Blowing Fuses on amp UNLESS I'm on the variac (?)

[Lectroid]

I could use some help figuring this out. My first build is a Bell & Howell projector amp converted to a guitar amp. It's a 6V6 push-pull with a 5Y3 rectifier, 6SJ7 pentode first stage and a 6SL7 dual triode driver/PI stage. I was shooting for something like a Fender Deluxe and used the original Deluxe tone circuit. The fuse is a 2A 5mm x 20mm slo-bio. The schematic is attached. Also a table of the measured voltages.

The amp works pretty well for a first build. It's loud and delivers a nice Deluxe distortion at higher volume. Speaker = Jensen CQ-12", 8 ohms.

I have two issues with it. Maybe they're related--I'm still pretty new at this.

1. Compared to my Marshall JTM-30 combo, the sound is overly treble, almost harsh. Also very percussive, LOTS of pick attack in an unmusical way. I can't seem to get anything close to that "clean" Fender treble sound. If I look at the waveforms, I don't see any obvious clipping. Do my filter cap values look okay?

2. The amp blows its fuse instantly when I turn on the amp, but ONLY when plugged directly into a wall outlet or a power strip. If the amp is plugged in through my variac, the fuse doesn't blow. The variac is set to 117 VAC, same as my wall current.

If I run it through the variac and then though a current limiter with a 100 W bulb, the bulb doesn't flare up. It just glows steadily dim and the amp runs. I can play it at max volume without blowing the fuse. If I plug the CL into the wall and my amp into the CL, the fuse blows instantly even though the CL is barely glowing. But when the amp is plugged in through the variac, the fuse doesn't blow whether I also use the CL or not.

Why would having the variac in the loop keep the amp from blowing a fuse?

I've searched forums and followed troubleshooting lists.
I've swapped out the rectifier tube with a known good tube but it didn't work.
I've tried a solid state rectifier tube replacement. Got lots more power but still blew the fuse, no real surprise there.
I've pulled tubes and added them back in one by one. I've swapped the two power 6V6's. Right now I'm out of fuses and waiting for more.

I'm hoping someone will have ideas about other things I can check, and/or maybe have some ideas about how to fix the trebly percussiveness of the sound?

Thanks in advance for any help.

[brewdude]

6SJ7 voltages look low to me... except the cathode voltage which seems on the high side.
Do you have a schematic?

[Stevem]

Your popping fuses condition points me to a low power supply resistance to ground issue like one of your new filters is leaky, but a 100 watt limiter bulb is likely to high to show surging.

You want to use a bulb to test with that has a wattage low enough to glow brighter when you play thru the amp, especially when cranked out!
Also output tubes if bad can be ok when the V+ is cranked on slow like with a veriac , yet when just flat out turned right on will arc for a second which is long enough to pop the fuse.
If you have 2 more spare fuses then I would yank out one output tubes one at a time and see if turning the amp flat out on still pops the fuse, if not then reinstall that tube and do that same test again with the other tube out.

Another big thing I would do while doing this test I just posted is to change over to a slow blow 1.5 amp fuse, or a fast blow 2 amp because every time you blow that fuse your placing a big ass load on the PT which will do harm to it sooner or later!!

[Lauri]

Why would having the variac in the loop keep the amp from blowing a fuse?

Does your VARIAC also provide isolation? If your variac is isolated and fuse only blows when your amp is connected to an outlet with safety ground I would check if there's a short or low resistance to the amp chassis on the primary side of the power transformer.

[R.G.]

Good point, Lauri.
@OP: Check very, very carefully on the AC mains wiring in regards to exactly where the line, neutral and safety ground go. If that doesn't turn up a safety/power issue, you might read through the sticky near the top of this forum entitled "Blows Fuses Debugging" for some tips on isolating what is causing the fuses to blow.

[Lectroid]

Thanks for all the help guys.

To update, I just checked the variac and RG is on to something. It's showing an open ground on its output socket. I can probably fix that myself and then I'll report back. RG, I have read and saved your whole article. Lots of good method in there for a new guy with just a little theory and even less hands-on experience.

As far as isolation, the old tech I bought it from swore it was also an isolation transformer and he's never steered me wrong before. It's a "Warwick Adjust-A-Volt Isolated Transformer, Type IV-4M."

I'll fix the variac output socket and see if that helps. If not, I'll start on all your troubleshooting advice.

@brewdude, there's a schematic attached and I think you've caught my mistake. It doesn't sound terrible the way I did it but I'll rewire it correctly. Thanks!

[Lectroid]

Okay. Inside the variac I don't fully understand what I'm looking at so I've attached some pictures--happy to take more if that will help.

Three wires come in from the variac power cord, go through a switch, and then into the variac main coil with a meter in series. There is also a thin black rectangular component you can see on the images--that's the circuit breaker/reset button assembly. The switch, meter, and reset are all the top of the front panel.

The variac output socket is at the bottom of the front panel as you look at it and there's a picture of it below also. I can see the ground wire attachment point but there is NO ground running to it from the incoming green wire from the mains earth. There is a green wire grounding the chassis but that's all I can see. Also the wires going to the circuit breaker are green but I don't think those wires are at ground--confusing.

Shouldn't the ground of my variac's output socket wired to provide a ground? Or is my ignorance showing?

Does your VARIAC also provide isolation? If your variac is isolated and fuse only blows when your amp is connected to an outlet with safety ground I would check if there's a short or low resistance to the amp chassis on the primary side of the power transformer.

I'm still working on understanding isolation in general so bear with me. Are you saying that the PT is grounded to the chassis, but not completely grounded? That there's some resistsance in the amp grounding? I seem to have good continuity between different parts in my ground scheme. I have continuity from my guitar cord sleeve to the ground plug of my amp.

I think Stevem is saying the same thing as Lauri:

Your popping fuses condition points me to a low power supply resistance to ground issue like one of your new filters is leaky, but a 100 watt limiter bulb is likely to high to show surging.

I'll buy some smaller wattage bulbs. I'll also test the filter caps.

Give a shout if anyone can think of anything else I should look at. Otherwise I'll report back here when I know something.

Thanks again.

[Stevem]

I have 2 variacs , both made by the Staco equipment co.
One runs at line voltage, the other also steps up to 140 volts, both however have the ground on its power cord connection to the ground on its outlet jack,

[Lauri]

I'm still working on understanding isolation in general so bear with me. Are you saying that the PT is grounded to the chassis, but not completely grounded? That there's some resistsance in the amp grounding? I seem to have good continuity between different parts in my ground scheme. I have continuity from my guitar cord sleeve to the ground plug of my amp.

I'm saying you might have AC mains voltage shorting to the chassis of your amplifier. It could be a faulty power switch or the power transformer. If that's the case be very careful with the amp so you don't electrocute yourself. The problem with isolation transformers and isolated variacs is that you might not realize your amplifiers chassis is live until it's too late.

[romberg]

Give a shout if anyone can think of anything else I should look at. Otherwise I'll report back here when I know something.

Check the resistance between the power transormer's primary winding and the chassis. To do this:

• *UNPLUG* the power cord and attach one lead of your DMM to the neutral pin of the power cord.
• attach the other DMM lead to the chassis
• measure the resistance.

The resistance should be very large (might be off the scale of your DMM). This would be normal. What you may find is that the resistance is low. If it is low, then that is your problem.

Mike

[Lectroid]

I'm saying you might have AC mains voltage shorting to the chassis of your amplifier. It could be a faulty power switch or the power transformer. If that's the case be very careful with the amp so you don't electrocute yourself. The problem with isolation transformers and isolated variacs is that you might not realize your amplifiers chassis is live until it's too late.

Thanks for the clarity on that. This is something that has to be right, the amp has to be safe.

Also, Mike, thanks for the explicit instructions on how to test this.

What about adding the ground connection inside the variac from incoming cord ground to the variac's output socket? Yes/no? I'd feel safer adding that pass-through for the ground but not if it's going to eliminate some other protection by adding it.

[Phil_S]

What about adding the ground connection inside the variac from incoming cord ground to the variac's output socket? Yes/no? I'd feel safer adding that pass-through for the ground but not if it's going to eliminate some other protection by adding it.

If it was meant to be there, the manufacturer would have done that. There is a reason why this shouldn't be done. I am not knowledgeable enough to be sure of the reason, but I can speculate. I suggest jumping the ground to the secondary defeats the isolation feature and on that basis, it is probably a bad idea.

[pdf64]

My view is that for 99.99% of the uses that it could be put to on a valve amp workbench, ground isolation is a drawback, not a benefit.