1935? National Dobro 6107A? Update!!!

[plexi50]

This arrive yesterday and it is quite a bit different than what i am used to working on. Mainly the pin out on the tubes is something new to get used to with very little info on this Model??? I want to call it a National Model 6107A. It seems some one has already tried to modify this adding an additional #79 tube at the rear of the chassis. It powers up and you can hear the speaker producing a low noise like when you have an input cable plugged in without a guitar. The volume knob has no effect on raising the noise level. Their appears to be an additional knob for possibly Gain? Any way i am checking the input grid of the input jacks and tube wiring. I have recapped the main power supply already but the rest came this way. I have the Model 6107A schematic. At least it gives me the layout before it was tampered with. My tube testers will not give info on these tubes to test them so i used a Multi Meter to find the heater pins. It's going to be a long year!

[plexi50]

Pic's

[plexi50]

Additional 79 tube chassis cut out.

[dogburn]

That's a big challenge to take on. It will be interesting to see what you are able to achieve with it and what it sounds like when done.

[tubeswell]

This arrive yesterday and it is quite a bit different than what i am used to working on. Mainly the pin out on the tubes is something new to get used to with very little info on this Model??? I want to call it a National Model 6107A. It seems some one has already tried to modify this adding an additional #79 tube at the rear of the chassis. It powers up and you can hear the speaker producing a low noise like when you have an input cable plugged in without a guitar. The volume knob has no effect on raising the noise level. Their appears to be an additional knob for possibly Gain? Any way i am checking the input grid of the input jacks and tube wiring. I have recapped the main power supply already but the rest came this way. I have the Model 6107A schematic. At least it gives me the layout before it was tampered with. My tube testers will not give info on these tubes to test them so i used a Multi Meter to find the heater pins. It's going to be a long year!

The verbal description of the existing work is difficult to follow. For instance, can’t tell if whoever modded it previously goosed around with a field coil speaker without changing the OT and speaker etc - what’s your best guess?


Suggest you try reverse-engineering a schematic to upload.

[plexi50]

It has the Original Transformers & Field Coil speaker. I have no idea what they were trying to do other than the additions i pointed out.The hard part is the tube sockets and wiring is under some of the transformers and difficult to get to.

[scstill]

This is a great find hope you can get it figured out.
Might be one of the few times where 2.5v and 6.3v tubes are mixed.

Assuming you have this info
http://prewaramps.org/nationaldob.htm
http://prewaramps.org/media/National%20Dobro%206107A%20Schematic.JPG

[plexi50]

This is a great find hope you can get it figured out.
Might be one of the few times where 2.5v and 6.3v tubes are mixed.

Assuming you have this info
http://prewaramps.org/nationaldob.htm
http://prewaramps.org/media/National%20Dobro%206107A%20Schematic.JPG

Yes i am marking the filaments now with a Green marker as i probe the socket pin's. Found 6.61VAC tube 79 & 2.63 VAC tube 56.

[plexi50]

Should i have continuity or Ohms between the driver transformer wire shown? I have neither. This is not my 6107A. Reference pic.

[shooter]

driver transformer

are there only 2 wires?
is that a choke/coil? 


measure ohms NOT continuity

[plexi50]

driver transformer

are there only 2 wires?
is that a choke/coil? 


measure ohms NOT continuity

On this forum their is a 9 year old posting of this same amplifier. They are calling this a driver or interstage transformer and has 5 wires. 2 on top and 3 underneath. https://el34world.com/Forum/index.php?topic=21314.0

[scstill]

You should post picture of your amp

Look at the schematic that I posted earlier.
It likely is the phase inverter transformer.
In the picture, it looks like the top wire goes to the 56 tube plate (pin2)
and the bottom wire to B+ after field coil

Should have continuity.
With all that dust are you certain your probes are making good contact?

[shooter]

so we don't have to bounce around


if it's the IT with 5 wires then yes there should be resistance between the 2 and also the other side, 3-wire, should have ohms between the 3.
usually an IT is a 1:1 so the ohms on the primary should be ~~~~ the same as the ohms on 2 "outside" wires on the secondary

[shooter]

FWIW;
i've bought a couple of these for my builds and repairs, can't recall who I got them from though. 
it can either be wired 1:1 or 1:2, hence the "extra wires"

[plexi50]

Yes i have scrapped down the pin's and wire connections to make sure i am getting a good connection. The two top wires (1) going to pin 2 of the 56 tube and the other to the B+. The only thing i have done to this is put in the PS capacitors and the single Mallory cap on the left of the volume pot/switch. I have to remove the transformer again to get two the wires underneath that go to the power tubes. I get a speaker hiss at idle when turned on but no input or volume pot function. One step at a time.

[scstill]

Did you measure the plate voltages on all tubes? What are they?
If that PI transformer (IT) is open (you measured no continuity)
then you would have no voltage on the 56 plate.

[plexi50]

Did you measure the plate voltages on all tubes? What are they?
If that PI transformer (IT) is open (you measured no continuity)
then you would have no voltage on the 56 plate.

Good idea on the plate voltage. See the transformer is just above the power tube pins, So i have to remove it and put on a small piece of wood and then ground it with a jumper. Checkin that now.

[scstill]

usually an IT is a 1:1 so the ohms on the primary should be ~~~~ the same as the ohms on 2 "outside" wires on the secondary

If the primary has ~250v to supply 56 plate, wouldn't a 1:1 IT put that same voltage on the 2A3 grids?
Or is it half with the center tap
Shouldn't 5:1 be more appropriate?
https://frank.pocnet.net/sheets/127/2/2A3.pdf

[scstill]

the transformer is just above the power tube pins, So i have to remove it and put on a small piece of wood and then ground it with a jumper. Checkin that now.

Alternatively you might be able to lift the tube slightly to get a probe to the pin from the top.
Just remember the tube pinout is reversed from the top.

[scstill]

Good idea on the plate voltage.  Checkin that now.

Check the grid voltages on both 2A3 as well

[shooter]

If the primary has ~250v to supply 56 plate, wouldn't a 1:1 IT put that same voltage on the 2A3 grids?Or is it half with the center tap

don't recall the V-drop across the primary, but the IT shouldn't have B+ on the secondary since it's not connected DC-wise.  The 1:1 is AC-volts, so whatever the driver produces, it should be ~~ the same at the secondary, but when I used one it was more like 80%, wired 1:2 close to unity.

[plexi50]

Tube PI (56) Plate is 65VDC. 2A3-Power tube plates 389 VDC. Grids are 66 VDC.

[plexi50]

Pulled back the cloth wire covering on the inter stage transformer and i am getting a solid B+ of 225vdc on the red feed wire.
The other wire to the 56 tube is fluxuating all over the place from 130vdc to 189vdc.

[scstill]

Sounds like the 2A3 voltages are close to the datasheet (maybe a little high)
The 56 plate voltage sounds low...
According to the schematic the 56 plate should be the same point as at the transformer that you say is fluctuating. Transformer shouldn't drop much voltage from the B+. 
Are the solder joints good? Have you removed tubes and cleaned the sockets and pins?
Does the voltage fluctuate when the 56 is removed?

[scstill]

Another thing that is curious is there are many orange drops in your amp
But not that many in the orig schematic.
Have you yet reverse drawn the schematic that you actually have?

[plexi50]

Another thing that is curious is there are many orange drops in your amp
But not that many in the orig schematic.
Have you yet reverse drawn the schematic that you actually have?

Some one else put these Xicon caps in here. What are the original values of those (2) caps? It looks like someone put Xicons in parallel to get an approximate value of the original caps that were in there.
I will put some Mallory M150's in there.. I have to re-solder this entire amp and pull all tubes and clean sockets and tighten the pins up.I'm going to pull the 56 tube and see what the voltage does to that transformer. Also is that a diode on tube 56? It has continuity both ways.

[passaloutre]

That’s definitely a cathode bias resistor, should not have continuity. Does it have any markings on it?

[plexi50]

That’s definitely a cathode bias resistor, should not have continuity. Does it have any markings on it?

Think i see Green, Red, Black ,Brown but barely and not for certain either. Badly faded and 90 years of degradation.

[shooter]

you can just cut the cap off the cathode til you get things straightened out.

The other wire to the 56 tube is fluxuating all over the place from 130vdc to 189vdc.

when power is off, short the grid on the 56 to ground and re-measure the plate wire
(same as vol set to zero but i don't know if it actually goes to zero)

[passaloutre]

That’s definitely a cathode bias resistor, should not have continuity. Does it have any markings on it?

Think i see Green, Red, Black ,Brown but barely and not for certain either. Badly faded and 90 years of degradation.

Body-end-dot for those ancient resistors. Is it red green black or red green brown? 25 or 250 ohm

[scstill]

According to the datasheet (for transformer load)
The 56 cathode resistor should be about 2700ohms (BED=red purple red)
The 56 grid voltage should be about 13.5vdc
https://frank.pocnet.net/sheets/029/5/56.pdf

[scstill]

BTW - does anyone know how the 2A3's are biased (to~66v as measured)?
There is no cathode resistor and there is no negative dc on the grid.

Is it possibly because the filament  transformer center tap has a resistor to ground?

[plexi50]

That’s definitely a cathode bias resistor, should not have continuity. Does it have any markings on it?

Think i see Green, Red, Black ,Brown but barely and not for certain either. Badly faded and 90 years of degradation.

Body-end-dot for those ancient resistors. Is it red green black or red green brown? 25 or 250 ohm

The only color that stands out for certain is Green. I can not guess with it. It is reading 27.5 Ohm.

[plexi50]

Voltages. Working on schematic.

[shooter]

from the link you provided earlier;

Replaced the cathode resistor on the 56 with a 1k, 1W metal film resistor and now the motor boarding is gone but still have the hum.

did you try this yet?

short the grid on the 56 to ground and re-measure the plate wire

[plexi50]

from the link you provided earlier;

Replaced the cathode resistor on the 56 with a 1k, 1W metal film resistor and now the motor boarding is gone but still have the hum.

did you try this yet?
Working on it right now. Getting schematic documented with what i we have now

short the grid on the 56 to ground and re-measure the plate wire

[tubeswell]

BTW - does anyone know how the 2A3's are biased (to~66v as measured)?
There is no cathode resistor and there is no negative dc on the grid.

Is it possibly because the filament  transformer center tap has a resistor to ground?

Yes. (What is the resistance? - out of interest)

(Edit: see bias notes on 2nd page
https://frank.pocnet.net/sheets/021/2/2A3.pdf

[plexi50]

1k 3 watt in place. Wait this is on the 79 tube.

[scstill]

here is another (better?) schematic from Rider
Has values that are missing from the earlier schematic
https://el34world.com/charts/Schematics/files/Valco/National_dobro_6107.pdf

On your voltage schematic also add the grid and cathode voltages.
It seems odd that you have 245v on both sides of the 100k

[TIMBO]

Hey mate,
I don't know if you remember this one 1948 2A3 AMPLIFIER
Really took me a while to decide what direction to go with the restoration.
It had too many wax caps and resistors that were past their prime.

[plexi50]

Reply.

[plexi50]

Tube positions

[Platefire]

I have always steered clear of the old 30's and 40's electronics even though I was tempted to pick up one now and again. So ya'll are diving in where I feared to tread. So I'm looking on with interest to see what you come up with---Go plexi50>>>>>>>>>>

[plexi50]

I'm sure at the time whoever added this extra 79 tube did so to increase the overall headroom of the amp. The work done was done so long ago from the looks of it using a period correct additional 79 socket. It would be easier just to build another amplifier in this. But that's not why it's here. We may wind up removing that additional 79 tube and getting this back to stock working condition. With your ideas and suggestions their should be no reason why this wont work again.

[plexi50]

Vote Time! Should we just build this amp back to it's Original 1935 Schematic posted by scstill?

[shooter]

since it's already 90% the same, fix what you have otherwise you'll just be making something "new" with untested stuff, compounding problems doesn't result in good solutions

[Platefire]

I keep wondering what this amp was originally intended to amplify, a lap steel, PA or Flying Pan? I would think get it working stock and then decide what to do to get it more guitar friendly.

[plexi50]

I keep wondering what this amp was originally intended to amplify, a lap steel, PA or Flying Pan? I would think get it working stock and then decide what to do to get it more guitar friendly.

A Flying Pan Guitar.

[scstill]

I keep wondering what this amp was originally intended to amplify, a lap steel, PA or Flying Pan? I would think get it working stock and then decide what to do to get it more guitar friendly.

Likely...
From an amplifier standpoint how would a lap steel design defer from a guitar design?

Agree to get it working original.

Sounds decent Original
https://www.facebook.com/TomsAmpsDetroit/videos/the-late-michael-hawkey-trying-out-the-1935-national-dobro-6107a/1037093861349200/

[scstill]

I keep wondering what this amp was originally intended to amplify, a lap steel, PA or Flying Pan? I would think get it working stock and then decide what to do to get it more guitar friendly.

A Flying Pan Guitar.

Not familiar with this - google only came back with fRying pan guitar
https://en.wikipedia.org/wiki/Frying_Pan_(guitar)#:~:text=George%20Beauchamp%20created%20the%20%22Fry,A%2D22%20%22Frying%20Pan%22

says it was the first electric guitar...  guy in this link plays it well
https://www.openculture.com/2016/04/behold-the-first-electric-guitar-the-1931-frying-pan.html