Dobro amp 6107

[maxkracht]

Don't see anything obviously weird in the pictures. Probably going to need a new rectifier. Check for any obvious shorts in the power supply. Check AC voltages with no rectifier, then add a new rectifier with power switch at the ready. If the lightbulb doesn't light, check dc voltages. Add tubes until one lights up the lightbulb or you have a working amp again. Can also measure for shorts on actual tubes with a meter if you wanna be really cautious. Lightbulb limiter is your friend.

Could tack on SS diodes for testing on the bulb limiter if you don't want to risk a tube or don't have a tube yet. Backup diodes on the rectifier and maybe ht fuses would add a bit more confidence with the tube rectifier.

Gotta let the smoke out sometime, I certainly have...

[angelodp]

You think I was seeing flyback voltage on that Rectifier and cooked it? So I can run the amp on variac w/limiter and just have the diodes in place? No tube. And then check AC & DC as you described.

AC voltages without rectifier or any tubes are in tact. I have 365VAC & 5VAC at the Rect. Pins and have the appropriate AC voltage at the heaters.

[maxkracht]

If you see a flash in a rectifier, at the very least I don’t trust it anymore. Might still be sorta functional, but it’s probably dead. I have had some that arc intermittently, and you really don't want that... A couple of 1n4007 or similar will work for testing. B+ will be higher, but lightbulb + variac can compensate for that. 100-200 ohm high wattage resistor after the diodes would be more correct, but not really needed for testing if you have a variac. After you figure things out, can use the same diodes to go permanently in front of the new rectifier tube for added protection. If you already have a replacement rectifier tube, use that + 1n4007s in front of it. I was just suggesting silicon in case you had that but no replacement tube rectifier.

Smoke could be from PT or choke insulation, an arc on a tube/socket, a dropping resistor. Could be lotsa stuff. Make sure there aren't burns on the tube sockets. Could also disconnect the choke to isolate the rest of the amp from the rectifier if that seems easier. I would guess bad rectifier, or bad rectifier and a bad power tube, but just a guess. Whatever you were messing with before that happened might give some clues, might not.

AC voltages are a good sign, won’t know for sure if it's fine until the PT is under load. PT, OT, choke, or field coil insulation breakdown can be intermittent and cause weird problems too, but tube failure seems more likely.

[angelodp]

Thanks Max, great insights. I was at the 57 tube and the cap, when it went down. The owner had mentioned that the amp was intermittent at times and a jiggle of the shield cap (the black cap with holes) would get it playing again. I will get those diodes in place... have no extra NC pins to hang them on. Could just put them in series or put a small terminal strip in. You think the 5Z3 is the way to go on the Rectifier?

[martin manning]

5Z3 would be fine, that's what the schematic shows, and they are available at reasonabe cost. So far so good on the PT health. Eventually you'll have to find out what got smoked. Maybe the shorted rectifier put AC on the reservoir capacitor?

[angelodp]

Thanks Martin, so that first cap prior to the choke might be suspect? If AC got on that cap will it now be shorted?

[maxkracht]

Sometimes I just attach diodes to the ht wires themselves then attach the diodes to the rectifier socket. Extra fuses are a bonus. You can tack in jumpers or another two diodes across the tube socket to B+ then cut the jumpers/diodes when you get the tube rectifier. 2 diodes in series on each side is probably better to get the right voltage rating, but I wouldn't worry too much if its a temporary thing. Don't mind the drawing, I don't know what the correct pinout for your rectifier is, but you should get the idea.

No opinions on what the correct rectifier tube is, I don't have much experience with the old four pin ones.

IMG_9825.jpg

[angelodp]

Max, thanks for that drawing and support. Think I found the culprit. The braided wire was hanging on by a single wire. I checked the caps no smell or short?? Will redo more solidly. Going through the entire amp to see if any other frayed connections exist. Still not sure where the smoke emanated from. Going nice and slow.

[angelodp]

Re-wired in the caps with frayed wire. Here is the plan for the diodes until a tube gets in. This would be with variac and limiter. Not sure how to determine where the smoke came from. I have been checking all the connections for burn marks and the R's for previous values. How would I know if the interstage transformer has an intermittent short? I reckon once I have the diodes in and bring the amp up slowly with bulb limiter and variac I'll see if any new shorts appear, then proceed to add tubes and one at a time bring them up.

I could also add these wired fuses to further protect as you recommend Max. 1A 250v time delay (slo-blow)?

[maxkracht]

As for smoke, the usual possibilities are: something burned open and won’t smoke again, or something was stressed because something else failed short and will keep smoking until you remove your shorts or it burns open. Lightbulb limiter should tell you in enough time to prevent further damage. Lower wattage bulb = faster glow and more protection.



I'm guessing this kind of Ht fuse should be in the 100-250 mA range for your amp. Whatever your total HT current is or double? I'm sure others have different opinions. Fast is good, smaller is more protection but more likely to be popped accidentally. Whatever you have is better than nothing, to a point.



If you add diodes like I showed, two permanently attached to the red HT wires, two going across the tube socket to the filament/output, you can just cut the two going across the socket out when you switch to the tube rectifier. No desoldering is necessary. The two permanently attached are an additional safety feature and take some stress off of the tube rectifier and protect the transformer against rectifier tube failure.

[angelodp]

Hi, so I added the solid state recto circuit. Good news - bad news. As I Brought up the amp with the SS recto in place (with fuses), I got up to about 60-70vac and could hear a buzzing crackling sound from the choke, maybe even a tiny bit of smoke before I shut it down. My guess is that the Choke is shorted to some degree internally.
Fuses are 300ma and did not blow.

What about eliminating the choke and putting in a 10watt 10ohm R to test, or should I try to source a replacement choke? Of what value?

The Choke reads 470ohms so I am off by a fair amount. Would a 400ohm R at 5 - 10 watts be a good sub. My electrolytic caps are at a higher value than the originals.

[maxkracht]

Careful not to bump those fuses! maybe it just looks extra precarious in the picture?

I would go a bit higher on the resistor, maybe somewhere between 1-5k? 2-5 watts would probably be fine. Just guessing at numbers, but 470 seems low to me. Could still be other issues that caused the choke to fail, but could just be stress from the ancient caps.

I'm guessing a fender deluxe style choke would work fine, they are common and only like $20. I would probably go with whatever fits the mounting holes, so maybe go up a size if necessary?

[angelodp]

Max, fuses are just temp and I need to be extra careful with that setup correct.

So on the replacement choke, any specs on the H or just go for a fender deluxe style?

It's ok/safe to try the Power R until I get a choke in?

Thanks A

[maxkracht]

Yep, you can absolutely test with a resistor instead of a choke, and you could leave that resistor in if you like. Just make sure it doesn't add too much hum and doesn't get too hot. If I had to guess, anything in the 4-10H, 50mA or more, choke would work fine. I have no idea what the original choke was, so just a guess.

[angelodp]

Ok subbed in a 1k 5watt cement R and limiter is clean and no crackle sounds. I reckon I should source the choke as you recommended and take it from there.

Max I have a Baldwin 512-30 K choke with the correct spacing for mounting almost identical size. You think it's wort popping it in? Online I see it might be 4-8 henries but no certain.

Best A