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 » Sat Oct 12, 2019 2:03 pm

Does anyone but me wonder what happened to xplorer and shakti and all the other stalwarts of this forum? Everybody must have found a hot new girlfriend to steal their minds from Marshalls, Jimi, and novel electronics.

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Re: The 6550 Experience

Post by gloeidraad » Sat Oct 12, 2019 5:54 pm

I don't know either. :(

shakti
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Re: The 6550 Experience

Post by shakti » Sat Nov 02, 2019 1:42 pm

Still around, but as I said in another thread I am very focused on Hiwatts at the moment. I will be back in Jimi territory soon enough, will be restoring a '66 tall bottom pinstripe cab among other things.
JTM45 RS OT, 1973 18W, JTM45/100, JTM50, JMP50 1986, JMP100 "West Coast", AC15, AC30, BF Super Reverb, Boogie Mk 1, Hiwatt CP103, DR103

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bill bokey
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Re: The 6550 Experience

Post by bill bokey » Tue Dec 10, 2019 10:35 am

I have a new girlfriend indeed !

I wish I still had the time to post here.... i'll be back ASAP ;)

revolver1
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Re: The 6550 Experience

Post by revolver1 » Fri Jul 03, 2020 1:34 pm

Hey hope everyone is keeping well with all the madness in the world at the moment. I got Covoid back in January, didn't know what it was at the time and got it again just as we went into lock down. It certainly kicks the shit out ya but I'm all good now. Luckily I didn't need hospital.

Another bit of luck was I had a whole bunch of builds waiting to be done and now all the time in the world. Lock down? It's been more like a Rock Down!

I'll maybe have to put some build threads together in the other amp section. I built a Tweed Champ a Tweed 5f4 Super an early 70's 4 hole Hiwatt DR504 and a Trainwreck Rocket. I'm really over the moon with every one of them. I think I'm done :lol: :lol: :lol:

So this gets me back round to the West Coast build. I've been doing loads of thinking about this and listening to Woodstock and BOG also going back to Monteray and Isle of White and I think I've hit on something here.

I had my 69 1987 an my 69 1959 side by side and there wasn't alot of tonal difference between the two so I decided to take the 470pf dogbone out of the tone stack in my West Coast and put it in the 1987 which did the trick. I then put a 500pf Silver Mica in the West Coast which its 50/50 if it was mica or dogbone but more to the point it definitely won't be 470pf gotta be 500pf.

Now the weird thing is with the 500pf in it got further away from the sound , I'm thinking how can this be?

So with alot of thought im thinking its gotta be the midrange response. The sound was so close in many respects but not quite right, I'd said before the drive characteristic seems a little off at times although it was doing alot of things right some times it just got a bit horrible sounding and I never heard it get like that in Jimi's performances so i know somethings not quite there yet.

So I'm thinking its gotta be 500pf in the stack so the only other place I can think of to alter the midrange content is the .0022uf plate cap on V1b. So then I went back through the threads and Shakti brought it up at one point I think was it Tazin who eluded to a slight change in this area, Dave mentioned it at one point and also Neikeel posted a clip of a guy and every one said how close his sound was and I think it was Tazin who said down in the comment section that the guy scooped out the mids with a graphic eq. Ah ha me thinks....

So heat up the iron I'm going in! So like I said I've got a bit of a theory going on here I've only half tried it out but it was hugely successful. I did change out the 10k tail resistor for a 15k unfortunately I didn't have the chance to test each change one at a time because of noise constraints but there probably isn't alot in that change although it may be helping to some extent. Any way I took out the .0022uf cap and stuck in a .0047uf and for me that's the one baby. No horrible distortion and it's got the thick clarity and articulation, just sounds more natural, woody and open like it should rather than the boxy mid thing that some times occured with certain combinations of vibe wah and fuzz and guitar volume.

Now here's the thing with the .0022uf cap it was almost there but at times I felt it needed a touch more gain for the wild stuff but I just couldn't see how to get it because adding more fuzz got into the horrible zone. Enter the V2 Cathode bypass cap.... Generally speaking I hate that cap, for me it just does not work with fuzz at all, once i tried it with out I've never built an amp with it in since but this has got me re thinking everything.

So with the .0047uf in i think it really sounds and reacts more like it but it's on the brink of needing a slight gain boost to really get into the wild side and also I think a slight lift in the highs would help. So my theory is .68uf sucks but what if i put a slightly bigger cap in there to get a gain and broader frequency boost. I think this is it, I'm going to get a few different values to tune it but I'm thinking maybe 1uf 1.5uf or 2uf it won't need much because the sound is so close.

So shelving less bass at the coupler keeping it bigger and warmer then introducing more highs and gain at the cathode but not letting it get into that horrible boxy mid thing. For me I think this might be it.

I'm going to get some caps ordered up now so I'll keep you posted with the experimentation.

On another aspect I went back to the 32/32uf can on the preamp because I really do prefer the feel of it compared to the 50/50uf also pedals seem to integrate better but listening to Woodstock I'd say that's 50/50 all round then listening to BOG I think the ambience of the Theatre is slightly softening the sound wich is why the 32/32uf sounds more like it to me.

Anyway I'll keep you posted, hope your all O.K and Rock On.

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Re: The 6550 Experience

Post by daveweyer » Fri Jul 03, 2020 8:51 pm

Have you got that 12AT7 in the phase inverter/driver slot? Depending on the make, the gain will vary. When you drive the amp hard the 12AT7 will turn the output tubes on harder than a 12AX7; this happens at about -5 volts of bias on the outputs when they begin to draw a little grid current.
You can also partially bypass V2 cathode with a larger cap to get more overall gain without the high boost thing, sounds like you only need a touch more.

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Re: The 6550 Experience

Post by revolver1 » Sat Jul 04, 2020 1:32 am

Hi Dave, good to hear from you. Hope you are keeping well.

Yes, I've got a Mullard 12at7/Ecc81 in the V3 position.

I'm going to try a few different value cathode caps. Also wanted to try a few different types, I wanted to try a tantalum as you had suggested to see what happens. Like you said I don't think it needs much just a little more highs and a smidge of gain.

It was a real buzz cranking it up yesterday, definitely the best its sounded for me so far. So very, very close just fine tuning.

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Re: The 6550 Experience

Post by revolver1 » Sat Jul 04, 2020 7:16 am

Dave, a quick question. The .68uf Mustard cap on V2 cathode bypass is rated for 160v, what's the lowest voltage rating I can go to? Is it the same for electrolitics?

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Re: The 6550 Experience

Post by revolver1 » Sat Jul 04, 2020 7:35 am

I've found a good source for Solid Tantalum caps but only in 6, 25, 50, 75, and 100 volt ratings. I know it would be ok on V1 but I'm unsure about the V2 cathode bypass?

revolver1
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Re: The 6550 Experience

Post by revolver1 » Sat Jul 04, 2020 8:31 am

Not so sure about the reliability?

daveweyer
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Re: The 6550 Experience

Post by daveweyer » Sat Jul 04, 2020 9:16 pm

There should only be a few volts on that cathode, like maybe 1 volt with an 820 cathode resistor. No worry about voltage ratings.'You can bypass half of that resistor by using (2) 400 ohm resistors--just bypass one of them to get a partial gain boost in the highs. A lot of amps didn't need that cap.

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Re: The 6550 Experience

Post by revolver1 » Sun Jul 05, 2020 3:19 am

Thanks Dave. Do you have any opinions on the Tantulum caps? I found a seller with loads of these Kemet mill spec caps, they look great on paper but I keep reading mixed opinions about Tantalum.

Some are saying it will last forever while others are saying they tend to fail and when they fail they Short out and make a real mess. Others said they only fail if polarity and operating voltages are not observed or if subjected to pulsing voltages.

Are they like electrolytic caps, as in they like to be operated near their rating to keep the dielectric happy?

Just trying to think what type of caps would have been around back in the day in the 1.5uf to 2.5uf range, I've got some tropical fish but not so sure they will sound any good in an amp.

daveweyer
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Re: The 6550 Experience

Post by daveweyer » Sun Jul 05, 2020 2:39 pm

Robert Hovland, former engineer at Vox, swears by them. They can add a whole new dimension when used as cathode bypass caps. Observe polarity and voltage rating. If they short on a preamp stage they will cause no damage. You have 4 basic choices, electrolytic, film, ceramic or tantalum. Sometimes you use an electrolytic, and them bypass that with a film cap, or a mica cap. I used to bypass with ceramic, depending on the sound I wanted to get. Definitely try a few values of tantalum and see if you get where you want to go.

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Re: The 6550 Experience

Post by revolver1 » Mon Jul 06, 2020 1:59 am

Thanks Dave great info.

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Re: The 6550 Experience

Post by daveweyer » Tue Jul 07, 2020 1:54 pm

Try a 12AZ7 in V-2 slot. It gives more drive for the tone stack among other things. Another Jimi secret most folks are unaware of.

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