70s Ampeg V4B Blows Fuses

[ca7922303]

Ok, with 2-5881s out, there is no smoke or smell of smoke and no hum with amp fully on. Also, I notice now both power lights are on.

[Enzo]

What I am seeing is that it burns something up and even blows fuses. SO whether or not it makes sound is irrelevant, at least at this point. We need to get the amp stable at DC voltages. SO that is why I suggested removing the tubes, and turning the standby switch ON, meaning the position like you would be playing. That means all the circuits are fully on. In that condition, does it still burn up or blow fuses?

If the amp sits there burning something and blowing fuses with NO tubes in it, what is the point of installing them?

I earlier suggested you add a light bulb limiter to your setup, that way even if you still over crank the variac, the bulb will prevent th further damage that occurs every time you smoke something.

And instead of edging that variac up and watching it burn, PLEASE add a AC current meter in the mains circuit. If you start turning up the variac and see the amp already drawing 2 or 3 amps when the voltage is still only 20v or something, then you know to stop and back off.

[ca7922303]

I was unable to see the tubes light up until I hit 100v on variac is reason I went to 100v.

[ca7922303]

I'm aware that it was built for 7027s but can use 5881s, 6550s, etc. I assume that person I got it from had it working at some point with 5881s. Can you give me more details on what you are thinking? Thanks for your help. Appreciate it.

[Enzo]

And yet we are stil cranking it up on the variac until it smokes???

[vintagekiki]

The Ampeg V4B has 7027 power tubes.
The Tube 5881 you mention has a different base connection, and diferent bias.

Consult the datasheet and check base connection.
https://drtube.com/library/tube-datasheets
https://drtube.com/datasheets/7027-jj2005.pdf
https://drtube.com/datasheets/5881-tungsol1962.pdf
https://drtube.com/datasheets/6l6gc-jj2003.pdf

http://web.archive.org/web/20120623045707/http://www.diyguitarist.com/GuitarAmps/PowerTubeBias.htm
Power Tube Bias Charts

[ca7922303]

Ok, here's what I know now. I just got this amp and was told that it was blowing fuses at power on, so I did not come out of standby until just now at 100v on the variac I have all preamp and power tubes lighting up and warming up.(2-5881s only installed), it is a 6 amp slo-blo and it did not blow but the humming increased and instrument cable being touched had no effect on output and then I smell and see smoke coming out from around 6k11 tube

[g1]

Many of these units have a 'safety' fuse on the inside. It is in series with the outside fuse. If someone puts in a larger mains fuse, the internal safety fuse will blow.

schematic: https://www.ampegv4.com/images/schematics/V4B.jpg

[Enzo]

I may not be asking right.

I'd like to know if a new good fuse STILL blows with all the tubes removed etc. I understand with a blown fuse or with no fuse tha amp would be totally dead and inert, that is any amp.

[vintagekiki]

About blown fuse
What type is fuse. Slow or fast?

[Jon Snell]

I would disconnect the two red HT wires from the bridge rectifier, if the mains transformer is not cooked and look for heaters.
No heaters = either the 6A ot 10A fuse has blown or your transformer is now toast.

[ca7922303]

I replace 6 amp fuse if blown, with power tubes out, no hum, no filaments, and the power on lights don't work with or without power tubes in.

[J M Fahey]

With no power tubes in stanby there is no hum and preamp tubes do not light up.

No hum for sure, but what about filaments?
And pilot lights?

And as Enzo says, what about mains fuse?
If blown, you will get NO signs of life whatsoever.

[Enzo]

And the blowing fuse?

[ca7922303]

With no power tubes in stanby there is no hum and preamp tubes do not light up.