Gain stages in Marshall & Soldano amps

[roberto]

I never thought about speaker performance before, so probably it has already been done by someone, but by serendipity I've just got a webinar invitation for some 3M filters:

http://www.filter-equipment.com/uploads ... heet_1.pdf
Look at bottom right of page 2. That one has the purpose to increase the filtering surface area of a flow passing through the filter, but...
Imagine that the flows doesn't pass through the membranes but tangentially... imagine it is air and imagine the round part is clamped on the magnet of the cone.

Has anyone ever done a "clampable heat sink" made in the HFR way, to be clamped on the magnet and with the same diameter of the speaker, in order to:
- increase the heat tranfer coefficient (when the cone moves, the convection goes from natural to forced (by the cone moving), so increase the capability to transfer heat;
- increase in the surface area of the magnet, so increase the capability to transfer heat.

This will give two scenarios:
- lower temperatures for the magnet, so higher reliability for the speaker;
- higher allowed power handling.

This could be a nice optional to sell with speakers.

Anyone has an idea on how it would impact the sonic performance?
By doing the "radiator" in Aluminum and thin, it will give optimal passage for the air, low weight, low costs and less sonic impact, but how can it be calculated?

[Scumback Speakers]

roberto: Regarding the voltage reading I do...I just use a standard multi-meter and set it to Volts and measure the input jack on the 4x12 cab for my reading.

[nworbetan]

DVMs usually measure AC in RMS, and speakers' impedances at 60 Hz are not usually the same as their nominal impedance.

[roberto]

Absolutely not the same, this is a Celestion G12H:


Why 60 Hz? I mean, except for the fact that is easily available from every socket at 115 Vac.

[nworbetan]

I'm assuming that's the freq the measurements are taken at, using a variac as a source. Some may not be, I can only assume.

[kanderson70]

Speakers, yeah, I know a little.

Jim...thanks for the ear worm. Now it's stuck in my head playing on repeat. At least it's a good tune

[pompeiisneaks]

I'm assuming that's the freq the measurements are taken at, using a variac as a source. Some may not be, I can only assume.

The AC measurement is based on the frequency of the signal input. It is limited though as most DMM's probably can only go between say the 50Hz common in europe up to about 500 or 1000Hz as a max. Therefore, your specific DMM is going to have the exact details. If using wall ac power then it would be at that 60Hz in the US and canada for sure, or 50Hz in a lot of other places.

~Phil

[pdf64]

roberto: Regarding the voltage reading I do...I just use a standard multi-meter and set it to Volts and measure the input jack on the 4x12 cab for my reading.

Jim, with regard to your comment over on TGP

My JTM 45 clone with Mercury trannies, NOS glass GZ34, NOS preamp and power tubes with EL34's and all the good caps put out 34w clean and almost 50w when cranked, no pedals. KT66's are a little more power

I was wondering if the above method, ie DMM set to Vac across the amp output with a real cab as the load, was how you measured your amp's power output?
As a cab's impedance will be above its nominal over much of the guitar bandwidth, the use of a cab as load will tend to result in a higher Vac than if the load was a resistive one of the nominal value. Hence deriving the power output from the Vac across a speaker load, squaring that voltage, then dividing that by the nominal impedance will tend to give a higher power than is actually the case.

Also the use of a regular, non true rms meter with signals other than a pure sine wave will cause a measurement error.
Cheers, Pete

[Scumback Speakers]

Well, pdf64, I've had the amp measured both ways. Once when it was built, sine wave, 1khz signal, 8 ohm input and measured at the output by my amp tech.

The second way was by me the DMM at the cab input jack.

The wattages I was given were done by the amp tech in test mode, the voltages I gave were with the same amp in "how a guitarist uses it" mode.

I've got 25 amps, the demo amps show up for demos and the LA Amp Show...which means they just don't get played all that much. I doubt there were any variations in the circuit, power tubes, etc between when they were tested by the amp tech, and when I tested them with the DMM voltage setting 11 years ago. The two amps built were assembled, tested, etc in 2008/2009.

I would have to deduce that the power wattage I was told is in direct correlation for what the amp puts out at the same volume to a DMM.

I don't pay much attention to the sinewave, etc tests...because all that changes when you strum the guitar, how high you set the pickups, etc, etc.

By the way, my 1987 Marshall clone with SS rectifier, standard specs, NOS glass puts out 55w clean (vol set at 4) and over 95w (vol set at 8 ). The DMM measures the cab jack at 36 volts with that amp at 8 on the volume setting.

I hope that answers your question, because beyond that understanding, I don't have any further testing criteria or methods for you.

[roberto]

By the way, my 1987 Marshall clone with SS rectifier, standard specs, NOS glass puts out 55w clean (vol set at 4) and over 95w (vol set at . The DMM measures the cab jack at 36 volts with that amp at 8 on the volume setting.

Speakers have a variable impedance depending on frequency, and at 1kHz the impedance is not the nominal one, but is higher due to inductance of the speaker (as can be seen in the plot I posted). This means that if you do the calculations at 1kHz, it is not correct to divide by the nominal impedance of the speaker, but it must be higher, so wattage is lower, and can be 2 to 3 times lower.
But on top of that, it must be understood what "Volt" means. Do you see Vrms? Vp? Vpp? This could bring the wattage 2 to 4 times lower.

I guess you calculated [(36V x 36V) /2]/8 Ohm. But the impedance of the speaker is wrong and the voltage is not sure, so the wattage is not correct.

[Scumback Speakers]

Again, don't assume or guess. Amp tech did the math on the amp when it was built. After I got it I took the Volt measurement of the cab's input jack.

I did no calculations, the amp tech did. just reported what the voltages read at the cab when turned up to where the amp tech said it made power at certain volume settings for clean maximum power and overdriven/10% THD.

[roberto]

Thanks for the explanation Scumback, what I'm trying to explain is that in AC demain "Volt" means nothing, because there's no information on what it is: rms, peak or peak-peak?
By omitting that information, you tell me a value that can be 1.4 to 2.8 times different (1 Vrms = 1.4 Vp = 2.8 Vpp).

May we ask you to report how the tech get these results? A copy-paste from an email is ok.
The only way I see to get that power with a pair of EL34s is to have 470Vdc and Raa 3.4 kOhm, then blast the signal up to have around 85Vpp on g1s.
But this is way far from "clean" and also far from 10% THD.

Just to be clear: I'm in no way arguing your competences in building speakers, nor I am attacking you in any way. I just want to understand how data has been calculated/measured.
If you don't want to dedicate time to it, and want to share your tech's references, I'll ask him directly.

Thank you in advance.

[Scumback Speakers]

roberto,
I'm sorry, but that tech and I are no longer friends, so I'm not going to shoot you his email. The simple answer is this...

Clean, 8 ohm, 1khz signal to the high input of Ch 1, the amp was 55w rms, around 4 on the volume knob of the 1987 clone.

Distorted, 8 ohm, 1 khz signal to the high input of Ch 1, the amp was 96w rms, around 8 on the volume knob. No master volume in this amp. All controls set to 10 except the volume knob.

With the amp set at 8/Vol, it had a 36 volt reading at the speaker jack input on my Digital Multii Meter. So 96w output translated to 36 volts to the speaker cab.

Past that, I have nothing else to offer, sorry.

Speakers are something I know a bit more about. I built this amp from a Metro kit with upgraded parts, previously mentioned.

If you want to get into all the minute details past what's above on the amp circuit, I'm afraid I can't supply those as the PT & OT are a custom spec I had Mercury Magnetics make me, and I'm not going to divulge that info.

The 1987 clone I built was based on a 1968 circuit. Here's the spec page for the JCM 800 version, looks similar in watts, etc.

[roberto]

Thanks for the detailed explanation Scumback! It is clear now.

By personal experience, the data shown is more a copy-paste rather than something else (same power at the same THD for two very different schematics is not so easy to be obtained).

Also, whilst I can agree with the low THD wattage, the THD at full wattage seems quite optimistic to me.
With such high B+ (around 500 Vdc) and Raa 3.4 kOhm, it is safer for most of the modern EL34s to lower B+ or raise Raa.

[Roe]

Thanks for the detailed explanation Scumback! It is clear now.

By personal experience, the data shown is more a copy-paste rather than something else (same power at the same THD for two very different schematics is not so easy to be obtained).

Also, whilst I can agree with the low THD wattage, the THD at full wattage seems quite optimistic to me.
With such high B+ (around 500 Vdc) and Raa 3.4 kOhm, it is safer for most of the modern EL34s to lower B+ or raise Raa.

yes, most marshall 50W amps turn out less power than this, due to low voltage largely. But the 100w amps are often capable of 115w/170w. One of mine blew a 130w cab during the first song