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 » Wed Feb 07, 2018 2:10 pm

Roe, the 2K2 OT will work great, it would really be super with KT120s because of their cathode current capability.
The Major I just did has about a 2K2 primary winding, and I found it would crank out almost 200 watts with plate voltage in the 550 range--you should come close at 530 volts, if the PT regulation is good. Some are, some aren't, you have to load them and test them to see the percentage of drop at rated power. I just use a resistor bank, simple and quick.

I got the KT120s for $30 each on ebay, that seemed like a very reasonable price--they were new, matched at 8000gm, and in boxes. They were made in 2017 in Russia of course.

You don't have to use 500uf caps as the main PS caps, I just had a bunch here from other jobs. I would go as high as possible though, it really helps keep that ghosting hum out of the amp at high volumes.

niknik
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Re: The 6550 Experience

Post by niknik » Wed Feb 07, 2018 6:35 pm

Ok Dave, thanks!
I will try to find a 650ohm 25w res, or put some 10w in parallel to get the value.
I also have got somewhere a marstran 100w 2k2 ot, so I could try a kt120 quad when all will be ok :D
Thanks
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niknik
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Re: The 6550 Experience

Post by niknik » Fri Feb 09, 2018 9:28 am

Dave,
In the Major screen supply, is the 27ohm after the mosfet, a fusible resistor? Is the wattage of this res important?
Thanks, and please excuse me if I am tedious, I'm really excited about this project, and I want to be sure to do things correctly.
Thanks again
Sammy who?!

daveweyer
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Re: The 6550 Experience

Post by daveweyer » Fri Feb 09, 2018 1:17 pm

The 27 ohm 1/2 watt resistor on the source of the HV mosfet has two purposes; it acts like a fuse, and will blow if there is a short on the output screens somewhere, AND, mosfets tend to oscillate if there is a capacitor directly on the source, and this varies from mosfet to mosfet. You could run the mosfet without a capacitor on the source in most instances, but for screen supplies the mosfet has to be protected from a phenomenon called negative resistance, something that occurs on the screens during switching in which the screens actually become a generator and try to push voltage back into the regulator. The electrolytic absorbs the pulses.
And also remember that a mosfet will oscillate if there is a capacitor directly on the gate, so be sure to use a resistor there--a 150 ohm 1/4 watt works fine in this situation.
Again, it varies from device to device. Be sure to use a 1000 volt mosfet, they are cheap and available at the big electronic suppliers. Try to find a mica insulator, and be very careful in mounting because there will be 600 volts on the drain and you don't want to short that to ground because there was a small filing accidentally on the insulator.

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Re: The 6550 Experience

Post by niknik » Mon Feb 12, 2018 8:39 am

Yesterday I did some works in my Major.
I installed 4 power tubes, and raised a little main filters.
like Dave said, I was going with 6550s, but unfortunately i found that my quad has one very weak tube :(
So, I went with a mix of kt88s and 6550s, I changed a bit the bias circuit with separate bias regulation, one each tube, so I was able to level the tubes mismatch.
B+ is 530v, bias is -69v, and I'm reading about 42mv each tube with 1ohm cathode res.
At the moment this amp is pretty stock specs, and it is very very loud and very very clean, the low E of my guitar is very big and tight.
Next step will be to disconnect UL wires, and go with the mosfet screen supply.
I ordered some components, but I can't find a 420v zener.. :shrug:
Dave, what diode are you using in your amp?
Thanks
Sammy who?!

Tek465b
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Re: The 6550 Experience

Post by Tek465b » Mon Feb 12, 2018 9:48 am

niknik

You can have Zener in serie(ie: 4x 100volt Z) OR replace the zener with a Vbe multiplier/amplified diode(make sure to use a transistor rated for that voltage!).
You you replace R2 for a potentiometer, now you have adjustable screen voltage ;)
Or use R1= 1M and R2 = 1.66k for 422 volt. Make sure R1 is 1/2 watt resistors.
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niknik
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Re: The 6550 Experience

Post by niknik » Mon Feb 12, 2018 11:43 am

Thanks Tek465b!
I miss some electronic basic :oops:
I have got some 100v zener diodes, so I'll go with serial arrangiament :D
P.s. I just received the pcb of Dave fuzz,
Many thanks!!
Sammy who?!

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Re: The 6550 Experience

Post by Roe » Tue Feb 13, 2018 8:05 am

Thanks, I just received the PCT too - looks great. Please do tell if you know of any suitable transistors that are available
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Re: The 6550 Experience

Post by shakti » Fri Mar 02, 2018 4:56 pm

I finally have the time to work on my 6550 WC Super Lead amp again. I am going to do the "Jazz" mod With the screen supply from the half HT supply, but so far I also tried the so-called "R/T" mod, which is a cascaded V1 mod. I was pleasantly surprised to find it actually sounded *more* like the BOG base sound than stock mode. I have it switchable, so comparing is very easy. The cascaded mode just has "more" of the same sound, but with more sustain and thickness (and obviously more gain available). Something about that BOG sound is so thick and complex yet with a lot of clarity, and this mod actually was even closer (and I'm sure the other WC mods help, such as higher voltages, 6550s, 12AT7 PI etc).

Getting closer still.... :rock:
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Re: The 6550 Experience

Post by Xplorer » Sun Mar 04, 2018 2:47 pm

you got me hooked with this mod. i hope that you'll make a video with the switch on and off !

shakti
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Re: The 6550 Experience

Post by shakti » Sun Mar 04, 2018 4:04 pm

You can hear lots of videos of the so-called "R/T" mod on Youtube, including on an unreal sounding Major. But the guy behind the mod typically uses humbucker guitars and lots of gain. The amps sound killer that way too, but I was very surprised how nice it sounded with the gain backed off and with a Strat.

Lower gain settings with a Les Paul also sounded surprisingly close to the Hiwatt Jimmy Page amp I had, which is not so surprising anymore if you study the schematic... 8) The other West Coast mods make the power stage behave slightly more like a Hiwatt too, so while it still has more of a Marshall EQ and slightly different midrange, the punch, bass tightness and response/touch sensitivity is quite close to that Hiwatt Jimmy Page sound. It has quite a bit more gain available at the highest settings, but once you back off the volume (I am using a dual 1M pot with a DPDT switch which controls both channel volumes as well as switching between cascade and stock settings) to around 6-7, it's Jimmy Page gain levels with a Les Paul and a G12H cab.

I will get to clips but it'll take a while yet.
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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Re: The 6550 Experience

Post by revolver1 » Fri Mar 16, 2018 1:21 pm

That's really interesting because that's exactly what I've been thinking for the last 6 months, thinking of the period his sound suddenly got a lot more head room and rumour of amps being modded in New York.

I definately want to do a WC 50watt but also I've been seriously thinking of building pretty much a West coast mod with KT88's as well to see if it came close to Page's tone.

Of course I'm only theorizing but I really have a strong feeling his fabled modded plexies would have been very close to a WC mod with KT88's.

It wouldn't surprise me one bit if Mr Page had contact with some one that had seen the inside one of Jimmi's amps.

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Re: The 6550 Experience

Post by Roe » Mon Mar 19, 2018 6:15 am

Page's plexis were introduced during the recording of LZ 2 in 1969, although he used a lot of other amps in this period, including Hiwatt and Rickenbacker amps. His plexis appear to be stock 1959s with 1982 cabs until they seem to have modded in order to get more clean headroom live for playing stairway to heaven in 1973 or so. I personally prefer his earlier tones, and thing that the right bridge pickup (UA5 magnet?!) and a decend 1959 with 5000pf bright cap does the trick
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Re: The 6550 Experience

Post by shakti » Mon Mar 19, 2018 12:50 pm

He only used Marshalls on rare occasions before late 1971. Sometimes in the studio (Heartbreaker solo, apparently, and a few other tracks and bits from LZ II), but not so often live. It was the Arbiter Power One Hundred or Rickenbacker Transonics(?) before the Hiwatt phase which began in July 1969 and lasted through 1971. By late 1971 it was often one custom Hiwatt and one Super Lead, and by 1972 the modding of the Marshalls began AFAIK.

But you'll see that the custom Hiwatt Jimmy Page preamp has a cascaded type setup (switchable via footswitch) on V1, which probably accounts for some of the similarity in gain and "feel" under the fingers.
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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Re: The 6550 Experience

Post by Roe » Tue Mar 20, 2018 6:33 am

Interesting - thanks Thorleif
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