Fairchild 670 DIY In Progress

[Kingston]

I can't recall if anyone has an actual working "670" with 6K4P? They're cheap, but I also heard that they're prone to failure.

Many builders even here on the forum have done it, but it's littered inside various threads and I could not find the posts now. Never heard of failures. In fact yours is the first comment on failure prone tubes I ever saw... 6K4P-EV is the military part which is  physically bigger and has stronger internal supports to protect from impacts and shakes. Like inside a missile you know.

 

[CJ]

ok i build a reveb tank, everybody says great but what about the 670?

i build a 45 degree rack and evrybody says great now let's see the 670,

ok so i better get back on the 670,

some guy in a uniform dropped this off today so now we have more toys to play with,



please do not steal the part number to look for a hidden hs-52 hiding for cheap under an alias,

i wanna do a search first, wtf, over? 

 

[jackies]

Please CJ, a few words on the quality of the existing schematics, like "Fairchild 670 Limiter.pdf" and such - incorrect values, maybe typos etc?
Any errors spotted?
:

 

[CJ]

the only way to really sort out the diagram is to get a real 670,

i am willing to bet that there might be additional parts or changed values on the real units compared to the schematic,

for example there is the RC network on the plates of the control amp that were added to suppress oscillation, it is penciled in, so i wonder if something did not get penciled in, there seems to be too may resistors on those rear turrets, unless they ran wires all the way over there from the pwr supply or something,

you know how Fender has the schemtic And the Layout?

it would be cool to have the Layout drawing of this beast,
but that would mean renting a 670 and where would you do that unless you live in a city of over 5 million people like LA, SF or New Jersey?

i bought that Triad and UTC iron from Doc Hoyer over at Audio Transformers

Doc knows everybody still alive from the golden age of audio, so he talks to George Alexandervich all the time,

i asked him to ask George about the crazy choke  in the negative power supply, chokes are not supposed to have that big a drop across them, looks kind of kludgy to me, too much current for that choke to have any inductance left due to polarizing DC, even with a gap,

think of using this un-used 6.3 @ 1.5 amps on the big chi town pwr trans as the neg supply, voltage doubler would give 12.6 which when rectumfried would be more than enuff current to feed the 100 ohm dual pot, this would save a transformer, and it might save a little hummmm since both B+ and neg supplyu would be coming off the same core in  the same can instead of having an open frame transformer located a ground loop away from the main supply,

knocked down by a cold for a few days but back at it full time now,

there is a schematic on the Drip site that i believe is up to date, the Sowter site still contains an error with the 100 K voltage divider resistor which s/b 17 k.

and Kubi needs some editing done on some of his 670 stuff,

circuit notes:  having 6 volts across  50 ohms (equiv R for both 100 ohm pots as wired) gives you 120 ma of current thru the balance pot, much more than the total plate current for all the 6386, around 4 or 5 times as much, why?

because as you adjust the cathode voltage with the 100 ohm dual balance pot, plate current changes, so you have an undesirable interaction of plate current to the balance pot,

by keeping the balance pot at a stiff 120 ma, the variations in plate current do not move the cathode voltage around much at that node of the pot/cathode resistor,



cj

 

[drask]

there is a schematic on the Drip site that i believe is up to date, the Sowter site still contains an error with the 100 K voltage divider resistor which s/b 17 k.

Hi Cj,

Do you have this schematics? I don't find it on the drip website? I asked in the past after it, but I received any answer.

Thank's you.

 

[bockaudio]

the only way to really sort out the diagram is to get a real 670,
you know how Fender has the schemtic And the Layout?

that was cool stuff.

but that would mean renting a 670 and where would you do that unless you live in a city of over 5 million people like LA, SF or New Jersey?

the streets are littered with them here.

i bought that Triad and UTC iron from Doc Hoyer over at Audio Transformers

CV also, right?

Doc knows everybody still alive from the golden age of audio, so he talks to George Alexandervich all the time,

i asked him to ask George about the crazy choke  in the negative power supply, chokes are not supposed to have that big a drop across them, looks kind of kludgy to me, too much current for that choke to have any inductance left due to polarizing DC, even with a gap,

think of using this un-used 6.3 @ 1.5 amps on the big chi town pwr trans as the neg supply, voltage doubler would give 12.6 which when rectumfried would be more than enuff current to feed the 100 ohm dual pot, this would save a transformer, and it might save a little hummmm since both B+ and neg supplyu would be coming off the same core in  the same can instead of having an open frame transformer located a ground loop away from the main supply,
circuit notes:  having 6 volts across  50 ohms (equiv R for both 100 ohm pots as wired) gives you 120 ma of current thru the balance pot, much more than the total plate current for all the 6386, around 4 or 5 times as much, why?
because as you adjust the cathode voltage with the 100 ohm dual balance pot, plate current changes, so you have an undesirable interaction of plate current to the balance pot,
by keeping the balance pot at a stiff 120 ma, the variations in plate current do not move the cathode voltage around much at that node of the pot/cathode resistor,
cj

Is that what you measure?

 

[CJ]

check out the Documentation section on the drip site, then go to Fairchild 670,

looks like it is broken into chunks but i thought i saw a complete schemo here,


lools like there  resistor is a 24 k resistor  missing on the orig schematic, look in the 660 section
it is attched to the threshold pot,

cj

 

[desol]

Hey Chris...just wanted to say that i got a chance to read the interview you did with Rein Narma the other night. Awesome read. Such a deep history behind so much of this equipment....wow.

I'm sure if anyone could've made the 670 sound the same with modern components, it would've had to have been him that tackled it. Project looks like it's coming together really nice btw!

 

[CJ]

Thanks! 

Alexandervich said that Rein was into the overkill thing when he designed the 660,

but i think he just was not happy with the specs on the Gates, Collins, RCA stuff,

thus the 3 millon volts control voltge and all that,

 

[desol]

Overkill seems to have worked.  ;D

 

[bockaudio]

Overkill seems to have worked.  ;D

Failure is ugly in tube compression.
CJ you got the CV xfmr from Hoyer as well?

 

[CJ]

i am using the sowters, i sprayed them black,

they are actually better than the original i believe, he probably reduced the phase shift on the tert windings as to reduce pos feedback at certain frequencies which was supposedly a problem with the early units,

 

[radardoug]

CJ what's happening on this build? We are all dying to see what happens when you fire it up.

 

[CJ]

we are all done with side projects and are ready to turn this bitch out, lets get the hs 52 and a 26 wired up before the next earthquake,

 

[radiance]

Aaand, we're back!

 

[CJ]

ok the reason for the stall out has been the 6BA6 thing,

why not try it first befor dropping 1000 bucks on some 6386 remakes?

if it does not work like the original, then we can drop 1000 bucks on the remakes,

sure, i have to rewire some sockets, but the savings is well worth it, plus, if this works good, it will pave the way for more diy fairchilds, right?

plus i have some cool mil versions of the 6ba6 which might mean a good match up between tubes,

so i will rewire for the t bar.

where is the t bar schematic?  ???

 

[alexc]

There's this one ..

 

[CJ]

ok, these guys have been burnin a hole in the tube caddy,

somebody even sorted some mil versions out,

the warranty card says 1942,

do i need 8 per channel? 

 

[DaveP]

CJ,
You need double the number of 6386 you were going to use, but you could get by with 6, just slightly less output and a bit more noise.  Even with Mil spec tubes, I would check each one first (run a graph) and match them up, easier in the long run than trying them in 6x6=36 variations
best
DaveP

 

[CJ]

ref. stuff

6386 data sheet

http://www.mif.pg.gda.pl/homepages/frank/sheets/142/6/6386.pdf


plate r-4250 ohms triode
4000 micro larrys 
9.6 ma plate I.

5749/6BA6

http://www.mif.pg.gda.pl/homepages/frank/sheets/087/5/5749.pdf

plate r- 1 meg-pentode  triode?
4400 micro moes 
11 ma plate I