Ampeg V4/VT22 blowing fuses

[ckeene]

Hi, I'm trying to triage an amp for a buddy that keeps blowing the main fuses (10A F + 6A Slo in series). It's an Ampeg VT22 (same as V4, but a combo). The build inside is tight, and frankly, it's a little more amp than I'm comfortable poking around too deeply with.

Mostly, I'd like to see if the problem is the PT, and then take it from there. What's the best way to test it - disconnect the high voltage secondary from the rectifier and disconnect the heaters, then turn it on and check voltages? Or can I leave the secondary leads connected and check with an ohm meter with the amp unplugged?

Thanks.

-Chris

[MrMike]

I would take out the chassis, yank all the tubes and power it up again. Keep an eye and nose on things. If not blowing the fuse, then check the voltages.

Is it a valve or ss rectifier?

[Manoteal]

It has a solid state rectifier and it puts out very high voltages. So high that the filter caps are in series for high voltage handling. I suspect that one or more of your filter caps has died, drawing enough current to pop the fuse. But take out at least the power tubes to ensure that it isn't a tube shorting out. I have one and when mine went due to a bad output tube, it took two others before smoking my screen resistors. Funny that it didn't pop the fuse, but I forget what fuse was in there.
M

[ckeene]

i lifted the secondary leads off the board and took a reading:
490VAC across the secondary, and it didn't blow any fuses. So I assume the PT is ok and there must be a voltage doubler at work in order to get 540VDC to the plates, correct?

So signs point to a filter cap that failed shorted. The caps are big in this and there are 3 500V ones (one that's 40-40-40uf, another that's 40-40-70uf and another that's 40-70uf). Where can I find replacements for these? I think even if I fond one that's bad, I should replace them all as a matter of course.

Thanks!

[kinmike]

Antique Electronics (tubesandmore.com) has some close replacement values in their multi-cap cans. Also try Fliptops.com for all things Ampeg.

Also a tip from another forum:

If you do a cap job, recommend re-building the bias supply at the same time. The bias circuit in these amps is built with caps of marginal ratings. In addition, the circuit is located in a part of the amp that gets VERY hot.
Replace the entire bias circuit, as well as the screen grid resistors, with parts of higher rating as a matter of course. MOST of the V-4 amps will have scorching on the PCB from the heat in those areas.

Mike

[ckeene]

Thanks for the help!

The main circuit board is installed with components facing down, but accessible through a removable panel in the underside. At some point, the bridge recto was rebuilt, but instead of being nicely soldered back onto the PCB, were ganged together (strangely, 3 per side, rather than 2) in sort of a rat's nest on the back of the board. I'm not sure why there's a third diode in there.

[bentaro_rifferashi]

fliptops.net

[Manoteal]

Thanks for the help!

The main circuit board is installed with components facing down, but accessible through a removable panel in the underside. At some point, the bridge recto was rebuilt, but instead of being nicely soldered back onto the PCB, were ganged together (strangely, 3 per side, rather than 2) in sort of a rat's nest on the back of the board. I'm not sure why there's a third diode in there.

The third diode is there becuase whoever put those in there was trying to do it 'safely'. Apparently the diodes he used weren't up to the voltage ratings needed, so he added more so they wouldn't fry. Maybe they did the first time, so he redid it with three. If you get high enough ratings on the diodes, you don't need to do that. But they are more expensive and harder to get, so he was either lazy, cheap, or in a hurry!
Manoteal

[ckeene]

Thanks for the help!

The main circuit board is installed with components facing down, but accessible through a removable panel in the underside. At some point, the bridge recto was rebuilt, but instead of being nicely soldered back onto the PCB, were ganged together (strangely, 3 per side, rather than 2) in sort of a rat's nest on the back of the board. I'm not sure why there's a third diode in there.

The third diode is there becuase whoever put those in there was trying to do it 'safely'. Apparently the diodes he used weren't up to the voltage ratings needed, so he added more so they wouldn't fry. Maybe they did the first time, so he redid it with three. If you get high enough ratings on the diodes, you don't need to do that. But they are more expensive and harder to get, so he was either lazy, cheap, or in a hurry!
Manoteal

Two 4007 diodes should be fine for just about anything you throw at them, right?

[Manoteal]

At 1000V per diode, 2 in series should be quite adequate.
M

[linusberg]

Hi Chris, I have a vt22 which is also waiting for a recap as soon as i find time to do it. I have done some research regarding the cans and found that fliptop as mentioned by Kinmike has some "good enough" cans that could be used without to much hassle, however looking over some of the schematics i figured i would try to do a "cheaper" recap and rip the cans out and use of the shelf caps instead, and as i found out i wasnt the only one to consider this approach. You can find some nice examples that will simplify your work here:

http://www.geocities.com/flintstudio/Re ... g_Amp.html


otherwise use the fliptop recap kit:
http://www.fliptops.net/catalog/product ... 6e6d6d3e6d


Good luck getting that baby up and running again.

/Peder

[linusberg]

I appologize for the incorrect link, the proper url is:

http://www.geocities.com/flintstudio/Re ... g_Amp.html