Blowing Fuses Unless I Use A 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.
• 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.

[tubeswell]

If it’s a modern PT with grain oriented lams, the startup will cause a surge that will probably blow underrated fuses.


Edit - I see you're running 2A Slo-blo - should be enough for an amp like that.


So its probably something between one of the secondaries and ground, where there's a low resistance pathway at cold temps that triggers the overload. At normal operating temps there is enough resistance for this not to happen.


It could be a faulty lamp. (Try taking the lamp out and switching the amp on.)


Or it could be a bad tube filament - filaments have low resistance at startup cold temps anyway, but you might just have a filament that takes longer to heat up and triggers excessive current. (Try a tube swap)