National-Dobro Amplifier, schematic??

Ken Fox · Post by **Ken Fox** » 19 Dec 2001 6:07 pm

I've got a very nice looking model 6439 to work on. It has metal tubes, 2 @ 6L6, 3 @ 6N7, one glass tube 5V4G. It's a field coil speaker, 12". Neat little rig. A friend had asked me to look it over. I'd like to find a schematic or link to same.

Blake Hawkins · Post by **Blake Hawkins** » 19 Dec 2001 7:46 pm

Ken, If you don't find a circuit diagram, you can easily draw one from looking at the underside of the chassis. While I have never seen the National-Dobro amplifier, I have worked on many with a similar tube line up and a field coil speaker.
The circuits are very straight forward and simple.
All you need is a tube manual for the pin outs of the tubes.
If you can't find the pin outs, E Mail me and I will send them to you.
Blake

Ken Fox · Post by **Ken Fox** » 20 Dec 2001 7:37 am

I drew it out last night. It a classic cathode boased circuit. Circuit is very similar to the early 50's Fender Pro. The fun part was the strage way they color coded resistors back then! I had three open 100K plate loads. Replaced two cathode bypass caps. Amp was biased way to hot! Someone had changed the cathode bypass at the 6L6's to a 125 ohm, should be 250 ohm. That had it running pretty much wide open (class A). Should have that fixed tonight! This is one cool little amp!! I saw a couple of similar ones for sale on the net. I did not even know such an amp existed. Just when you think you have seen it all!!

Blake Hawkins · Post by **Blake Hawkins** » 20 Dec 2001 10:40 am

Ah Ken, we are kindred spirits. Glad you are having fun restoring the amp.
I usually replace all the tubular caps as well as the electrolytics.
Yes, the old resistors with the "Body, End, Dot" codes are interesting. Usually I find several that are still within 20 per cent of the value and just leave them in the circuit.
The usual problem with the old carbon resistors is that they increase in value with age.
It is unusual to find metal 6L6 output tubes in an audio amplifier.
They may well be original equipment.
Blake

Ken Fox · Post by **Ken Fox** » 20 Dec 2001 1:24 pm

I've got a sourse for NOS american tubes. I'm get the metal 6L6, matched at $10.00 each and a 5V4G, coke bottle rectifier for $11.00. The rectifier that was in there was a 5U4, too much V-drop according to tube specs. The old filters caps had just had it, replacing them as well. It was a two section, 8mfd at 450VDC, Not original, and not enough filtering for the circuit. I'm gonna set up the filtering more like the early Fender Pro amp. Hope to have it all up and performing in a week. If you need that source on tubes just look up Roundup Electronics with a search engine, he is in Pendleton, Oregon. Great guy to deal with.

Blake Hawkins · Post by **Blake Hawkins** » 20 Dec 2001 5:25 pm

Good prices on the tubes! Thanks for letting me know about Roundup Electronics.
8 mfd was typical for amps of that vintage with a speaker field coil as choke.
I usually go up to 22 mfd. The original caps may have been as low as 4 mfd.
The tolerance of the original caps was -50,+100 percent. That may be hard to believe with the tight specs. we have today, but that's the way it was. Technology for electrolytic caps was just starting to develop in the '30's and 40's.
Slightly audible AC hum was expected.
Ah...but you are rebuilding to Fender specs..that should give you a very interesting amp.
Thanks for sharing the info.
Blake

Ken Fox · Post by **Ken Fox** » 21 Dec 2001 10:41 am

Appreciate the info and help. The unit has a choke for smoothing the DC. The field coil speaker is tied to B+ and ground on this one. I ordered all new .1 @ 600WVDC for the inter-stage audio coupling (the old ones were very leaky). The Grid resistor to ground on the 6L6's appear to go up in value when heated up, drive the bias way high! Looks like I'll replace most all of the resistors in the amp as well. Not bad though, a hand full of parts and she out to be good for another 60 years or so!!

Blake Hawkins · Post by **Blake Hawkins** » 22 Dec 2001 6:11 am

Ken, Good to hear about the field coil being in shunt rather than series.
That way you'll have a constant flux density and can adjust the voltage across it independent of your other modifications.

How do the transformers in the National amp compare with those in the Fender Pro?

Blake

Ken Fox · Post by **Ken Fox** » 26 Dec 2001 2:48 pm

The only Pro I have worked on was a 1958, different animal than the early 50's model. The OT is fairly small, not much larger than I saw on a '57 Deluxe that I restored. The choke is quite large (much larger than anything I have seen even on a Blackface amp) and the power transformer seems adequately large. I imagine this amp is less than 20 watts RMS. I've got the speaker in for a recone, more NOS tunes (6N7) asand caps on the way. Should have it up in a week or so.

Ken Fox · Post by **Ken Fox** » 9 Jan 2002 9:00 pm

Got it all done! Speaker reconed with exact replacement cone. Eagle Works in Portland did it for me, you can't even tell it was ever reconed! I found an "Old Bassman" schematic in one of my three Fender tube manuals. Very similar amp, 6N7 driver/inverter to two 6L6 tubes. I changed my grid leak resistors at the 6L6's and changed the cathode resistor to 250 ohms. Now the power tube bias is to spec. The amp puts out about 13 watts RMS clean, an improvement over the original design (about 9 watts). I also changed the B+ line to Fender design, set up the filters like in a Fender, and beefed up the plate voltage to the 6N7 and the screen grids (like the old Bassman). Neat little amp! The sounds is very similar to a 55 Fender Deluxe, 5D3, I brought back to life earlier this year. I saw one on Ebay the other day that was a little ealier than this one. I think this one is mid to late 1930's.