Federal Build: From Octals to Miniatures

[schmidlin]

So I'm wrapping up my second Federal clone, and the first one was a real success!  So they ask for a two-channel unit.  I don't see how to do this other than going from octal to 9/7-pin miniatures and fit it into one rack unit.  http://www.montagar.com/~patj/am-864u-sch.gif for reference.

I couldn't find a reference online that translated octals to the miniatures: seems someone could have used something like that at one time.

Here is my guesstimate:
6SK7 --> 6BA6
6SN7 --> 6AU6
6SQ7 --> 6AV6

Please tell me if I'm off my rocker here, please.

All are pretty close by my eyes, but maybe I am missing something critical, or just plain obvious.  I think I'll keep the 5Y3 as is, maybe go SS.

Thanks in advance!

 

[lassoharp]

Addressing the space issue - have you given any thought of powering two units with a single  outboard supply? 

Also, 7/9-pins are smaller than octals but not by a terrible lot. If you study any of the consolette preamp modules from RCA, Langevin and Altec you see stuff jammed together in super tight proximity on small narrow chassis.

Don't know what your layout style is aiming for but using arrangements as seen on the old modules could net you some economy and maybe solve the problem without switching tube types.

Post some pics if you can.  Never get tired of seeing a good homemade tube limiter.

 

[schmidlin]

Surely:

I moved the 2 rear pots to the front as part of the "updated" design.  I also potted the IT & OT, mostly to add a coolness factor.  Screened face.  I lent it to a pal whom I consider to be the best audio engineer in the city and warned it was best for bass and vocals.  "So what are you using it on?"  "Everything."  Expanded the bandwidth to 20-20K and viola!  A fun build for sure.  Oh, and DC heaters: quiet as a church mouse.

I hear you on the outboard PS, but after dealing with both the Ampex 350 & 351, all-in-one is a dream!  Besides, I like to get PRR scratching his head....

 

[emrr]

Stay octal and do whatever you have to do to the layout.    There's tons of room in there.  

Seen my Langevin rack?  Four 10W power amps.  16 octals in about the same space.  
Stay octal and do whatever you have to do to the layout.    There's tons of room in there. 

 

[pH]

Wow, that looks great! I like how you kept mostly to the original layout. For a true clone you just need to coat the whole guts in amber shellac  ;D

If I were you I'd stick to the octals. Some of those minis can be tough with noise.

You're making me want to build one, as I'm still kicking myself for selling my real one. What did you use for iron?

 

[lassoharp]

Wow!  That thing looks awesome - very nice work!    It nods nicely to the 30's.

You gotta tell me where you got that chassis - custom?  I've been looking for one that style.

Engraving on the dials?

How did you go about potting the transformers?  Hard to beat chrome -Looks like old RCA mystery iron.

This thing doesn't want to be shrunk - tell 'em to get a bigger rack.

I do think you could get two (octal) channels on that chassis - may need to use potted PT(s) and go an inch or so deeper front to back.

Congratulations on a first rate build !

 

[mad.ax]

"6SN7 --> 6AU6"

You'd probably want to check that again... 6SN7 is a dual medium mu triode, while 6AU6 is a sharp cutoff pentode.

Axel

 

[Gordy]

6SN7 --> 6CG7

 

[Piedwagtail]

6sn7 wouldn't hurt being 12au7 or 12bh7 or 5687 with a few changes of opt/bias/loadline.

Immaculate build there!

 

[schmidlin]

You guys are spot-on about the tube, I was thinking 6SN7-->12AU7.

And you've convinced me about the octals: I do like them better.  But now I'm contemplating just making 2 regular units: it would look much cooler stacked in a rack, I think.

Thank you for your kind words, I'll see if I can address your Qs.

I started will all Edcor iron, hoping the grounded potting shield would keep it quiet.  But the IT was causing LOTS of noise so I swapped it for a UTC A-11, and that did the trick. (boy those are getting pricey!)  Would prefer an A-10 or A-12.

And if you have LOADS of time:
Go to Home Emporium or some other interior store, find the gifty-foodie aisle.  There they have these SS spice cans from Spice Appeal that are perfect.  I had to cut off the lip with a wet tile saw.  Now I ran the leads out, dipped it in wax twice, supended it in the can propped with one of those 3/8" hollow lamp screws.  Then poured in a 2-part polymer resin and they're done (didn't do the UTC, tho).  Still wondering if potting screwed up the Edcor IT. 

Standard Hammond chassis, stock textured black.  No engraving on the dials: 7" face panel from Nebraska Surplus painted and silkscreened.

And to push it over the top, I re-created the original artwork on the dial, hit print and stuck it on the inside panel: you really can't tell other than the bezel cover isn't quite the same shape.

 

[lassoharp]

Thanks Schmidlin

Nice resourcefullness with the spice cans.

One last question on the chassis - Where did the side support 'L'- brackets come from?  I've never seen them at Hammond. They're kind of the key to making that whole arrangement work.


Cheers

 

[stitch-o]

Beautiful box!

Whats the word on the 20-20k bandwidth mod?

Cheers!

 

[emrr]

Whats the word on the 20-20k bandwidth mod?

use full bandwidth transformers; simple as that.

 

[emrr]

But now I'm contemplating just making 2 regular units: it would look much cooler stacked in a rack, I think.

Assess your real estate.  Sounds cool until you get so much big footprint stuff that you can't rack it all.  Doesn't take long.  I have (7) six foot racks, and there's stuff I can't rack and use.  

 

[schmidlin]

Hi lasso, I have a local metal fab guy that bends up chassis, etc: threw those on with an order.

And stitch, it may be 30-20K.  I assumed the trannies were the narrowing factor and I used wider spectrum than the original spec, which is 300-5K IIRC, which is why some claim these give an old-timey sound.

Thanks!

One more thing: I (H) padded the crap out of the input and output, around 15db on one side and 20db on the other, as suggested by a very fine gentleman on this very board.  Stock they are real hot.

Duly noted on the rack space emmr, guess I should speak with the potential customer...

 

[gemini86]

Tubes don't care how you mount em. You could fairly easily fit two in there if you think three dimensionaly.

 

[syn]

Hey Schmidlin
                      not much help from me on this one, but I had to say it is beautiful build.

 

[emrr]

Tubes don't care how you mount em. You could fairly easily fit two in there if you think three dimensionaly.

Some rectifiers do care. 

 

[Joechris]

Thanks Schmidlin, lots of inspiration. What meter did you use? Is it a 1ma, or smaler?
20 db att on the output sounds like a lot, but if it works...

joe

 

[gemini86]

Tubes don't care how you mount em. You could fairly easily fit two in there if you think three dimensionaly.

Some rectifiers do care. 

I forgot about that. Thanks for the correction!