5f6a w/el34's and adjustable bias popping fuses

[leydenjar]

I've checked everything I know to check in this amp and it still blows it's 2amp fuse after it's turned half way up or so.

Caps test under voltage fine.

Heater voltages good.

I'm attaching voltages on everything else.

Cathode current looks good when biased at idle.

[52 Bill]

I've checked everything I know to check in this amp and it still blows it's 2amp fuse after it's turned half way up or so.

If you are saying that when you turn the volume control up past 50% the fuse blows, the only thing that I can think of is some sort of parasitic oscillation that is drawing a lot of current. Do you have an oscilloscope available?

[pdf64]

What is your line voltage?
I'm thinking that a 3A line fuse may be more appropriate on a 120V supply.
To pick up on the previous point, have you fitted grid stoppers to the power tubes?
Just because it worked to omit them with USA made 5881 doesn't mean it will work with current production EL34.
Try 10k.
Pete.

[leydenjar]

If you are saying that when you turn the volume control up past 50% the fuse blows, the only thing that I can think of is some sort of parasitic oscillation that is drawing a lot of current. Do you have an oscilloscope available?

Yes I have an oscilloscope and would love to acquire some knowledge on how to look for a parasitic! Hint, hint It works well (the o-scope) as does my signal generator.

The amp blows fuses after about 12 oclock and when I push the amp a little (by playing).

@pdf64

Line voltage has been at 122.5 to 125. around these parts lately. Right now it's 124.4.

This amp has grid stoppers of approximately 6k and I'm told it worked fine in the past.

[pdf64]

Try a 3A line fuse.
Have you got a dummy load (or guitar amp attenuator)?
Using a scope, parasitics are usually apparant as noise / 'hash' on the waveform, it may only appear above certain signal level, on certain parts of the waveform, with certain control settings etc.

[leydenjar]

These are the pics of my 'scopes crt.

I don't have experience interpreting.

Input is .3vac at approximately 500hz.

@pdf64 - Are you sure it's safe to put a larger fuse in? It does make sense considering this amp has 2 x el34, 3 x 12ax7 and 1 x gz34 but I've never had to do that with any other amp I've fixed. Not sure.

[Diablo]

The Fender 5F6A schematic calls for a 3A slo-blow fuse, and that's what's in my 5F6A clone.

[leydenjar]

The Fender 5F6A schematic calls for a 3A slo-blow fuse, and that's what's in my 5F6A clone.

What concerns me most w/the idea of putting in a larger fuse is that the builder of this amp designed it with a 2 amp fuse.

You would think with the el34's that it's designed to run it would have a larger fuse?

A new rectifier tube seems to be solving the issue. My current meter in line w/the wall ac now goes over 2 amps when the amp is almost all the way up, but just for a second, and the fuse isn't blowing.

Has anyone else had this experience?

[Diablo]

What concerns me most w/the idea of putting in a larger fuse is that the builder of this amp designed it with a 2 amp fuse.

You would think with the el34's that it's designed to run it would have a larger fuse?

A new rectifier tube seems to be solving the issue. My current meter in line w/the wall ac now goes over 2 amps when the amp is almost all the way up, but just for a second, and the fuse isn't blowing.

Has anyone else had this experience?

Swapping from the original 5881 to EL34 tubes in a 5F6A isn't going to reduce current draw. EL34s have higher current heater filaments and also are higher watt power tubes. If the rest of the amp is a 5F6A in design, it should require at least a 3A fuse to keep from popping the fuse. Maybe the original amp builder stuck in the wrong fuse by accident or the owner did?

[leydenjar]

Maybe the original amp builder stuck in the wrong fuse by accident or the owner did?

That's what I would have thought but this is one of a small boutique company's production amps. It says 2 amp on the chassis. Maybe they got it wrong? It sure seems so. But the amp is working now, even cranked, so I'm gonna' leave it be.

Thanks Diablo and everybody else for the help.

One other question; I'm just learning how to use a scope to troubleshoot and I wanted to know if it looks like I have the right idea from the pictures I posted of my oscope measurements?