C*rr*m Mod?

[leadguy]

Twistynecks clips sounded like they had limited dynamic range and not much bloom.

It reminded me a bit of a amp sounding like a giant EQ pedal with all the frequencies maxed out.

This would really be expected because the usual varying output impedances that can alter for all the different frequencies are effectively in parallel with a non frequency varying impedance (resistance).

So the resistor just flattens out the frequency dynamics, so all the frequencies get a more equal part in the output and the output has more limited dynamics and not as much bloom and sounds more direct and immediate and less varying.
The output tubes end up working with a different impedance as well, depending on the value of the resistor so that affects the sound as well.

It wouldn't be my choice but horses for courses.

[jerrydyer]

I downloaded a couple of his cover songs from another site and I hear a nice bloom. hhhhmmmm?
psycho-acoustics I guess.

[mr.twistyneck]

Leadguy -

Think organic. There are many factors at play, including (but not limited to) the player, what the player is playing, what guitar the player is playing, effects (if any), the ampflier, any mods, speakers used, microphone and recording gear used, any post-production (if any).

I had a pretty good shot at recording the mod when I did the last Boodogs recording. All the Boodogs recordings were done with Bill L. my 100 Watt Super lead (1971, with replaced trannies and parts and such, but with essentially a stock circuit). I was running at somewhere between 92 and 105 volts (i don't remember what) and had an MXR phase 90 in between me and the amp. the amp had a 1K resistor between the plates. BUT - the amp was in an iso booth and was mic'd with as i recall some sort of AKG mic. there was probably EQ, compression, verb, possibly delay on some or all of those recordings. the mastering was all over the place because we were at the end of the band's lifespan, nobody was interested, the engineer was more into Audio Adrenalin than hard rock and ultimately the freaking studio burned to the ground. i almost went over there and poured salt on the remains. so http://www.twistyneck.net has the examples of C erre m's Mod on at least the covers we recorded ( no vox on those tracks), and http://www.myspace.com/theboodogs has a few of the originals, also with C erre m's Mod.

The Youtube clip detailing the Mod http://www.youtube.com/watch?v=I7HAknjipE0 was done with a much lower ohmange. lower ohmage equals more gain, etc. it was also recorded in a bedroom with hardwood floors using the mic that came in a hundred dollar digital camera. You're not going to hear any bloom in that because the itty bitty mic is limited what it hears.

Again, recorded with another digital camera, http://www.youtube.com/watch?v=gy2YTufLukM this was live at the rock bar in nashville, and I was running the mod. i don't really care if i blow up the head, because i'll just rebuild it - and when you play out perhaps once a quarter on average, why not take a chance? at stage volume, it is VERY sensitive. it was squashed so much ( i think that was with a 75 or 100 ohm resistor) that i ran a hotplate, unattenuated just so i could grap a line out, run it thru another power amp into the bottom half of my 4x12. (the top 2 with tone were celestions, the bottom two were 200 watt EVM L-series; ie: wet/dry split in one cab).

i'd love to get in a studio here in nashville and record some examples, but that's unlikely to happen (job, baby, blah blah blah). What i would encourage you to do is, rather than judge based on mp3 clips and YT vids, to actually try this out yourself, or find somebody you know who is willing to try it and let you fart around with it. In the end, it's just a mod among countless mods. the allure for me is in the simplicity, the risk, and the sound.

[leadguy]

The speakers impedance at any given frequency gets reflected back via the turns ratio of the output transformer and basically becomes the primary impedance that the output tubes see at any given frequency.

This primary impedance varies depending on what frequency someone wants to use to measure the impedance.

So shove a big resistor in parallel with this varying primary impedance and the resistor is going to stop the primary impedance that the output tubes see, varying as much as it would with no resistor.

Dynamic variations happen in the output tubes and output transformer/speaker combination (variable impedance depending on the particular frequency) and the resistor is limiting the variations.

If a resistive load was used instead of a speaker, then the impedance reflected back to the output transformers primary would be mostly resistive and less variable and combine that with a resistor in parallel with the primary and there would be even less impedance variation going on.

If someone wants a flat output section that is basically constant and doesn't vary much then this is the way to go.

[vanhalen5150]

I just learn more by doing. It's an amp forum. Some like to actually try this stuff out.

[leadguy]

Do whatever you want.
Knowing a bit of theory gives an idea of what to expect though.

A Powerbrake for instance is trying to emulate a speakers varying impedance that gets reflected back to the primary and becomes the varying primary impedance because not everyone likes a non varying resistor impedance (resistor load) reflected back to the primary.
How well the Powerbrake does it and how well the actual virtual speaker impedance works is really a matter of taste but that's what it's modeled on.

[vanhalen5150]

A hotplate also has a fan that is directly related to the output of the amp. It reacts with the more output, either with turning up the amp or hitting the strings harder, similar to a speaker. Even with slaving from a resistive load your OT is still going to see both a reactive(second amp) and a resistive(resistor). What exactly the ot sees? I don't know.

[leadguy]

With slaving the first amps output transformer just sees the load whatever it may be, a resistor, a speaker emulator like a powerbrake and the second amps output transformer will see a varying speaker load.

The output transformer primary has no impedance on it's own.
What sets the output transformer primary impedance is the load connected to the output transformers secondary and this load is a speaker or a resistor or a powerbrake etc.
A resistor load is the same ohms for all frequencies, a speaker is not the same ohms for all frequencies and the same goes for a powerbrake.

So this low ohm load connected to the output transformer gets reflected back to the output transformers primary and sets the primary impedance.
So if the speaker load is say 3.2 ohms at 1000hz then the 3.2 ohms gets multiplied by the output transformers turns ratio to become the output transformers primary impedance at 1000Hz.

If the speaker load is say 6.8 ohms at 500hz then the 6.8 ohms gets multiplied by the output transformers turns ratio to become the output transformers primary impedance at 500Hz.

So the outputput transformers primary impedance is different in this rough example for the 500hz and 1000hz frequencies and the output tubes are seeing different output transformer primary impedances for the 500hz and the 1000hz frequencies.

With a resistor load of say 8 ohms, the 8 ohms always gets reflected back to the output transformer primary exactly the same for all frequencies so the output tubes are seeing a non varying primary impedance for all frequencies.

A big ass resistor in parallel with the primary impedance will tend to nullify the impedance frequency variations of a speaker load.

The speakers different impedance values at different frequencies will get reflected back to the primary impedance which is in parallel with a resistor that doesn't vary with frequency so the primary impedance that the tubes see is more resistive and the total primary impedance varies less no matter what the speakers impedance is at a particular frequency.

Normally the speaker load at say 3.2 ohms at 1000hz and at 6.8 ohms at 500hz will produce pretty different primary impedances that the output tubes see, but with the resistor in parallel with the primary impedance, the output tubes will see the different primary impedances produced by the speaker load at say 3.2 ohms at 1000hz and at 6.8 ohms at 500hz and the output tubes will also see the primary resistor in parallel as well and the overall primary impedance that the output tubes will see will be more resistive and be more independent of the speakers varying load value which depends on frequency.

To make a long story short, the output tubes end up working into a more constant primary impedance with a big ass resistor across or in parallel with the primary, so the overall dynamics will be different because a lot of the dynamic response depends on a varying primary impedance produced by the speaker.

I don't think I've really explained it well, but I didn't set out to write a book so t is what it is. It's pretty hard to explain all this stuff without hitting some math.

[leadguy]

Short version.

Usually the output tubes see a varying primary impedance caused by the speakers load variations.

With a big ass resistor across the primary, the output tubes see a more constant primary impedance.

The output tubes work into the primary impedance and this produces a lot of the dynamics etc.

[leadguy]

really short version

Big ass resistor = more constant primary impedance.

[Dunkmop]

lol.

Sry, im just wanting to learn a bit of speaker theory here; do the speaker impedances really fluctuate that much? I mean, will you see a few ohms difference when the speaker is moving? I know heat plays a large role too.

Sry to jack the thread Shannon.

[vanhalen5150]

Put your meter on a unhooked speaker. Move the cone a bit. How much does it change? We are talking about primary and secondary coils here so some of is not directly related to the Cerem idea.

[leadguy]

The impedance curves of speakers are usually around at the manufacturers sites.

The impedance value of a speaker varies with frequency and a resistors impedance doesn't vary with frequency.
A resistor doesn't really have a impedance just a resistance because it's value doesn't vary with frequency.
Impedance values depend on frequency and resistance values don't, so speaker impedance values change according to a particular frequency.

Impedance can be loosely described as resistance whose value depends on frequency.

[jerrydyer]

so did you end up trying this on a 50?
thanx

[jerrydyer]

Hey Twistyneck
if I wanted to try this in a sublte way, what res do you recomend