The 6550 Experience

His guitar slung across his back, his dusty boots is his cadillac.

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bill bokey
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Re: The 6550 Experience

Post by bill bokey » Mon Oct 05, 2015 11:00 am

I have a 45/100 clone with Marstran PT and OT so I might just turn it into a 'West Coast' ...
I didn't build it so I will take it apart and start from scratch again (just like I did with the SLP). I might as well go for a West Coast straight away.

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Re: The 6550 Experience

Post by Xplorer » Mon Oct 05, 2015 11:32 am

I have a 45/100 clone with Marstran PT and OT so I might just turn it into a 'West Coast' ...
I didn't build it so I will take it apart and start from scratch again (just like I did with the SLP). I might as well go for a West Coast straight away.
it looks like we have almost the same dilemma, and same parts.

i might go for a brand new SL build, with the 45/100 transformers . will you too ?
transforming a 45/100 is a bit a pity, when you remove the whole board for another one, when you have to put some big 100/100 uf cans on an earlier chassis, etc ... not accurate historicaly speaking.
+ building another amp in a new chassis gives you .... two amps :) , better than one
+ you just built or restored one SL i see ( nice one ! ) , and you've got the hand now ...

EDIT : i realize that i have more parts missing than just the PT OT and cans. so i'll have to wait until the list is complete.

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Re: The 6550 Experience

Post by Roe » Tue Oct 06, 2015 2:18 am

I have a super 100 amp with ca. 540v on the plates of the RI gold lion kt66s. I use 1k screen resistors in series with a RS 690R (20H) choke. The screen voltage drops below 400v (and plates below 500v) at full power. This setup doesn't seem too hard on the tubes and its historically correct (although most originals had even higher voltages). The amp sounds a little cleaner than my other super 100 amp (3h, 110R choke, 510v), with tighter bass. Also, the high voltage amp sounds brighter when running flat out (even though it is not much brighter sounding at the point of breakup)
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Re: The 6550 Experience

Post by Roe » Wed Oct 07, 2015 11:55 am

For those who use a 1204-43 (marstran/classictone/******): try the 560vdc secondary on the plates and the 490v (****** 455) on the screens. A dual rail system is a good option, isn't it?
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Re: The 6550 Experience

Post by daveweyer » Wed Oct 07, 2015 4:32 pm

If that transformer has dual secondaries or just taps on the main winding, then that is a perfect system for using separate screen supplies. You can use a choke input on the low voltage taps to get about 300 volts for the screens.
This system cannot be a bridge rectifier like in the old SLs; it has to be a full wave center tap system to use separate taps.
With the bridge you don't need separate taps anyway because you have the midpoint. If you don't want to punch extra holes in the chassis, you can use triple electrolytic cans (ones with three caps in them) and get a couple extra capacitors to use on the screen supply.
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Re: The 6550 Experience

Post by daveweyer » Wed Oct 07, 2015 10:26 pm

One other thing to mention here is that if the screens are fed with a separate supply from winding taps on the PT secondary HV winding, their power supply, if set up as a choke input system will have much better regulation than the equivalent capacitor input system. If you end up with 300 or 350 volts on the screens, when the power is at full, the screens will still have nearly 300 or 350 volts on them. Unlike the capacitor input supply, the voltage does not drop from its peak value to its RMS value. In effect this means that the tubes have a robust dynamic at full power. The screens happen to like a steady voltage that does not drop by a hundred volts when the current rises to maximum.
Just another small advantage of the separate screen supply system, especially if that screen supply is regulated, either by a choke input filter, or a solid state regulator of some kind.
Just babbling on about it, if nobody gets it just write and I'll explain it better.

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Re: The 6550 Experience

Post by Xplorer » Wed Oct 07, 2015 11:13 pm

you're currently explaining how to have a well regulated screen voltage ?

if i understand correctly, usualy is it fed by B+ going also to a "derived circuit", which lowers the screen voltage ? the screen resistors + the choke, right ?

why didn't the marshall amps directly use another tap of the power transformer, to feed the screens better ?

it makes me think that i didn't even measured the screens voltage in the modded sl. i should verify it.

and with the benefit of this non droping voltage , how would you describe this "robust dynamic at full power" ?

i'd have another question please : you certainly tried such 600 v high plate voltage + 300 v screens voltage ... with EL34 , right ?
how does it compare tone wise, to a standard 500 v el34 amp ? is it less saturated ?

thanks.

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Re: The 6550 Experience

Post by daveweyer » Thu Oct 08, 2015 12:28 pm

The main point was how to make the amp more reliable and to operate the tubes within their manufacturer's specifications. The other advantages just happen to be inherently a part of a separate screen supply system. Marshall never implemented that system, but they did implement the ultra-linear screen tap system on their higher powered amps, which was in effect a way to keep the screen voltage always lower than the plate voltage.

The difference in sound between the standard system and a separate screen supply system is in part less saturation of the output stage as a result of screen current. Overall, the lowered screen voltage makes the output tubes less sensitive; the higher screen voltage gives the tubes more current change for the same amount of grid 1 voltage variation. That's why on the high screen voltage output stage you have to have so much negative grid voltage to keep the tube turned off or at low current.

With regulated screen supplies, the tube current goes higher at zero bias than with unregulated screen supplies, in other words when the tube is full on it will draw more current than if the screen supply can drift downward. This is what gives it that dynamic feeling, no sag in power during peaks.

And since you don't have to have so much negative bias to keep the idle current down, the PI driver stage adds less distortion to the mix because it is not swinging all the way to the rails to get the output tubes turned on all the way.

If you have 60 volts of negative DC bias on the tube, the driver has to swing over 100 AC volts to get the output tube grid to zero. That's a pretty good feat for a 12AX7 on a 250 volt supply and 40 to 90 volts on the cathode.

Neither Marshall nor Fender bothered with separate screen supplies per se, and it was all about profit margins. But other companies did, including Guild, who made the Quantum amps that Jimi started with and that I have an example of here. If you recheck the schematic we posted of Jimi's Guild Quantum you can see the regulated screen supply. (It used a tube regulator)

Anyway, it's an old West Coast tweak that makes a nice mod and has a cool sound.

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Re: The 6550 Experience

Post by shakti » Thu Oct 08, 2015 1:47 pm

This is some very cool info that slowly is beginning to sink in. One thing that has always struck me about the BOG sound is how dynamic and responsive it is, much more so than a lot of Hendrix' stage sound which is often very saturated and compressed sounding. The BOG stuff has a unique clarity and snap to it. High voltage on the plates and lower voltage on the screens might be one way to get that response?

Just to clarify; on a standard '69 Super Lead, you are proposing to run the screens from that midpoint between the two first filter caps? Running through the choke? But what about the downstream items (phase inverter, V2 and V1)? WOuld the be running in the traditional Marshall manner downstream from the screens? So in effect, only the power tube plates see a high voltage? I imagine in that kind of setup it would be a must though to have a high voltage PT, otherwise the preamp would be running on a very low voltage?
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Re: The 6550 Experience

Post by Xplorer » Thu Oct 08, 2015 2:51 pm

just a little parenthesis () , i'll come back to the subject in another post later, after i'll read your latest posts.
or i'll cut and paste this post in the right thread, not sure what it was.

someone , maybe thorleif was looking for a fuzz recommendation if i'm correct, for bog.
since roger mayer claims that his axis fuzz was used on this show, and since my blue fuzz didn't work so well with a 6550 setup, i tried my axis fuzz clone, and i think it works very well...
even with the volume at 9 o' clock ( huge boost already , i can't imagine what it gives at maximum ... the endless notes could probably be on the way ... ) ) and the fuzz gain not so noticeable at 3 o' clock , it gives some very interesting shades, and this fuzz which has the reputation to be very trebly works nicely with the 6550's , compared to a classic fuzz which really dar'kens the tone in such setup. so why not ... the combination of Dave work and roger's work might be something to try. it's worth trying this axis fuzz.one other thing that points me in this direction, added to the fact that it's quite transparent and not so fuzzy when it's played with this class B 6550 setup , is that when he plays the roger mayer octavia , there's no big tonal difference except for the octave doubling..... let's remember that the octavia is ( at least today, maybe not on the original ) the axis fuzz + and octave section added ..
anyway, it's a whole different beast when it's played with Dave's amp, not what you'd expect, not so gainy, while played in a standard el34 SL, it gives me some van halen stuffs lol ..
i think that the modern axis fuzz and octavia are just an attempt of adaptation to the standard amps , while the originals were simply adapted to Dave's work on Jimi's amps. that's pure conjectures but it could be a possible story i guess.


EDIT : = the axis fuzz isn't made for every amp. and Dave's amp isn't made for any fuzz if you're targeting bog.

on the record, i start to feel that many frequencys and dynamics can't be heard, you had to be there to feel it. for example, the high notes, they're pardoxaly trebly and powerful while remaining typical of this 6550 low end tone : in the real life, i'm sure that it felt more ear piercing ( just imagine a second how it must have sounded in the real life, for the intro of "power to love" , just when he starts with the wah .... do you see what i mean ? ) . this giant transparent volume boost of the axis fuzz should be great in that perspective.
with my tests, it sounded like it, just a bit brighter than our past tests. with the wah it ,sounded like it too.

now i'll certainly find it interesting to discover what shades the west coast fuzz brings, when we'll get back to it later.

end of the parenthesis.
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Re: The 6550 Experience

Post by daveweyer » Thu Oct 08, 2015 6:13 pm

Shakti, the rest of the tubes run at high voltage, only the screens run from the midpoint of the caps. This may require another small choke, but you can just use resistors on V1, V2, V3 supplies if that's more convenient.

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Re: The 6550 Experience

Post by Xplorer » Thu Oct 08, 2015 6:24 pm

guys, you have to Watch this ;) ...... ha ha

https://www.youtube.com/watch?v=dwEsjN6BEaE

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Re: The 6550 Experience

Post by daveweyer » Thu Oct 08, 2015 8:04 pm

I made a pedal for Hitler's son, maybe that was it.

There is an excellent possibility that the BOG concert in question used a modded 6550 amp and a Roger Mayer pedal; in fact the odds are extremely high because a lot of concerts used just that combination. There were probably a few with the West Coast Fuzz, the West Coast Wah Wah, and the West Coast 6550 amps. And there were a lot with the West Coast Wah, the Mayer pedals, and the West Coast 6550 amps.
Probably three different sounds to consider.

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Re: The 6550 Experience

Post by Xplorer » Sat Oct 10, 2015 8:26 am

thanks Dave ;)

yes,i tried your west coast 6550 mod with the axis fuzz, going for Woodstock too, and it was it, definitely. i could get some amazing "hear my train coming", izabella , red house, Villanova junction etc with this setup. the vox846 and univibe liked it a lot too.

6550 testers ! give it a try !

i should have tried earlier since we can see this fuzz at Woodstock. i mean ... the enclosure is like it, with strat knobs. no way to know what it is Inside for sure but anyway, it sounds like it.

surprising how it can help achieving both the bog and Woodstock tones !!
this roger mayer fuzz only works well with your mod, for Hendrix tones.

Now back to the discussion about the screen voltage supply, do someone think about trying it ? it would be cool to hear the impressions about it, and clips maybe.

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Re: The 6550 Experience

Post by Xplorer » Sat Oct 10, 2015 11:05 am

just thinking of something : it would be cool to have some west coast white stickers re made, don't you think ? 8)

one could put it in the back of his plexi if it's modded.

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