Dobro amp 6107

[angelodp]

Primary wires are clean on shorts or ground connections. I have 2.3ohms between the leads so that's good.

[maxkracht]

Gotta be something connected to something on the primary side before the transformer then.

[angelodp]

I don't know. Primary side is two wires. The intro of the IEC is causing the amp to not conduct??

One side of primary to Neutral, the other side of primary to switch, back to fuse, back to Hot on IEC. Ground is ground.

I am gonna try this one last time tomorrow when I am fresh. I am mindful to not stress this very old amp.

[sluckey]

One side of primary to Neutral, the other side of primary to switch, back to fuse, back to Hot on IEC. Ground is ground.

That works on paper. It also works in all my amps. You must be overlooking something.

[angelodp]

Steve, so the cap on the input circuit is a vestige and not a worry? With or without?

[sluckey]

One side of primary to Neutral, the other side of primary to switch, back to fuse, back to Hot on IEC. Ground is ground.

You want this and nothing else.

Remove that silver cap from the on/off switch.

[angelodp]

OK, whatever I was doing was wrong and now I have it right and it works. A nice nights sleep and fresh eyes. And oh by the way the old Marshall fuse holder was blitzed

Waiting for new holder to remove the in-line fuse.

Now I can address the old caps. Still sounds good, very good.

Thanks for hanging in there with me on this dear old amp.

Ange

PS this should have taken a hour to do.... oh boy!!

[maxkracht]

That's good, sometimes you just need a reset. Cap to the right of the PT looks hollow from that angle, any chance someone already stuffed all the cardboard/wax caps?

[angelodp]

Hi Max, looks original, hard to tell if stuffed. It reads only .14uf and the can says .5uf

I need to order some replacement caps.

I see .5uf @400vdc for the Bias arrangement w/800 ohm R

2 4uf @ 400v - replace with 10uf @ 450V (both)

2 8uf @ 450v - replace with 16uf @ 450V (both)

2 10uf @ 25v - replace with 10uf @ 50V (both)

Does that seem ok for this older amp.

All the wire wound R's have drifted up about 15-20%, amp sounds pretty good so might just leave it as is. '
It's not running overly hot so dissipation seems in line.

[maxkracht]

All seems reasonable to me.

[angelodp]

See that cap under the parallel R's, it is the bias cap. It reads .5uf at 400v. - I believe they used an electrolytic for size (those days). I assume it's safe to sub in a non-polarized cap. .47uf 400v Mallory?

[maxkracht]

See that cap under the parallel R's, it is the bias cap. It reads .5uf at 400v. - I believe they used an electrolytic for size (those days). I assume it's safe to sub in a non-polarized cap. .47uf 400v Mallory?

That should work fine. I don't think you would have anywhere near 400v on the cathodes unless I am really misunderstanding things. It's on a 2.5v heater, right? I'm guessing you could treat it like any other cathode biased amp and put in a larger cap for a bit more bass/volume. Say, 22uf/50v as a nice standard value?

[angelodp]

Ok, great, makes sense. So keep to an electrolytic and use the standard value.

Have almost all the old caps out.

Onrush of voltage at the choke is 360vdc max so I have a 450vdc rated cap I can use there.

So all caps are in (used on hand stuff all 105), Gain increased quite a bit with the fresh caps. It's stable and loud now. I'll run for a while. I still may get some F&T's for the main caps but it's running very well as is.

[angelodp]

I am sourcing some backup tubes. Any issue with the 80 vs 5Z3 rectifiers. This amp has the 80 and seems no issue. They have a similar max voltage but the 5Z3 can handle more current.... but I don't think that is an issue with this amp. Anything to gain from switching to the 5Z3 as per the schematic?

The 57 tube seems a tad microphonic by way of the cap cable which rings at times until I repo the shielded cable.

I re-tensioned the 57 tube pin sockets, which are unique metal loops. That socket is trouble and goes intermitant at times. I think the re-tensioning will solve it.

Amp this old, you look at it and it stops. So far I am keeping it going.

[angelodp]

Well, I am going to need some help. The 57 tube was making noise and then saw some flash in the rectifier. Then a bit of smoke coming out of the amp. Shut it down. Pulled all the tubes. Put it on the bulb limiter and variac with no tubes and got no shorts no smoke. Put in the rectifier and brought it up with the variac and the bulb lit up, no smoke, shut it down quickly. Thats where I am at. I could not tell where the smoke was eminating from, but it seemed to be from the PT end of things.

First time letting the smoke out. Could use some recommends on best way to proceed.