katopan wrote: ↑Tue Jun 09, 2020 2:20 am
Sorry, I was away for a few days and couldn't post any reply.
Where this happens on the Express volume knob depends on how hot the guitar pickups are, the pot taper and to a lesser degree what valves you are using (brand and individual differences, not type). So let's talk about three regions of different operation. Firstly is the clean region - Clean symmetrical response with very minimal EL34 screen compression and grid clipping. The important thing here is the output waveform is symmetrical both in the peaks and balanced mark to space ratio. Second is the magic Express overdriven tone - Transition clean to mean is over a narrow band of adjustment, to the point where it almost flips between the two modes of playing (that's what happens with a real guitar input, rather than a test waveform). Just under the transition point you have the clean region, which can be very mildly clipped. Just over and the mechanism I've described in the other post immediately flips to a blended distortion where the top half of the waveform is power valve grid clipped, the bottom half is purely PI clipped and the mark to space ratio is uneven. The magic area and tone of an Express is not "pure output valve distortion" despite what is commonly written/understood. Then we come to the third region - The 3rd stage starts to clip with cut off and actually ruins everything. The blended distortion mechanism falls apart, the output waveform turns into balanced power grid clipping on both sides, the tone gets buzzy and the feel of the amp changes for the worse.
It is fine, Katopan. No worries.
I thought at the maximum volume, it was a blended distortion of power tubes, PI, and the third preamp stage. But actually, we are back to pure power tube distortion just like before we turn the volume knob pass 12 o'clock? If at the maximum volume we finally return to pure power tube distortion, is that what a very distorted power tube sounds like at its most extreme? Like a square wave fuzzy distorted sound produced by a fuzz pedal? Then a pentode like an EL34 basically turns into a big diode thing like those of distortion pedals with that fuzzy square wave sound.
katopan wrote: ↑Tue Jun 09, 2020 2:20 am
An Express, old non-master Marshall and many other classic amps are not going to give you a modern metal tone without modifying and putting that feature in the preamp. Overdriving the EL34s even more than is normally possible will not do that.
Ditto. Then, that's where preamp stages come to play if we want more distortion.
katopan wrote: ↑Tue Jun 09, 2020 2:20 am
So what about the Marshall JTM45 which uses an 8K output transformer. Why isn't a JTM45 even better than an Express if the 'secret' is the output transformer? Yes, the 6.6K impedance means more screen current on overdrive, which is then used to drive the power node sag that makes the blended distortion mechanism possible. But it's the rest of the amp circuit around that OT impedance.
Great amps! I've never built one but have played one. But anyone can listen to the clips to hear what they sound like. Pure output valve distortion! So why do these Marshall 20 Watt amps sound different to the Marshall 18 Watt amps? They are both output stage overdrive dominant distortion with the same EL84s. And why does a Fender Bassman (or any of the other amps I mentioned earlier) set up for pure output valve distortion sound different to a Marshall 18 Watt? There is no single benchmark for pure output valve distortion, but there are recognisable characteristics like a smooth singing quality to the distortion and a level of sustain. Even those characteristics vary depending on the power valves, the OT, the rest of the circuit, and even the speaker it's driving.
It is always a combination of many things when it comes to generating distortion sounds.
katopan wrote: ↑Tue Jun 09, 2020 2:20 am
In theory you could do that and with the right design you could probably build a modern metal sounding all EL34 preamp. Pentode overdrive creates a different distortion tone to triodes even in single ended use like this would be and you might have to set things up in a particular way to get the same cutting tonal response. But this is different to a EL34 push-pull output stage, which will never by itself give you a modern metal amp. There are proven ways to get that tone.
In order to get what will an EL34 sounds like in comparison to a preamp tube like 12AX7 in terms of distortion, did we just have to listen to an amp with small signal pentodes as the preamp tubes? Is that accurate? Do all pentodes such as EL34, EL84, those small signal pentodes, etc. sound the same? And what are the distinguishing sonic characteristics of EL34 tubes compared to 12AX7 tubes?
According to these links,
http://www.valvewizard.co.uk/pentode.html especially around the first paragraph it said "However, pentodes still offer a different quality of tone to triodes, with a little more odd-harmonic content and a 'glassy' sound. When overdriven they can also produce an expressive compression effect as the average screen current increases.
In some ways they can be thought of as simulators of power-amp distortion, but at lower volume, and they have begun to creep back into boutique designs". Power-amp distortion sounds less distorted and less aggressive compared to the preamp distortion in general.
Then, from another link
https://www.premierguitar.com/articles/ ... bes?page=2 in the first paragraph, "The only pentode preamp tube seen with any regularity in amps today is the EF86 (or 6267), which appeared in early Vox amps and has more recently been used in models from Matchless, Dr Z, 65amps, and a few others. Another less frequently seen, but much admired, pentode preamp tube is the 5879, notably used in Gibson’s GA-40 Les Paul amp of the late fifties.
Both of these pentodes fit the same 9-pin bottle as the dual triodes but require very different circuitry, and are known for their thick, robust sound. Both have higher gain factors than even a 12AX7, but aren’t prone to distorting the way that dual-triodes can, and instead pass their fattened-up signal on to the next stage. They also have a reputation for handling effects pedals very well. Drive a 12AX7 hard, however, and it will induce quite a bit of sizzling, slightly fizzy-voiced distortion of its own. This can be a great thing if you’re looking for a super-fried overdrive tone that’s cooking at all stages, but not at all desired if you want more headroom and clarity, or the fatter distortion that’s generated in the output stage of the amp when a cleaner preamp signal is driven into clipping at the output tubes (more of which in the next installment)."
Thus, a pentode sounds less distorted compare to a triode when used as a preamp tube in a guitar amp. But does that applies to a bigger-sized pentode like EL34? So, generally if an EL34 sounds less distorted than a 12AX7, the preamp-like circuit with six EL34 will sounds less distorted and less aggressive than the usual preamp with 12AX7? Thoughts?