The 6550 Experience

[Frank1969]

I forgot the bias I set it to 80%

[Tazin]

I forgot the bias I set it to 80%

Are you running EL34's or 6550's?

[Frank1969]

EL34 JJ
and E83CC TESLA

[Veritas]

Hey Dave, I'd like to start a 73' ptp SL 100w west coast conversion
To your specs I've been a fan of jimi a long time, the amp was owned by george lynch and sounded horrible when I liberated it from the interwebs. We installed big bottle Ruby's and a good bias years ago I've got a list together from your posts on what I could gather. Too get as close a possible upto include a transformer swap too the corrects specifications, could I ask you directly for the mod list, I dont know if you are in buisness still, i contacted the shop.

Thx
Myles

[shakti]

Checking in after a long time....wow, that was a terrific clip! Really, really close! Cool to hear you can get it with a stock amp too. The right Strat and not least that reverb does help in getting it to sound BOG-like.

Tell us about the Strat and the cab. Anything else in the signal chain? Biased at 80% huh, that’s pretty hot.

[Frank1969]

I used a Fender stratocaster 69 custom shop. signal chain: Guitar-TS808 Tube Screamer- amp and 4x12 Marshall 1960 AX cabinet, nothing else. No reverb, the recording has not been mixed with any other effect, the reverb is the natural one of the rehearsal room in which I recorded. But above all a great inspiration for the Hendrix sound guided me on the touch that I believe is essential to get the right sound.

[daveweyer]

Hey thanks Frank for clarifying that situation for us. It makes a lot more sense in my mind now. Good of you to share your insights.

[revolver1]

I've been doing a bit more experimenting with the amp lately. First off I came accross a 330uf cap for V1 so I've swapped out the 220uf that was in there just to get the circuit 100% right.

In the same vein I also swapped the 5k6 grid stops for 1k5 and this I think made a really big difference.

I was then messing with the .0022uf cap and the V2 Cathode cap again. I tried some paper in oil and also some polycarbonate types of various values. I was really trying to think of what caps techs of the day would have to hand for the mods because I doubt mustards of odd ball values would have been laying around a techs workshop bench.

Theres that amp on the archives, the one Tazin made comment, something about another image mixed in of a different amp. Not even sure if it is verified as a Hendrix amp, any way it has the shared cathode mod and the two caps I've been playing with are not stock. It's very blurry but the .0022uf cap looks to me like a tantulum but I dont think I'd feel comfortable using a tantulum cap as a coupler. I think bypass positions are safer and besides I feel tantulum is too aggressive for this the amp is aggressive enough with Mustards. The .68uf on V2 Cathode looks like a polycarbonate type, obviously cant be sure of anything.

I really feel that the mods Dave gave us at least in my amp sound bang on for Woodstock but there is something just slightly different about BOG. This is purely speculation but I cant help thinking the amps may have been slightly moded again (or added to) between Woodstock and BOG.

The polycarbonate cap definitely had something about it, kind of an ambience that you hear in Hendrix tone but the best sounding came back to a .0047uf Mustard and a 2.2uf Mustard clone cap.

I wasnt so sure about the 2.2uf cap before but now with the 1k5 grid stops it's working great for me anyway.

I think I'm done for now it's as close as I'm going to get and that's pretty damn close.

I mostly play with two amps in Parallel so I know how much of an effect it can have when two amps are set up slightly differently and with mixed speakers how one amp can fill in whats missing from the other and vice versa. I think this is a big, big part of it. Maybe I need to build another.... I need to have a go running it daisy chained with an El34 amp as well, I think Shakti was on to something there.

[daveweyer]

You may recall the photo essay Xplorer directed us to see, of one of Jimi's amps which was rebuilt to stock at Marshall and which had some of the original 6550s still in from the West Coast days. Tazin thoughtfully noticed that the highlighted 6550 in the picture had a 1970 date code; this means that the amps were back to West Coast in the spring of 1970, or else that particular amp did not get modded until that date.

Point being that the Woodstock sound is different from the BOG sound because there had been more work done to the amps during the period, and, the effects chain had been upgraded as well, some rather extensively, as in the three transistor fuzz.

As techs of the day, we had access to mostly polyester caps if we had replacements in mind. There were exceptions like Vitamin Q caps. I liked earlier series Sprague caps, but we had Mallory, Cornell Dubilier, Astron, American Radionic, Good-All, Aerovox, Kemet, and others, mostly all plastic encased by then. Some of these would look like mustard caps. West Coast only tried tantalums as bypass caps, mainly because Bob Hovland advised us on the matter.
All these caps had different sounds, and if they ended up in a Jimi amp, it likely had more to do with supply inventory than a scientific study of the effects of each brand. The more times an amp would come back to West Coast, the more time we had to fiddle with it and aim for Jimi's preferred sound, or at least his description of what he thought it should sound like. The results would have been a combination of the two facts.

The stopping resistors on the output tubes limited the HF response, and created a trap for parasitic oscillations which occur easily on paralleled pentodes. If you look at the older Marshall schematics you can see where they did not use them at all. But it depends on the stability of the particular circuit design--look in a higher powered Ampeg and you will see 50K stopping resistors. If you can hear a difference great, but most of us who had to keep these beasts running preferred higher values, mainly because they were safer. Same for the screen stopping resistors; these were also used to prevent oscillations, but larger values would also limit screen current, which had several positive effects.

The difference in sound between a 220 uf and a 330 uf on a preamp cathode bypass network has a lot more to do with the construction of the cap than the value of the cap. Cathode impedance in parallel with the resistor value determines the roll-off frequency, which with a 330 uf is WAY down there. So whether it is 5HZ or 8HZ, it hardly makes any sonic difference, even for my "golden ears" friends. But the cap construction can be heard.

[revolver1]

Alot of great info there Dave, thanks. That kinda confirms alot of my thoughts thinking about the first shipment of equipment to you and then Woodstock followed by BOG then back to the UK for the Isle of Whight and a different set of equipment that sounds like it's been similarly modded to what ever he had going on in the States.

I tried some Vitamin Q and they did sound quite nice. I also tried the polycarbonate wich also had a great quality to it but when the guitar volume was full up it got kinda undefined. I wouldn't write it off it's just maybe that particular brand of cap I had I think wasnt right.

Either way it sounded better with film foil, all the throbbing bottom end came back with more focus. I know it's not a BOG tune but it's got the throb for Killing Floor.

I know alot of people say there is no difference between 220uf and 330uf but I can definitely notice it. Its probably more about the interaction of the guitar and amp in a room so I can feel it when playing but anyone listening probably wouldn't notice it.

I've been messing with screen resistors and grid stops on two other amps I have, a 5f4 and a Trainwreck Rocket. Again alot of people say there is no difference with grid stops until you get to higher values. I could not disagree more with this. For me there is a huge audible difference from 1k5 to 5k6 especially when using dark sounding fuzz. Probably more in the way it affects the drive characteristic.

From the point of protecting the valves from oscillations and self destruction I'd much rather use 5k6 (all my other Marshall's have 5k6) but the extra band width of the 1k5 seems to soften it slightly and sounds better with the fuzz in this amp. It was always riding a fine line between sounding grunty or harsh with this circuit.

Strange because my assumption was that 1k5 might be harsh but to my surprise the 5k6 sounds ballsier just in the way it affects the drive characteristic probably more apparent midrange. Maybe I could better describe it as having a harder edge. The 1k5 sounds more rounded and helped tame the harshness I was getting.

I had always found with the stock circuit I could either get the cleaner tone or the balls to the wall foundation shaking tone but there was no way of transitioning one to the other. This set up seems to be doing it for me right off the guitar volume.

I checked out some of that Maui footage that was remastered the other day. Watched it on YouTube on my TV and sound system, man they did a great job of it. Just like being there, perhaps not quite as loud....