How do I check output voltage on my Marshall Super Lead 100 vintage 1971 head?

[firehouserecording]

Dr. Gonz, you have motivated me. I totally disagree with your fondness for room mics. Every time I get a guitarist or intern that likes to apply their classroom sound theory and put distant mics up, I have to record twice, and show them why my recording is so much better. Guitar, to me is about as direct a sound as possible. Now, maybe guys can use a distant mic with a better plan than me, and make me look silly. I haven't seen it yet.
You did motivate me, though. I've often wondered about isolating just one speaker for the purpose of miking. Right now, I am waiting for three speakers to be re-coned. I am going to record through my one speaker cabinet today in the new guitar isolation closet that I have built. Then i can compare that to the 4x12 when it's operational.
I still haven't quite grasped the watts/ohms thing. I was running 100 watt/16 ohm into 4 25 watt/16 ohm speakers- series parallel. If I run through just one 25 watt/16 ohm speaker, I am guessing that the speaker will receive twice the wattage as it did in the group of four. Is that right?

Thanks,

Paul

[g1]

No, you will be giving it 4 times the power (at equal volume settings). Speaker wattages add in a cabinet. So with four 25watt speakers your cabinet could handle 100watts. I think you will really be pushing your luck .

[Gtr_tech]

Room mics are for capturing the sound of the room....providing the room sounds good enough. You use *both*, each on a seperate channel and mix in the room to taste for ambiance.

This is a very old trick and can go a long way to add character to a guitar sound.

[Steve Conner]

Yeah, if you record in an isolation closet, room mics are pointless, the best way is to close mic and add digital reverb later.

[firehouserecording]

I have an awesome drum room. I just personally don't care for a room sound on guitar, unless it's acoustic. That's why I built the iso closet. To get the room out of the recording. I rarely put reverb on a guitar, either, unless it's clean or acoustic. Of course, I am recording a 100 watt Marshall at 10, through 25 watt speakers. It's full of life, and very vibrant. It stands on it's own.
I was running a tone generator through the rig the other day, and hearing how the volume was dropping at 200 hz. Obviously, a guitar speaker is not designed to put out much volume at that low frequency (or higher than 10kz for that matter). That confirms up my practice of high passing all that mud out the instrument. If anybody out there is interested in recording techniques, consider visiting Fab Dupont's website at puremix.net. He's the best teacher in the world, a grammy winning producer/engineer, and an awesome guy. He's got a lot of free instructional videos, and plenty that are worth paying for.

Thanks,

Paul

[Gtr_tech]

10kHz...are you playnig thru tweeters? Do you think a guitar speaker has a freq response that high? Really? Have you looked at the datasheet for a G25M?

[pdf64]

Paul, you may want to consider the potential for damaging the Marshall when the speakers blow open circuit; the sudden change in load impedance under heavy drive is likely to cause a voltage spike on the output transformer primary circuit, which could burn through insulation, thereby wrecking the transformer.
There's mitigation in that the 4x12 has 2 parallel speaker circuits. The potential for damage would be greatly exacerbated if the second pair blew open in short order, which seems feasible given the conditions of use.
It would be beneficial to run the two 4x12 together when using this amp; they don't both have to be in the (same) isolation booth.
Given your discerning ear, you may find that any form of dummy load will impact on the tonal response, whereas a second cab (whose speakers have a similar frequency/impedance response, eg the Celestions in your 300W 4x12)) should have a minimal impact, apart from reducing the SPL coming from the cab you're miking.
Pete

[g1]

Paul, you may want to consider the potential for damaging the Marshall when the speakers blow open circuit; the sudden change in load impedance under heavy drive is likely to cause a voltage spike on the output transformer primary circuit, which could burn through insulation, thereby wrecking the transformer.

Especially due to the vintage value of this amp, it's a real good candidate for a safety resistor across the speaker out jack. It could save the OT if the load ever goes completely open.

[firehouserecording]

Thanks, g-one,

I will run that by my tech. Don't be afraid to be more specific.

Paul

[pdf64]

The Vox AC50 implements a 470 ohm permanently wired to the 16 ohm tap, 3 or 5 watt rated, presumably to mitigate for this scenario (bare in mind it was intended for use with fragile Vox Blues).
Given that it's getting heavily overdriven, 470 ohms at >12 watts should cope with this 100W JMP.
Pete

[firehouserecording]

Thanks, Pete. I emailed your thoughts to my tech. I would like to do that.

Paul

[Enzo]

Paul, I don't think anyone was suggesting a room mic instead of a close mic. It is always an additional sound to blend in.

Of course it all depends on the sound YOU like, as with anyone. But when you say direct sound is best, remember that you don;t listen to the guitar with your ear in the cone, you are standing a few feet away as a rule, and like it or not, you are hearing not only the speaker cones, but also the sound of the cab within the room.

Engineers with lots of free channels will even run one mic straight onto the cone, one at a 45 degree angle of to the side of the cone pointing into the far wall of the cone, as well as a room mic. Room mic can be anything from a mic 4 feet away to one across the room. A little of this, a little of that, and your recorded tone resembles the live tone better than any one source.

This isn't classroom theory, this is tricks of the trade. They also do things like record the guitar itself through the board, so they can then go back, if they want to, and play the same guitar performance through a different amp, or the same amp at different settings. With the guitarist in a booth, or otherwise not where the speaker is, they can put a mic on the guitar strings, just to pick up some of the percussiveness of the pick on the strings. A little of that adds just a certain something... in some cases. And let us not even get into things like dropping or raising pitch a cent and panning each left and right to fatten the sound, and other processing tricks.

But to comparing experienced engineers with a recent school grad is not fair. Even with a good technique, his experience may not get the best results from that either. Make him record it your way, then compare you recording it your way and see if they sound identical. I bet they don't.

[firehouserecording]

I'm familiar with those techniques, Enzo. Thanks for the input!

[soundmasterg]

It would greatly lower the vintage value of your amp to add this, but you could also consider adding power scaling to it. With this added to the amp, you can get the sound of the amp running flat out but it is at lower volumes. Since it is at low volumes, it does not have the speaker distortion effect but the amp is still touch responsive like it is at loud volumes. This is a better approach than using an attenuator to my ears and fingers. You can still play it at your normal loud volumes when you want to also. You can check out the kits here >> http://www.londonpower.com/catalog/a...ur_ps_kits.php

Greg

[firehouserecording]

Very interesting suggestion, Greg. Thanks. I seriously doubt I would ever mod my head to this, but I would love to hear it. My only issue to this point is a pair of blown speakers every 7 years. What kind of rig do you run this through?

Thanks,

Paul