Hoffman 5F6A build issue

[sluckey]

That looks great! Enjoy...

[Apexelectric]

Thanks Steve

[tubenit]

Well done!  What a great job.  Looks terrific.

With respect, Tubenit

[Apexelectric]

Well done!  What a great job.  Looks terrific.

With respect, Tubenit

Thanks, Tubenit.

I have a couple finishing touches to do to it this weekend before I call it good but its pretty much there.

Put casters on the speaker cabinet this morning. Need to do something on the back of the head to wind the power cord on since there is no opening on the back to stick it in. Probably going to change out the screws/washers on the front of the head to blend in a little better and maybe add some to the front of the speaker cabinet to match the look of the head.

Mike

[Apexelectric]

Finally had some time to play the amp for more than 10 minutes and still had an issue with the signal dropping. Took about 30-45 minutes of playing it at more than 75% of max volume to get it to act up. Played it some more while bench testing it and all plate voltages as well as the output tube screen voltages all stayed relatively stable. Just minor drops under heavier playing.

I turned the lights out and watched the tubes and noticed a flicker/arcing in one of the output tubes. In swapping the tube positions, the arcing followed the tube. The output tubes glow orange during this entire process and no blueing or red plating occurred. The tubes test ok in my Orange tester. The tubes are dissipating right around 22W each. 440V between ground and pin 4 of the output tubes with 50mv across the 1ohm resistors. Preamp tubes seem to not show any issues. I still have the Weber rectifier installed.

This seems to be my third tube issue on this build. I’m wondering if the problem I had with my bad filament solder connection or the tube rectifier arcing problem would explain my other tube problems?

I still need to invest some time in familiarizing myself with the operation of the scope I recently acquired before I can use it as a diagnostic tool. It’s on my priority list.


I have another set of output tubes arriving early next week but before I swap them out I figured I’d better see if there’s something I might be missing to look at as an issue.

One other thing I checked, under hard playing the MV reading across the 1 ohm resistor jumps as high as 100 and the plate voltage drops to only about 400.  Is this normal? 

[PRR]

> arcing followed the tube.

Bad tube.

> under hard playing the MV reading across the 1 ohm resistor jumps as high as 100 and the plate voltage drops to only about 400.  Is this normal?

Yes. It is a class B stage. It idles "cool", 40mA-50mA/tube. Under signal, it sucks more power. My benchmark for an amp like this at FULL test power is around 400V 200mA, which is indeed 100mA per tube. So that's right. Arcing is bad.

[Apexelectric]

I knew the arcing was not good. I’m pretty sure it’s what was causing my signal drop this time. I dialed the bias back a little more and couldn’t get the signal to drop out again, even though the tube would still arc slightly. The thing that really has me curious is if my issue with the filament string or the rectifier or both would have lead to all the tube problems I’ve had. Just trying to ascertain what to be wary of in the future so that I don’t run into an issue like this again.

I really don’t mind making mistakes as it’s part of the learning process but making them twice is not something I don’t particularly care to do.

Many thanks!