Info on a NATIONAL amp needed

[Valvehead]

I just acquired an old National combo amp. Wondering if anyone can ID it and date it ? Not sure on the model . I would also love a schematic if anyone has it ??

I will post 3 pics of it.

The circuit is :
2 6n7
2 6v6
1 80 rect tube

2 inputs
1 volume/power switch
single 10" national dobro speaker .

[Valvehead]

Well , so far Im finding it might be a Model C , dated 1938 ?

any National experts here on the forums ?

ive clipped in a couple caps--and believe it or not....the thing works ! The speaker is rotted though... also has old Tung Sols in it !

[olddawg]

Looks like a field coil speaker.

[madkatb]

Sounds like you may have already found this:
http://www.valcopages.com/Schematics...al-dobro_c.pdf

[Valvehead]

Heres a weird issue-- i put a new bulb in for the power light. It works, but it goes on and off randomly. I cleaned the socket and all the solder joints re- flowed.

it does it with or without any tubes. almost like some sort of current is causing it ? IVe poked around a wiggled wires--but that didnt change anything. Its a new, generic 6V bulb i got today .

also watched a AC meter with amp on---its off the 6v heater tap---the meter doesnt change when the bulb is going on/off .

[Valvehead]

Sounds like you may have already found this:
http://www.valcopages.com/Schematics...al-dobro_c.pdf

yea buddy --thats the amp !

[olddawg]

Heres a weird issue-- i put a new bulb in for the power light. It works, but it goes on and off randomly. I cleaned the socket and all the solder joints re- flowed.

it does it with or without any tubes. almost like some sort of current is causing it ? IVe poked around a wiggled wires--but that didnt change anything. Its a new, generic 6V bulb i got today .

also watched a AC meter with amp on---its off the 6v heater tap---the meter doesnt change when the bulb is going on/off .

That's why they call them filament lamps. Anyway it could be an intermitent lamp (it happens even new) or more likely just bad contact. Sometimes adding a little solder to the end of the bulb will fix it. Also a little Caig Deoxit in the socket. Btw, if you don't re-cone that speaker you will have to substitute the field coil with a choke to use a modern speaker with that amp.

[Valvehead]

wow- i didnt know that . so the choke would go between the power tubes and OT ?

what kind / size of choke would i need---mH , etc...

please explain the field coil speaker and why i have to add a choke with a reg speaker --- Ive never seen one of these !! Interesting !


also-- i did clip onto the OT and ran that to a 4x12 cab....the amp worked fine.

[olddawg]

wow- i didnt know that . so the choke would go between the power tubes and OT ?

what kind / size of choke would i need---mH , etc...

please explain the field coil speaker and why i have to add a choke with a reg speaker --- Ive never seen one of these !! Interesting ! i did clip onto the OT and ran that to a 4x12 cab....the amp worked fine.

That's because you just paralleled the cab to the existing speaker. The existing speaker is a field coil speaker. It doesnt have a permanent magnet. It has an electromagnet on the B+ supply. If you look at the schematic it shows the field coil. That is in the speaker. If you take the existing speakerout and replace it with a modern one, you usually have to substitute the field coil with a choke or a resistor. I have no idea what the value should be, bit the guys that repair old organs on this board will.

[Valvehead]

That's because you just paralleled the cab to the existing speaker. The existing speaker is a field coil speaker. It doesnt have a permanent magnet. It has an electromagnet on the B+ supply. If you look at the schematic it shows the field coil. That is in the speaker. If you take the existing speakerout and replace it with a modern one, you usually have to substitute the field coil with a choke or a resistor. I have no idea what the value should be, bit the guys that repair old organs on this board will.

ok , id rather use a resistor --if anyone can come up with a value, and where exactly it would go