My Next Project: Insanity

glpg80

glpg80

Well-known member
Next year I plan to build a Marshall style amp with 6x KT120’s using Mercury Magnetics aftermarket Marshall Major Power Transformer and a custom OT. I’ll use a stand-alone unit for DC heaters since each KT120 is almost 2A/tube.

Let me know what features you think it should have? I’m thinking a steel chassis from somewhere, maybe valvestorm. I wouldn’t trust aluminum with the weight of the iron.
 
I'm no tech guy, so I want go anywhere near there lol, but fwiw I have a Schroeder Dozer 200 that came stock with 4xKT120 (also have an Alessandro Red Bone with KT120's & another with EL34's (prefer that one)) and I much prefer it's tone with 6550's, KT90's or even KT88's (it can take all those tubes). The 120's IME are more bland/sterile tonally. They just have imo more efficiency/headroom in sound that I hear as advantages (but not like a night & day difference). Just my 2 cents. I heard KT150's sound even worse in that regard, but never tried any myself

Also, while it may be different since those are Mercury's rather than original Major tranny's, I was pretty disappointed with the '68 Major 200 I had and 2 others that I tried, so I'd be a little skeptical on that. They didn't have the huge sound I was expecting like say the Hiwatt's (not even close)
 
braintheory said:
I'm no tech guy, so I want go anywhere near there lol, but fwiw I have a Schroeder Dozer 200 that came stock with 4xKT120 (also have an Alessandro Red Bone with KT120's & another with EL34's (prefer that one)) and I much prefer it's tone with 6550's, KT90's or even KT88's (it can take all those tubes). The 120's IME are more bland/sterile tonally. They just have imo more efficiency/headroom in sound that I hear as advantages. Just my 2 cents

Also, while it may be different since those are Mercury's rather than original Major tranny's, I was pretty disappointed with the '68 Major I had and 2 others that I tried, so I'd be a little skeptical on that. They didn't have the huge sound I was expecting like say the Hiwatt's (not even close)
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I could do a design for 6x 6550s either way although it will change the OT I’d planned to use. I appreciate the input too.

Done correctly it adds a big sound that only many power tubes in parallel can offer.

I might design a new master volume circuit.

I also want to drive these 6550s with an EL84 instead of a 12ax7.
 
glpg80 said:
I could do a design for 6x 6550s either way although it will change the OT I’d planned to use. I appreciate the input too.

Done correctly it adds a big sound that only many power tubes in parallel can offer.

I might design a new master volume circuit.

I also want to drive these 6550s with an EL84 instead of a 12ax7.
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Yeah just thought worth noting. Of course, some amps like other tubes more than others, so I can't predict that. They do sound big for sure, but I didn't find my 88's or 6550's in the Schroeder to sound much smaller and to me the trade off for tone was worth it. Maybe in a more modern amp aimed primarily at oomph or punch (like my Megalith Beta, Wizard or Fryette) I'd possibly like 120's more, hard to say

That sound's very interesting with 84's. Big fan of those. I've also wondered about the sound differences with using pre's like EF86's or octal tubes. Some of my amps use only those in the pre and they're all EL84 amps
 
What's this thing going to be 360watts? Also can't those tubes take ridiculously high PV? I don't know if it's possible but a mono and stereo power section would be way cool.
 
Chubbs said:
What's this thing going to be 360watts? Also can't those tubes take ridiculously high PV? I don't know if it's possible but a mono and stereo power section would be way cool.
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The PT I plan to use is 247-0-247 which is 700VDC rated at 1A. I’ll still bias the 6550s at 60% but that’s still 210W.
 
braintheory said:
Yeah just thought worth noting. Of course, some amps like other tubes more than others, so I can't predict that. They do sound big for sure, but I didn't find my 88's or 6550's in the Schroeder to sound much smaller and to me the trade off for tone was worth it. Maybe in a more modern amp aimed primarily at oomph or punch (like my Megalith Beta, Wizard or Fryette) I'd possibly like 120's more, hard to say

That sound's very interesting with 84's. Big fan of those. I've also wondered about the sound differences with using pre's like EF86's or octal tubes. Some of my amps use only those in the pre and they're all EL84 amps
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Cameron designed an amp that goosed the front of a Marshall with a pentode type of smaller tube which I think was the EF86. Someone here played it and tried to buy it off of him on the spot and he said no lol.

Yeah I’ve always wanted to use an EL84 for this purpose. I believe it to be more efficient.

I’m wondering what kind of preamp? SLO?
 
glpg80 said:
Cameron designed an amp that goosed the front of a Marshall with a pentode type of smaller tube which I think was the EF86. Someone here played it and tried to buy it off of him on the spot and he said no lol.

Yeah I’ve always wanted to use an EL84 for this purpose. I believe it to be more efficient.

I’m wondering what kind of preamp? SLO?
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What kind of preamp for your build you meant?

My only experience with EF86’s has been in Vox type amps, so would be very curious to hear how they’d translate in other amp types. I feel like by hearing them in only Vox’s I don’t really know yet what their character/flavor is

The 12BZ7’s also sound more efficient than 12AX7’s, but more hollow sound. Seems to usually be a trade off with efficiency vs tone
 
braintheory said:
What kind of preamp for your build you meant?

My only experience with EF86’s has been in Vox type amps, so would be very curious to hear how they’d translate in other amp types. I feel like by hearing them in only Vox’s I don’t really know yet what their character/flavor is
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I looked into designing a clean boost using one and didn’t like the sound they imprint on high gain. It’s too sweet and narrowband. A lot of overhead in design for not a lot of benefit but that’s my opinion.

Yeah for my build. My thinking is that because it’s 210W of power I could get away with some stupid aggressive diode clipping lol
 
This is indeed an insane project...following
Will be interested in seeing what you do for DC heaters on power tubes
Hasn't been done on but a few amps. Peavey Valve King is one I know of.
 
T Wilcox said:
This is indeed an insane project...following
Will be interested in seeing what you do for DC heaters on power tubes
Hasn't been done on but a few amps. Peavey Valve King is one I know of.
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Something similar to this that I designed in my 74. DC on all tubes with floating heaters on the power tubes. It also supplies my relays and footswitch LEDs. It will just need to be beefier and maybe the same one dedicated to power and the PT running the preamp on AC only.

600CA84A-8D28-4433-B5F4-9C6800648CCD.jpeg
 
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I’ll have to go dig up my equations but I’m certain I’ll have to increase the filtering to supply transient currents to a sextet of 6550s running at around 650V. I may also have to go two banks of 3 in series to get the proper voltage headroom of 1500V for 2x B+. That’s part of the reason I want a steel chassis - lots of Swiss cheese will have to happen.

No suggestions for a pregain? I’m thinking tube buffered effects loop and 5 stages of gain. Maybe something plate fed for a change.
 
You could make it a Wizard crusher with a Wizard pre. They are already known for being solid/stout. Or you could do what it seems nobody does, put a IIC+ pre with GEQ.:whistle:

What would the end-to-end length be for 6x properly spaced KT88? An over-sized SLO style chassis would be great for this. With the transformers on opposite ends, power tubes in back, preamp tubes in front right over the pots.
 
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