Recently Repaired 1968 Blowing Fuses... What to do?

[sntz2]

Hey y'all,

Recently I got my Sunn Sonaro repaired. I got it for a steal in a beat to hell state:

• mismatched 6L6 tubes, no idea about biasing
• ripped off a combo and in a custom too-short enclosure for modern EL34s or 6L6s
• ungrounded plug and death cap, tons of buzz

So I got it repaired and worked w the tech to settle on this:

• new JJ 5881 tubes, rebias, and a new JJ GZ34
• new power section caps, bypass the multicap w regular caps
• new plug, get rid of the death cap
• replaced some burnt resistor in the power section

Overall, was pretty happy with the work. I understand circuits, but am not experienced with higher voltage power sections so I stick to pedals and bring amps to techs.
I try it in and the shop and it works great. I take it home, it doesn't want to turn on. Okay. I wait one day, try again, works fine.

I play it for about a month and yesterday I get the same thing, except waiting a day does nothing.
I check the fuse and it's blown. Not super burnt or anything, but no continuity with a multimeter. Visibly the wire is broken.
I ask the tech what they think and they think it could be faulty switch wiring. Not sure how that relates to the fuse blowing, but also unsure why the fuse suddenly blew after working (if it was bias or something I'd expect that it'd blow everyday I switch it off standby, right away)

What do y'all think? Any harm in trying new fuses? Does it seem like it could be a bad transformer?

[loudthud]

A "Light Bulb Limiter" will keep fuses from blowing and assist in troubleshooting. Not likely to be a bad transformer. Lets try to figure out what caused the fuse to blow.

This should be the correct schematic for your amp. If your amp has a 7199 tube, it's the correct one. If your amp has a 6AN8, it's the 60W version like the 200S. Is there a date or inspection sticker anywhere on the amp ?

A chronic problem with these amps is the way terminal strips are grounded to the chassis. The pop rivets can get loose over time. I recommend that you get a large soldering iron and solder any terminal strip that is used for a ground to the chassis. The way those new caps are grounded is likely the source of any buzz, but you should fix the fuse blowing first.

As you can see from the schematic, the amp came with EL34s. 6L6GC or 5881 are not as strong in this application. Can you get your old GZ34 back from the people that replaced it. It will probably last longer than any currently manufactured replacement.

Keywords Sonaro schematic

[loudthud]

Is the burned resistor on the terminal strip circled in red on this pic ? That's the bias supply. When the bias supply fails, the power tubes draw maximum current until something breaks, hopefully the fuse. Question is, what made it fail ? In normal operation it doesn't receive much stress.

Is there a red wire with a yellow stripe coming out of the power transformer ? I didn't see it in your photos.

[loudthud]

Do you have a DVM with a DC Volts range of 500V or higher and a diode check function ?