Dobro amp 6107

[angelodp]

Hi, I am looking over a Dobro amp for a friend. It's a hand-me-down from his Grandfather. Lo and behold it plays very nicely, no appreciable hum and sounds good. I cleaned it up a bit and tightened up bolts and nuts. It was worked on by a tech a while back and has some newer caps. It does not have a grounded plug. Some of the older wax caps are leaking a bit. My thoughts are to either to remove the AC receptacle and install an IEC or
a grounded cable with plug. This amp gets used lightly in studio. Any recommendations appreciated.
The speaker is in tact and sounds good. The cab needs some TLC, while keeping the general feel in tact.

What type build should I source please.

thanks A

[maxkracht]

Fuse looks a bit sketchy. Definitely replace any/all caps and double check those resistors. You would think the wirewound looking things would be fine, but sometimes those are really off. Be careful with field coil stuff, I've been bitten from unexpected places...

[angelodp]

Thanks Max for the heads up. Can you elaborate on the coil dangers.

I could put in a fuse holder by opening up those previously drilled holes? The wax caps are a wash.

What about on the case wear and tear. Is it worth salvaging this bottom piece or cut it away and glue in a matching piece.... If I can find it.

[Stevem]

It's cool to see one of these in such good shape!
The last one I did looked like it came off a WWI battle field .

Those solder connections on the parts that have been replaced looks like a 3 year old did them!

One very important thing to focus on with old amps this is the use of the bolt down / rivet down lug on any terminal strip being used a a ground .
These can get darn funky over the years and give you a call back on the repair.

The same goes for any lugs or rings that are used for a ground.

I would tell the owner to stock up on the tubes that this amp uses while he still can, all the other issues that may lie in there can be dealt with.

[sluckey]

The color code for those old resistors is the same for modern resistors. The order is "body, end, dot" (BED). The body color is the first digit. The end color is the second digit. And the dot is the multiplier. For example, that green resistor with a black end and yellow dot is 500,000 or 500K.

That schematic does not match your amp. Your amp has many more resistors than shown on the schematic.

[maxkracht]

Thanks Max for the heads up. Can you elaborate on the coil dangers.

Nothing specific, just be overall even more cautious than normal. There is full b+ near the speaker, if not on the speaker bell/housing itself from leakage or sloppy/old wiring. This can also lead to a hot chassis situation. I usually expect everything can bite me, but the really old stuff can surprise you.

I would try to save the cabinet. Old box is half the charm.

[angelodp]

Thanks for input, dealing with a number of cosmetic issues..... Handle - trying to salvage it.

[angelodp]

Looks like my tube compliment is

2- 2A3 power tubes

1- 56

1- 83V Rectifier

and in the 56 position I have the tube shown but has no markings. As Sluckey indicated this schematic is not my amp, close but not exact. I am working on making a schematic.

Can anyone ID this tube - Sylvania and says 5ER on base

No luck on Google picture search

6C6 looks like a match. ??

[Stevem]

A 56 is a medium mu single Triode, whatever that tube is with a plate cap is not.

[martin manning]

Have a look here. The model 2A3 LA278 looks pretty similar to what you have.
http://prewaramps.org/natdob2a3.htm

The tube with the plate cap looks like a 57, as used in the 2A3 LA278. https://www.radiomuseum.org/tubes/tube_57.html

[angelodp]

Martin, right you are. Upon closer inspection, I can just make out a 5_ and it does look like it's a 57 Tube and you have me on the correct schematic.

Thanks

[angelodp]

Can I ask how this specific amp is biased?

[martin manning]

It’s cathode biased. The 1k at the filament winding center tap is a shared cathode resistor.

[angelodp]

Ah, I see that now, thanks Martin.

I am about to install a fuse holder and IEC socket. the amp had a 3A fuse tacked in. It seems that a 1.5A fuse is more appropriate. Data shows 15-20 watts for 2A3 in PP. Is 1.5A the way to go?

A

[martin manning]

The start-up surge current will probably determine the value. 1.5 seems more appropriate than 3, but I think you’ll just have to try it and see if it holds for a number of cold starts.