1935? National Dobro 6107A? Update!!!

[plexi50]

Right now the plate voltage on (56) is running up and down like a vibrato. From 130vdc and up to 187vdc and back down over and over again. Tomorrow i will pull (56) and check the pin/wire plate voltage again. I am trying to rule out or condemn the driver transformer because that just does not make any sense what the plate voltage is doing on (56). Tomorrow is another day. Before too long i will know this amp inside and out and what things i may be overlooking.Theirs really not a lot to over look. It's weird looking at these tubes with the cap wire on top of them (Grid?) Your whole thinking has to be changed or say relearned just trying to figure out the pin numbers themselves. Once i found the Heater Filament pins the rest is Clockwise. It's just a different animal.

[plexi50]

If i need this driver transformer it's a big help knowing which one to get from another posting from years ago by PRR. Thankyou! https://www.tubesandmore.com/products/transformer-audio-interstage-10-ma

[passaloutre]

Theirs really not a lot to over look

lol no kidding…the amp has more tubes than resistors!

[scstill]

Yes one grid on top cap of 79 other grid is pin 3(recommend studying the tube datasheets)
Here is a car radio I converted to guitar amp with grid top caps...
https://stillampd.com/phirty6

Can you get your hands on a good 56 to try

[scstill]

Just for giggles, I asked Gemini, Here is that reply

"Wild plate voltage swings in an interstage transformer circuit: tube or transformer?

Wildly swinging plate voltage in an interstage transformer circuit could indicate problems with either the tube or the transformer.

Potential tube issues
1) Weakened tube: A tube that's nearing the end of its life can exhibit unstable operation and contribute to plate voltage fluctuations.
2) Gassy or faulty tube: Internal shorts or gas in the tube can lead to erratic behavior, including unstable plate voltage.
3) Incorrect biasing: Improper bias voltage on the tube can cause incorrect current flow and potentially lead to plate voltage swings.

Potential transformer issues
1) Loose internal contacts or connections: Over time, internal connections within the transformer can become loose, leading to intermittent contact and fluctuating voltage.
2) Partial short circuit between turns: Insulation breakdown can cause partial shorts within the transformer windings, resulting in voltage instability.
3) Winding faults: Unequal turns, resistive imbalance, or shorted windings in the primary of the output transformer can cause issues, including voltage fluctuations and potential redplating.
4) Degraded insulation: Damage to the transformer's insulation can lead to electrical failure and contribute to unstable operation.

Troubleshooting steps
To diagnose the problem, consider the following:
1) Visual inspection: Look for any physical damage to the transformer, burnt components, or changes in insulation color.
2) Check tubes: Visually inspect the tubes for signs of gas (purple glow), red plating, or white getter (indicating a cracked seal). You can also try the "chopstick test" to identify microphonic tubes, according to YouTube.
3) Test the transformer: Use a multimeter to check the continuity and resistance of the transformer windings. You can also conduct an excitation current test or measure the turns ratio to assess the transformer's health.
4) Check wiring and connections: Look for loose or damaged wiring and ensure all connections are secure, according to HVAC School.
5) Monitor temperatures: Check the temperature of the transformer and surrounding components during operation. Overheating can be a sign of a problem.
haha - Consider expert assistance: If you are not comfortable troubleshooting tube circuits, consider taking the amplifier to a qualified technician for diagnosis and repair. "

[Platefire]

Yeah Frying pan guitar was what I was thinking about by Rickenbacker. I got Dobro and Rickenbacker mixed up:>(
I listened to the recording of the amp, not bad at all. Reckon it was the original field coil speaker? Couldn't help but wonder howcranked the volume was. 

[plexi50]

Yes one grid on top cap of 79 other grid is pin 3(recommend studying the tube datasheets)
Here is a car radio I converted to guitar amp with grid top caps...
https://stillampd.com/phirty6

Can you get your hands on a good 56 to try

Looked through my boxes of tubes but not a single 56. Yes i have the tube data sheet from Franks Tube Data. We would be lost without his site. That's one heck a car radio back then. I don't remember what kind of radio i had in my 1949 Dodge Split Windshield. Fluid Drive Tranny. Shift on the column or just push the clutch pedal quick and it automatically shifted. Sort of didn't make sense at the time. Got Doc appointment today. Will get back on this soon.

[plexi50]

Just for giggles, I asked Gemini, Here is that reply

"Wild plate voltage swings in an interstage transformer circuit: tube or transformer?

Wildly swinging plate voltage in an interstage transformer circuit could indicate problems with either the tube or the transformer.

Potential tube issues
1) Weakened tube: A tube that's nearing the end of its life can exhibit unstable operation and contribute to plate voltage fluctuations.
2) Gassy or faulty tube: Internal shorts or gas in the tube can lead to erratic behavior, including unstable plate voltage.
3) Incorrect biasing: Improper bias voltage on the tube can cause incorrect current flow and potentially lead to plate voltage swings.

Potential transformer issues
1) Loose internal contacts or connections: Over time, internal connections within the transformer can become loose, leading to intermittent contact and fluctuating voltage.
2) Partial short circuit between turns: Insulation breakdown can cause partial shorts within the transformer windings, resulting in voltage instability.
3) Winding faults: Unequal turns, resistive imbalance, or shorted windings in the primary of the output transformer can cause issues, including voltage fluctuations and potential redplating.
4) Degraded insulation: Damage to the transformer's insulation can lead to electrical failure and contribute to unstable operation.

Troubleshooting steps
To diagnose the problem, consider the following:
1) Visual inspection: Look for any physical damage to the transformer, burnt components, or changes in insulation color.
2) Check tubes: Visually inspect the tubes for signs of gas (purple glow), red plating, or white getter (indicating a cracked seal). You can also try the "chopstick test" to identify microphonic tubes, according to YouTube.
3) Test the transformer: Use a multimeter to check the continuity and resistance of the transformer windings. You can also conduct an excitation current test or measure the turns ratio to assess the transformer's health.
4) Check wiring and connections: Look for loose or damaged wiring and ensure all connections are secure, according to HVAC School.
5) Monitor temperatures: Check the temperature of the transformer and surrounding components during operation. Overheating can be a sign of a problem.
haha - Consider expert assistance: If you are not comfortable troubleshooting tube circuits, consider taking the amplifier to a qualified technician for diagnosis and repair. "

That Gemini has been coming in handy over the past few months. I thought it was weird at first but it's handy.

[shooter]

haha - Consider expert assistance: If you are not comfortable troubleshooting tube circuits, consider taking the amplifier to a qualified technician for diagnosis and repair. "

headlines;
......"we regret to inform you, the last human with knowledge died of neglect having been starved to death by a one n a zero"
grok that!

[scstill]

Here is another way National powered that 56
http://prewaramps.org/media/nd2a3schematic.JPG
Wonder what this difference results in....

Confirms my suspicion that the 56 cathode resistor should be 2500ohms (per datasheet too)
Do not understand why there is a 25ohm in this amp (BED =red blk blk)
And the likely value of the 2.5v filament lift resistor 1k, Thinking it is the large ceramic sitting next to the PT

[tubeswell]

Is the voltage swing on the 56 a ‘low frequency’ oscillation? (I.e. is it slow enough for you to attempt to time/count it?) I merely ask this because you say the plate voltage is bouncing up and down, but you haven’t said how fast it’s moving.

[shooter]

don't believe he's grounded the 56 grid to insure nothing creeping in that way also

[plexi50]

Is the voltage swing on the 56 a ‘low frequency’ oscillation? (I.e. is it slow enough for you to attempt to time/count it?) I merely ask this because you say the plate voltage is bouncing up and down, but you haven’t said how fast it’s moving.

Fairly fast on the voltage bouncing around. 1 second from 180 to 300+ or - Minus.It's too fast to get any actual top reading. Pulled the 56 tube and no change in the voltage on the plate pin.I ordered the Driver Transformer from the link PRR posted from 8 or more years ago on this same amp.Pulled the added 79 tube and cleaned up the chassis to get ready for the new Driver Transformer.Labeled the wire connections and tube socket pins for quick install. It's Hot as H**l outside today!

[plexi50]

don't believe he's grounded the 56 grid to insure nothing creeping in that way also

The 56 tube was grounded and no change. The tube is NOS by the way. I Ohmed out the Driver Transformer wires going to the tubes and to ground and i get a reading of 2.45 Ohms & 3.04 Ohms

[plexi50]

Ordered the Driver Transformer. Labeled my wire connections. Solder is a mess and i have to flux clean it up still.

[plexi50]

2A3 Power Tubes and fresh solder on the 5Z3

[tubeswell]

Is the voltage swing on the 56 a ‘low frequency’ oscillation? (I.e. is it slow enough for you to attempt to time/count it?) I merely ask this because you say the plate voltage is bouncing up and down, but you haven’t said how fast it’s moving.

Fairly fast on the voltage bouncing around. 1 second from 180 to 300+ or - Minus.It's too fast to get any actual top reading. Pulled the 56 tube and no change in the voltage on the plate pin.I ordered the Driver Transformer from the link PRR posted from 8 or more years ago on this same amp.Pulled the added 79 tube and cleaned up the chassis to get ready for the new Driver Transformer.Labeled the wire connections and tube socket pins for quick install. It's Hot as H**l outside today!

I'd categorise '1 second oscillation' as low frequency oscillation. And if it is - you need to isolate where the signal feedback is occurring that is causing this.
(Maybe try and capture the time period of the oscillation on a scope.)

[passaloutre]

1 second might just be the sample rate of the DMM. Some meters have a “Hz” setting, which may be useful here, but I agree on the value of a scope in this situation.

[plexi50]

I will have a new Driver Transformer Monday. In the meantime i will cleanup the soldering mess and verify all the tube pin outs on 79 & 56 tubes. Heater voltage has already been established on all tube sockets. In the process of making a Schematic with pin out and voltages to come.

[Platefire]

This may have already been referenced but I'm always interested in the history of things and found this article from Vintage Guitar about the 6107A. So here is the link:

https://www.vintageguitar.com/20982/national-dobro-6107a/

This is one of, if not the only first commercially available guitar amp ever. I thought it was funny saying(in the article) the speaker grill looked like a hub cap or also the speaker grill was made to resemble a Dobro resonator guitar sound chamber.

[plexi50]

This may have already been referenced but I'm always interested in the history of things and found this article from Vintage Guitar about the 6107A. So here is the link:

https://www.vintageguitar.com/20982/national-dobro-6107a/

This is one of, if not the only first commercially available guitar amp ever. I thought it was funny saying(in the article) the speaker grill looked like a hub cap or also the speaker grill was made to resemble a Dobro resonator guitar sound chamber.

I saw that a few days ago and it really is one of the  very few articles online about the 6170A and the Dobro guitar. The whole thing here with this amplifier is Historical as it gets! Awesome!

[Platefire]

Looking forward to when the new driver transformer is installed to see what that does

[plexi50]

Looking forward to when the new driver transformer is installed to see what that does

USPS has it being Delivered today but it's 5:30 now and it hasn't arrived just yet. I can't wait until it gets here. Oh by the way after i removed the old Driver Transformer i again tested for Ohmage and got nothing this time with my Beckman DVM. Interesting!

[plexi50]

8:30 and it finally got here. Man that was a long day. Putting it in now/

[plexi50]

Long day.

[plexi50]

In and wired up. Some more to do tomorrow. I am wiped out. On the road most of the day.

[plexi50]

Ok i have to start all over again. Been overwhelmed with too many things the past few days. First to clarify the cathode pin for the 79 tube. And the cathode for the 56 tube. I am spinning in circles and getting nothing accomplished. I have the Driver Transformer installed and have a solid plate voltage of 255vdc on tube 79 & 56. I have 392vdc on the 2A3 power tubes. I have Heater voltages of 6.6vac on 79 & 2.5vac on 56 & 2A3 tubes. That leaves the Grid which i thought was pretty simple but does not seem to be functioning. If i tap on the chassis i hear it through the speaker.

[scstill]

see reply #30 should have about 13v on the grid
see reply #59 cathode resistor should be about 2.5k

[shooter]

what VDC do you have on the 56 cathode?

[scstill]

per 56 spec
should have about -13v grid to cathode (0v relative to gnd)
+13 v cathode to gnd
right?

[plexi50]

see reply #30 should have about 13v on the grid
see reply #59 cathode resistor should be about 2.5k

Tube 56 No resistors or cap. Tube 79 pic coming.

[plexi50]

what VDC do you have on the 56 cathode?

I have 65.5 VDC on the Cathode of 56.

[shooter]

if you have a 2.2OHM resistor on the 56's cathode to ground you have ~~30AMPS going through the tube


IF you look at your schematic, the cathode of the 56 does NOT go to the input jacks, per your notes


looks like you made a change to the pic

[plexi50]

if you have a 2.2OHM resistor on the 56's cathode to ground you have ~~30AMPS going through the tube


IF you look at your schematic, the cathode of the 56 does NOT go to the input jacks, per your notes


looks like you made a change to the pic

I am just adding voltages and other finds to the pics as i go. Yes i have been looking the whole time at these Xicon caps going to the input jacks terminal but it is a grounded stud on the input jacks. I haven't made any changes other than to put a 1K resistor on the Cathode pin of tube 79.
So that Whiteish wire on the Cathode of 56 going to that big Ceramic resistor 2.2 Ohm should not be there? It's been their since i got the amp. This thing is a real mess but getting sorted out.

[plexi50]

Big Ceramic Resistor.

[shooter]

Cathode of 56 going to that big Ceramic resistor 2.2 Ohm should not be there? It's been their since i got the amp.

all I did was math;      I = 65vdc / 2.2ohms = 29.5A
comes in handy for head-scratching, and question asking 

[plexi50]

Got rid of the (2) wires coming from that Ceramic resistor and just put a 1K with a .047 Coupling cap parallel on the 56 tube Cathode. Have a 1K resistor and .1 Coupling cap on tube 79 Cathode. I have 15.05 volts on the 56 cathode tube. 1.776 volts on the Cathode of Tube 79. It's ALIVE and sounds really decent. The .1 Coupling cap on 79 really brought out the definition of the bottom end so to speak. I am Ecstatic and very happy with the tone and volume level. Posting pics in a minute!!!!!!!!!!!!!!!!!!!!!!!!

[plexi50]

Have to make some sound clips later in the day. Owner is Happy as can be! He's an old Friend and i couldn't not settle for this not working.

[passaloutre]

Happy to hear the news! Looking forward to hearing the sounds!

[shooter]

 


15mA is much better than 30A   


that might be a 'lil hot, you'll have to look at the datasheet for tube current at idle.

[plexi50]

What controls the Bias of the power tubes? Is it the Driver Transformer?

[Platefire]

Yaaaa!!! Glad you got it going  

[plexi50]

Yaaaa!!! Glad you got it going

No Hum Either!

[plexi50]

2A3 power tubes Data says Self Biasing Resistor 750 Ohms. Where? In the TUBE? Ha!!!!!!!!

[plexi50]

Ok i think i understand the biasing of the 2A3 power tubes. Can be either cathode Biased or Biased using driver transformer using (2) separate windings one for each tube which we have using the Driver Transformer.

[shooter]

I was talking about 56's cathode bias being maybe a 'lil hot


as for the 2a3, look at the snippet from your schematic, that "Black resistor" is your "bias R" since they are directly heated cathodes

[scstill]

Black resistors are defined in this schematic link
This schematic has been referred to a couple of times earlier - but apparently not read

Yes 56 is way too hot - again cathode resistor should be 2500 ohms...
The lift resistor on the 2A3 heaters is 1000 ohms...

http://prewaramps.org/media/nd2a3schematic.JPG

[plexi50]

Black resistors are defined in this schematic link
This schematic has been referred to a couple of times earlier - but apparently not read

Yes 56 is way too hot - again cathode resistor should be 2500 ohms...
The lift resistor on the 2A3 heaters is 1000 ohms...

http://prewaramps.org/media/nd2a3schematic.JPG

I changed tube 56 cathode resistor to 2500 Ohms and now have 20 volts on the cathode. Not as loud either.

[shooter]

8 milliamps is probably good, you reach a diminished return eventually.

[plexi50]

Black resistors are defined in this schematic link
This schematic has been referred to a couple of times earlier - but apparently not read

Yes 56 is way too hot - again cathode resistor should be 2500 ohms...
The lift resistor on the 2A3 heaters is 1000 ohms...

http://prewaramps.org/media/nd2a3schematic.JPG

I have a 2500 r on 56 Cathode now. What a difference in volume though. It does not have the attention / presence in volume it had.