The 6550 Experience

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

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daveweyer
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Re: The 6550 Experience

Post by daveweyer » Thu Nov 05, 2015 9:21 pm

Well I figured someday some way somebody would fiddle with the West Coast mods and come up with an amp that pretty well captured the Jimi Hendrix amp sound from the 68/69 period. At least it's a fairly good indication that the West Coast mods were influencing his sound, or at a minimum following what he wanted to do musically and what he aimed to get from his equipment. I'll never know if I caused it, or just followed the clues he gave me--it did all fit rather well together with the direction he seemed to be heading with his playing.
I hope this doesn't put off all the Jimi aficionados who think he just played dead stock SLs. I still think their voices are important.

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Re: The 6550 Experience

Post by Xplorer » Thu Nov 05, 2015 11:07 pm

this kind of humility is very classy :thumbsup: , the signature of the best usualy.

yes, they should come arround and discuss a bit. i think that it reveals now a focus that should be interesting to do about Jimi's EL34 amps, between the early kt66 and the later 6550, and maybe here and there some el34 use, even in 68 - 70.

another point would be perhaps to experiment with some el34, high plate voltage, lowered screen voltage ... who knows, some amps maybe had some high voltage even with el34, stock, or once modded .. and you said you saw some earlier amps which had some burned tube socket ...
if jimi used some el34, it never really sounded like a stock el34 amp.
at least one blackflag, and ( correct me if i'm wrong ) two 67 super tremolo may be the transitional amps he had, and may very well have used el34 but ... they don't sound stock to me when i hear the 67 and early 68 studio and lives.

+ he had several fender amps ... dual showman, bassman i think, etc ... and yesterday at a friend's place i played on a nice fender concert amp. using the presence at max and the settings like those you saw on jimi's amps made it sound pure Hendrix too. very good for little wing etc ..

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Re: The 6550 Experience

Post by Roe » Fri Nov 06, 2015 3:24 am

clips sounds good to me, reminding me of BoG
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Re: The 6550 Experience

Post by daveweyer » Fri Nov 06, 2015 2:07 pm

Now I'll add another element to the 6550 discussion, something I may have mentioned earlier in the West Coast topic;
There was another type of tube I used in Jimi's amps called the 6CA7, it was the American counterpart to the EL34 and would drop-in replace the EL34 with no mods because it was electrically identical, as far as specs go.
I've sent Xplorer a couple of pics of the tube so you can see it if he posts them.
What you may notice right away is that the plate looks identical to a 6550; American companies used a shorter plate structure in their tubes for audio because that was what all the companies were geared up to produce, and this tube was no different, avoiding the taller plate structure of the El34. (America didn't use the taller plate structure until the advent of plate cavity sweep tubes in television circuits but they never crossed it over into the audio tube market--amp builders like myself did, but the sweep tubes were different beasts and required different circuitry)
There are other differences too, if you look real close at the bottom mica, you can see a metal plate right at the place where the suppressor grid posts are on an EL34. This is a beam plate; the tube is a beam power tube, not a power pentode like the EL34, and the internal structure of it is almost identical with a 6550, completely different from the El34. The EL34 is a true power pentode, and the suppressor grid is wound on posts just like the screen grid and control grid. Another example of a power pentode is the EL84.
The point of this is that there may have been 6CA7s in Jimi's amps during the time period discussed here so much, and that the sound produced will be more like the 6550 than the EL34, even though they are considered direct replacements for each other.
The 6CA7 used the same tube envelope as the 6L6GC tall black-plate, about the same height as the EL34 but a bit bigger around, so it is possible, where fuzzy pictures are the only evidence for the 6550 and some folks think the envelope is too big to be that tube, that these 6CA7s may be the reason the size doesn't seem to look right.
The 6CA7s don't sound exactly like the 6550, sort of in between the El34 and the 6550.
This may partly explain the sound on certain concerts, and also why the tube size seems off.
In the end, the 6550 was more rugged, and I think by the time it was all over I had used them almost exclusively in Jimi's gear.
The 6CA7s would fit in the amps better and use the same tube holders--the 6550s were always too big around for the sockets, no matter which style you used, and you had to bend the tube holders to use them.

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Re: The 6550 Experience

Post by Roe » Fri Nov 06, 2015 2:29 pm

great info again - thanks! what kind of running conditions would you recommend for the 6ca7s? I seem to like running the 6ca7s slightly different than a typical marshall, prefering a higher primary impedance
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Re: The 6550 Experience

Post by Xplorer » Fri Nov 06, 2015 5:38 pm

here , the 6ca7 , sorry for the delay Dave , i see that you already elaborated about it.

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Re: The 6550 Experience

Post by daveweyer » Fri Nov 06, 2015 7:08 pm

Roe, the 6CA7 will work fine with a higher plate resistance, I've mentioned too many times I liked 3 to 4 K. I always biased them way down until just before the point where they cut off the tail of the guitar sustain.
I just threw that subject in here to make matters even a little more mysterious regarding Jimi's sound.
One more possibility for why he sounded the way he did at any given venue since 6CA7s are actually beam power tubes like the 6L6 and 6550.
American manufacturers wanted their share of the market, but not bad enough to actually make a real EL34. They all knew tubes were dead at that time and refused to do any more capital investments in them except for a few Improvements on horizontal deflection amplifiers--they only did this because TVs were getting bigger and no solid state device could handle the stress or breakdown voltage required. Transmitting tubes were also continued because of the same reason, still used today as a matter of fact.

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Re: The 6550 Experience

Post by C J H » Sat Nov 07, 2015 4:42 am

Dave, 6ca7:s sounds interesting indeed! About half price compared to 6550's over here as well.. Will give them a try in the future!
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Re: The 6550 Experience

Post by shakti » Sat Nov 07, 2015 6:11 am

Yes, very interesting about the 6CA7s. Since they don't require any mods to install other than a rebias that could explain why purported Hendrix amps don't show any significant mods.
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Re: The 6550 Experience

Post by daveweyer » Sun Nov 08, 2015 12:06 am

I hope this is not too arcane, but the 6CA7 has a beam plate instead of a suppressor grid, it has a non-alined screen grid like the EL34, hence it can over-dissipate like the EL34 does when the screens are run at the B+ voltage.

The Sylvania tubes were the best sounding, unfortunately the NOS 6CA7s are selling for about $100 to $150 each.

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Re: The 6550 Experience

Post by badfinger » Wed Dec 23, 2015 1:30 pm

:jimi:
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Re: The 6550 Experience

Post by badfinger » Wed Dec 23, 2015 1:34 pm

:hide:

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Re: The 6550 Experience

Post by shakti » Sun Mar 27, 2016 6:53 am

Just wanted to say this; I had my 69SL build reverted back to use EL34s for a while. Yesterday I put the 6550s back in again, no other mods except a 12AT7 in V3, and bingo...Instant BOG tones! I've been trying to remain skeptical and thoroughly test things, and this is not to be taken as evidence of anything Dave Weyer has said about Jimi's gear, but if you want BOG tones out of your Super Lead, they really are the way to go.

Previously I liked it with the OT mismatched to get a higher primary impedance, but I found this time I really, really liked them with the standard 1.7k Dagnall OT impedance, and negative feedback at 8 ohm. I had about 515V on the plates and didn't really feel the need to go any higher, biased at about 30 mA. One little thing I did change was I put in two old 1M pots from a 1970 Super PA in volume 1 and the bass pot. Something about the way those old pots work just make them sound just *right*...the taper, the relaxed breakup... Really it was the icing on this particular cake.

edit: I must add that I haven't tried 6CA7s, and if they do sound similar to 6550s then those could work well too.
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Re: The 6550 Experience

Post by Xplorer » Sun Mar 27, 2016 2:39 pm

i'd love to try a set of 6ca7 too, maybe in a black flag, as Jimi perhaps did this on his 67 amp. he got some tones in 67 that sounded very close to what the west coast amp delivers, for example with "all along the watchtower" ( as heard in CJH's clip about the west coast mod. too bad that he removed the clips and doesn't appear here anymore ) recorded in the beginning of 1969. even little wing.
+ on a supposed Hendrix amp, black flag, with the JH.EXP stencils on it etc, i could see some long 10 watts green screen résistors, as described by Dave. the rest looked hacked, and the owner certainly changed the tubes a bunch of times.
wether it's accurate to Hendrix or not, i think i that there's something arround this idea that should deliver something very interesting, and perhaps the missing part about jimi's 67/68 tones. just a lot of conjecture but .... it should be fun.

since one of my 45/100 might be used to feed a new superlead West coast build ( the current one will get back like yours to el34 stock specs cause i like it too ) , with its transformers, it can become a blackflag if i can get a set of transformers that would be appropriate.

can i ask you if you measured these old pots ? because the old RS pots measure way higher usualy. do you think that more than just a higher value may explain this icing on the cake ? how would you explain that ? that should be interesting.

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Re: The 6550 Experience

Post by shakti » Sun Mar 27, 2016 4:07 pm

Actually I just remembered something I read in another thread on 6CA7s, but in relation to those tubes possibly being used by Duane Allman in his 50W Marshalls...and that was that supposedly the 6CA7 didn't come into production before 1970 or 1971. I don't know if that is true or not, but if it is, then it certainly puts it into question whether Hendrix could ever have used those tubes. Certainly not in 1967 when AAtW was recorded. That song is so heavily processed in the studio with lots of compression so I really wouldn't read too much into what you're hearing in the final mix as far as tubes in the amp being used.

About the pots I put in; I did measure them and wrote about it in the thread where I compared my 69 with a real 69/70 Marshall and found the pots to be one of the significant differences. I don't remember their values now, but in the 1.3-1.5M region. I don't know if it's the overall value, the taper or the composition of the pot, or a combination of all those things. But I certainly did find them to sound less cluttered in the bass and with a more relaxed and "natural" sound if I can use those words.
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