I'm tired of anemic guitar amps!

[scruffy]

Well, my PI design worked, there's exactly 90 volts on my cathodes.

So that's all you gotta do, if you want a cleaner and slightly more aggressive PI in a 12ax7 amp, put a 12at7 in the PI slot and change the 82k/100k load resistors to 47k/51k respectively.

This will give you more drive into your output tubes, and stay cleaner longer as you raise the volume. When you start getting grit, it will be authoritative, not anemic.

 

[scruffy]

Oh - use 450 volts on the PI. Don't go any higher. Lower is okay though.

Higher will work, but you'll exceed the 90v cathode spec.

 

[soul musician]

Panasonic has always been the best for my money. I even have my granddaddy's TV that he got in 2006. Still works like a charm.

 

[scruffy]

Oh, and the PI solution assumes you have an adequate power supply, the AT is a higher current tube.

I have no idea why Fender decided to use a 22k tail which raises the cathode above spec and puts a lot of stress on the tube. Nor why that idiotic 100 ohm resistor is below it, with an 820 ohm feedback resistor. Kinda dumb design. Leo must have been having a bad day.

You can do some quickie math on this, for an 8 ohm speaker 1% of your speaker current is going through the feedback loop. At 100 watts full power, speaker voltage is about 28v so your current is 3 amps and some. 1% of 3 amps is 30 mA. That's more current draw than the entire preamp section, with all the tubes combined!

You don't need that much current, for sure. Besides, the tube is a voltage device, it's the ratio of the resistors that matters, because they're a voltage divider. Continuing with the math, you're going to use about 1/9 of your speaker voltage as feedback, at 28 volts (100 watts) let's say that's 3 volts. Compare that to the 12ax7 ratio of 100k/15k which is 6.6:1 instead of 8:1. No big diff. So you're injecting 3 volts at a point in the tail that represents about 30 volts, so about 10%, and we can calculate what that is in dB (but I won't). You can easily prove to yourself that a 100k NFB resistor will work just fine with a 12at7.

 

[scruffy]

Panasonic has always been the best for my money. I even have my granddaddy's TV that he got in 2006. Still works like a charm.

This amp was made by Panasonic

 

[scruffy]

Well, so far in this exercise we've fixed the cathode follower and designed a better phase inverter. We could be clever with voicing too. If you take a peek at the Super Lead Overdrive schematic you'll see the differences between the clean channel and the gain channel. There are mainly two types of differences: cathode caps, and voicing between stages. And most of both is an effort to remove bass from the signal. The cathode caps cut everything below 500 Hz, and the voicings generally make things brighter.

For a bass amp, you don't want to cut frequencies below 500 Hz, so you switch in larger (25 uF) cathode caps. Amp stages 1 and 2 can be handled with a single pull switch. That leaves voicing between stages.

You'll notice that between stage 1 and stage 2, in advance of the gain control, are Marshall type treble peakers. And at the input to stage 2 there is a huge 470k grid stopper (cuts highs) which isn't present in the clean channel. (Mesa uses the same trick in their Dual Rectifier, just with different part values). Those two things combined are an additional pull switch - so we have a "caps" switch and a "voicing" switch.

Now, if you just bypass the cold clipper, you have a clean channel. You can bypass the cathode follower too, for "super clean". If you want gain, switch out the cathode caps and switch in the voicing. We could in theory put these functions on the same switch, if we had a 4PDT. But this way you have more flexibility.

 

[scruffy]

The ultralinear amp needs twice as much drive and twice as much bias.

The normal amp has -56v bias and 30v drive, the ultralinear has -88v bias and 80v drive.

 

[scruffy]

Tone stacks are part of voicing. What exactly do tone stacks do? The Duncan Tone Stack Simulator (free!) will show you. Mostly it's about cutting mids. There are other ways to cut mids though, besides just the TMB (Fender/Marshall) tone stack. One way is by using a bridged T or twin T notch filter. Gibson did this in the early days, they put the notch between the bass and treble controls.

https://www.schematicheaven.net/gibsonamps/ga30rvt.pdf

This is really the best way to approach tone in an amp, and it's why the gain amps need 5 stages to do the same thing piecemeal. Put the bass control up front, like right after stage 1. Then the mid notch can be after stage 2. Treble should be last in the circuit. There's no reason to put them all together.

Bass is first because of the intermodulation behavior of tubes. If you cut bass up front in an overdriven amp it'll be filled in later by intermodulation subharmonics. Treble is last because the amp can generate its own noise and oscillations and stuff you want to get rid of.

Notches are easy because you just find the frequencies you want to get rid of. 200 Hz is a favorite, that's "mud". For a guitar, 600-800 Hz is another.

 

[scruffy]

In a tube amp, first off, you want your PI to be adequate. That's what creates the sound coming out of your power tubes. An anemic PI means an anemic amp. If your output tubes are rated at 100 watts, but your amp is only getting 80, how come? Chances are good your PI is misbehaving. Not enough drive voltage, or not enough current. A 12ax7 simply can't provide the current required by a pair of KT-88's. No matter how good the output section is, the amp simply won't perform like it should. You want adequate clean drive feeding your output section. Let the output tubes provide grit, not the PI.

 

[scruffy]

The little practice amp is working out better than I thought. Reducing all the tone shaping circuitry to two switches gives the player some intuitive options.

One of the two switches cuts bass, the other one increases treble. If you're in clean mode and you're finger picking some bluegrass, you probably like the bass cut. It's also great for disco, when you need thin guitar sounds.

On the other hand, you don't "have to" use the voicing for overdrive. If you don't, you get deep dark Black Sabbath sounds with a well defined low end and not much on top. Great for metal rhythm, you can use the tone stack to cut the lowest bass notes so you avoid the bumblebee sounds.

With two switches you get 4 combinations in clean, and 4 combinations in crunch. Combined with the selection of preamp stages, this becomes a versatile little powerhouse. The amp is loud as hell, it makes my windows rattle. It's an honest 100 tube watts, just as loud as the Marshalls that usually deliver about 115 watts and much louder than a Twin Reverb that tops out at 85.

I should be receiving the new power switch when I get home in about an hour, if it arrives I'll install it and put the amp in the case and take pictures of it. Looks like a narrow JCM-800, very pretty.

 

[Fort Fun Indiana]

The little practice amp is working out better than I thought. Reducing all the tone shaping circuitry to two switches gives the player some intuitive options.

One of the two switches cuts bass, the other one increases treble. If you're in clean mode and you're finger picking some bluegrass, you probably like the bass cut. It's also great for disco, when you need thin guitar sounds.

On the other hand, you don't "have to" use the voicing for overdrive. If you don't, you get deep dark Black Sabbath sounds with a well defined low end and not much on top. Great for metal rhythm, you can use the tone stack to cut the lowest bass notes so you avoid the bumblebee sounds.

With two switches you get 4 combinations in clean, and 4 combinations in crunch. Combined with the selection of preamp stages, this becomes a versatile little powerhouse. The amp is loud as hell, it makes my windows rattle. It's an honest 100 tube watts, just as loud as the Marshalls that usually deliver about 115 watts and much louder than a Twin Reverb that tops out at 85.

I should be receiving the new power switch when I get home in about an hour, if it arrives I'll install it and put the amp in the case and take pictures of it. Looks like a narrow JCM-800, very pretty.

Have you thought about including a wattage cut setting or two?

 

[toobfreak]

a well defined low end and not much on top.

Sounds like my ex-wife.

It's an honest 100 tube watts,

I think what you are saying is that it is a real 100 watts r.m.s. and not that fakey 100 watts rated IHF.

 

[scruffy]

Have you thought about including a wattage cut setting or two?

Just a master volume.

There are many good options for cutting watts. Cathode bias will take you down to 70 watts, triode mode will give you 60-ish at best, and there's always the poor man's class A, just pull one of the output tubes and you're down to 50 watts.

50 watts is still very loud. For clubbing the master volume works well. It's plenty loud at a setting of 3, that's maybe 30 watts.

 

[scruffy]

Can I upload a sound file?

 

[Fort Fun Indiana]

Just a master volume.

There are many good options for cutting watts. Cathode bias will take you down to 70 watts, triode mode will give you 60-ish at best, and there's always the poor man's class A, just pull one of the output tubes and you're down to 50 watts.

50 watts is still very loud. For clubbing the master volume works well. It's plenty loud at a setting of 3, that's maybe 30 watts.

Hell yes it's loud.

I gig at 20 watts. And the master never gets to 12 o'clock.

 

[Fort Fun Indiana]

Can I upload a sound file?

You can link to one in Google Drive and make the link "read only".

 

[scruffy]

Here's the practice amp.

You'll see it's missing a knob, it's on the way, should be here tomorrow.

So let me explain how this works, and how exactly it differs from a Marshall.

Every knob is what it says it is. Except, every knob pulls out for an additional function. So, the leftmost four knobs are the tone stack, presence on the left followed by bass, middle, treble.

Out of the box, all the knobs are pushed in like a regular amp. In this configuration, you get what amounts to a very versatile Marshall, with two extra gain stages. The master volume and preamp volume controls are in their usual and correct places. The knob on the far right, above the input jack, dials the "amount of crunch".

If you pull out any of the leftmost 3 knobs (presence, bass, middle) you're getting successively cleaner, by disabling (bypassing) stages 5, 4, and 3 respectively. In this amp, stages 1 and 2 are your ordinary preamp, stage 3 is the cold clipper, stage 4 is the cathode follower, and stage 5 is "more gain please". So with all three of the leftmost knobs pulled out, you're in "clean" mode, the preamp goes straight to the power amp with only the tone stack in between.

The rightmost 3 knobs are volume controls. From left to right you have your master volume, preamp volume, and crunch control. The crunch control only works when the cathode follower is active (bass knob is "in" - if you pull out the bass knob you bypass the cathode follower and disable the crunch control).

With the CF disabled there are several other ways to get crunch. The cold clipper is crunchy and so is stage 5. Both are regulated by the preamp volume control. If you want crunch at low volumes, turn up the preamp and turn down the master, just like in a Marshall.

Removing the other input jack means the voicing has to be done somewhere else, because the two jacks on a JCM-800 have different voicings. There are two differences, the cathode bypass cap and a resistor in front of the second stage grid. In this amp, the voicing combinations are provided by pulling out on the master volume, preamp volume, and crunch control. Pulling out the master is a "bright" switch for the tone stack. Pulling out the preamp volume puts 25 uF caps across all the cathodes for maximal bass response (you'd usually do this in clean mode). Pulling out on the crunch control bypasses the large grid stopper resistor and removes the treble peaker from the first stage of the preamp. (Again, you'd usually do this in clean mode).

On the back of this amp are the two speaker jacks, and the hum balance control is next to them. You can use either 4 or 8 ohms total speaker load, the amp doesn't care.

This is a practice amp. The idea is, you plug it into whatever speaker happens to be available, and you have "enough" different sounds, and you can get to any of them without too much trouble.

 

[scruffy]

So, back to the mother amp. It differs from the practice amp in that it provides "precise control" over each stage. There's an up-front tone stack to prep the sound going in to the gain stages. Each stage has its own tone control. And there's reverb.

The concept of a dial-an-amp could be taken even further, it could go into full-on dial-a-Dumble mode where individual component values get selected. It would require more switches and more knobs and more front panel real estate.

For example - the cathode resistor in the cold clipper, is "selected". Standard values are 10k and 39k. But my ear says 4.7k sounds the best, and what if someday you found a sound that wants 22k? You could just as easily put a 50k pot there, that way you can dial any value you want. This adds a knob to the front panel, and it's only useful when the cold clipper is active. Otherwise you can use a toggle or one of the pull switches if you don't need the full range.

Voicing the mother amp becomes a question of finding the useful ranges. And that is best done interactively, because the calculations never find the sweet spots. Hang a couple of alligator clips and a pot off the amp, find the useful range, then measure it with a meter. Then you can replace with fixed values and set the range as desired.

 

[scruffy]

A discovery - it's not possible to get the mondo killer overdrive without a third gain stage in front. With more gain, you really start hearing the cold clipper and the cathode follower. Without the input drive, you have to turn the preamp gain all the way up to get the creamy overdrive sounds, and that's not the desired behavior. There should be at least 10v input into the gain stages. With 20v you get all the way up to blocking, but 10 volts is adequate.

My two-stage preamp tops out at about 8v, which is the most I get remaining "completely clean" and allowing a little extra slop for tone shaping. So the third input stage is pretty essential for any kind of high gain activity. You could boost the input with pedals, but it would be better to have the capability inside the amp.

 

[Etymology gaslight]

I'm going to build an amp. And post the whole thing on YouTube. Show the world how it's done.

My ear has had it with anemic amps. I like thump, I like an authoritative sound. I like the power tubes to break up at full volume, none of that saggy blues stuff.

My other requirement is it has to be bulletproof. It has to be able to fall out the back of a truck and survive. So no PC boards, all old school point to point wiring. The worst thing that happens is a tube blows and then you replace it and you're done.

If you're into guitar amps, check out the schematic of this 200 watt Marshall. Notice the 12AU7 driver, in front of the power tubes. That's there because the KT-88's require 50 V rms to reach full power. They sound great when they do, they're thumpy and they have great dynamics, but they need some beef backing them up.

https://schematicheaven.net/marshallamps/major_1967u_lead_200w.pdf

So I'm going to have a 400 VA toroid that can supply almost an amp at 560 volts (the tubes draw 640 mils at full power), but it weighs less than half as much as a big metal power transformer.

And I'm going to make it a dial-an-amp, so you can get any sound you want just by flipping a few switches. If you want a Fender sound with reverb and the tone stack up front you can get that, and if you want a Marshall sound with the tone stack in back you can get that too. And anything in between, and above and beyond.

By using a 12AU7 as a phase inverter, ahead of the driver, I get a combined gain of about 60 for the power amp, which is just about perfect, that means about 0.8 volts will drive it to full power. With a long tailed pair, it'll have the same sparkle as a Marshall Major about halfway up, and then it'll get really aggressive when it's cranked.

I want to blow some windows out this year. It's one of my New Year's resolutions.

Buy a vintage Mesa boogie bro.
Find your sound
& let sound reinforcement do the rest.