How would pure power tube distortion sounds like?

[thinkingchicken]

Wattage rating on a tube power amp is a measure of how loud it can get before it begins to distort. It is not a measure of how distorted it can get.

Take a look at this quote I got from a different forum,

"In some respects, a tube is just a tube: we could cascade 5 or 6 EL34s in a preamp-like circuit to mimic a master-volume preamp, where each stage contributes a little distortion per stage to build up a smooth high-gain sound (just like a Mesa preamp, or a SLO preamp, or a Marshall preamp, etc). Nobody does this because it would cost a lot more than using the usual small-signal preamp tubes, power supply requirements would be obscene, and a preamp for our "power-tube preamp" would be required to get signal levels large enough to drive each tube to distortion."

Given that we have enough power supply requirements and enough signal levels to drive each EL34 to extreme distortion, will it actually produce a similar high gain preamp distortion sounds that are produced by modern Mesa preamps and other high gain amps despite the differences in characteristics between pentodes and triodes?

[katopan]

Sorry, I was away for a few days and couldn't post any reply.

Ok let see if I understand what you're saying, a Trainwreck Express is only producing pure output valve distortion until we drive the volume pass 12 o'clock where the PI and the third preamp stage started to change in waveforms. Correct? What is the function of the third preamp stage when we started to overdrive the power amp? Does it act as a clean boost?

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.

Speaking in layman's terms, if I strictly want to get that modern metal high gain distortion tone by using EL34 Trainwreck Express amps or clones, there is no way I can get the tone of modern metal by only relying on the distortion of the EL34 itself even if someone had modified it to have highest maximum gain between the input jack and the power grids so I have to drive it to the max where blended distortion of output valves, PI and third preamp stage eventually happens and then put a distortion or boost pedal into it? Isn't it?

No matter how high the gain is available for output valves i.e EL34 or EL84, is the distorted output valve sound always going to sounds like those of Marshall 18 watt? Remember, here we are talking about purely output valves and this excluded the PI and preamp stages.

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.

Another thing to add, "The secret to a Trainwreck is the output transformer. The impedance ratio is about twice that of other amps. I.e. typical 50W Marshall has a primary impedance of about 3200 ohms. A Trainwreck is about 6500 ohms. The causes the power tubes to clip much sooner."

What is the highest possible impedance ratio for EL34 Trainwreck Express amps?

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.

However, I built a clone Marshall 2061X

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.

"In some respects, a tube is just a tube: we could cascade 5 or 6 EL34s in a preamp-like circuit to mimic a master-volume preamp, where each stage contributes a little distortion per stage to build up a smooth high-gain sound (just like a Mesa preamp, or a SLO preamp, or a Marshall preamp, etc). Nobody does this because it would cost a lot more than using the usual small-signal preamp tubes, power supply requirements would be obscene, and a preamp for our "power-tube preamp" would be required to get signal levels large enough to drive each tube to distortion."

Given that we have enough power supply requirements and enough signal levels to drive each EL34 to extreme distortion, will it actually produce a similar high gain preamp distortion sounds that are produced by modern Mesa preamps and other high gain amps despite the differences in characteristics between pentodes and triodes?

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.

[thinkingchicken]

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.

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.

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.

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?

[katopan]

Do all pentodes such as EL34, EL84, those small signal pentodes, etc. sound the same?

No. EL34 and EL84 sound different to each other. They sound different to 6V6 and other power valves. Small signal pentodes sound different too.

In some ways they can be thought of as simulators of power-amp distortion

Small signal pentodes in a preamp sound fantastic! But if they are being thought of as simulators of power amp distortion, then it's in single ended mode. A small signal pentode gain stage is still different to a single ended power stage in a number of ways and they certainly don't simulate a push-pull power stage.

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?

An EL34 based cascaded preamp would be less aggressive than the usual 12AX7 cascaded preamp. The gain of each stage is lower and the overdriven characteristic completely different. The whole transfer characteristic of power valves and preamp valves is different so it would be it's own thing.

[thinkingchicken]

No. EL34 and EL84 sound different to each other. They sound different to 6V6 and other power valves. Small signal pentodes sound different too.

Please read this entire excerpt if it is not bothering you ""In some respects, a tube is just a tube: we could cascade 5 or 6 EL34s in a preamp-like circuit to mimic a master-volume preamp, where each stage contributes a little distortion per stage to build up a smooth high-gain sound (just like a Mesa preamp, or a SLO preamp, or a Marshall preamp, etc). Nobody does this because it would cost a lot more than using the usual small-signal preamp tubes, power supply requirements would be obscene, and a preamp for our "power-tube preamp" would be required to get signal levels large enough to drive each tube to distortion.

- Just because nobody does it because "it's a bad idea" or "too costly" doesn't mean no one will ever build a one-off for themselves that uses power tubes for a "high-gain preamp." I knew a guy who built an inverted mode amplifier where the input to the power stage is applied to the plate, and the output is taken from the grid, then sent to the speaker. It's a "stupid way to operate a tube" because available power output from the tube is severely restricted, but the guy did it because it 1) eliminated the requirement for an output transformer, and 2) he wanted to see if he could make it work. After making it work, he tore down the amp & re-used the parts for some other project."

Pay attention to the second paragraph. In that way, EL34 tubes will act like small-signal pentodes when used on a preamp. Right? If EL34 tubes act like small-signal pentodes, will they sound exactly like small-signal pentodes too? If they do, we just have to listen to the distorted sound that small-signal pentodes produce to get the idea of how will EL34 tubes sound like if used on a preamp. Is that correct?

Small signal pentodes in a preamp sound fantastic! But if they are being thought of as simulators of power amp distortion, then it's in single ended mode. A small signal pentode gain stage is still different to a single ended power stage in a number of ways and they certainly don't simulate a push-pull power stage.

So, the distortion generated by small signal pentodes in a preamp especially push-pull type should sound different from those that are used on a single ended amp? Is that what you mean?

An EL34 based cascaded preamp would be less aggressive than the usual 12AX7 cascaded preamp. The gain of each stage is lower and the overdriven characteristic completely different. The whole transfer characteristic of power valves and preamp valves is different so it would be it's own thing.

I think we are on the same page. Even if we operated at a plate voltage of about 500-800 volts or more as would be normal for EL34s on the multi-gain stages, we might actually get even less distortion. What do you think?

[katopan]

I read the whole thing the last 3 times you posted it.

Yes you can make a voltage amplifier gain stage out of an EL34 because a valve is a valve. No it won't sound like an EF86 stage because it's a different valve with different characteristics. Then again a 6AU6 stage won't sound like an EF86 stage either because it's a different valve. Likewise a 6 stage EL34 preamp won't sound like a SLO preamp because it's a different valve.

The distortion generated by *any* valve in a single gain stage will not simulate the distortion generated by a push-pull power stage because the circuit operation is completely different. A small signal pentode preamp stage will be closer to the output of a single ended power stage, but still not the same.

I agree it would give less distortion because the gain of an EL34 is lower and also the clipping threshold of an EL34 is higher.

[thinkingchicken]

I read the whole thing the last 3 times you posted it.

Yes you can make a voltage amplifier gain stage out of an EL34 because a valve is a valve. No it won't sound like an EF86 stage because it's a different valve with different characteristics. Then again a 6AU6 stage won't sound like an EF86 stage either because it's a different valve. Likewise a 6 stage EL34 preamp won't sound like a SLO preamp because it's a different valve.

The distortion generated by *any* valve in a single gain stage will not simulate the distortion generated by a push-pull power stage because the circuit operation is completely different. A small signal pentode preamp stage will be closer to the output of a single ended power stage, but still not the same.

I agree it would give less distortion because the gain of an EL34 is lower and also the clipping threshold of an EL34 is higher.

Sorry about that Craig, anyway thank you for clearing it up!