1940 national field coil ?

[fred]

My first post . I have a old national 1940 with no guts or wiring in the chassis. I tested and rebuilt the field coil speaker , new cone . I'm having a problem finding a wiring chart for this old girl . Could I wire up the chassis as a fender deluxe 5e3 amp and use the field coil speaker on the new wired up amp . Thanks for any ideas for this . Fred.

[sluckey]

Sure. But you need to know more about the FC. But in this case I have to ask why? You have a gutless chassis and a FC speaker and no schematic. Sounds like curb stuff to me.

[fred]

Well she came with a 1939 Gibson L4 + a ga 20 in a deal about 20 years ago . It's been setting there with my other stuff looking for some fixing . The cab is in nice shape . The amp has all  the tube sockets , just missing the pt , wiring , I reconed the FC  . It tested 2.7 ohm vc , 500 ohm  coil  and bucker coil is good . I have all the tubes for it . I found a wiring diagram but it's to hard to read . I'm not familiar with the old type 6J7 , 6N7stuff . I like the 60s and 70s fender stuff. This is just a Winter project .
So can i hook up a fc to a 60s style deluxe 5e3 . I guess it just would work as the choke . Whats your thoughts .
Thanks Fred

[sluckey]

Give it a try. You will probably need to have the plate current flowing through the field coil in order to get enough magnetism to operate a speaker. IOW, connect the field coil between the rectifier and the first filter cap (plate node).

[PRR]

> I found a wiring diagram but it's to hard to read

Let us see. Even squinty it may hold clues.

[bmccowan]

In looking for a schematic, are you searching only for "National?" There are many schematics out there for old Valcos. The schematics may be identified as Valco, National, Supro, Gretsch, Ohahu, Airline - lots of crossover although I don't believe all those brands date back to 1940. I have repaired/rebuilt a few old field coil Valcos, And have spent time searching out schematics. I may be able to help you find a proper schematic. Number of sockets, tubes you have, and size of the speaker, number of potentiometer holes, are all good clues. Although nostalgia ain't what it used to be, I think its worth rebuilding that with a Valco circuit.
Based on your info so far - this is a likely candidate:

[66Strat]

In looking for a schematic, are you searching only for "National?" There are many schematics out there for old Valcos. The schematics may be identified as Valco, National, Supro, Gretsch, Ohahu, Airline - lots of crossover although I don't believe all those brands date back to 1940. I have repaired/rebuilt a few old field coil Valcos, And have spent time searching out schematics. I may be able to help you find a proper schematic. Number of sockets, tubes you have, and size of the speaker, number of potentiometer holes, are all good clues. Although nostalgia ain't what it used to be, I think its worth rebuilding that with a Valco circuit.
Based on your info so far - this is a likely candidate:

Thank you for posting the schematic. IMO, there is a lot to like about this schematic, regardless of the speaker. From a personal opinion, I don't like high B+ voltages outside the chassis. With this schematic, there is likely 300 to 350 VDC on the wires that excite the speaker field coil. My question is; what is the benefit of a field coil speaker over an alnico or ferric speaker in this circuit? IMO, an appropriate alnico speaker combined with a suitable choke in the power supply can achieve similar (period correct) sonic results as achieved with a field coil speaker.

[fred]

Hi , thanks I believe that is the wiring diagram for this chassis. I have 5 tube sockets  6j7 ,6n7 , 2 - 6v6 , rec 5y3 . I guess my question is should I use the 6j7 ,  6n7 . Or go with 2-12ax7 preamp section to the power section . I don't think it would effect the power to the field coil 500ohm . I would 1 wire to the first cap off the rectifier and the other to the center of the ot back to second cap.
I don't think I understand the older type preamp section. The plate, grids + heaters are way different than the 12ax7 type .
I'm not sure about the resistors value ? Is m in the millons ohms . It's a project thanks to all.

All my other amps are alnico or ceramic.
That's y i what to try this.

[sluckey]

For that time period...

M = kilo = thousands when associated with resistors.

M = micro  = millionth when associated with caps.

[fred]

Thank you , it was confusing looking at it the first time.
You just made it all make sense .
I've been reading Doug's pages for years . And you always help people to understand .

[PRR]

> what is the benefit of a field coil speaker over an alnico or ferric speaker

#2 If you must use a field coil, you can make it help filter the B+.

#1 Alnico was not much available until WWII, so post-WWII for commercial use, and then delayed while old and surplus products were sold-off. The principles of Alnico go back to Tokushichi Mishima 1931, and G.E. demoed a disk in 1935, this was all special fancy chemistry, while a field coil is plain iron and copper. Radar magnetrons needed massive Alnico magnets for very steady field. Surplus maggie magnets were sold for fishing lost iron from lakes. Lansing was one of the first to design speakers around Alnico. Most other speaker makers waited until the end of the 1940s. Bottom-barrel amp makers were getting good deals on "old fashioned" field coil speakers. Ferrite comes like 1960s.
https://www.researchgate.net/publication/304676716_MAGNETS_HISTORY_THE_CURRENT_STATE_AND_THE_FUTURE

[PRR]

> I don't think I understand the older type preamp section. The plate, grids + heaters are way different than the 12ax7 type .

6N7 is a low-gain 12AX7 with both cathodes tied together and all the pins switched around. That part should be clear.

6J7 is a Pentode, so has an extra grid to bias. It's one to a bottle. Look at EF86 preamps.

[bmccowan]

6J7 is a Pentode, so has an extra grid to bias. It's one to a bottle. Look at EF86 preamps.

And further, the 6J7 has a top cap for a grid connection.  The 6SJ7 is the same/similar tube with all connections on the socket. You could use that tube since you would be rewiring the circuit. To see how the 6SJ7 is typically wired you may want to look at the Champ 5C1 schematic and layout, as well as the tube data sheets.

what is the benefit of a field coil speaker

In Fred's case a benefit would be that he reconed the speaker. Some say there is a magical sound to FC speakers, but I can't hear the magic, and usually replace with a choke and a PM speaker.
As far as rebuilding the amp all octal or converting to 12V noval 9 pin tubes. This can be argued either way. I would rebuild it all octal, reasons:

Its cool to have something period correct.
I like the sound of octal tubes - especially with a pentode in V1. I like the sound of 12v noval preamp tubes too - they are just different.
You already have the right size socket holes
Octals are easier to wire (my eyes are old and I have some arthritis)
When you are done and like the amp, you will likely have the bug and you can buy all the parts and build yourself a modern amp too!
And Damn PRR, you know a lot about a lot of stuff!

[fred]

Thanks for all the lnput . I've worked on my own amps for40 some odd years , and a few friends amps , always  fender 60 s , 70s . Ampeg  j 12 , Gibson ga 20 from 1956 that girl sings all original but a few electrolytes . I'm ahorader when it comes to amps and guitars . Have my first 66 fender mustang with a 66 fender vibrolux reverb purchased about 1969. I quit playing out I can stay up that late , getting home at 3:00 am , that for the young .back to my project , after all your input I have ordered my pt 325 -o- 325 V , 70ma that should be enough for the tubes. 250 ohm 5watt bias resistor , 1meg vol pot . And some 22 mf 500vdc ele caps. I had tone caps ,resistors , tubes .
Now that I have some understanding of the old preamp tube section I will start building when I get the parts .
Give me a month and I'll be back to tell my tail of smoke
I'll use my varic + my light bulb , glowing bright hope not
Thanks

[fred]

Thanks to all . your info helped me bring this old national back  to life . bmccowan I used the valco diagram you suggested with the field coil it sounds real nice . I can't say better than any other pm speaker . this was my first complete build and I really enjoyed it . I did have many times I had to do some reading , thinking , reading and more thinking .
Thanks to all you smart brains out there .