The 6550 Experience

[daveweyer]

With Jimi's amps I mostly just used Marshall transformers except for a few builds. It all depended on the B+ voltage. I liked the Dynaco because it would work with real high B+ voltage and not break down. As you know, Marshall experimented with primary impedances starting at about 4400 ohms and then started working down to about 1750 ohms as they lowered the B+ voltage to try to make the amps more reliable.
I found that 4400 ohms worked well at lower B+ voltages too, because with (4) 6550s, you can still get 100 watts with a 4400 ohm P-P impedance, and the tubes just cruise along without too much dissipation.
The Stancor transformers sounded the best with ripping guitars, but you just can't find them anymore.
So you can use the Dynaco, the Heyboer, the Tube Doctor outputs, and if you have some money to burn, Mercury Magnetics.
With real high B+ stick to 4K primaries.

I always check Ebay to see what old relic might be available, every once in a while something shows up.

[Xplorer]

thanks Dave for the explanation about marshall process over the years with transformers, and for the infos.

i saw some stancor transformers on Google but probably not the right ones, with high voltage.

ok, so for the output transformer i'll go for a standard 69 SL OT with marstran or ****** or mercury magnetics or this german builder, i don't remember the name but he makes the most perfect replicas from what i've been told. he also makes some nice partridges transformer replicas , for the hiwatts or sound city.

[daveweyer]

Mercury has some higher primary impedance models, most of the others are lower as a rule. The Tube Doctor models were hard to find information about, I couldn't find the specifications page for his transformers, but they looked like really nice replicas.
What will your B+ voltage be for the amp you want to use this OT on?

[Xplorer]

Mercury has some higher primary impedance models, most of the others are lower as a rule. The Tube Doctor models were hard to find information about, I couldn't find the specifications page for his transformers, but they looked like really nice replicas.
What will your B+ voltage be for the amp you want to use this OT on?

since it'll be a 45/100 Marstran power transformer i can re use , it would be 560 volts, or a lot more if i use the 220v tap, but i don't think i will unless it's really Worth it.
( this 45/100 will become a 67 black flag, with el34, and another power transformer )

i don't know about these tube doctor transformers , i know "tube amp doctor" brand though, in germany. is it what you're talking about ?

[daveweyer]

It's the German company.
560 volts and 4 EL34 tubes would be best with about 4K ohms P-P impedance, so you'd probably be safest with the Dynaco. 4 EL34 tubes with 450 volts like about 3300 ohms P-P impedance.

[Xplorer]

no, sorry i maybe wasn't so clear about my project. here it is :

the new superlead :

i'll build a whole new Superlead 69 , dedicated to the west coast mod, with 6550. this SL will use my second 45/100 PT. 560 volts.
all NOS parts i have in stock. it's just missing an OT in the list, and some 100/100 uf caps

the second 45/100 :

since i'll remove his PT for the benefit of the west coast SL, i'll transform this one into a 67 black flag with EL34 . i'll need some new transformers for this one, since they aren't accurate if it's supposed to be a 67 black flag clone. the OT will be a spare for i don't know what, but not for an SL, right ?
but i'm still thinking ... if i really want to do it or not ...

the first Superlead 69 clone :

this is the metro build that currently serves as a basis for tests with the west coast mod, at 530 volts.
i'll put this one back to stock, with EL34.

the first 45/100 :

i won't touch this one.

>> in the end i'll have :

a 66 45/100 clone - kt66
a 67 blackflag clone - EL34
a SL 69 metro - EL34
a SL 69 clone - west coast mod - 6550

[daveweyer]

4 KT66 systems also used a 4K ohm P-P impedance. The Black Flag tried both 1.9 K and then 2.2K for more reliability with 4 EL34s. They often had 490v to 500v B+ supplies, and the tube manual recommends about 3.3K ohms with that kind of plate voltage. So Marshall was still loading the tubes pretty hard in 1967, too hard actually, and that led to more changes later on.
Many of the SL amps had quite low plate voltage, which allowed Marshall to use a 1.7K P-P impedance OT, but they still burned up from over-dissipation of the screen grids. The lower P-P impedance OTs were cheaper to make because there was less copper used and less turns to make. This was a business oriented decision that proved a bonanza for the service shops like West Coast Organ and Amp but a disaster for Marshall.
If used hard, it is quite risky to use a 1.7K ohm OT with 4 El34s. That's where the 2.2K screen resistors came from; you could increase reliability by 50% with no other mods. Too bad this often meant that you could get two nights out of your amp instead of just one! We replaced thousands of EL34s.
If you used a Dynaco OT and connected the screens to the screen taps on the OT, the tubes, even EL34s, would last and last.
I seriously doubt that Jimi used ANY unmodified amps. They just didn't last; probably the first technician to work on any of them changed things out to get more reliability. I found tons of different mods in his amps.
Point being that any original Marshall clone is likely not going to be what Jimi used.
Still, for Marshall lovers, all the different models are like lovely girls, you just can't tell from outside what their problems are--but they sure look good and are fun to play with.

[bill bokey]

So as mismatched C1998 would be pretty close to 4k or is it best to use a 45/100 OT ?
I'm pretty sure we've been through this before but which stand up transformer did you use to get 560V on the plates and who currently makes a decent PT for this job ?
Or should we just use a 45/100 PT as well ?

I am currently turning on a 1959SLP into a '69 Super Lead clone and if it doesn't sell too quickly I'll turn it into a West Coast amp. Then if it does I'll build one from scratch anyway.

[shakti]

Still, for Marshall lovers, all the different models are like lovely girls, you just can't tell from outside what their problems are--but they sure look good and are fun to play with.

[neikeel]

all NOS parts i have in stock. it's just missing an OT in the list, and some 100/100 uf caps

I have some 100/100 NOS LCR cans.

[Xplorer]

all NOS parts i have in stock. it's just missing an OT in the list, and some 100/100 uf caps

I have some 100/100 NOS LCR cans.

cool ! good to know
how much are they ? maybe do you need a few mustards and piher ?

Bill, Dave mentioned the dynaco PT as a good one for such project, and the 45/100 is fine too if i'm correct.

a c1998 made by mercury magnetic or marstran should be fine, correct me if i'm wrong.
i saw your current build, nicely done !

but you mention the 45/100 output transformer for a Superlead west coast build ? 4k with this one, is that right ?
i'll never understand these output transformer impedance things .. gotta do my homeworks about it.

[neikeel]

all NOS parts i have in stock. it's just missing an OT in the list, and some 100/100 uf caps

I have some 100/100 NOS LCR cans.

cool ! good to know
how much are they ? maybe do you need a few mustards and piher ?

[daveweyer]

Bill, Xplorer mentioned the Marstran for the PT, and Mercury has one too. The original 45/100 would probably give you 600 v or better on the plates, and in that case getting the Dynaco OT would be a good idea. The early Marshalls used an 8K P-P OT primary impedance, so it came a long way down over the years as they went to lower and lower plate voltage.
You can mismatch a 4K OT with no problems.

[daveweyer]

BIll, doesn't Metro-Amp sell the Heyboer 45/100 transformers? If so, the replacements for the Drake 1202-84 and the Drake 1204-43 should be just right for the mods. According to the original specs, the 1202-84 was supposed to get 560v of B+, even though I used to find them delivering over 600v. Maybe the clones are closer to spec. The C1998 would work mismatched, but would not be the most desirable situation.
The mods are the most fun when you bump the plate voltage on the SL amps.

[Xplorer]

BIll, doesn't Metro-Amp sell the Heyboer 45/100 transformers? If so, the replacements for the Drake 1202-84 and the Drake 1204-43 should be just right for the mods. According to the original specs, the 1202-84 was supposed to get 560v of B+, even though I used to find them delivering over 600v. Maybe the clones are closer to spec. The C1998 would work mismatched, but would not be the most desirable situation.
The mods are the most fun when you bump the plate voltage on the SL amps.

i think that the metro heyboer 45/100 transformer doesn't deliver 560 volts. rather arround 500 v , or correct me if i'm wrong.

on my side i have good marstran clones of the 1202-84 and 1204-43 into my 45/100. so ... will i hack this beautiful amp and put these in a new superlead build ...... it's a tough decision. i don't know if i want to transform it later into a black flag bad enough.