Bogen Db212 output transformer replacement question

[6l6gmike]

Greetings.

I have been working on a new to me Bogen DB 212 the past few nights. I didn't bother trying to turn it on I just went ahead recapping it. Some of the modifications I made were repining out the 7199 to 6GH8a and I had problems with the stereo mono switch so I removed and cleaned up the wiring on that. While testing I noticed one channel was weak and the sine waves looked poor and choppy. I tested the driver circuit and that all looked good. Then I checked voltages on the output tubes and discovered that in one channel, one of the plates had weird voltage readings. Switching tubes did not help. So I did an ohms check across both plates and plates to B+ wire and found that the transformer was open on one the side where I had the weird plate voltage. It's the same side where there is a polarity switch some maybe that has something to killing the transformer.

On to the main question. I measured the good transformer by injecting 10vac in to the primary and read the voltage output from my digital O scope.
10/.260= 38.46 so 38.46^2 x 8 = 11834.3 Ok So 10k is a typicall 6v6 trans.

Here is the kicker that is bothering me and my main question. On the Sams it says it's a 5.6k center tap trans. Is that plate to plate or plate to B+? Of course measuring to B+ makes sense in the readings I measured. Am I overthinking this? Should I just get a new pair of 10ks? Or is it supposed to be a 5.6k? See attached picture of the replacement transformer chart. Also attached is the schematic.

 

[Tom Bavis]

I've never trusted Sams on output transformers... so many seem far from reasonable.

I no longer have a DB-212 to comparet - but I have a 8K Bogen output transformer from a DB-115 (as I recall). 2 3/4" tall. 2 7/8" between mounting holes - does that sound close? Might be bigger...

 

[gadget73]

Are you measuring that off the 32 ohm tap, or the 8? I'd use the 32 ohm just to rule out any possibility that the tap isn't precisely at the right spot

~10k sounds reasonable though. Nothing about that output stage looks unusual enough to make me think 5.6K plate to plate is likely.

Problems in the phase reverse switch could have allowed that channel to run open circuit, which could have fried the winding. I'd also look at the cap + resistor if it happens to be on the same side as the open winding. If that shorted you could lose the winding.

Honestly I'd be tempted to open up the transformer to see if there is any chance of repairing it. Maybe you'll get lucky and it broke near one of the connections to the wires. At this point you have little to lose.

 

[6l6gmike]

Thanks for all the tips. I did test it at the 8 Ohm tap. I can try testing again tonight on the 32 ohm tap but I think it's safe to assume that it should be 10k. I will attempt to disassemble the bad transformer. Like you said I have nothing to lose and who knows maybe it can be fixed. I will do that before I order the custom replacement transformer pair. Also I did replace that cap on the output tubes and I checked the resistor and it was in spec so I know they are good..

 

[6l6gmike]

I did pull apart the transformer last night and I couldn't find a broken wire where it was soldered to the lead wires. Bummer. So I when ahead and ordered the replacement pair. They won't be in until Jan and I will report back with install pictures when I get them installed.

 

[gadget73]

just keep in mind that if your replacements don't have a 32 ohm tap (and what does?) you'll need to change the value of the feedback resistor to keep the level the same. Also possible it will need some HF tuning to get it to come out right.