Blows a fuse at high volume

[jewishjay]

Hey amp experts, I built this Orange inspired amp and it works, and even sounds pretty good up to half volume BUT with the master volume above 75% it howls a low tone (I donno, maybe 100hz, I haven't scoped it) and before the master volume reaches max, the fuse pops. I've probed all over for shorts, but haven't found one. Anyway, I suspect its related to the MV and the wolf tone. A low freq oscillation maybe?

[tubeswell]

Not much to go on there.


First thing that springs to mind is a microphonic preamp tube. Do the fingernail flick test. If it jangles like a bell, swap it out.


If it’s random oscillation, try a few grid stoppers soldered right onto the signal grid socket pins. Same with screen grid resistors.


Chopstick the caps and resistors throughout to see if there’s a culprit there.

[sluckey]

Swap the OT blue and brown primary leads. Any joy?

[GAStan]

With or without something plugged into the input?

[jewishjay]

Swap the OT blue and brown primary leads. Any joy?

Thank you! In the past when I've accidently had positive feedback, amps whistled like a tea kettle. This was more subtle, it didnt howl until the master volume was above 75 or 80% so I didn't connect it with that.

But that part is solved. No more PFB howl. Thanks Slucky!

However, at max volume it still pops a 5 amp fuse! It draws a LOT of current. I search and search for a short, but can't find one.

If I plug it in through the light bulb limiter the bulb glows brighter than any other amp I have. And without the light bulb, you can now max the volume and gain if you leave the EQ at noon, but when you max all 4 knobs it blows a fuse. All this is with or without a guitar by the way.

So why is it drawing so much juice?

[sluckey]

Maybe you need a bigger fuse or slow blow type. What kind of power tubes? Measure your actual line current.

[jewishjay]

Maybe you need a bigger fuse or slow blow type. What kind of power tubes? Measure your actual line current.

It's blowing 5A slows. I ordered some 6.25A slow, they'll be here at the end of the week.

4 Electro Harmonix EL34

What do you mean by line current?

[sluckey]

What do you mean by line current?

The current that is flowing through that fuse. Just remove the fuse and connect your meter leads across the fuse terminals. Measuring the current usually involves moving the red meter lead to another jack on the meter. READ YOUR MANUAL AND BE SURE YOU UNDERSTAND!

[nandrewjackson]

Looks to me that you do not have the output tubes wired like the schematic shows.


In the photo you do not have pins 1+8 wired together to ground like the schematic shows.


I realize an el34 has suppressor grid, and a quick search only shows me suppressor grid either being used with an ultra linear OT , or connected to pin 8.

[sluckey]

In the photo you do not have pins 1+8 wired together to ground like the schematic shows.

That's not good. Using pin 1 for a tie point for the control grid stopper works fine for 6L6 type tubes but not for EL34s. So, the suppressor and control grids are tied together. Not sure what that would do, but if I had an amp that was blowing fuses, I'd make it right.

[jewishjay]

Oh snap! Youre both right. Good catch!

I was thinking of 6L6 and 6V6. I always use pin 1 for the stopper and pin 6 for the screen. Well...I feel dumb. Thanks guys. Let me work on that, and hope I didn't do any lasting damage.

[Greg]

If I understand correctly, the whole negative supply sits on the supressor grid? That shouldn't cause any fuse problem. In fact, the amp might be running cooler at high volume with this set-up. If you consider that the role of the suppressor is to reduce emissions to the screen grid. I don't know how it affect the tone though. Life of EL34's might be improved. I saw this hack on Traynors. Popular hack is to put more series resistance than typical on the screen grids. 

[Greg]

Never mind my last post. I misread. I understand now that the signal voltage on the control grid was also applied to the supressor. Can't be good.

[jewishjay]

Ok. Update. It was three completely different issues.

1. Thank you for finding the wiring mistake. That was my flub.

2. Positive feedback was corrected by swapping blue and brown primaries.

3. It seems that the fuse failures were a bad JJ GZ34 with intermittent internal shorts.

I will keep testing, but I ran it for 30 minutes this morning with a different GZ34 without issue. The kill-a-watt meter says its drawing about 1 amp. Plate voltage is higher than I would like, so I might try a 5U4.

[dude]

5U4 draws more filament current than GZ34,  can your PT handle it..? 5U4 draws 3A, GZ34, 2A

[jewishjay]

5U4 draws more filament current than GZ34,  can your PT handle it..? 5U4 draws 3A, GZ34, 2A

Yes, and I did think of that. So far I like the 5U4 better, same bias at a slightly lower PV. The amp is running well, no more fuse failures, and only drawing around 1A of current. Next step is voicing, and I might ask for help with that soon....

[dude]

Put the 5AR4 back and use a Bucking Transformer to lower wall voltage, close to the 115vac we had then, this will lower B+ close to original. Then start tuning the tone to suit.