Old Elk Viking Amp Head Scratcher

[jcollins]

I've got an old Elk Viking Bass Amp on my bench right now.

The schematic is here:

http://www.drtube.com/schematics/elk/vkb-100.gif

From the schematic, it's got a transformer phase inverter that drives two push pull pairs going into two separate transformers.

On this amp one transformer has been replaced.

The problem is the amp hums fairly loudly. I did a cap job, but it still hums.

Well, through trail and error, I found out that only one side of the power amp
produces the hum. The side with the replacement OT. If I pull the pair of tubes driving the replacement OT, the hum goes away.

Both pairs run from the same B+ source. It doesn't show it in the schematic,
but the heaters for each pair are run in series and the sets are wired in parallel. So if you pull one tube of a pair and power up, the remaining tube
doesn't light but, the other pair both light.

It doesn't matter which pair of tubes I put on the bad side, I still get a hum.
The bias is about the same at each socket of the humming side.

Not sure where to go from here, except to swap in another OT, although I've never heard of a bad OT causing hum.

Any insight is gladly accepted.

[Enzo]

Both 150 ohm grid resistors measure the same?

I would dewire that OT in a heartbeat and clip in another just to see. COuld be a shorted turn in it.

[Twist]

It appears that the outputs of the speaker trannys are wired in series to drive an 8 ohm load. I would first wonder if the replacement OT had the right primary and secondary impedance. It may be that the primary wires on the replacement OT are backwards. A typical amp with negative feedback would howl with the wires backward but this amp is not a typical amp.

[Steve Conner]

Yikes, that is one crazy amp.

My guess is that the amp has another fault in the preamp that makes it hum, and then in addition to that, the *other* OT, the one that wasn't replaced, is shorted. So when you pull the tubes driving the good OT, the power amp stops working completely and you can't hear the hum any more.

If both OTs were good, then pulling either pair of power tubes should have stopped it working, because the secondaries are in series.

I wouldn't care to try running this amp with missing tubes anyway, since the series connection could generate high voltage at the empty sockets. Maybe that was how the OTs got roasted, from someone trying to reduce the power by pulling two tubes.

If I had this amp, I'd parallel the OT primaries to stop stuff like that happening. It's a bad design. I bet it sounded good while it was working though. It's like a mini Fender 400PS without the EQ.

[Enzo]

If both OTs were good, then pulling either pair of power tubes should have stopped it working, because the secondaries are in series.

Really? The lack of power tubes on one primary would render the associated secondary open? I would have thought the current from one secondary would simply flow through the other. The inductance might affect the freq response a little. Seems to me either transformer by itself would still make sound come out the speaker.

It's a dumb design though, I agree. I suspect expedience raised its ugly head. They'd rather just use two 50 watt OTs they already stocked than to spec a new 100 watt OT. Just a guess.

[custom_amp_30]

Well, Now on Dr. Tube, there is the correct schematics posted from an engineer who worked at the plant during the sixties. I know this is a old post, but for anyone still out their who owns a Elk, The answers are finally out there. Good Luck.