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

[Doug H]

No that was probably it, the half supply to screens that way does sound like the ticket as long as you have enough voltage in the first place.

Thanks

[daveweyer]

It works great and has a less saturated kind of distortion. Probably one of the forum members will build one of those mods eventually and we can hear how it reacts compared to the more basic mods. If you check the tube manual for suggested points of operation for El34/6CA7 and 6550, you find that 600v on the plates and 300v on the screens is the sweet spot for getting 100 watts per pair in a reliable configuration. The engineers who designed the tubes had a pretty good idea of where the tubes would function the best, and give long tube life.
One mod I did for Jimi had 710 v on the plates and 300v on the screens, and it turned out to be quite reliable, and just as loud as a model with 500v on both plates and screens.
It's well worth doing as a super mod.

[bill bokey]

I recently serviced a Musicman combo which had 750V+ on the plates and about 350V on the screens, and it sounded great ! So I'll definitely give that center tap screen supply a go

[daveweyer]

It works well to come off the center tap with a choke and then into a 100 or 200uf cap. This gets rid of some power supply hum. The real beauty of the circuit is that you don't need much negative bias on the output tubes--it's the screen voltage that determines how negative you have to run the first grid to get the current down to 25-30ma in the tube.
With less negative bias, the driver tube has way more ability to turn on the output tubes because it doesn't have to swing very far. This can saturate the outputs much more fully and get more power besides. The plates don't really care how high they are at idle, but you do have to consider the dissipation of each plate with more voltage across it.
Generally this manifests in a higher P-P winding impedance in the OT and less cathode current at full output on a per tube basis.
There is something about high voltage amps and the way they sound, their dynamics.

[daveweyer]

Did we get too one-sided or boring on these threads, what happened to Tazin, Frenchie, Tone Seaker, and all the other Marshall lovers who used to keep the discussion going?
Have we covered every last detail?
Just curious.

[shakti]

Can't speak for them, but I am still all ears and currently scratching my head and poring over my build to decide if I want to mod it further.

Dave, I know this is probably hard to answer, but do you think you modded any of Jimi's SLs with this lower voltage screens supply? And in that case; would you have kept the stock 1k screen resistors? And would you have run the screens through the choke, but the rest of the amp just downstream from the plates and through resistors?

The sound I have now is very close, but I seem to want it to sound as clean and linear as it does on volume 6, but want the drive and sustain I get on 8. There's still a little bit of "clutter" I want to get out. Mind you, I am currently at around 540V on the plates, and with the stock 1k screen resistors. Do you think I would gain much by getting to 560V and lowering the screens with 2k2 resistors?

[Xplorer]

Can't speak for them, but I am still all ears and currently scratching my head and poring over my build to decide if I want to mod it further.

Dave, I know this is probably hard to answer, but do you think you modded any of Jimi's SLs with this lower voltage screens supply? And in that case; would you have kept the stock 1k screen resistors? And would you have run the screens through the choke, but the rest of the amp just downstream from the plates and through resistors?

The sound I have now is very close, but I seem to want it to sound as clean and linear as it does on volume 6, but want the drive and sustain I get on 8. There's still a little bit of "clutter" I want to get out. Mind you, I am currently at around 540V on the plates, and with the stock 1k screen resistors. Do you think I would gain much by getting to 560V and lowering the screens with 2k2 resistors?

+ 1

and yes, give it a try, with 560v and 2k2 !! don't hesitate, Dave said it's the sweet spot.
i don't know if it's the 2k2 but my amp , at 530v with the 2k2 really is a lot cleaner than a standard SL.
maybe cleaner than your clips i think.
and for some boost and a bit of drive , grab an axis fuzz and tell us how it goes for you.

i'm sure that they read the thread when they can but yes, if they could write a bit their ideas it'd be great for a richer dynamic here.

Dave, do you still have pictures to post here ? about the workshop west coast , souvenirs etc , and maybe does your friend with the originaly modded amp allow you to post some pictures ?

[daveweyer]

Shakti, I did mod a couple of Jimi's SLs with the lowered screen voltage. I think I got some Drakes for the PT, or something very similar. I remember there was over 600 volts on the output tubes and around 300 on the screens.
You don't have to use the 2.2K screen resistors with the lowered screen voltage, the 1Ks will work fine.
EL34s also work well in that amp configuration because they are in a safe operating zone.
I mentioned before that I hand picked transformers for high voltage, and had a stock of Marshall transformers at hand for use in the mods. Sometimes the "stock" transformer was just right for the mod, so you could pull the one out of the chassis and install the hand picked model in its place and no one would ever know the difference. Just wide variations in transformer tolerances.
Some of the mods only used two screen resistors, the supply connected directly to the first tubes' screens, and the resistors went from there to the second pair's screens. I did the same with the plate resistors. This was just a way to get away from what they call "lumped constants", the place in the frequency spectrum where all the little capacitances and inductances add up together at the same place to make a tank circuit and turn the amp into an oscillator. Better to have the places where this happens divided up so they happen at different points and with much less Q.
It's an old Robert Hovland trick.

Suddenly Tone Seaker is back!

[shakti]

Cool, I might go ahead and try it. I'll just be scared to wire something wrong and blow up something, so I'll need it by the teaspoon...

So I can do this with a stock 69SL which has a Dagnall T2562 type PT? From the midpoint between the two first filter caps, where the yellow center tap wire for the secondaries is? As far as I understand it, I run the choke from that point, then to the screens, with the screens filter caps also connected to the screens? And then run the rest of the preamp through resistors from where the choke was originally inserted?

[daveweyer]

You can do it with the stock 2562 Dagnall, you just will lose the advantages of the extra high voltage supply.
But why not try it to see how it grabs you, then if it is heading in the right direction you can do an upgrade.
So you just come off the CT, the mid-point between the filter caps, go through the choke to a 100uf 450v cap, and then to the screens. Measure the DC resistance of the choke, maybe around 150 ohms. Use a 150 (or whatever it is) ohm resistor in place of the choke. (put it where the choke was 'electrically' in the circuit). Then leave the (2) 100uf caps that used to be connected to the screens and going to the preamp section just as they are, they'll get their juice from the 150 ohm resistor which is connected to the B+ caps. So you only have to add a 100uf cap to make the circuit work.
But you will need to raise the negative bias supply up onto the -25 to -30 volt range, whatever you need to get the output tube cathode current to about 25-30 ma in each tube.
The mod can be removed without a trace. Just hang the extra 100uf @450v cap anywhere under the chassis for the time being until you see how it works. If it does what you want you can get a three section can to install in one of the existing holes in the chassis so you don't physically alter the original hardware. If you hear power supply hum from the pre-amp section you can add another pair of 100uf caps after the 150 ohm resistor to help out.
But to just hear what it does for now, it should be okay to go with what's there.

[shakti]

I like these suggestions, especially as they are quite uninvasive.

Just one thing; would it not be possible to use the 50+50 cap that is currently used for the phase inverter on the "new" screens supply instead? Then rewire as you suggest with a 100-150R in place of the choke, with the "old" screens filter caps connected there and serving the phase inverter. The only problem I see with this is that the phase inverter filter caps would be connected before the 10k+8.2k voltage droppers, and the phase inverter plates would draw through these resistors, with the filter caps being connected on the other end of the resistors. Any problems with that?

It would be a cool way to check out these mods. As mentioned, when I mismatch slightly (230V tap, 240V wall voltage) I get around 530-540V on the lates, so I am not that far off. I must say that if I really like it, I am beginning to be tempted to build a "full on" West Coast amp, complete with high voltage Drake power transformer, Stancor 4k output transformer, and keep this 69SL build stock with EL34s.

[daveweyer]

If it's an easier wiring job you can use the 50/50 on the screens directly after the choke. Then just take the 20K 1W resistor from the B+ and run it to the 100/100, and run the PI plates from that, and then on to the pre-amp stages with the 10K just like stock. You won't need a 150 ohm anywhere in the pathway.
So, a 100/100 off the positive side of the bridge, then through a 20K or 18K 1W to a 100/100 for the PI, and then through a 10K to a 50 for V2 and another 10K to a 50 for V1.
The screen supply comes off the midpoint of the first 100/100 through the choke to a 50/50, which connects to the screens.
Now, you'll probably get a little power supply hum on the screens, but that's okay for a test amp. I used a large capacitor there, like a 200uf, right after the choke.
You can worry about that later if the amp tells you anything you need to know.

[C J H]

Been a while, life caught up but I´m back.

Just skimmed the evolvement on this forum since I last checked in. I still have not recorded any new BoG or machine gun clips and still have not built my 'always on fuzz'. However I did some work on my amps the past few days. Removed ppimv on both the JTM100 and the 45 -- the JTM45 is now running EL34s and the JTM100 is still juicing on 6550's but now with a 12at7 in the PI.

Made some (very quick) clips of noodling you'll recognize to give you a taste of the changes. Iron is still heyboer, tubes are all new production and I'm playing on my newish strat I finished in spring.

CLIPS:
https://soundcloud.com/pippo1448/sets/n ... -b12at7-pi


Read someone commenting on my old MG-clips, can't remember who it was now but thank you!

These latest clips were, as the soundcloud-description states, recorded using DI-techniques. While I find it useful in some cases this amp looses some balls while not running proper speakers, or I'm just to much of a novice using cab/mic simulators.. Anyway, when I get the time and an empty house I will record some clips with a full stack -- will try to get the MG-chops flying by then as well.

Peace

[Xplorer]

AATWT sounded scray close ! it makes me wonder how it could sound like this in 1968 on electric ladyland, before Dave modded Jimi's amps ? the other clips were nice too. But it's funny how all the west coast modded amp that we have sound different from one to the other, but still with some similar things Inside of course.

maybe a few components, bias , speakers, fuzz, settings, pickups involved ...

i think that it now sounds different from the MG clips you've made before, i liked it a lot.
about fuzz, sorry to repeat myself many times on different subjects, i don't know why i do that, but the axis fuzz worked fine for me, with this amp.

pluging a little jack from the top right to the bottom left inputs , playing with the normal channel vol a bit made some interesting things.

did you mismatch the speakers , like 8 ohms into 16 ohms like shakti did ? i think it works great.

[C J H]

Thanks for the compliment!

I, as usual, contribute alot of the obvious difference in colouring to different miking and speakers. Ofcourse all the factors you mentioned Xplorer are important as well. What I mean is that they rather detail the sound differently (if we assume the amps are of the same model, running the same tubes etc.) where as guitars, speakers, miking and mixing, to my ears, makes day and night differences that are hard/harder to compensate for using EQ. A big difference might also be the fact that the new clips are recorded through DI and not using speakers. I find that DI, for some reason, hides less ghosting than real speakers.. Maybe it is time to give up on JTM filtering ?

No mismatching going on, running 16ohms in these clips.. The old clips are on 8ohms if i remember correctly so that might be causing a change as well. On AATWT I plugged in to Normal 2, the other two, Bright 1.

I have read that you are totally hooked on the Axis fuzz. Inspired me enough to take out my old one, it needs some TLC but I will definitely look into this.

I should also say this, BOG-amp or not, these are the closest sounding amps (or rather amp-configs) to what, in my ears, was the essence of Jimi's late tone: ballsy, clear cleans with alot of thump and VERY smooth and dynamic overdriven tone with a certain snarl and gnarl to it. Its no one-trick pony, not in any way.


Peace