70s Ampeg V4B Blows Fuses

[ca7922303]

Pulled two bad 5881s when received amp. Using two tested good 5881s, amp powers on. In standby getting a hum and no power or preamp tubes lighting up. When taken out of standby, 6 amp fuse blows. This occurs using variac at about 60v.

[Enzo]

Well, what does it do with no 5881s at all?

But mainly, you are mis-using the variac. The variac is not a see how high you can get it game. What do you learn by blowing fuses with it?

Put a current meter in the mains feed through the variac, turn the amp on, and slowly ramp up the variac. if the current starts to ramp up, STOP, turn it off.

Leave the standby switch on, in the operate position. That way whatever the variac might reveal will do so right away.

I would measure resistance to ground from pin 3 of each power tube socket just to check for a shorted protective diode.


It might serve you to add a light bulb limiter to your variac setup.

[SoulFetish]

Enzo is spot on here. A variac is a nearly useless tool to an electronics tech without a means to monitor the mains current. To be clear, you need to be able to monitor the current fed from the variac wiper, which provides the mains to the device under test (your V4 in this case). The best way to do this is using an analog panel meter because it will respond much faster than a digital meter to any changes in current, which is what you need. Digital true RMS meters are much more accurate in giving steady state measurements, but when your monitoring for a fault you don't need dead on accuracy, only that there's a problem. When you get familiar with the response and behavior of an analog meter, you can often see signs that a fault might be about to happen.

[ca7922303]

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

[Enzo]

And the blowing fuse?

[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.

[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.

[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.

[vintagekiki]

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

[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.

[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

[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

[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

[Enzo]

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

[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.