National Dobro Amplifier

[dpm309]

Just got a 1935 National Dobro Amplifier to work on. Says it was Manufactured to specifications for NATIONAL DOBRO CORP. by Webster Electric Company. Had a hard time reading the model number but I think it is 61V7A. Owner wanted a new 3-prong power cord installed and checked out. Cannot get the unit fired up so I am going to look into it a little further. Fuse is not blown. This amp has 2 2A3, 1 56, and 1 5Z3 rectifier tubes plus one other tube in there that I have not pulled yet, it in enclosed in a metal tube with the shielded wire running from the top of it. Cannot find any schematics on this so if anyone knows where I can find one, I would appreciate it. I have included a few pictures. This looks like it could be a fun project as I have never worked or seen an amp this old.

[HotBluePlates]

... one other tube in there that I have not pulled yet, it in enclosed in a metal tube with the shielded wire running from the top of it. ...

A grid-cap tube.

... Cannot find any schematics on this so if anyone knows where I can find one, I would appreciate it. ...

You'll almost certainly have to draw your own.  There are National websites with lots of ephemera on the instruments, but very little on the amps (at least until Valco came along).

... Owner wanted a new 3-prong power cord installed and checked out. Cannot get the unit fired up so I am going to look into it a little further. Fuse is not blown. ...

Not much in the amp.  Good chance the switch on the Volume pot is faulty if no tubes light up.

Hopefully the field coil is good once you do get the tubes lit up.  And the 2A3's (they're now expensive due to the Hi-Fi crowd).

... This looks like it could be a fun project as I have never worked or seen an amp this old.

I'd love to hear it!

I've got a Craigslist seller near me with the 40's-era cousin to this amp, with output tubes updated to 6L6G's (not a typo).  But they think they have "a treasure" and want far more than I'm willing to pay for an amp that "powers up but only hums; no audio."

[dpm309]

I found an article in the Vintage Guitar Magazine that talks about this very amp. The model number is 6107A (not 61V07) and the grid-cap tube is a "57". HPB you are correct in that the switch is not working. Looks like a 500K pot with an SPST switch that I can get at AE (Used the same one for a 1947 Magnatone M-197-3-V that I just repaired). In the meantime, I will wire a 500K pot to see if rest of the amp is working while I wait for the switched pot to arrive. Also I am going to have to sketch a schematic as you suggested.

Will post a sound clip when I get this working.
Thanks,

[sluckey]

I would just solder a jumper across the switch for now. Unplug from the wall to turn it off. I would also plug it into a lamp limiter first.

[Platefire]

Those paper capacitors look really rough. It's strange the metal cap over the speaker, the holes aren't very large--seems like it would be restricting! Platefire

[PRR]

Tube lineup: 57 56 2A3 2A3 5Z3

http://www.mif.pg.gda.pl/homepages/frank/sheets/021/5/57.pdf
http://www.mif.pg.gda.pl/homepages/frank/sheets/021/5/56.pdf

'57 is pentode, good gain. '56 is a low-Mu triode, good for driving the interstage transformer. So 1st '57, 2nd '56.

'57 is better known as 6J7 6SJ7 types.

'56 is similar to '76.
http://www.mif.pg.gda.pl/homepages/frank/sheets/021/7/76.pdf

Taking 14V peak at '56 grid, and gain near 140 for 6J7/'57, we need ~~100mV at the input jack to reach full output. Compare to ~20mV for classic Fenders, and 1mV for hot hi-gain headbanger amps. This is for guitarists with ARMS trained to play large rooms un-plugged, and who did not want much nasty distortion.

Power out of a couple 2A3 is 10W-15W. If you put a booster in front so your modern arms can push to 15W-"20W" overload, the irreplaceable speaker will be destroyed. While you "can" put a 2016 speaker and a resistor in, it won't sound like 1935 any more.

5Z3 seems to be a fat-filament rectifier in the 5U4 class.
http://www.mif.pg.gda.pl/homepages/frank/sheets/021/5/5Z3.pdf

Full set of tubes runs $250-$400+. Not too many caps. The OT can be 6V6-like iron. The interstage could be a problem.

Suggest you jump the bad switch, lamp-limit, and see where it is *really* at. Maybe it worked recently. Maybe it hasn't worked since WWII, and massive repair is needed. Don't get ahead of what the client is prepared to pay.

[PRR]

> strange the metal cap over the speaker

What is a National Dobro? An acoustic with a metal resonator under the bridge, typically with slot metal over the resonator.


This is not a genuine original, but shows the type.

So when an outfit known for "grille" branches out, they "have" to keep the grille theme, whether or not it is best sound.

And knowing the SotA in 1935, some blocker over the cone may be wise.

Also the "Electric Guitar" had to have a "different" sound, not like an acoustic Gibson, not like a good radio (which is probably what the speaker was made for). A little boxy and tin-tone is "different!".

Also the resonator makes little sense on a string-pickup electric guitar. And the pickup patent situation was murky in 1935. So how could NatDobro cash in on the e-guitar fad? By selling a speaker with the famous resonator. Perhaps a manual Dobro player, asked to step-up his game with an amplifier so the band could play larger rooms for more money, could be told that a standard electric guitar through the Dobro Amplifier would "be" the Dobro sound in larger spaces. Some BS there, but nothing new in this racket.

[PRR]

That multi-section main filter cap is sure not original. (Someone installed that well after WWII.) Original would be rectangular wax cardboard box. 4uFd was large, 8uFd very-large, so it may have been a 4u+8u or something. Since the field coil was also the choke, this could give ample filtering, possibly without a 3rd node for the preamp. Or the preamp could use a high-value drop resistor and a 0.5u wax-cap to filter.

[dpm309]

I soldered a jumper across the switch and plugged the amp into my light limiter. Bulb dimmed at first and then got brighter so I must have a short somewhere. Also none of the heaters in the tubes lit up. Will first need to install another 5Z3 as I accidentally dropped it  . Can get another at AE or the local electronic reclamation shop.

[PRR]

> Bulb dimmed at first and then got brighter so I must have a short

If the bright-up happened 3 to 10 seconds after turn-on, that's tube heat-up time. First suspect is shorted B+ cap, which is not revealed until the rectifier hots-up and tries to charge it. Second possibility is badly failed bias or even internal short on 2A3 so they SUCK hard, only after heat-up.

I suspect this is not even close to a "3-pin plug" job. Closer to restoring the 1935 Hudson found in a barn. It is probably worth it. Whether the client will pay to do it right, or you are the tech to do it, is the question. I see you have done serious restoration on post-WWII stuff, good; but the further back the more things are different (1935 Hudson had dippers) and proper parts may have to be whittled out of unobtainium.

(OTOH, you could mount Octal sockets in the 4/5-pin holes and do any 2-6V6 amp in there. 99% of players would not know the difference, though true Collectors would object.)

[dpm309]

Thanks for the feedback PRR. Upon further investigation, the grid cap tube is actually a 79, not a 57 and I believe this acts as the phase inverter. Also if you look at the attached photo, you can see 2 wires that were cut from pin 3 (grid) on both 2A3 tubes. Looks like mice may have gotten inside this as the wires going to the speakers looked chewed up.  Another question I have is that according to the datasheets, the heaters for the 2A3s and the 56 are 2.5V (AC or DC) while the 79 is 6.3V. I am going to try to trace everything out to come up with some sort of schematic that will help me get everything wired up correctly. One other issue is the cap has come off of the 79 and I am wondering if I could somehow reconnect the grid cap. I don't even know if I can find a 79 anywhere to purchase.

[sluckey]

One other issue is the cap has come off of the 79 and I am wondering if I could somehow reconnect the grid cap

Do you still have the cap? If so, remove the solder. Then set the cap back on the tube with the wire stud from the tube poking through the hole in the cap. Resolder. The cap probably also had some high temp glue also, but solder should hold it pretty good unless you're gonna be removing the tube every day.

[dpm309]

Sluckey, yes I still have the cap. From what I can determine, this grid cap goes to ground. I will try to resolder.
Thanks

[sluckey]

I'd be surprised if it goes to ground. Grounded grid circuits are rare in guitar amps but I guess it could be.

Remove all the old solder. Flux helps. Clean the old cap good. Same with the wire stud on top of the tube. I would use a lot of flux. Then the solder will flow easily on those old parts. Clean flux with denatured alcohol after solder cools.

[PRR]

> I don't even know if I can find a 79 anywhere to purchase.

https://tubedepot.com/products/79
Precious? Six bucks. Supply must be low but apparently demand is lower.

$3 at ESRC (bottom of this page):
http://www.esrcvacuumtubes.com/vacuumtubes_tubelist_tubes_13CW4-90c1.html

> 79  .... I believe this acts as the phase inverter.

I thot I saw an interstage transformer, but a '79 paraphase would be coming into fashion in '35. The cap is the signal input, possibly from the volume control. The other grid would come from a small resistor in the paraphase and not need to be routed "outside".

In octal (no cap) the '79 was called 6Y7 or 6Z7. But with NOS '79 at $1-$19 there's no urge to re-socket.

> the heaters for the 2A3s and the 56 are 2.5V (AC or DC) while the 79 is 6.3V

The 2A3 don't have separate cathode. They would have a dedicated winding with the "cathode" resistor on the 2.5V CT tap. I doubt '56 would be heated from this winding, we usually avoided "any" heater-cathode voltage like the ~~50V on the "cathode" of 2A3. The '79 may have become such a "better" (cheaper) alternative to an interstage transformer that nobody minded yet another 6V winding just for that.

These will all be heated with AC. 2.5V is not a common DC battery voltage. DC works too but no easy way to get that much DC from the wall. The 6V types originated in cars (DC) but all used the "AC heat" construction so are agnostic.

[dpm309]

The more I look into this amp, the more issues I have uncovered. Sluckey was correct in that the cap for the 79 tube should not go to ground. The shielded cap wire was soldered directly to the cap at some time in the past therefore, the wire inside is grounded to the shielding.  Have also discovered another 2 wires which have been removed or cut. Also as PRR observed, a new multi-section cap was added also sometime in the past. With all of the wires that are removed it is very difficult to try to draw a schematic. Have scoured the internet for a schematic with no luck. The closest I came was this article: https://www.vintageguitar.com/20982/national-dobro-6107a/ which shows the inside of what looks like an un-modded Dobro amp. If you look closely at the shielded cap wire, you can see that the cap is not grounded. Anyway, I will get with the owner to see how much he wants to put into this and if he feels it is worth restoring. Thanks for everyone's observations and tips and will provide any updates.

[dpm309]

Right after I posted, I did one more search and lo and behold I found the schematic.  Now I have something to work with. As you can see, the PT has 4 secondaries, 2 of them for the filaments - 2.5 and 6.3VAC.

[dpm309]

There is also a missing transformer that connects tube 56 and the pair of 2A3s. See picture below that I got off of one of the National Dobro websites and compare with picture NatDob3 up above. The schematic does not give any values or type. It looks like it is acting as a PI.  What can I use in it's place? 

[PRR]

There's a lot of irons here.

I assume the OT is on the speaker? Field Coil there of course.

The big chassis-hole lump must be the PT? (Lots of leads.)

Chassis lump next to it logically the main choke? (2 leads).

That leaves your internal transformer as the "missing" interstage. It would have 5 leads.

Yet you have a picture of another amp with another internal iron, and you do seem to have screws to mount such a part.

I think your latest schematic is "reasonable", but not for-sure-just-like yours. This is really a Hi-Class radio output end. Designs evolved quickly in the 1930s. Parts-kits were made up by distributors and offered to radio factories. If a new kit costed-out a buck cheaper than the way they were doing it, it might get adopted and never documented. (Because servicemen of the day grew to know all the variants and didn't need perfect docs, or expect them from minor manufacturers.)

Key question: '79 likes to be a "two". Two inputs or two outputs. Do you have two inputs? Then '79 makes some sense as an input stage. If you do not have two inputs, maybe '79 makes sense as a 2-output driver. I see two jacks so maybe '79 is the "two input channels" (but one volume knob). And the '56 is born to drive a transformer to power tube grids.

[dpm309]

Yes, you are correct on all of the other iron. This amp has 2 inputs with one going to pin 3 and the other to the cap grid on the 79.  The missing transformer hardware is still shown in one of the above pictures. The mounting holes are 3" apart. I have found a few interstage transformers on line but need to determine what value I need.

[PRR]

> you are correct on all of the other iron.

I've lost count. Is your OT on the speaker? Or that thing on the 2A3s?

If you are missing a choke, that's trivial.

[dpm309]

PRR, the OT is mounted on the chassis and connects to pin 2 on both of the 2A3s. There are 4 wires going to the speaker, 2 from the OT (Lighter colored ones) and the other 2 (red and black) coming from the multi-section cap can so I am guessing that they are connected to the field coil attached to the back of the speaker. The 5 cut wires, 2 from pin 3 on the 2A3s, one from pin 2 on the 56, one to the cap can, and one to ground appear to be from the missing transformer.

[shooter]

The 5 cut wires

Like PRR speculated, it's probably your inter-stage trany, PI

I've used a couple from here;
http://www.triodeelectronics.com/unintrui.html

If it is the missing tranny, you'll have to "spec out" current through the tranny cause they are limited on how much they can take.

[sluckey]

I would try this Hammond 124B available from AES.

https://www.tubesandmore.com/sites/default/files/associated_files/p-t124b.pdf

Or maybe this cheaper one...

     https://www.tubesandmore.com/products/transformer-audio-interstage-10-ma

[PRR]

The OT "belongs" on the speaker, since it shares wires with the B+ and field coil. Maybe they had doubts about the speaker's OT mounts in nightly drop-abuse (much worse than a parlor radio which never moves).

So you think the layout should be this?

[dpm309]

Yes, that looks correct. All 5 wires that were cut about one inch from where they connect and are pointing towards that direction. I don't know why this was removed. The owner thought the amp worked and only needed a fuse, cord and new pot. Must have gotten a raw deal. Next step is to source out a replacement transformer. Have not been able to find one with 5-wire configuration. Hammond series 126 has only 4 and series 124 has at least 6.
Thanks,
Dan

[sluckey]

Did you look at the two links I posted??? Both have 5 wires. Both are 10K primary, 90K CT secondary. Both are available from AES. One is expensive. The other is cheap.

[dpm309]

Thanks Sluckey.  I must have overlooked your previous post. Will definitely go with the P-T156 from AE. Need to order a couple of other parts and a tube so I will hopefully get this finished soon. Will post my progress in a couple of weeks.
Thanks again everyone.
Dan

[PRR]

Triode Electronics UIS-1 has 11 leads but they tell you several ways to connect it for different options. You have Single-Ended with DC, and probably want Step-Up.

125B has just 5 legs.

Stancor A53-C clone P-T156 sure seems to be 5 legs, great price, and probably ample.

I would go by mounting centers, for ease, but also to get about the same core size. Then price.

> The owner thought the amp worked

This is the hardest part. He thought the 1935 Packard needed tires but it really needs pistons, brakes, carb, etc. My neighbor takes on jobs like that. A recent Pontiac and a Hudson, while "complete", were very far from drivable, and the owners had to be "educated".

[PRR]

I forgot this instrument-rated part.

ClassicTone Part Number 40-18093
40-18093  Interstage Driver, 15W  026847         
Amps Used In: Musicmaster Bass** amps
http://www.classictone.net/40-18093.html
$38

Used with the 10K Rp triode instead of the 60K 12AX7 in the Musicmaster it should have more bass extension, though I don't see that we need even the bass of the small Musicmaster. It may not have the step-up we really want for driving the sluggish 2A3s.

[dpm309]

Just got all of the tubes and parts and am ready to work on this again. I have one question on the interstage transformer hookup. The green secondaries will go to pin 3 on the 2A3s, and black to ground. On the primary, do I connect the red to the power rail and the blue to pin 2 on the 56, or vice versa?
Thanks,
Dan

[dpm309]

Here is the spec sheet for the Hammond 124B which is similar to the P-T156 which did not have a spec sheet.

[sluckey]

Convention says blue usually connects to plate (Blue Plate Mayonnaise) and red usually goes to B+.

[PRR]

It does not matter. Sluckey's colors are very traditional.

[dpm309]

OK, finally got around to installing the transformer, new switched pot, 3- prong plug, re-wired the tube cap for the 79 and reinstalled all of the tubes. All tubes are new except for the pair of 2A3s. Fired it up and am getting some sound out of it but now what I would expect for a 5W amp. I get the same sound out of either inputs. When I turn the volume up, I get some crackling and a couple of dead spots as I turn it. This is a brand new pot so I would expect that is not the problem. All of the tubes light up and my next step is to check voltages which is difficult to do since this will not fit in any of my amp chassis cradles. Will have to rig something up. Will check the filter caps and coupling caps also. Anything else I should be looking for? If the 2A3s weren't so expensive, I would try a new pair.

[dpm309]

After replacing the coupling cap (w/.1 600V mallory 150) and cathode cap (w/ 0.47/50V e-cap) I am now getting decent volume but when I dime the volume, I am getting motor-boarding. Will need to replace the filter caps next. The cap can is a 20-30-20/400V. Do I need to use exact values or would a 40/20/20 - 500V JJ cap can work? Would rather use a cap can instead of 3 separate e-caps.

[PRR]

> would a 40/20/20 - 500V JJ cap can work?

Yes, but put a 20u at the rectifier, put the 40u at the FC-OT node.

[dpm309]

That's what I thought. Thanks,

[PRR]

The schematic (If I have the right one) shows the Field Coil only powering the small tubes, not the large tube. This is odd. Usually a FC takes "all" the current of the power stage. A big power stage needs a big-magnet speaker and this works out well.

Just saying in case you discover that the Choke and FC are wired opposite of what is on that schematic. I would not "change" it, but if the amp disagrees with the plan I would go with the as-built connection.

[dpm309]

Thanks for the feedback PRR. It appears to be wired as the schematic but will keep that in mind when I install the new cap can.

[dpm309]

Installed the new cap can and am getting some hum and it is still motor-boarding when I dime the volume. Cleaned the jacks and rechecked all of my connections with no luck. Looks like both heater secondaries have a ground tab according to the schematic. Tried moving wires around in the chassis also with no luck. The 2.5VAC secondary has a 750 ohm resistor connected to the center tap which goes to ground. Overall, I am getting good volume but cannot find the source of the hum or motor-boarding. All the caps and the potentiometer have been replaced. Don't know where to look next.

[dpm309]

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. Checked both legs of the 6.3VAC secondary and am getting about 40 ohms so it appears the center tap is working for that winding. On the 2.5VAC winding, I am getting 750 ohms which is the value stamped on the resistor connected to the center tap so I am assuming that winding is good also. Am wondering if I need additional filtering? I have one more 20UF leg on the new cap can I installed. Also could all of this old iron be causing the hum? This amp is from So. California and looks likes it has been sitting in a fairly humid environment for many years. Trying to get the history on this amp but I suspect it was discovered in someone's basement or attic where it had been sitting since after WWII.

[dpm309]

Went back and measured the AC voltages on both sides of the filament. On the 6.3VAC winding I am getting 3.4VAC on both sides. On the 2.5VAC winding, the AC voltage fluctuates between 1.8 and 3.8VAC on both sides. Also getting 69VDC on both sides. Don't think this is normal and am now thinking I could have a bad secondary. Next step is to see if I can hook up a 2.5V power supply to the 2A3s and the 56.

[DummyLoad]

the resistor connected to the CT of the 2.5V winding is the cathode bias resistor for the 2A3s. it will fluctuate with signal. measure across it and not to chassis. if you use DC power supply you MUST float the PS output and connect the cathode bias resistor to a balance network across the filaments. cathode bias with tubes with directly heated cathode can be bitch with DC power: see attachment.


--pete

[dpm309]

Thanks DL. I remeasured the filament voltage like you suggested and am getting a little over 2.5VAC so I am assuming that the secondaries are good. Added a 470 Ohm resistor between the field coil and the unused 20UF leg of the filter cap and the preamp power supply for more filtering. This lowered the hum a bit but not to where I like it. Also tried lifting the ground from the power cord with no change in hum. Don't know if this is the best I can do for this 80-year old amp. Sounds pretty decent playing my strat through it but am still not happy with the hum.

[DummyLoad]

hum could be one of the 2A3 getting weak. also, as you already state, it's an 80+ year old amp and that layout may just be the culprit. try a matched set of 2A3s, that may quiet it down some, but remember you DO have AC on the cathodes and amps that use directly heated cathode tubes are prone to hum.




--pete

[dpm309]

DL, the 2A3s are the original tubes - replaced the other 3. Don't have a spare set of these as they can be pricey (close to $200 for a pair) and might not solve the hum. Will talk to the owner to see if he wants to throw any more money into this.
Thanks,