Gain stages in Marshall & Soldano amps

[Scumback Speakers]

Hi Jim

Hope you're well. Been a long time mate!

Try this resource:

https://www.ampbooks.com/mobile/classic ... ldano-slo/

It's great write up for understanding the preamp section of the legendary SLO.

Cheers

Kurt

Hey Kurt! We can always stuff your gut with lasagna next time you're in So Cal! I'll get around to that article in the next few days. Thanks for the link!

Jim

[Scumback Speakers]

From an electrical standpoint, I'd guess one would want a speaker rated for at least twice the power rating given by the amp because this power spec almost certainly was done with sin waves and not square waves. So the amp could deliver double that.

+1! I'd hedge that a bit more and say two to three times.

Hey Martin! How are you doing, buddy?

romberg, thanks for the info.

I appreciate all the comments, links and thoughts!

Jim

[wpaulvogel]

The area I can see to be a problem for the user is that when a square wave is fed to the phase inverter and then into the power amp, it’s hard to distinguish when the power amp reaches its “50” watt rating. In a Fender or early Marshall the power amp clips before the preamp or phase inverter and gives that wonderful transition. We as the player know exactly when we surpass the RMS output of the amp. We also intended to push past into peak operation. With such a high gain preamp it’s probably easy to push the amp ignorantly. I don’t know why..... anyone would get satisfaction from 100 watts through a single speaker but it’s apparently happening. I prefer to use any amp, 10 watt to 100 watt through multi speakers for a more open sound. I always thought the greenback sound required at least 2.

[Scumback Speakers]

wpaulvogel: yeah the two amp builders I talked to said basically the same thing as you about the square wave from the preamp increasing the signal to the PI and beyond, well past clipping, etc.

One amp maker measured 122w with dual EL34's, the other measured 118w. I always figured no one would go past the 90w plus measured in a Komet or the JCM 800 series, but I'm obviously wrong, and it can be pushed even harder with the 4th preamp tube / gain stage setup.

It's not going to be the tone most players want, but I'll have to start recommending something like the EVM 12L, JBL or some other high power handling (non 1.75" Greenback form/tone factor) type speakers for a single 1x12 setup. All of those speakers are different than the old Greenback G12M / G12H tone, though, and that's when players will have to get two 65w speakers to cover their power output.

Ok, I got a bunch more orders to get out, going to look at all of those links everyone posted later and see if/what I can get out of it with my limited IQ. Thanks!

Jim

[wpaulvogel]

I think it’s more of a matter of transformers. If the power transformer doesn’t sag down much and the output transformer is efficient and has low DCR, there will be some tonal indication of power tube clipping but the problem is that power tubes clipping takes the high gain sound into nervana and I as a player would push into this area. Many people claim that they get their distortion sound from pedals and that’s possible but I’ve heard too many tracks that were a pushed power amp and man it sounds awesome.

[nworbetan]

One amp maker measured 122w with dual EL34's, the other measured 118w. I always figured no one would go past the 90w plus measured in a Komet or the JCM 800 series, but I'm obviously wrong, and it can be pushed even harder with the 4th preamp tube / gain stage setup.

I'm just going to point out once again that the important differences between amps that are able to put out 90w vs the amps that can put out 120w are in the output stage.

The specific answers to your question about the fourth gain stage really aren't about the fourth gain stage. A power amp with an output transformer with extremely wide bandwidth for high fidelity high frequency response will let the power tubes deliver a cleaner square wave with higher THD. I can't imagine that the speakers would be happy with that arrangement though.

One thing that needs to be elaborated on is that clipping as hard as possible as late in the amp as possible is going to create a more square square wave. A square wave injected at the input of an amp is subject to varying degrees of phase shifting at different frequencies and won't be square any more by the time it gets to the power section. An amp that clips hard in the inverter or driver or power amp stage will create the opportunity to push more watts into the speakers, but that extra class D-esque wattage can still be held back by a "vintage" transformer or inadequate power supply.

[Scumback Speakers]

nworbetan: Apparently there are players boosting the front end of their 4 stage amps with the usual pedals (bosst, phase/flanger, fuzz, etc) and then if the amp has an fx loop in front of the power amp (but after the preamp / PI) they'll put a boost pedal for 10-20db there to create a huge increase in volume for solos.

The amp I was referring to, the Splawn Super Sport, apparently has a +10db boost at the end of the signal chain right before the power amp.

So I can see why a "50w four stage" amp setup like this would fry a single 65w speaker after 9 months, two power tube failures and a preamp tube failure, too.

[kdmay]

I'll take you up on that Jim!

Somehow me thinks it will be a while before that happens though

Hey Kurt! We can always stuff your gut with lasagna next time you're in So Cal! I'll get around to that article in the next few days. Thanks for the link!

Jim

[Roe]

Jim, I'd suggest the following minimum speaker wattage:
jtm45: 50w
jtm45/50/ jtm50/jmp50/2204: 80w
jtm45/100 w. kt66s and 500v: 120+w
jtm45/100 w. kt66s and 560v: 160w
jtm100/jmp100/2203 (el34s): 160+w
Major 200: 260+w

In most cases this should work well.

[roberto]

I would like to add that the extra gain stage it is not there to reach square wave, otherwise there would be no more dynamics available.
Then of course a square wave will have more power considering the same amplitude (can an output transformer fro a guitar amp reproduce square waves? Up to what frequency? Then?), but who plays that way?

Let's see the issue on the opposite way: how to you declare the power of your speakers?

[pdf64]

If the designer is happy (or oblivious) to massively breach the EL34 plate dissipation limit, given a super stiff HT up near 500V and a typical 3k4 OT, it's theoretically possible to get maybe 80W clean out of a pair; so at full squarewave overdriven 120W seems feasible.
Point being that the '50W' Splawn may have an actual output power way over 50W, especially if the wall voltage is high.
Which raises the question of how the stock 65W Celestion speakers are surviving that
Perhaps Celestion are underrating them?

[Snicksound]

If the designer is happy (or oblivious) to massively breach the EL34 plate dissipation limit, given a super stiff HT up near 500V and a typical 3k4 OT, it's theoretically possible to get maybe 80W clean out of a pair; so at full squarewave overdriven 120W seems feasible.
Point being that the '50W' Splawn may have an actual output power way over 50W, especially if the wall voltage is high.
Which raises the question of how the stock 65W Celestion speakers are surviving that
Perhaps Celestion are underrating them?

You know, I was testing my 2204 build all-out into a single Creamback 65W the other day... it didn't sound like it was a good idea. Speaker was clearly struggling and I backed off.

But I also can't imagine a use case for that much volume from a single 65W medium magnet speaker. If you need that much volume, this is not the right cabinet for you. If you want a small lightweight amp but expect it to sound like a 412, you're gonna have to mic it.

[roberto]

If the designer is happy (or oblivious) to massively breach the EL34 plate dissipation limit, given a super stiff HT up near 500V and a typical 3k4 OT, it's theoretically possible to get maybe 80W clean out of a pair; so at full squarewave overdriven 120W seems feasible.

I totally agree, but guitar as most instruments is not a constant signal source, and for sure it's not a full square signal at its output, so mean power is way low. I think (but Scumback can answer better than me) that there are heat related issues on speakers (due to continuous excess of power) and mechanical related issues (due to peak power and/or low frequency), and the speaker can most probably withstand the former, but in this case not the latter (the ratio of the two is chosen by the builder).

Point being that the '50W' Splawn may have an actual output power way over 50W, especially if the wall voltage is high.
Which raises the question of how the stock 65W Celestion speakers are surviving that Perhaps Celestion are underrating them?

That's exactly why I asked how the producer her determines the power of its own speakers.

Celestion only says:

Generally, you can safely run a 60-watt Celestion speaker at 60 watts and it'll keep going all day long.* Connect it up to 100 watts and it might work for anhour or more before it incinerates. "Over-power" any speaker and it'll workfine for a while; just don't bank on it lasting.

(* Extreme use can break a speaker at lower-than-rated power levels. Forexample a sustained drop-tuned Metalcore pummelling through vintage-typespeakers would almost certainly cause damage.)

https://celestion.com/speakerworld/guit ... _to_know!/

[Scumback Speakers]

Speaker power handling is based on the melting point of the voice coil former material. The signal power sent has a voltage to it, roughly 36w on a 50w 1987 Marshall cranked up to 8-9 on a NMV amp circuit. in my 50w, that's around 95w output, resulting in 36 volts at the cab speaker jack. I've measured that myself. 100w will be more, but I haven't done that measurement in the shop, so I can't say exactly what that voltage is.

I break in speakers with a variac set at about 12 volts for 20-36 hours (M's around 20, J's around 25, H's around 36) since the heavier the magnet, the more magnetic force on the coil/cone/etc and it takes longer to break them in due to the cone not moving as far, or as easily in a lighter magnet.

Of course, there's a limit of how much power a 1.75" voice coil can handle based on the former size/wire, etc. You're not going to be able to handle 150-200w from a single G12M/Scumback M75 reliably. Also, some voice coil formers sound awful, such as aluminum, etc.

But a square wave using fuzz/boosts/similar pedals often presents a signal that will make the speaker "hang" in it's excursion and not return to it's base position. That leads to overheating the voice coil former, it warps/deforms/goes out of round and then you have voice coil rub, the typical "blown, scratchy" speaker failure.

Typically, the bigger/heavier the magnet, the power handling goes up slightly since the magnet acts as a heat sink to soak up some of the power in your signal to the speakers, but usually no more than 10-20% more.

Hope that answers your question about how speakers are rated for power.

[roberto]

Speaker power handling is based on the melting point of the voice coil former material. The signal power sent has a voltage to it, roughly 36w on a 50w 1987 Marshall cranked up to 8-9 on a NMV amp circuit. in my 50w, that's around 95w output, resulting in 36 volts at the cab speaker jack. I've measured that myself. 100w will be more, but I haven't done that measurement in the shop, so I can't say exactly what that voltage is.

Thanks, but I don't understand your calculations. Are you talking about Vrms and Wrms? I ask you because a sine wave gives:

36 Vrms into 4 Ohm makes 324 Wrms.
36 Vrms into 8 Ohm makes 162 Wrms.
36 Vrms into 16 Ohm makes 81 Wrms

36 Vpp into 4 Ohm makes 162 Wrms.
36 Vpp into 8 Ohm makes 81 Wrms.
36 Vpp into 16 Ohm makes 40,5 Wrms

I break in speakers with a variac set at about 12 volts for 20-36 hours (M's around 20, J's around 25, H's around 36) since the heavier the magnet, the more magnetic force on the coil/cone/etc and it takes longer to break them in due to the cone not moving as far, or as easily in a lighter magnet.

Thanks again, assuming it's 12 Vrms you break in speakers at 18 Wrms for 8 Ohm speakers and 9 Wrms for 16 Ohm speakers at 50 Hz.
I will sketch an idea on how to do it reducing the noise emitted outside, I'm curious to know if it's something already done.

Of course, there's a limit of how much power a 1.75" voice coil can handle based on the former size/wire, etc.

I guess this is due to the thermal power dissipated by the coil and the capability of the coil to dissipate the power outside (forced convection due to the speaker moving, and irradiation towards the magnet.
So there will be a limit of temperature at which the wire of the coil melts because the delta temperature needed to dissipate the power between the coil and the magnet is too high.

But a square wave using fuzz/boosts/similar pedals often presents a signal that will make the speaker "hang" in it's excursion and not return to it's base position.

I guess that this is due to the fact that the power of the signal is higher (assuming the same Vpp), so the excursion of the speaker is higher, so it goes out of the admissible "swing".

Typically, the bigger/heavier the magnet, the power handling goes up slightly since the magnet acts as a heat sink to soak up some of the power in your signal to the speakers, but usually no more than 10-20% more.

Thanks, yes by increasing the magnet you increase the surface area that can dissipate heat through (almost natural) convection, and due to the fact that convection is proportional to the heat transfer coefficient that is about 5 to 25 W/(m^2 xC), to the surface area and to the delta temperature, by increasing one of the three you reduce the others (considering the same watts), so being the heat transfer coefficient stable, by increasing the size of the magnet you reduce the temperature of the magnet and then the temperature of the coil.