NAD: DG 30 head!

[ruger9]

But no, it's not better. I'm not even sure it's newer.

Finally, I'm not angry or hot over this. We're having a conversation, not an argument.

Ah, good.

Well, as I said.. from all the reading I've done on it over the last 20 years, it seems to me the post- design is more generally accepted as better, or at least at was when it was (is?) all the rage in these designs.... for the reasons stated above. I remember when many were moving to the post-PI design, in the effort to get a more "transparent" MV design that did not affect the gain structure as much... I never meant to say it's objectively better, as I do agree- there is very little objective about tone.

Same goes for the mythology of "I don't like NMV amps", and "I don't like channel switching amps"... both valid opinions, but neither is objectively a statement of truth.... for example, Grissom has "never gotten along with 2-channel amps, because the clean channel is too clean and the dirty channel is too dirty".... I couldn't disagree with him more, but it all comes down to what works for us. I've owned (and still do) NMV amps.... and it depends.... my Princeton is fine NMV. However, my Supersonic 22 and 60 would be very difficult to use w/o it (the 60, impossible). Ditto my Hot Cat. The Dr Z I owned had a post-PI MV, and it was a fine design... it was only a 15-watter, but w/o the MV it would have been a gig-only amp.

As Mr. Scott from the Enterprise always used to say, "the right tool for the right job!"

As for the DG30, if the MV is good on it, I believe you.... I'm almost more intrigued with that 2x12 cab design than I am the amp itself anyway lol.

 

[László]

Ah, good.

Well, as I said.. from all the reading I've done on it over the last 20 years, it seems to me the post- design is more generally accepted as better, or at least at was when it was (is?) all the rage in these designs.... for the reasons stated above. I remember when many were moving to the post-PI design, in the effort to get a more "transparent" MV design that did not affect the gain structure as much... I never meant to say it's objectively better, as I do agree- there is very little objective about tone.

Same goes for the mythology of "I don't like NMV amps", and "I don't like channel switching amps"... both valid opinions, but neither is objectively a statement of truth.... for example, Grissom has "never gotten along with 2-channel amps, because the clean channel is too clean and the dirty channel is too dirty".... I couldn't disagree with him more, but it all comes down to what works for us. I've owned (and still do) NMV amps.... and it depends.... my Princeton is fine NMV. However, my Supersonic 22 and 60 would be very difficult to use w/o it (the 60, impossible). Ditto my Hot Cat. The Dr Z I owned had a post-PI MV, and it was a fine design... it was only a 15-watter, but w/o the MV it would have been a gig-only amp.

As Mr. Scott from the Enterprise always used to say, "the right tool for the right job!"

As for the DG30, if the MV is good on it, I believe you.... I'm almost more intrigued with that 2x12 cab design than I am the amp itself anyway lol.

The DG cab is cool, but I like it best with the DG amp. That seems to me to be its made-in-heaven match, though I've tried it with all of my amps. I've found it's voiced to sound its best with the DG amps.

I tend to prefer single channel amps, but that's because with most I find the transition from clean to dirt a bit 'wider' in range of attack and volume that will trigger the transition, and I can more easily control it from the guitar.

I've found that transition can be done with lots of two channel amps as well, if they're set up right; it's just a bit more difficult for me to find the sweet spot with them.

But that's just a personal preference. There's no right and wrong to any of this, and I also have the Lone Star and Fillmore which have 2 channels, and for many years played Two-Rocks, so I'm not against two channel amps. I don't often switch channels on them, but the option is there, and it's not a bad thing, given that clients ask me to do an awful lot of things I wouldn't ordinarily do on my own accord.

The DG's MV is as useful to me as any, which in my case means, "ehhh...kinda-sorta useful." I run the DG's MV at around 3 o'clock, and set the volume with the gain control. On the other hand, I probably play louder in my studio for recording than most here do.

On the Lone Star I switch the global master, the extra gain stage, and effects loop completely out of the circuit. I can't switch off the individual channel masters, but I run them as open as conditions allow. The Lone Star is my high-headroom clean amp. The Fillmore breaks up pretty early, so the volume is relatively sane, but I mainly use it in clean mode as a lower-headroom amp.

My favorite amps for my own purposes are still the PRS single channel models. I use the Mesas when the clients are looking for a specific Tweed or Twin sound. That happens from time to time, so it's worth having them around.

 

[Em7]

While it may sound like heresy, the power tubes have very little to do with the harmonic complexity of an overdriven guitar poweramp, as most of the harmonic content is generated by the phase inverter, which clips asymmetrically under heavy load. A Marshall 1987 50W JMP amp and a Tweed Bassman have the same basic circuit topology, as they are both based on the 5F6-A tweed circuit. A lot of people will point to the use of EL34 tubes being pushed into distortion that differentiates the Marshall sound from the Tweed Bassman. However, the truth is that while they share the same circuit topology, they do not share the same voicing. The power tubes help, but it is the difference in interstage coupling and a better power supply that makes these amps sound different.

5F6-A Tweed Bassman Circuit

Marshall 1987 Schematic

As I mentioned, the circuits are basically identical from a topology point of view; namely, gain stage, volume pot, gain stage, cathode follower, tone stack, phase inverter, power tubes. One thing to note here is that this schematic is for a US export amp from the seventies. Marshall sent JMPs to the US with 6550s to increase reliability. That being said, what really makes this amp sound different is capacitance. For one, the cathodes on the bright and normal channel input gain stages are tied to the same cathode resistor and cathode bypass capacitor on the base 5F6-A circuit whereas they each have their own cathode resistor and bypass capacitor on the 1987 circuit. The bright channel on the 1987 has a 2.7K cathode resistor and a 0.68uF bypass cap. This change alters the bias point and the frequency at which local negative feedback is decoupled. In effect, the gain above the RC time constant is boosted. The next difference is the use of a 0.0022 instead of 0.022 interstage coupling capacitor between gain stages 1 and 2 on the 1987's bright channel. Once again, the change exists to cut bass. Now, two interesting differences lie in the tone stack. The slope resistor has been reduced from 56K to 33K and the top capacitor has been increased from 250pF to 500pF. This change is an often overlooked part of the 1987 (50W JMP) and 1959 (100W JMP) circuits. It makes the tone stack more responsive to a guitar's voice by shifting midrange frequencies into the bass control. The final critical change are the coupling capacitors between the phase inverter and the output tubes. Marshall reduced this value from 0.1uF to 0.022uF, which reduces the amount of bass that is fed to the power tubes. These circuit changes coupled with more power supply capacitance and a silicon-rectified power supply are what are responsible for a JMP's in your face tone when cranked, not the power tubes.

That is not to say that power tubes do not contribute to the tone, everything in an amplifcationsystem affects tone. The EL34 and EL84 are a bit unique in the guitar amp world in that they are power pentodes (five element tubes) whereas almost all of the other power tubes are beam tetrodes (four element tubes with beam forming plates). Historically, the different designs were driven by a patent. Philips and Mullard were the first to solve secondary plate emission (basically electrons hitting the plate and bouncing off) by introducing what is known as the suppressor grid to a tetrode. Because this design was patented, EMI engineers came up with an alternative design that suppressed secondary emission via electron beam forming plates. The result was known as the "kinkless tetrode" (i.e., the KT in KT66), which is known today as a beam tetrode, or as my father used to say "beam tube" (my father was a practicing electronics technician from 1951 through 1994). The KT66 was originally manufactured by the Marconi-Osram Valve company, which was owned by EMI and GEC. Tube geometry affects frequency response.

With that said, what Marshall did was to lay the groundwork for a common practice in overdriven tone today; namely, cut bass before going into a gain stage and cut highs after exiting a stage is that is pushed into clipping. The main reason why the HXDA sounds so fat and juicy is due to the horrible frequency response of the Cinemag transformer, which acts like a low-pass filter that removes undesired higher order harmonics from the output signal (i.e., the harmonics that sound like angry bees or crushed glass).

 

[László]

Interesting, Em7!

One thing I noticed back in the day when Mesa offered a rackmount preamp that could also be used for direct recording, and taking direct other amp preamp outs into a recording console, was that bypassing the output stage of an amplifier sounded just awful. I couldn't use it at all in my work.

Whether that's the PI, the tubes, the output transformer, and a variety of other parts that affect the tone - whatever the reason, all that back-end stuff matters.

It's good to know why it matters! So, thanks!

 

[Em7]

Now, for another twist, the Marshall amp used on Van Halen's debut album did not have EL34s, nor did it have 6550s installed in the power tube sockets. The power tubes installed in that amp were 6CA7s. The 6CA7 is drop-in EL34 replacement tube with a twist; namely, it is a beam tetrode like the 6L6. What the EL34 and EL84 have over the other power tubes is that they have higher transconductance values. Vacuum tubes are transconductance devices, that is, a voltage is used to control current flow through the tube much like a valve controls water flow through a pipe, which is why the Brits call them "valves." Mathematically transconductance the change in current over the change in voltage. With respect to tubes, it is measured in milliamps over volts (mA/v). The typical EL34 has twice the transconductance of an 6L6, which means that EL34 are easier to drive into clipping. The same difference in transconductance occurs between the EL84 and the 6V6.

As an aside, I have seen quite a few posts on music forums where the authors have claimed that the 6L6 has the kink in its voltage-current characteristic curve that was eliminated in the KT66. That is not true. The 6L6 is a kinkless tetrode and so are 6V6, 6CA7, and 6550 power tubes. Marconi-Osram Valve licensed the kinkless tetrode patent to RCA because they did not believe it could be mass produced, which was one of their less than stellar business decisions. RCA made a mint.

 

[Deleted member 5962]

While it may sound like heresy, the power tubes have very little to do with the harmonic complexity of an overdriven guitar poweramp, as most of the harmonic content is generated by the phase inverter, which clips asymmetrically under heavy load.

When you say this, you mean because of the circuit, not the PI tube, right?

When I was building pedals, it was very easy to manipulate the degree of asymmetric clipping. My reading showed the same thing was done with amps.

There was a big discussion in one of the forum I used to be in. A guy kept insisting that people should stop saying to "buy a balanced PI tube" because he had one that tested stronger on one side than the other, and it was by far his favorite PI tube. I told him that first, it was an NOS tube and perhaps was just a better tube than his others. I told him that if you buy a balanced PI Tube, you're getting the type of clipping that the amp designer intended, but that doesn't mean that to your ear, some other unbalanced tube might not sound better. He never seemed to grasp that and just argued that nobody should recommend balanced PI tubes because they weren't needed and didn't sound better. And, this guy was single and lived alone, so he did push the amp loud enough to get the PI stage clipping.

I discussed this at TGP and several big time amp makers, and a couple tube guys were in the conversation (Doug from Dougs Tubes was one of them). The amp builders said they built in the degree of asymmetric clipping in the PI stage by changing values of the two stages feeding the two sides of the tube, and all agreed that to get exactly what they designed for, you needed a closely balanced PI tube. But, they also agreed that a "happy mismatch" could sound better to an individual than a perfectly balanced tube could. If you think about it, if you design a certain degree of mismatch and the PI tube just happens to be mismatched the other direction and by the same amount, the PI tube could make the clipping symmetrical though it was designed to be asymmetrical. It would be a fluke of sorts, but it could happen. One guy at TGP even said he looked for PI tubes that were mismatched by a certain amount because he'd had his amp for years and knew that amount of mismatch was what sounded best too him.

So while I agree that you should buy balanced PI tubes, I think you should still test other tubes and see which one you like the best. And, to some guys I used to hang at the other forum that obsessed over this stuff while never turning their amp up past 2...

 

[Em7]

When you say this, you mean because of the circuit, not the PI tube, right?
I discussed this at TGP and several big time amp makers, and a couple tube guys were in the conversation (Doug from Dougs Tubes was one of them). The amp builders said they built in the degree of asymmetric clipping in the PI stage by changing values of the two stages feeding the two sides of the tube, and all agreed that to get exactly what they designed for, you needed a closely balanced PI tube.

Basically, every amp that is based on the 5F6-A circuit has an asymmetrically configured phase inverter. If we examine the circuit I posted, we see that the triode on the top in the phase inverter has an 83K plate resistor whereas the triode on the bottom has a 100K plate resistor. The top triode is fed directly from the tone stack; therefore, it has a stronger signal than the bottom triode which receives its input signal via an AC grounded grid. This phase inverter topology is known as a Schmitt Splitter (a.k.a. long-tail pair). It is a differential amp in which the outputs are 180 degrees out of phase with each other. Anyway, as I mentioned tubes are transconductance devices, that is, they take a voltage as an input and output a current. What plate resistors do is translate current swing into voltage swing. Ohms law states that voltage (E) = current (I) * resistance (R). The top triode receives a stronger signal than the bottom(grounded grid) tube; therefore, it has a larger current swing for any given input signal excursion, which is why it has a smaller plate resistor. In effect, the circuit designer built a current conduction imbalance of ~= 20.5% into the circuit, that is, if the top triode is conducting 1.205mA of current, then the designer is assuming that the bottom triode will be conducting 1mA of current.

top triode voltage swing (E) = 0.001205 (I) * 83,000 (R) = 100VAC
bottom triode voltage swing (E) = 0.001 (I) * 100,000 (R) = 100VAC (however, this voltage swing is 180 degrees out of phase with the top triode voltage swing)

The same asymmetry occurs in the AB763 circuit phase inverter, which is the circuit on which most of the desirable blackface amps are based.

Remember, no one was thinking about guitarists diming the controls when these amps were designed. They were shooting for clean, hi-fi like tones on a budget.

 

[Deleted member 5962]

Yes, that's what I was talking about. The topology is much simpler in OD pedals though. I have lots of simple schematic drawings on my old computer from OD designs and ideas I either copied to modify, or drew up myself.

Ha, I had an idea about 15 years ago when I first got into building and modding pedals for what would have to be the greatest OD ever. First, I'd choose my 2 favorite clipping options and maybe 3, and have them switchable. But everybody was doing that. What would make it special, was that I found every guitar/amp/speaker I used would have a different "optimal" frequency and Q for the mid hump. My great idea was to design a full blown Parametric mids control into the pedal. So, you would have Q, frequency and level adjustments. You could choose where the boost was centered, how strong it was, and how wide it was. GENIUS!

I not only talked about this, with one of my circuit guru friends, I started drawing it all out and had it drawn in a circuit design program. And then.... I discovered that it had been done YEARS before (the para EQ mids part only). With my dreams crushed, I never did anything else with the design. 2 years after I completed it, BYOC came out with a kit to make a new improved version of the pedal, which I could have easily modded to make have clipping options... but I never even bought one.

I still think with the EQ and several clipping options it would be the best OD pedal ever, simple because every user could TUNE it to their rig and tone preferences. When David Barger came out with the Gain Changer, I figured it was close enough, and never bothered building one.

 

[Deleted member 5962]

Here (more or less) is what was going to be my big original pedal design that would take over the world and I'd be in the pedal business.

https://buildyourownclone.com/collections/overdrive/products/parametricod

The original "Pearl Drive" wasn't even popular enough that I'd ever heard of it, and thought I had one HE77 of an original idea for a great design. Oh well. I still might buy a kit some time. BYOC is good stuff.