The 6550 Experience

[shakti]

BTW, I hope to make a clip A/Bing between these two amps through the same cab to show you the differences. I have one of George's amp switchers, so it will be a true A/B clip with instant switching.

[C J H]

Shakti, I started collecting pics with the purpose of showing micing from different shows.

The pictures I have on hand from BOG show all 3 stacks are atleast "on" but its hard to see if theres a daisy going to the amp on the far left (facing the amps). However, on ALL the other shows from mid-late 69 through 70 (again, that I have found pictures of) micing on Jimis cabs is quite inconsistent as far as placement goes (center of cab, somewhat on axis on specific speaker, type of mic, which cab is miced and so on..) On BOG; The middle stack is miced; bottom cab with whats either a dynamic mic or a small-diaphragm condenser (pencil) facing the center of the cab pretty close to the grill. Here´s the interesting part; the top cab of this middle stack is also miced (looks like the top left speaker). From the pictures I have on hand I cant tell what type of mic it is (.. its hard to distinguish from the drum mics due to angle).

So basically you have "stereo" or "doubletracking", I frankly do not know the correct term but when recording this way, especially if mixing mics, you get a much bigger sound and more depth on tape. Again, Im not claiming that tone is built on micing alone, just trying to work out a good foundation (or common ground) how to record these amps to get as close as possible to the lustrous BOG tone.

I guess the only one of your questions I answered were if all stacks for on, shakti. Dont have pictures good enough to see details like logos on amps and so on. What I see in other pictures (Rainbow bridge and IoW) where 3 stacks are rigged though is that all are used/plugged in to. Those were both outsidegigs though, so no guarantee that this was the case at BOG.

[bill bokey]

That's all very interesting, guys ! Thanks !

When you use more than one mic you can get phasing issues. If they are all exactly at the same distance from the source then all is fine but if not, thats when you get weird sounds...sometimes good sometimes not so good...

Can you share some of the pics you have, regarding mic placement ?

[C J H]

That's all very interesting, guys ! Thanks !

When you use more than one mic you can get phasing issues. If they are all exactly at the same distance from the source then all is fine but if not, thats when you get weird sounds...sometimes good sometimes not so good...

Can you share some of the pics you have, regarding mic placement ?

With you on the phasing issues, as I said I will use this to get a rough idea how to place the mics, then go by ear from there. Sure, Ill find somewhere to upload them and post a link here!


EDIT;

Here you go, nothing more than a couple of pictures at the moment so dont get your expectations up too far..
https://drive.google.com/file/d/0BxpdI- ... sp=sharing

[bill bokey]

Great stuff ! Thanks !

One thing for sure : Woodstock was recorded w/ a Shure SM575

[Billy Batz]

This is a very worthwhile line here on this topic. I dont think Ive ever given any thought, or seen any topic on how different amps and cabs would effect the sound via interfering with each other sonically and the phase effects on the mic. Engineers use tricks like taht all the time. Periods of SRVs sound came from running fenders and marshalls at the same time which are out of phase with each other at the speakers and if you set the mic at the right spot, or mix mics on each cab just right, it sounds totally hollow or like sound is getting sucked form your ears. Not suggesting this is the case with Jimi just its a cool effect of having different amps going. These Marshalls wouldnt be out of phase with each other without and extra gain stage in the preamp of one of them but the sound waves interfere with each other in certain positions for phasing effects. It would be audible for the audience as well.

One thing Id really like to know. Are there any seemingly bootlegged recordings of the first night?

[daveweyer]

Here's one little item to consider: cords were often reversed, largely because of roadies quickly soldering ends back on that had broken off. On speaker cables, the polarity is often hard to see, not that they really cared, because they just needed to get things going. When Jimi's big black box of cables and effects would show up at West Coast Organ and Amp we were instructed to go through all the cables, that's when I noticed the kluged cords with reversed polarity. We'd put new ends on them, but road life is rough and they'd all get torn up again rather quickly.
So you really never know whether a Marshall is in phase with the one next to it, or a Fender is in phase with a Marshall. This undoubtedly confused the recording techs.
There was probably a 50% chance of a funky cord being used on any given night, unless the gear had just come from West Coast Organ and Amp, or another repair house.
All this to say that there is this big variable to consider regarding the phasing of any two boxes on any given performance. I've even seen a Celestion wired in backwards in one of the cabinets, the result of a field replacement of a blown driver. So there are lots of things to factor into your line of thinking.

[shakti]

Indeed, lots of possible strange variables.
Listening to the DVD with the video of the infamous Jan 1 show that yielded Who Knows and Machine Gun from the original BOG album, the sound and tone is different still from the original album or the Fillmore East 2 CD mixes. It's a different and useful perspective in that you can hear different nuances still. IMHO, we are all just about as close as we are going to get with the actual amp tone and circuit, because there are so many different variables beyond minor amp tweaks that we simply have no way of controlling. That video is also valuable in that you can see (and then hear) which parts are without a fuzz and which ones are with a fuzz. Stop, for instance, looks to be without a fuzz throughout (except the Octavio at the end).

It would be extremely welcome with a complete release of all four nights with *consistent* new mixes...that would cast some interesting light on any possible differences from night to night and between shows.

I am still very anxious to see and hear more about the three transistor West Coast fuzz though!

[shakti]

Dave,

what would increasing the PI tail resistor from 10k to 18k or 22k do, from a tonal point of view? More headroom/cleaner sound?
I feel like I am getting a touch too much breakup from the amp in its current state. Even though we have no pictorial evidence of the 10k resistor being changed, I understand this was one area where you frequently made modifications?

[Roe]

Dave,

what would increasing the PI tail resistor from 10k to 18k or 22k do, from a tonal point of view? More headroom/cleaner sound?
I feel like I am getting a touch too much breakup from the amp in its current state. Even though we have no pictorial evidence of the 10k resistor being changed, I understand this was one area where you frequently made modifications?

a little less punch, if I remember correctly

[Carbia]

Less headroom if you change 10k to 22k in the tail resistor.

This is what Valve Wizard says about:

A larger tail resistor improves balance, although too large and it will limit maximum output signal swing and even lead to frequency doubling ('swirl')

[daveweyer]

In order for the first section to effectively change the current in the second stage through alteration of its cathode current, the common cathode impedance should be a constant current source; this could be an infinitely large resistor down to an infinite negative supply, or a high AC impedance like a coil, or a pentode plate or mosfet drain. The most inefficient method is a resistor with some voltage across it. The higher the resistance the better the coupling between the stages. With more voltage across the resistor, more plate voltage is needed because the stages will have effectively a lower voltage supply from plate to cathode when sharing a larger common cathode resistor.
The lower the common cathode resistor, the more of the first stage's cathode current is used up driving the resistor, and the less there is available to change the cathode current in the second stage.

Higher resistor equals better coupling, better balance, and the need for more plate voltage.
A 12AT7 will drive the output stages harder than a 12AX7 because it has more plate current, but as the gentleman said, don't let it run out of headroom or you will end up with less swing. Try to get 400 to 450v on the supply to the PI, and about 300 to 320 volts on the plates if you use a 22K cathode resistor. Then you can lower the values of the PI plate resistors to 68K and 56K to get even more current in the stage to drive the output tube grids.

[shakti]

Thanks for that very thorough explanation, Dave.

While it seems that most of the guys in this thread are now working on their fuzzes, I am still fiddling with the amp a bit. The last few days I got really frustrated and thought the tone sucked, and I was nowhere close to BOG tone. The truth is probably that as I grew more familiar with the 6550 sound and the few mods I did, I am hearing more clearly the things that are *not* spot on. So I went back to fiddling around a bit.

I tried mismatching again (8 ohm setting into 16 ohm load) to increase the plate load/primary impedance. This time I am liking it a lot more! Maybe it has to do with the other mods I had done, but the tone is a little smoother, warmer and clearer. I am guessing that I am hearing less power amp breakup, which in this case was a good thing. In some respects, this means taking it closer to a 45/100 sound with the high primary Z. Sweeter, cleaner, clearer, less highs and bass, less "raggedy" and easier to control the breakup IMO. Now I want to try a higher plate voltage as well (I have about 510V now), but am a bit concerned about aproaching 550-560V with just 1k screen resistors. Do you think I would be safe with 1k?

I also went back and forth with the .68uF V2 bypass cap. I must say I am liking it...the combination of a mids/highs boost and thus more preamp overdrive and less power amp breakup with the higher pirmary Z does sound quite sweet and adds sustain and breakup in the right places for BOG tone IMO. Very easy to get the opening tones to Them Changes or Power of Soul, for instance. But if the BOG fuzz is an "always on/clean fuzz/buffer" thing, then I might think otherwise.

With these small changes, I could back off a bit on the presence and still get a clear and present tone, resulting in less hiss and a less harsh breakup.

Sooo...how are the fuzz builds coming along?

[daveweyer]

If you want your tubes to last, you'll be better off using 2K2 screen current limiting resistors with a system using 560 plate volts. You might find the slightly smoother breakup in the output stage to your liking as well.
All this is assuming you will be able to crank the amp a bit, that's when screen failure occurs, not at low volume.
The screen resistors are really used to limit the current that the screen can draw during the part of the cycle when the plate voltage is below the screen voltage and the screen subsequently attracts all the electrons. As the voltage drops across the resistor, the electrons are less attracted to the screen, especially secondary electrons emitted by the plate.
This action has a compression effect on the total current drawn from the cathode and hence through the output transformer, smoothly rolling off the peak output power.
If you make the resistors large enough, you can obtain a high level of power compression; in this process you also get negative feedback at the screens because an AC current is developed across the resistor, essentially providing an out of phase voltage to the screen grids, lowering the gain of the output tubes. This dynamic is what occurs in a greater fashion with the ultralinear output system where the screens are run from a tap on the output transformer, like in the Marshall Major amps. In these configurations there is a very large out of phase signal driving the screens providing a lot of negative feedback to the tubes, and lowering distortion at the same time. These systems are the safest for setups where the output tubes are run in pentode mode, because the screen voltage is never above the plate voltage, and therefore the screens don't take on the role of the anode and try to over dissipate.

If you look at the recommended plate and screen voltages for output tubes run in pure pentode mode at the highest voltage range, you'll see advisable screen ratings at about half to 2/3 of the plate voltage.
So even with 2K2 screen resistors, most guitar amps are exceeding the recommended ratings for the tubes and living in the danger zone. A 6550 for instance with 600 plate volts, should run with about 300 screen volts; it can still deliver 100 watts per pair at these voltages, and have a much longer life.

[Billy Batz]

Here's one little item to consider: cords were often reversed, largely because of roadies quickly soldering ends back on that had broken off. On speaker cables, the polarity is often hard to see, not that they really cared, because they just needed to get things going. When Jimi's big black box of cables and effects would show up at West Coast Organ and Amp we were instructed to go through all the cables, that's when I noticed the kluged cords with reversed polarity. We'd put new ends on them, but road life is rough and they'd all get torn up again rather quickly.
So you really never know whether a Marshall is in phase with the one next to it, or a Fender is in phase with a Marshall. This undoubtedly confused the recording techs.

Wow I never even considered this. I was always looking at amp phasing through a totally schematic lens but your right. Regardless of the amp architecture, if the conductors are swapped or the speaker polarity is not respected you could have phase effects between amps and between speakers in a single cab. I dont think most guitar players understand the lareg effect that can have. Especially on recordings but it would be an audible effect live as well.... talking myself into a polarity switch on all the speakers...