Gain stages in Marshall & Soldano amps

[Scumback Speakers]

Hey TAG members!

I've got a couple of questions regarding gain stages in amps. I'll make it simple, since I'm a gain stage newbie.
I've been told it's basically something like this.

Marshall Plexi's = 2 gain stages

Marshall JCM 800/DSL/etc = 3 gain stages

Soldano SLO = 4 gain stages

According to 2 amp makers I've talked to already, the 4 gain stage Soldano SLO design adds on a fourth preamp tube to
run the 4th gain stage, provide saturation, and some more power. They say if you run the plate voltage or the gain/volume
too high that you can produce a square wave in the amp before it is sent to the speakers.

I know a Marshall JCM800 can put out 90w at 16 ohms with 10% THD, and over 50w at 3% THD.

How much more will the 4th gain stage add for power and THD?

Thanks in advance!

Jim

[Phil_S]

Jim, there is a long sticky thread on gain stages. Maybe it's just too long?

[Scumback Speakers]

Phil_S, there may be a thread, and I missed it. I just need a couple of questions answered about the 4 gain stage stacking and how much square wave it can produce. Speakers, yeah, I know a little. Gain stages...not so much. I never needed more than a Plexi and a wah or clean boost in front of it for my tone.

[nworbetan]

This first part of my answer isn't directly to the question, but it may be useful to you. The extra power above and beyond the rated clean power happens because the output tubes are being swung from cutoff to clipping quickly, and the amp is operating similar to class D. When the power tube is at cutoff the plate voltage is at its highest but the current is at a minimum (whatever the idle current is), but when the control grid is clipping the plate is at max current and the plate voltage is at its minimum. Switching between those two states as quickly as possible is the most efficient way for the power tubes put the most possible watts into the speakers that the power supply can supply, in theory. But that leads to a more direct answer to your question.

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.

[pdf64]

...How much more will the 4th gain stage add for power and THD?...

Adding more gain stages doesn't make amps more powerful, per se.
Multistage rigs have been around forever, classic amps have been turned into multistagers by means of boost etc pedals. Super high gain preamps are no different than that.
Once the power amp is squarewaving, adding more gain stages, whether outboard or within the amp, is immaterial.
The gain may be used to make the signal more continuous, so reducing the opportunity for the speakers to cool down.
But Parisienne Walkways shows that a regular, classic amp degree of gain is plenty to generate a continuous signal

[Snicksound]

...How much more will the 4th gain stage add for power and THD?...

Multistage rigs have been around forever, classic amps have been turned into multistagers by means of boost etc pedals. Super high gain preamps are no different than that.

This is a good point, a SLO is basically a JCM800 2203 with a boost pedal built-in (yeah sure, it has a colder biased clipper stage, and an extra DC cathode follower for the loop, but still), and people have been boosting their 2203s for a long time. Well, also it uses 6L6s.

Anyone who is running a SLO with both the gain and the MV dimed probably needs his ears checked though.

[Scumback Speakers]

Well, the square wave being produced is the problem for a speaker. I've had some recent failures with a 50w SLO style amp and a 65w speaker. Granted the way it was being set was easily passing the 65w speaker power handling, and I'm surprised it lasted 9 months the way it was run before it fried. There was a power tube failure it survived, and a few months later a preamp tube failure, at which point it took out some other amp parts and the speaker. The client also had boosts, delays, and the 10db boost in his Splawn Super Sport engaged.

I've discussed this with a couple of amp builders, and they think the 4th gain stage (with 4th preamp tube) adds a degree more power, plus gain and the square wave. Both of them have said that if the wall voltage was over 122-129, plus the 4th stage/4th preamp tube setup could easily blow past 110-122w at full volume/saturation with dual EL34's.

I'm trying to figure out if even a 100w rated speaker will live in this 1x12 setup reliably, and I think it's borderline at best.

Mostly the problem is the square wave which does fry voice coils, regardless of the volume...but then I'd have to figure out the minimum # of speakers,and power handling for these 4 gain stage setups to be reliable.

Too many players want a single 1x12 setup, but want the old Greenback sounds, which the 1.75" voice coil will be hard to handle reliably on it's own.

I'm thinking it's going to have to be a minimum of two 65w speakers from now on for reliability.

[Roe]

the gainstages in the preamp won't matter much as long as they drive the power amp into full distortion.

[Scumback Speakers]

Hey Roe!
Apparently with the SLO design the 4th preamp stage and tube add on in a way that increases RMS, too. It may not be as intense as a fuzz box, but it approaches that level of square wave signal. The extra strength/heat/etc in the square wave signal will cause a voice coil to hang open long enough that it can't cool properly and cause premature heat failure.

I'm trying to figure what the safe level is, if there is one, so I can advise players using boosts/fuzz/etc what they need to order to have their speakers last.

[pdf64]

...Apparently with the SLO design the 4th preamp stage and tube add on in a way that increases RMS, too...

There's no doubt that a high wall voltage will increase the amp's HT voltage and its output power, but I'm sceptical that the 4th gain stage can affect it.

What's the process for determining the power rating of your speakers?

If customers choose to ignore your warning about needing the total cab rating to be twice that of their amp's rated output, I don't see what more you can reasonably do

[Scumback Speakers]

There's a melting point from heat applied to the voice coil that determines the power rating, also coupled with coil diameter, former material, whether it's a heat treated nomex paper or plain paper coil, etc. That all goes out the window with a square wave, though.

[romberg]

The RMS average power of a pure sin wave is:

Prms_sin = Vpp^2 / 2R (Vpp = peak to peak voltage, R = load)

Which is more or less the area under the curve of the sin wave. Amps are usually measured for output power right before clipping and the signal is still more or less a sin wave.

When the amp is producing square waves then the area under the curve is now fully filled. The formula is now:

Prms_square = Vpp^2 / R

So, an amp putting out a 20Vpp sin wave into an 8 ohm load will be deliver 25 watts RMS into the load. That same amp delivering 20Vpp square waves will deliver 50 watts RMS into the load.

It does not really matter how the square waves are produced. They could come from driving the power tubes into clipping, multiple gain stages in the preamp eventually clipping a stage or a distortion pedal in front of the amp. Once you are no longer dealing with sin waves then the RMS power is now using the formula without the 2 in it. I imagine that these square waves do other interesting mechanical things to the speaker. But electrically the voice coils have to deal with twice the power.

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. And even a beefy one gain stage amp could do that if driven by a distortion box feeding it square waves.

Mike

[pdf64]

There's a melting point from heat applied to the voice coil that determines the power rating, also coupled with coil diameter, former material, whether it's a heat treated nomex paper or plain paper coil, etc. That all goes out the window with a square wave, though.

Bear in mind that heat is heat, 25 watts of square wave is no hotter than 25 watts of sine wave.

[kdmay]

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

[martin manning]

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.