Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plexi50 on July 13, 2025, 04:58:50 pm
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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!
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Pic's
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Additional 79 tube chassis cut out.
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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.
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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.
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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.
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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
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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/nationaldob.htm)
http://prewaramps.org/media/National%20Dobro%206107A%20Schematic.JPG (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.
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Should i have continuity or Ohms between the driver transformer wire shown? I have neither. This is not my 6107A. Reference pic.
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driver transformer
are there only 2 wires?
is that a choke/coil?
measure ohms NOT continuity
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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 (https://el34world.com/Forum/index.php?topic=21314.0)
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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?
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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
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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"
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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.
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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.
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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.
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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
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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.
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Good idea on the plate voltage. Checkin that now.
Check the grid voltages on both 2A3 as well
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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.
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Tube PI (56) Plate is 65VDC. 2A3-Power tube plates 389 VDC. Grids are 66 VDC.
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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.
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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?
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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?
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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.
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That’s definitely a cathode bias resistor, should not have continuity. Does it have any markings on it?
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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.
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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)
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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
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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
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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?
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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.
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Voltages. Working on schematic.
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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
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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
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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 (https://frank.pocnet.net/sheets/021/2/2A3.pdf)
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1k 3 watt in place. Wait this is on the 79 tube.
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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
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Hey mate,
I don't know if you remember this one 1948 2A3 AMPLIFIER (https://el34world.com/Forum/index.php?topic=19150.msg213451#msg213451)
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.
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Reply.
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Tube positions
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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>>>>>>>>>> :happy1:
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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.
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Vote Time! Should we just build this amp back to it's Original 1935 Schematic posted by scstill?
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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
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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.
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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.
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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/
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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
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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.
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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
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Theirs really not a lot to over look
lol no kidding…the amp has more tubes than resistors!
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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
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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. "
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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.
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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 (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.
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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.
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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!
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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
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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.
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don't believe he's grounded the 56 grid to insure nothing creeping in that way also
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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!
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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
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Ordered the Driver Transformer. Labeled my wire connections. Solder is a mess and i have to flux clean it up still.
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2A3 Power Tubes and fresh solder on the 5Z3
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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.)
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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.
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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.
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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.
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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/ (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!
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Looking forward to when the new driver transformer is installed to see what that does
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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!
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8:30 and it finally got here. Man that was a long day. Putting it in now/
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Long day.
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In and wired up. Some more to do tomorrow. I am wiped out. On the road most of the day.
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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.
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see reply #30 should have about 13v on the grid
see reply #59 cathode resistor should be about 2.5k
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what VDC do you have on the 56 cathode?
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per 56 spec
should have about -13v grid to cathode (0v relative to gnd)
+13 v cathode to gnd
right?
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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.
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what VDC do you have on the 56 cathode?
I have 65.5 VDC on the Cathode of 56.
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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
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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.
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Big Ceramic Resistor.
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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 :icon_biggrin:
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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!!!!!!!!!!!!!!!!!!!!!!!!
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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.
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Happy to hear the news! Looking forward to hearing the sounds!
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:headbang:
15mA is much better than 30A :icon_biggrin:
that might be a 'lil hot, you'll have to look at the datasheet for tube current at idle.
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What controls the Bias of the power tubes? Is it the Driver Transformer?
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Yaaaa!!! Glad you got it going :bravo1:
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Yaaaa!!! Glad you got it going :bravo1:
:worthy1: :worthy1: :worthy1: :worthy1: :worthy1: No Hum Either! :worthy1:
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2A3 power tubes Data says Self Biasing Resistor 750 Ohms. Where? In the TUBE? Ha!!!!!!!!
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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.
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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
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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
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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 (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.
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8 milliamps is probably good, you reach a diminished return eventually.
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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 (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.
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:laugh:
when you beat tubes to within seconds of life they tend to produce exceptionally well for a few minutes at full roar
what does the datasheet have listed for tubes max idle-NO signal current????
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:laugh:
when you beat tubes to within seconds of life they tend to produce exceptionally well for a few minutes at full roar
what does the datasheet have listed for tubes max idle-NO signal current????
Right i read that on no signal current on the Tube Data sheet. Dam it sounded so much louder with the 1K Cathode in their. Really nicely louder and defined. It still sound good though. Can't complain and buck the Data.
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Did you ever measure the resistance of the primary side of the driver transformer?
This could help determine the plate current and allow you to draw the load line for the 56.
Whats interesting is you have 395vdc on the plate and 392vdc at B+
Maybe a remeasure is in order - should drop voltage thru the xfmr
BTW - 20v on cathode (-20v grid) might still be too hot for the 56. 13.5v is ideal per spec sheet.
Remember in 1935 they had not yet heard of Rock n Roll, Things were not so loud in those days.
But if its really to low maybe you have a gain issue on the 79 or the 2A3's
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Did you ever measure the resistance of the primary side of the driver transformer?
This could help determine the plate current and allow you to draw the load line for the 56.
Whats interesting is you have 395vdc on the plate and 392vdc at B+
Maybe a remeasure is in order - should drop voltage thru the xfmr
BTW - 20v on cathode (-20v grid) might still be too hot for the 56. 13.5v is ideal per spec sheet.
Remember in 1935 they had not yet heard of Rock n Roll, Things were not so loud in those days.
But if its really to low maybe you have a gain issue on the 79 or the 2A3's
I will remeasure tomorrow. I work to fast sometimes. Here is the 56 tube data. I measured the Driver transformer before and after it was remove. After it was removed i could get no Ohms reading from either side of the transformer wires. Zero nothing. Edit: The 56 tube is wired with a 1K resistor on it's cathode pin with .047 cap.
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when I used an IT in a couple builds, i did some quick n dirty reading on them, the short answer, they don't take much current to smoke 'em.
my "fix" was to scrap Gibson's design n steal another design so my IT was capacitively coupled, in n out.
the original amps tone gets altered using 2 reactive parts, the upside;
from the buyer;
"I've been chasing That sound for 17 years, a dozen amps, how did you do that??""
I asked if he wanted the really hard math explanation or the simple... um, it was random chance, glad ya like it. :icon_biggrin:
EDIT: the data sheet shows 5 mA plate current at 250vdc that's your "target" then you change based on tube life, amp sound. so if you have many on hand, runner hot n see, just know when the tube goes, it might take the "load" your new IT with it.
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when I used an IT in a couple builds, i did some quick n dirty reading on them, the short answer, they don't take much current to smoke 'em.
my "fix" was to scrap Gibson's design n steal another design so my IT was capacitively coupled, in n out.
the original amps tone gets altered using 2 reactive parts, the upside;
from the buyer;
"I've been chasing That sound for 17 years, a dozen amps, how did you do that??""
I asked if he wanted the really hard math explanation or the simple... um, it was random chance, glad ya like it. :icon_biggrin:
The top Ground Cap on the 79 is making a good connection. :think1:
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Found this online.Pa Max for 56 = 1.3W Pa Max for 76 = 1.4W
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Once you get it tweaked operating best you can tell as a 6107A was designed to work, I would appreciate a good general performance report from your perspective.The description I've read on this amp is "very warm" that to me is saying "not much brightness". So using a present day guitar it would be more suited for a single coil (strat of tele) guitar than say a Les Paul. The video with Michael Hawkey playing a strat through one had pretty good tone IMHO but I kept thinking, I wish he would turn it up! Also I kept thinking, "I would like to hear it coupled with a good reverb unit or reverb pedal. Oh yeah, I would like to know how it takes pedals?
The reason for my interest in this amp is I use to do a lot of research trying to determine what was the first commercially available tube amp designed for electric guitar. I never could quite pin it down. Now I think this is probably the first! I know there was some experimenting going on with this by individuals at this time but none of them put it out as a product for sale. So if there is anybody with info of a commercially available tube amp made for magnetic PU guitars prior to this one, I'd like to know :dontknow: :icon_biggrin:
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Once you get it tweaked operating best you can tell as a 6107A was designed to work, I would appreciate a good general performance report from your perspective.The description I've read on this amp is "very warm" that to me is saying "not much brightness". So using a present day guitar it would be more suited for a single coil (strat of tele) guitar than say a Les Paul. The video with Michael Hawkey playing a strat through one had pretty good tone IMHO but I kept thinking, I wish he would turn it up! Also I kept thinking, "I would like to hear it coupled with a good reverb unit or reverb pedal. Oh yeah, I would like to know how it takes pedals?
The reason for my interest in this amp is I use to do a lot of research trying to determine what was the first commercially available tube amp designed for electric guitar. I never could quite pin it down. Now I think this is probably the first! I know there was some experimenting going on with this by individuals at this time but none of them put it out as a product for sale. So if there is anybody with info of a commercially available tube amp made for magnetic PU guitars prior to this one, I'd like to know :dontknow: :icon_biggrin:
Ok i understand your interest and will over the next few days try and upload a video of it's sound, It sounds more suited for a Stratocaster to my ear. I wont forget. Just so busy with Doctors this week. I am sick of it. You go for one thing and the next thing you know they want to send you to a dozen other places. Anyway it's nice to have had your and others help in getting my brain in order on this amp. It really was a cool journey and learning some new things about Ancient tubes and such.
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The reason for my interest in this amp is I use to do a lot of research trying to determine what was the first commercially available tube amp designed for electric guitar. I never could quite pin it down. Now I think this is probably the first! I know there was some experimenting going on with this by individuals at this time but none of them put it out as a product for sale. So if there is anybody with info of a commercially available tube amp made for magnetic PU guitars prior to this one, I'd like to know
This Dobro is very early, but this article discusses a few amps even earlier...
https://www.vintageguitar.com/1804/antique-guitar-amps-1928-1934/
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Thanks scstill! :thumbsup: That kind of info is just what I've searched for. It really was quite a bit of electric amp/guitar creations prior to the 6107A. The problem is there was not really any documentation on any of these early amps and the ones build didn't survive until this present time. Also the early stuff didn't have brand tags, serial numbers or user manuals. So if one turns up in the present time, it would be very hard to tell what your got :dontknow: Thankfully, there is pretty good documentation on the 6107A including some pretty good schematics
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when you beat tubes to within seconds of life they tend to produce exceptionally well for a few minutes at full roar
Indeed. They sound especially incredibly sweetly unbelievably fantastic just as they’re redplating- the instant before they implode
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when you beat tubes to within seconds of life they tend to produce exceptionally well for a few minutes at full roar
Indeed. They sound especially incredibly sweetly unbelievably fantastic just as they’re redplating- the instant before they implode
And Boy did it sound just so. I put a 2500 Ohm resistor on 56 and it still sounds nice. I had it on for 30 minutes and played a bit and no red plating. It passed the test. Voltages never Flucuated.
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And Boy did it sound just so. I put a 2500 Ohm resistor on 56 and it still sounds nice. I had it on for 30 minutes and played a bit and no red plating. It passed the test. Voltages never Flucuated.
This a classic example of why we should follow the tube datasheet.
Draw the load line, bias in the middle and you will have a happy tube.
What is the bias voltage for 2.5k? the datasheet actually likes 13.5v with a 3k
Asked earlier - why is your measured voltage higher on the 56 plate than at B+??
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And Boy did it sound just so. I put a 2500 Ohm resistor on 56 and it still sounds nice. I had it on for 30 minutes and played a bit and no red plating. It passed the test. Voltages never Flucuated.
This a classic example of why we should follow the tube datasheet.
Draw the load line, bias in the middle and you will have a happy tube.
What is the bias voltage for 2.5k? the datasheet actually likes 13.5v with a 3k
Asked earlier - why is your measured voltage higher on the 56 plate than at B+??
Ive measured the plate voltage many times and sometimes it will vary a voltage or two. I will remeasure tomorrow when i get the amp back from the party that owns it. He took it to a place to see if they will, or know how to re-cone a Field Cone Speaker. I am not done with it just yet. I need too change a cap on (56) Cathode. I screwed up and used a coupling cap in place of an electrolytic capacitor. Look at the bottom of the caps pictured.
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how to re-cone a Field Cone [sic] Speaker.
Reconing a speaker isn't difficult (with a little practice). The tricky bit is finding a speaker coil with the optimal impedance for the field coil. (Of course, it could be the field coil is open, which would make restoration trickier.)
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the coil is "in circuit" so it's doing it's thing since the pre n driver have B+
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The speaker itself is working good. It's just got a 5" tear in it.
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I used to re-cone speakers until the re-cone kits became as expensive as a new speaker. Altec Lansing and Bose 901 and Infinity speakers were awesome to listen to.
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The speaker itself is working good. It's just got a 5" tear in it.
I use Arlene's Original Tacky Glue to repair speakers. Very strong transparent and flexible.
Small brush glue onto both sides of tear edge.
Carefully align the edges, press lightly, remove excess glue with a wet Qtip..
Dry it only night
viola..
BTW - if you thin the tacky glue it is a great dope and speaker renew size...
BTWW - rehydrate a speaker - if you have a dry speaker (old) put the speaker in a garbage bag with a wet sponge in a bowl not touching speaker. Seal bag. let it sit overnight, paper cone absorbs the humidity and gets renewed, repeat as necessary....
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I have not posted a video to youtube in years. Do they make you wait a day now before it is live?
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That's really a nice clean tone and I didn't hear any hum. Great Job!! :thumbsup:
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That's really a nice clean tone and I didn't hear any hum. Great Job!! :thumbsup:
I took every pic possible except for the Front and rear of the finished product. I got a bad case of DA these days. I think i have the onset of Dementia. It's getting in my way and messing up my memory. Thankyou Platefire for the comment. :worthy1:
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Your welcome! Dementia---I'd be lying if I said I didn't have the same thoughts sometimes as my forgettor is working real good these days. However, dementia will have to drag me off kicking and fighting all the way cause with the help of the Lord I'll whop him good when he comes a calling.
You know I was just thinking about the 6107A you just brought back to life. That thing has been through World War 2, Korean war, Viet Nam war, Gulf War and all the other crazy stuff going on in this world and is a vet of bringing some nice guitar tones into the situation and now, more to come. It would be nice to know the history of this amp's journey through the years but at this point we can only imagine
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Your welcome! Dementia---I'd be lying if I said I didn't have the same thoughts sometimes as my forgettor is working real good these days. However, dementia will have to drag me off kicking and fighting all the way cause with the help of the Lord I'll whop him good when he comes a calling.
You know I was just thinking about the 6107A you just brought back to life. That thing has been through World War 2, Korean war, Viet Nam war, Gulf War and all the other crazy stuff going on in this world and is a vet of bringing some nice guitar tones into the situation and now, more to come. It would be nice to know the history of this amp's journey through the years but at this point we can only imagine
I will ask him when i call him again. He has had the Guitar he bought new in 1955. He was 12 years old at that time.History is an amazing thing and like you said with the help of the Good Lord we will continue to battle our issues with getting old.It's Good & Bad and a real Nuisance. :BangHead:
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I have not posted a video to youtube in years. Do they make you wait a day now before it is live?
Man that thing is 90 years old!
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I have not posted a video to youtube in years. Do they make you wait a day now before it is live?
Man that thing is 90 years old!
90 years old. It has seen more than i will ever see.
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Did you ever measure the resistance of the primary side of the driver transformer?
This could help determine the plate current and allow you to draw the load line for the 56.
Whats interesting is you have 395vdc on the plate and 392vdc at B+
Maybe a remeasure is in order - should drop voltage thru the xfmr
BTW - 20v on cathode (-20v grid) might still be too hot for the 56. 13.5v is ideal per spec sheet.
Remember in 1935 they had not yet heard of Rock n Roll, Things were not so loud in those days.
But if its really to low maybe you have a gain issue on the 79 or the 2A3's
Typo on the plate voltage of tube 56. It is 390vdc.