tada! Phase cancellation and filament hum reduction. Clever. Unfortunately I can't remember any details of the circuit. It's basically what I described. The length of the lead in the Mesa amps was about three or four inches IIRC.

EDIT: At the very least you should go through the wiring and be sure you're grid leads are not parallel to or too close to filament leads.

[Chuck H]

Ok... My head hasn't let this go yet. Why is 120Hz on the filament circuit. If the voltage is a really bad wave form then it could be a harmonic I suppose, but I would still expect the 60Hz fundamental to be prominent.

How is the filament elevation wired?
Was it always there in the design, or did you add it trying to solve for hum?
Where is the voltage from and where is the filter grounded?

120Hz is typical of rectified power. It might be getting into the filament string through the elevation circuit.

[Mr. Bill]

Chuck, The elevation circuit is gone. I tried it and it made no difference. Maybe I did it wrong. I clipped in a voltage divider off the screens cap to make about 40 volts and bypassed it with 1uf to ground. The current center tap for the filaments is grounded at the PI cap's ground lug. The only non-standard Marshall thing going on right now is that the preamp cathodes for the first 3 gain stages are mounted to the rear panel on switches. Their grounds are tied together and ran to the preamp filter cap ground lug. I could redo the grounding fairly easy on most things. I left myself enough space to get to the under board can cap in case I needed to. Perhaps I could also reduce noise by giving my added stage it's own cap as well since there are 2 in phase signals off of that one side. ( 1 tube plus 1 triode ). This would be a much bigger pain in the rear than changing grounding layout.

Now that I'm getting into higher gain stuff, my next build is going to be an exercise in grounding and noise reduction, so I might as well start with this one. Funny, I've heard of people inducing hum on purpose to make it cancel and gluing the wires down with silicone. I thought it sounded like a neat idea. Maybe that's how the SLO gets away with those straight heaters and being so quiet, it's being induced equally throughout the chain while filpping phase.

As far as the frequency, idk, it's pretty overdriven while checking for noise, so maybe it is harmonics. Also like I said, maybe it's the bandwidth limitations of the speaker cab. I have clips of every change recorded it you would like to hear them.