Finally Finished 2x6550 Head w/Tube FX Loop

[David Root]

After many iterations, here it is. This is in a Gromet headbox, with one of Allyn Meyer's heavy duty aluminum chassis.

The turretboard is a turretboards.com large HiWatt board.

The PT and OT are both custom toroids from Trafomatic.

6550s are old Sylvanias out of a mid-'70s SVT. Still good! You can't beat 3 getters.

The FX loop is right out of TUT 1, the Best All-Tube FX Loop.

This is the most complex amp I have built and I won't do it again, should be largely done with PCBs. Sounds good though, but due to the LC tonestack it doesn't sound instantly recognizable to me, which is probably a good thing.

It's 50W the same way an old 50W Marshall is 50W, i.e. it ain't, it's 38W.

[mhuss]

Looks nice. Clips!

--mark

[David Root]

Have to go on the road tomorrow thru Dec 21. I'll have time over Christmas.

I also will have to haul the chassis out of it and do a jpeg of the interior. Then you'll see why I don't really want to do another one!

[drhulsey]

Then you'll see why I don't really want to do another one!

I can't wait It sure looks great on the outside
Have a safe trip

[RJ Guitars]

Nice Work.... love to see those interior shots and hear the clips... thanks

[David Root]

While I'm out of town I can add a few things from my odyssey with this amp that may be helpful to others.

Originally I started out with a 5k6 OT, but I changed it to 3k4 because I didn't care for the 5k6 tone that much and it wasn't putting out the watts either. When it came in it was 1/2 inch bigger diameter than the 5k6, which gave me a problem with the V6 power tube. I could no longer use reissue Tungsols because the bigger OT physically interfered with its coke bottle belly. (Remember these are custom ordered transformers from Trafomatic in Europe). I never thought to ask and they never told me.

So I hunted thru my bag o'6550s and found four old used "RCA" pieces with straight glass only 1 1/2" dia., which fit V6 if I shaved down the phenolic on the base a little. Turns out these are Sylvanias labeled RCA which sound pretty good. Unfortunately when these 4 wear out I'm looking at NOS $150/pr because I can't use anything else.

Moral is don't skimp on clearances I guess!

I also changed the PT to a 400-0 VAC sec. and fed it to an IXYS bridge rectifier module (soft hexfreds) for 565VDC out. Because of the bias point drawing 62 mA cathode current each 6550 plus 25 mA draw from the others I get 490V plate at idle, and about 420 on the screens with a 2k7 resistor right after the in-line 12H choke. 380V on the PI, 335V on the FX loop, 345 to 300 on the preamps.

I have a 12H choke in line in the PS and a 15H choke directly in the screen feed line. Very clean screen supply. All the PS caps are Black Gates or Cerafines. I have 75 uF/700V on the plates and 47/500V on the screens. The rest have 22uF/350V BGs in series at each node except the FX 12AT7, which only needs a single. It looks a bit messy but it is a good PS. The ripple on V1 supply is 5 mV on 300VDC, yet I have only 5.5uF on each triode.

Max power measures 38W RMS. All preamp stages have their own pot, so it is very flexible. It also responds very well to rolling back the guitar volume pot, cleans up very well.

Interestingly, Zout is about double Rl, which I guess accounts for the Po being considerably less than predicted graphically. Zout =Rl is necessary for maximum power transfer. I need to pay more attention to this kind of thing.

The PI is the standard Marshall/Soldano/5F6A values w/82 and 91k plate resistors. Balance is about 5% or so in both AC and DC.

12.6VDC regulated heaters on all the small tubes, and capacitor coupled bias supply, like old Ampegs, because the rectifier is a full wave bridge. Independent bias supply thru the post-PI MV with test jacks and adjustment pots. This allows me to use heavily mismatched pairs like the two remaining Sylvanias I have.

Double pot post-PI MV, with a conventional pre-PI MV also. Most pots are 2W RV4 PEC, same as Randall Aiken uses. The best. The Mexican Clarostats are close and slightly cheaper, I have a couple 1M Clarostat pots in stock that I'm using up that are in this amp.

Chassis is Allyn's very excellent heavy duty 1/8" aluminum, you can't beat it with two sticks.

Galactic star grounding with a single chassis connection at the preamp end of the chassis, per drz400. Very important, should be further up the list!

You have to check your wiring very carefully as you go, this is pretty complex to do on a circuit board, and much of the FX loop is hanging off pots and switches.

Tonestack is Steve Bench's LC design. This is quite different than Marshall or Fender stacks as it is dead flat with the pots at noon. Mids cut/boost is switched at 450Hz or 250Hz for more variety. It doesn't sound like a Soldano as I didn't cut the bass as much, nor a Marshall, although it will get closer with full mid cut. With the tonestack flat it sounds a bit like a deranged Fender Super on the edge of tweed.

One negative of this tonestack, it really sucks signal even fed from a cathode follower, and if I use it again it would only be with a recovery stage into the PI.

The FX loop is completely bypassable when not in use. I think this is better than putting the signal thru it, even though it is designed to be neutral, when it's bypassed you know it's not in the tone.

[skyboltone]

I also changed the PT to a 400-0 VAC sec. and fed it to an IXYS bridge rectifier module (soft hexfreds) for 565VDC out.

Hi David. I'm using the Fairchild Stealth 8 amp soft diodes for the full wave rectifier in my two hole Liverpool. It's my first experience with these so I really can't say much except the amp went together perfectly with no more than 10 minutes of "What the Heck" time on first powerup. The only thing I notice is that pick attack seems maybe just a touch less clicky/harshy than other EL-84 builds I made with 1N4007s. The sustain is literally forever and the slide into harmonics is so smooth and sweet it defies description. I'm too lazy to do an a/b test with 4007s on this build but maybe down the road? I plan on making a few more of these 18 watt monsters.

Looking forward to gutshots and clips of your new masterpiece.

Dan

[David Root]

I have used the Fairchild Stealth 4A diode in this amp as the bias supply rectifier. I figure anything that goes on the grid should be as clean as possible. I also used 3 x 10 uF caps. Sure they cost $3 instead of $.03 but what the hell, I don't do mass market.

I guess if I were doing a Hi Fi monobloc I'd try to find a 60V regulator chip too.

They also make a double 8A Stealth in the same TO 220 case, which I used on my previous amp, a brown Vibrolux recreation w/reverb, for the HT supply, conventional fullwave w/grounded CT.

[Tubetwang]

Nice Job David...

Looks like lots of work:shock: ...you should be proud of yourself...!

I'll second Allyn's chassis as being the best...

Are those red resistors PRP (Precision Resistive Products) on the Hiwattish/Coyote board ?

Louis

[David Root]

Yes Tubetwang, those are PRP resistors, regular grade. I have bought a few of the PR9372 audio grade to try out too. And that is the large HiWatt board from turretboards.com

[Tubetwang]

Thanks David.
Louis

[David Root]

Finally, my wife got me a new digital camera for Christmas, so here's apic of the guts of it. This is why I won't do another one.

[VacuumVoodoo]

Finally, my wife got me a new digital camera for Christmas, so here's apic of the guts of it. This is why I won't do another one.

Why not? Now that you've built first one the second one will only take half the time

[David Root]

Actually about 20% of the time this one took!

I changed both the PT and OT, amongst other things. This PT has 400 mA capability in the HT winding, and is a 400-0 secondary (FW bridge rectifier). Overkill, but it don't break a sweat either when abused!

[skyboltone]

Well, it's a beauty anyway, even if it did drive you nuts.