BC Two tube preamp

[Drawbar Dave]

I have received my preamp and am now waiting for the B+ supply to arrive. I’m curious about the two boxes on top of it. I will sit down with the schematic over the weekend but what are the two boxes for? I know one must be an oil block capacitor but what is the other and which is which?

Before it arrived, I always assumed that the smaller one was the transformer for the heaters. Now I realise that the transformer is underneath the left hand side. I hadn’t seen underneath one of these until last night. I am told it’s a version two because of the 50/25 can.

Edit: I’ve managed to establish that the large box is the dual oil block cap but what is the smaller box? It looks like a third transformer from below where the wires come out but I can only see two transformers on the schematic.

[Drawbar Dave]

What do you make of this 30mfd cap? I can’t see it on the schematic!





- - - Updated - - -

[David Anderson]

One of the best ways to determine what a component is is to note what it's attached to. If it's attached to other identifiable components, you should be able to narrow down what it is from the schematic.

The schematic also shows 1uF/200V and 1uF/400V capacitors. In the 1930s, either or both of those could have been oil-block capacitors. I've seen chassis-mount metal-case capacitors in RCA gear from the 1940s.

Components from the 1930s don't necessarily look like what you'd expect based on standard packages for resistors and capacitors from later decades.

[Drawbar Dave]

Ok thanks. I’ve already identified C5 as the large double oil block cap. People refer to “the oil block” as if there is only one double oil block cap which is why I was curious about the other smaller protrusion. I have printed off several copies of the schematic at work so I can unravel this preamp over the holidays.

[David Anderson]

If I had to guess, I'd guess that that mystery component is the 1uF 400V capacitor and that there's a 1uf 200V cap under the chassis. But I've only seen a few two-tube preamps.

[Drawbar Dave]

If I had to guess, I'd guess that that mystery component is the 1uF 400V capacitor and that there's a 1uf 200V cap under the chassis. But I've only seen a few two-tube preamps.

It certainly looks that way having now accounted for every component against the version two schematic. The small block is C13 1uf 400V and C4, the 1uf 200V is mounted underneath.

[dunderola]

I do have such an amp but was years since i looked at it. But, the same type of caps are in my novachord preamp as well and is those that leak ”pcb goo”.

[Drawbar Dave]

They’re not so rare then! I’m looking forward to rebuilding this amp during the holidays :-)