1935? National Dobro 6107A? Update!!!

[shooter]

 
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????

[plexi50]

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.

[scstill]

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

[plexi50]

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.

[shooter]

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.   





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.

[plexi50]

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.   

The top Ground Cap on the 79 is making a good connection.

[plexi50]

Found this online.Pa Max for 56 = 1.3W Pa Max for 76 = 1.4W

[Platefire]

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

[plexi50]

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

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. 

[scstill]

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/

[Platefire]

Thanks scstill!   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   Thankfully, there is pretty good documentation on the 6107A including some pretty good schematics

[tubeswell]

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

[plexi50]

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.

[scstill]

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+??

[plexi50]

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.

[tubeswell]

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.)

[shooter]

the coil is "in circuit" so it's doing it's thing since the pre n driver have B+

[plexi50]

The speaker itself is working good. It's just got a 5" tear in it.

[plexi50]

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.

[scstill]

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....

[plexi50]

I have not posted a video to youtube in years. Do they make you wait a day now before it is live?

[Platefire]

That's really a nice clean tone and I didn't hear any hum. Great Job!! 

[plexi50]

That's really a nice clean tone and I didn't hear any hum. Great Job!! 

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.  

[Platefire]

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

[plexi50]

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. 

[tubeswell]

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!

[plexi50]

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.

[plexi50]

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.