Jimi Hendrix' Gear and Mods at West Coast Organ and Amp

[daveweyer]

Frenchie, I forwarded your comments to that brainy hippy chick, the former "Psychedelic Mary", who sang and wrote "I Can't Say Fuck. Needless to say her response was very very positive. She asks how she might thank you for your astute observations. I hope others found the time to hear what a contemporary of Jimi Hendrix is still able to do.

[Xplorer]

She's not only brainy , she really doesn't sound like a 70 years old woman , are you serious ? she sounds more like a 35 or 40 yo woman max. Bravo to Mary Redick !
So she's a 60' legend ? cool

Dave, i played along most of "Electric ladyland" album the other day, and i realised that i was never wrong with this west coast amp ..
i was surprised to sound totaly "Voodoo child slight return" , when dimed , and played with the bridge pickup.
"long hot summer" , "Moon Turn the Tides Gently Away" .... all sounded right.

sometimes i wonder how Jimi's amps were , between the 1966 45/100 era and the late 1968 - 70 west coast era. is it possible that some previous tech modded some amps a bit the way you did, in class B or something, to allow more headroom and more power to Jimi ? do you remember some older mods maybe, on some amps that arrived to you ?
Because hearing some 67 records and playing along with it, i can't really think of some other way to obtain such sound.
it's really cool. i' mean, it's not quite the west coast mod, but it sounds not very far from it . little wing and castle made of sand have some accents like it .

[daveweyer]

I did see some mods in Jimi's early JTMs when they first came into West Coast Organ and Amp Service for repairs. Usually one of the tube sockets had been replaced and the chassis was all blackened in that area indicating a blown output tube and its screen resistor. If I remember correctly, the screen resistors would usually use the same value. I do remember measuring the plate voltage on these amps and being surprised at how high it was, prompting me to replace the screen resistors with 2K2 10 watt units. I also seem to remember seeing a .01 cap to the PI grid on one of these amps. As I have mentioned before, I modded one of these high voltage units to a separate screen supply; something like 640 volts on the plates and 320 on the screens, and I think I left the EL34's in it to see if it would hold up. I thought if it held up I would demonstrate it to the factory as an improvement over their design, but soon after, the SLs came out, being their solution to the reliability issue.
I probably used a 12AT7 in these amps too. I do not remember any amps with the bias set as low as I set them, but I think they had their bias lowered somewhat by other techs for better output tube life.
On the amps with super high plate voltage, I might take them down to 15ma, just slightly over the point where you hear crossover distortion at low volume and the amp cuts off the tail of the guitar sustain.
I finally gave up on the EL34s and started using 6550s most of the time, unless we were out of stock and the amp had to get out in a hurry.

If Brandon's answer is really a West Coast mod, maybe George would make one of them for folks who want Woodstock tone right out of the box. I notice that the thread about Woodstock tone has gotten a lot of interest. The West Coast Mod doesn't really cost any more than a stock SL.

[Xplorer]

this is very interesting, thanks !
great for the Hendrix archeology !

i'd love to give a try to a west coast mod with el34 ! and see how it sounds.

i gave another try to little wing and castle made of sand, but in some virtual cab .. it sounds quite close anyway to my real cab :

https://soundcloud.com/xplorer80/little-wing-west-coast

so you see, it's not totaly accurate, but it can make you think that why not, with the el34 based amp and high plate voltage .that you describe, on the earlier Hendrix marshalls like the blackflag maybe, in 67 ..

i know the guy who has this blackflag by the way, but the Inside looks like it's been hacked, with orange drops everywhere, but some 10 watts green screen résistors still there apparently. it's tough to ask him infos, he doesn't talk much, too bad, or i would have asked him the output voltage of the main transformer !

[Xplorer]

double post

[Roe]

great info on the jtms - thanks. My (limited) exerience with fuzz faces suggests that it is easier to get redplating with the fuzz on than off and that the fuzz is also hard on the speakers

[Xplorer]

Dave,

i was discussing about this thread with JC Maillet, who loves building univibes, and very strange circuits.
he thinks he could easily make a transistor version of the tube modulator, to experiment, it could be fun.

he looked at the schematic and noticed "chora tone projector", i didn't see that before. what is the relation to the tube modulator ?

http://imgur.com/a/mOQr2

thanks !

oups, edit , sorry, you already explained it :

K: Deniz tells me you invented several pieces of gear, including the Choratone [pictured right], the first chorus unit.

D: The Choratone originally came from the Baldwin Piano Company, and it was a device conceived in their electronic laboratories during the fifties. It was a rather large box with quite a number of tubes on it, and it was supplied to their dealers to install on the church organs of the day. Its purpose was to recreate the effect of the viol and the viol celeste on pipe organs, particular string-like sounding pipes that are on practically every pipe organ. And that chorusing...only the pipe organ could do it. The Choratone was invented as a way of getting around having to have a complete extra set of generators to de-tune to get the chorusing effect that was so popular and used in so much of the liturgical music. Of course, my history with repairing organs and working for Baldwin (which I did, at the factory outlet there in L.A.) put me in contact with these boxes. At first, I simply felt they were sort of a novelty; the way that they had them being installed in churches, I didn't think really added that much to the sound of the organ, but it certainly piqued my interest, and I was determined to get ahold of one of those things and take it home and analyze it to see what it was and what could be done with it.

I eventually had my opportunity; I got one out of a church in North Dakota...they didn't even know what it was for. They got a new organ and I ran off with the Choratone. The first thing I thought was, "What can we do with this thing? How can we make it actually chorus?" I determined to come up with a modification (which I did) that allowed it to be used with a guitar and gave it two outputs instead of one output, one of the outputs being a non-chorused sound and the other output being the chorused sound, and having the phase between those two switchable. Once that was done, and the resulting signal came out of two speakers and two amplifiers, and UNBELIEVABLE chorus was obtained. I took my Choratones to L.A. when I went, thinking, "Somebody's gonna LOVE this," and they did -- every guitar player that ever plugged in wouldn't leave me alone. Everyone wanted to buy the Choratone, but I couldn't really sell them because I only had two of them. I let them use them, but you can probably imagine a guitar player who has never experienced an effect like that plugging his guitar in and hearing, for the first time, a brand new kind of a sound, with their own strings and their own guitar and their own musical expression, now having access to it. It gave them a tremendous feeling of power. And it was years yet before chorus units would appear. Everyone that I played it to wanted it. Of course! Why wouldn't they? It would set them apart.

We used the Choratone on Den's Equinox album to a great extent, a number of times, and Den had never liked chorus, he said. He always thought it was too fruity. But when he heard the Choratone, he said, "That's the first chorus I've ever liked. Unbelievable." It has a tremendous depth. In the '70s, with the bucket brigade chips that you had on the market, they were able to produce small chorusing units. They started out as phasers. It was the obvious place to start, because that was something that no one had heard before either, except from the flanging of tape recorders in the studio. Gee, I think I heard that when I was just a kid, and I was told that it was Les Paul that got that one going. But the Choratones...I still have one of the ones that I had down in L.A.; I keep it operating in the studio. Of course, they haven't been made for many, many years; I don't know if there are any still in existence. They could be made again, but with the power of microprocessors and the digital delay, there's certainly no reason to have one.

It could be recreated; there's something about plugging into this thing that's all tubes from front to back and the way that it sounds. There's a certain character about it that doesn't seem to be available, at least not in any digital equipment that I've ever come across. I know they're working on it. Tubes are a rather complex device from a sonic standpoint. From a strictly mathematical standpoint, as far as designing amplifiers goes, they seem to be rather simple. There's a family of curves, and there's a family of formulas, and there's a family of functions and relationships that seem to occur consistently, which gives you the ability to design things with them. Sonically speaking, there's complexities that aren't addressed in any formula or any mathematical function. The audiophile people will attest to that.

[daveweyer]

Xplorer,
Sometimes I look at what I wrote and wonder if I actually said that. There is obviously so much more to the story than what was in the interview. At least I gave the proper credit for the tube modulator, that's actually what it is; Baldwin just wanted a clever marketing name and that's where the Choratone Projector moniker came from. I changed the oscillator frequencies and added a preamp section, ran the heaters on DC, and several other things to adapt it for guitar use.

If your friend wants to he could indeed build this device with transistors although some of them would have to be Jfets because of the necessity of gating the phase shifted signals without an audible pop.
If you like I could publish some letters between Robert Hovland and myself regarding the theory of the unit and other patents which were being developed at the time Hovland was at Vox. Brad Plunkett himself was working on a unit which used 90 deg phase lags instead of 60 deg like the tube modulator.
This might be too much for the forum members.

[Xplorer]

Hi Dave, thank you, yes that would be sweet if you have some documents and more infos about this effect.
JC really seems to be excited by this project and that could help. he told me that he never saw such idea before although he might have come up with a variant which has an oscilator speed control.

i advised him to come over here to discuss , maybe he will. anyway, i'm transmitting, he might build this one and he mainly writes on his own forum : https://www.viva-analog.com/forum/index ... icseen#new

i first talked to him about this thread and you because i was sure he would love it. if you two discuss, hopefuly here, there wouldn't be the troubles you're facing with newbies like me , he's an advanced tech and he understands the univibe like no one else.

what other things did you do to adapt it to the guitar ? maybe do you have your own modded schematic from back then or you built it directly ?

so you changed the original 1Hy 375Hz, .56Hy 750Hz, .27Hy 1.5Khz, .12Hy 5kHz inductors frequencys to some others ?
Yes, maybe let's hear how JC's attempt will sound, compared to what you remember of the original, it could be very interesting ! Do you like this idea ?

[daveweyer]

It seems like a fusing of the two forums on certain subjects would be a cool idea, sort of networking--maybe it would widen the audience of participants. I have a feeling that a lot of folks check out the forum topics but feel like they don't have anything of importance to say, or maybe just feel a bit overwhelmed by some of it, especially the real technical stuff. Even basic concepts can be a bit daunting to respond to, more so if you think you'll come across as dumb or something.
Remember Crlyguitr and Frenchie getting into it, the forum lit up with activity; all unproductive of course, but it was sure fun to see everybody jump in, that was a real bit of enthusiasm.
So I think it would be fun to have your friend do some thinking on this forum if he thinks it would work out for him.
This winter I'll send you some more of Jimi playing through the tube modulator to post, that way everybody can get a chance to hear what it sounded like with the main man. Some of this will be post processing. (Yes I know there will be those who say it can't be so and it's not him, just like last time.)

[daveweyer]

On those center frequencies, I changed the capacitor values to lower them. There's a whole theory behind these values which we'll hopefully get into. Two of the frequencies are added to the original signal and two of them are subtracted from the original signal.

[Xplorer]

some more Jimi playing this effect ? super cool !!

yes, the forum ... you can't predict how it'll react, anything can happen. it can be a good thing , depending on the way you look at it.
carlygtr ... if only he knew what he's missing ... too bad for him who's after such tones ha !
if you wish, don't hesitate to bring anything different, documents, pictures of the workshop from 1969 etc .. you can't predict how or when peoples will react, but you see, infos from the past can pop up back in the flow of the thread.

i know you have tons of storys, also on some other famous artists with a specific sound. maybe a new thread about the other musicians you took care of ?

yes, i'll try to convince JC to do a jump here. he invented some very cool effects and mods. and he could maybe do his version of the tube modulator on his forum, and links between the two, it could be an idea.

[Eb7+9]

HI Dave, and all ...

first of all, many thanks for all the info you've provided us Dave
incidentally, I had always wondered whether or not Jimi used the Fuzzface as a booster rather than an all-out fuzz // thanks for mentioning that in the your interview ...(!)

thanks to Xplorer here I became acquainted with your work and existence only a few days ago ... first with the West Coast mod to Jimi's amps (BF 12at7 PI circuit and 12ay7 Tweed Bassman front end, and low bias 6550's // almost re-Americanizing the circuit as it were) ... and then the Choratone circuit, which has piqued my interest

---

let me give you my take on how I think the Choratone circuit works and you ca tell me how far off I am

simply peaking, we see at the input a filter network sandwiched between the input (differential output) gainyInverter-buffer pair and the later 3 "feeder" gainyInverter-buffer pairs ... the network produces low-pass and high pass outputs ... these are then passed on to two gainyInverter-buffer circuits, and their outputs are summed together and sent to a third one ... the output of these three feeder circuits are sent to four oscillator/mixer circuits ... running at double frequencies, starting at 375Hz ...

the oscillators are three phased by nature, and not necessarily linear in operation ... actually, quite likely not since there is no non-linear element (eg., diodes) to kill gain in both directions ... the main thing here being a 120deg out-of-phase operation ... signal from the 3 feeder circuits are injected in the cathode circuits, indicating common-Grid operation (current followers in essence) // and in this case, we could say modulated-Gain common-Grid operation (modulated current followers ...?!) ...

I find it very interesting how the circuit serves a dual purpose in this case ... I would be interested to read the patent on this circuit if it exists ... at the output of each oscillator lies a tank impedance that, I'm guessing, will try to null out the triple-frequency oscillator "clock" appearing at the common outputs (between the individual 3k9 plate loads and common 10k load) ... leaving a residual audio program outside of this frequency //

---

first of all, I find this a curiously brave design ... it reminds me a little bit of the Tremulant circuit in this rare Ampeg harmonica amplifier I once wokred on

I was thinking, maybe for the fun of it we could try exporting the idea to a different technology ... either keeping it all single ended (n-chan jFEt's or Darligton BJT's). or go op-amp // maybe simple 9v battery operation if we're extremely lucky ...

as far as "new" goes, the front end lends itself well to a differential output op-amp circuit ... and the feeder circuits could be inverting op-amps ... now the mixing part // here's what I was thinking for a modern take

my assumption is that we want to oscillator frequencies to match the tank circuit resonance frequencies ... or use an adjustable NOTCH filter ... in so doing, modernizing, and maybe even enhancing the response of the effect (you never know!) ... either way, we have the option of staying somewhat close to the original design, or pushing things a bit for a tweaked response

as well, for the oscillator/mixer stages I was thinking the following ... the circuit does two jobs, it has to oscillate and it has to mix signals to a common output ... it seems to me that we could keep the mixing arrangement the same (common Collector/Drain loads connected to a common top load, as in the triode version) but having tri-phase SINE waves directly driving the inputs of the transistors from an external PIC circuit ...

I'm just thinking of options at this point ... but, advantages are readily apparent here

one advantage of running PIC's is we can fine tune to match the tank frequency // tho, I'm not sure if the design forbids this ... the other advantage is having a nice clean faux-sine wave (from a table lookup) to modulate transistors with, something the Baldwin circuit won't do ... not saying it's good or bad // just as an option ... ultimately, we could program any shape of LFO pulse we wanted from the PIC if we wanted

if we use PIC's for 3-phase frequency generation, and active notch-fiters at the output of the ANALOG modulator blocks ... I'm thinking that might be a cool approach to take if we wanted to explore a modern version of the idea

otherwise, a completely single-ended jFET approach might work as well, ... with a little tinkering

I'm attaching two TINA-TI simulation examples of the oscillator/mixer blocks in action (sort of ...)

looking forward to discussing more
best ...
~jcm

[daveweyer]

There are 4 low frequency phase shift oscillators in the tube modulator running at 1HZ, 2HZ, 4HZ and 8HZ respectively.
These frequencies can be changed to create a different effect, all lowered by half for instance.
The most important idea in the unit is the all-pass phase shift network at the front end which outputs signals 60 deg out of phase with each other. These signals are what the other circuits act upon, the low frequency oscillator signals adding and subtracting 1HZ, 2HZ, 4HZ, and 8HZ from the original signal in each of the four bands.
Okay so far?

[Xplorer]

sorry for not replying earlier. yes it makes sense so far. it'll make even more sense to JC , but he tried to post here and apparently his first post requires some moderation checking... he posted since a few days already, nothing happend yet.

so it's adding and substracting those frequencys to the original signal, and i suppose that it can be made in a certain depth for each ?

i should re read some techs though on wilkipedia about phase shifting, all pass,and the degree angle related to the phase.

you modded the preamp too, to make it fit a guitar ?