Just Finished my new Vibroverb build. Popping fuses...

[Blackdog]

I need some troubleshooting ideas...
Just finished this TAD BFVV kit. It's built pretty much like a clone: eyelet fiberboards, cloth covered wires. The layout is pretty much the same as the old Fenders. The quality of the components is top notch, and everything is brand new: TAD tubes and xformers, ceramic sockets, carbon composite Rs and OD caps.

It has already blown the fuse (2A) 4 times.
First time was when I was when I was starting to power on, only the 5AR4 and the 6L6s where in their sockets: switched the Standby to ON, a brief "BZZZT" sound with a hint of arcing inside the rectifier and fuse blown. At the time I thought it was a case of reverse OT wires, oscillation and such, so I disconnected the NFB loop and fired up again without problems. Later on I discovered this was not the issue at all, but that's a different story that has been already sorted out.

After that episode the amp worked fine, I biased the tubes and called it a day.

Yesterday I set the scope and signal generator up for further testing. First turn on of the Standby, same issue as before: Bzzt, arc, fuse blown.
New fuse and everything worked again. Could see that I had no spurious oscillations and that I could get a solid 39Wrms before clipping on a resistive load. I assembled the amp and played for a while at very moderate levels. The amp sounds loud and bold, I really love the tone it has. The hum is nil, there's a bit of background hiss but I think is well within reason. For all these operations I turned it on and off several times, obviously.

Eventually left it on, unattended for about 15mins and when I came back the fuse had blown again (this time I can't confirm that there was the Bzzzt/arc thing, though).

So it does it when you engage standby to ON, but not always. And it seems it does it spontaneously too while running idle.

I will try replacing the rectifier tube, I have a 5U4 in the parts drawer and I could borrow a 5AR4 from my other amp. But somehow the culprit seems to be "after" the standby switch (it never blew the fuse while in standby, so far). Could it be the switch itself ?? It's a Carling and the power switch that came with the kit (same brand, dpst) already failed on me and had to be replaced... I thought about the first stage filter caps, but these are the usual dual 70/350 in series, it means that BOTH should fail at the same time...

I need some more troubleshooting ideas, can you please help ??

I have two other small issues remaining with the build and one solved, I'm mentioning them here in case someone thinks there's a relation:
- When the tremolo oscillator is running (shorted plug in the pedal jack) I have a pumping sound in the speaker. Not too bad but noticeable.
- Listening to the amp in idle there are some coming and going subtle "frying" sounds on top of the background hiss. This should not happen with all new components, right ??
- There were some intermittent low volume "wailing" noises that were physically associated with the AX7 I had in V4. Thy would change appear/disappear when moving the wires around and even moving the metal shield !! I moved the tube to the V5 position and eventually replaced it with another AX7 and this problem seems to be solved.

Any help will be greatly appreciated.

[Blackdog]

The voltages on the tube pins are here:
http://www.el34world.com/forms/valvedata/BD_BFVV_1.htm

[tubeswell]

Could be a short somewhere from the B+ to ground - check all B+, plate, and screen to ground with your R-meter for unwanted continuity.

Otherwise biasing the output tubes way too hot can also cause fuses to blow.

[pzung]

hey,
i hate locating current leaks. if you think it's that switch, replace it so you won't be suspecting it.

-do you remember soldering anything stubborn repeatedly mercilessly cooking the part or melting it's insulation like a shielded wire?

-look all around inside at the chassis, any burnt spots? post a gut shot.

-did you tie off and insulate the center tap for the rectifier tube filaments where it can't short to anything?

-does it blow fuses with all the tubes out even rectifier? some kind of wiring mistake or bad PT

-does it blow the fuse with all the tubes out except the rectifier and switched to hi-volts on? something in the B+ power supply. if all the parts are new look for dull cold solder joints, somewhere where the solder is just sitting on top of the connection.

-does it blow fuses with the power tubes in but no pre-amp tubes? bad signal coupling cap, shorted cathode resister, shorted pre-amp tube.

-did you use any NOS coupling caps? could be leaking.

- right after it blows a fuse, unplug the amp,drain the filter caps and feel around for a hot part.


hope that helps

[HotBluePlates]

And if you suspect rectifier arcing, it probably won't hurt anything to install the 5U4 to see if that keeps the fuse from popping. If it does, the 5AR4 was likely bad.

Always suspect wiring error or solder drips, stray wire on your part. Everyone makes the same dumb errors; the experienced guys just assume they made a dumb error, and therefore find it faster. I know I've made plenty of them. I just know to look for mistakes on my part in a new build.

[bnwitt]

Well if I understand your posts correctly the fuse popping is intermittent.  That is a tough one to find.  If it was instantaneously popping the fuse I'd use a light bulb limiter and Gerald Weber's amp test procedure for an amp blowing fuses.  If you don't have a light bulb limiter, than you should build one.  It's just a few dollars in parts and if you don't want to buy new transformers when starting up new construction amps with wiring errors, they are worth their weight in gold.  I never start any amp up that comes into my shop without first running it through the limiter.  The limiter may still work for your intermittent issue if the amp will do it's thing while you're watching it on the limiter.  You'll see the light get bright and the fuse will not blow so maybe you can then poke around with your meter and find the culprit.

Gerald Weber’s instructions for using a light bulb current limiter in an amp blowing fuses are below:


Step 1. Plug the amp into the light-bulb limiter's outlet. Turn the amp's "Power" switch on and leave the "Standby" switch in Standby mode (if available).  Observe the intensity of the light bulb’s glow.  If the amp is ok, it will probably come on brightly at first as the in rush current flows and then dim down considerably.  If it does not dim, the amp has a short and it is probably the rectifier or the power transformer unless there are filter caps up stream of the standby switch and then it may be a shorted filter cap.

Step 2. If the lamp is dim (no short) in standby mode, switch from "Standby" to "On/Play". If the lamp lights brightly and stays bright, this indicates a short somewhere in the amp downstream from the standby switch.

Step 3. Remove power tubes, one at a time. If the light stays bright after a particular tube is removed, that tube can be replaced in the amp as it is not defective. If the light dims dramatically when a particular tube is pulled, the tube you just removed is shorted. If none of the tubes appears to be shorted, go to the next step.

Step 4. Remove all power tubes together. If the light dims, the problem is probably in the bias supply. If the light remains bright, the problem is (probably) a bad rectifier, shorted power transformer, or shorted filter cap - see next step.

Step 5. Now switch the amp to back to "Standby" mode. If the light dims dramatically or goes out, the problem is in the filter caps downstream from the standby switch. If the light keeps shining bright, the problem is in the transformer, rectifier or any filter caps upstream from the standby switch - see next step.

Step 6. Remove the vacuum tube rectifier, or disconnect the diodes if it's a hard-wired solid-state rectifier (with the amp unplugged from the power source and drained of all power of course). If the light dims when the amp is turned on without a rectifier, the rectifier is bad. If the light stays bright without a rectifier in place, the problem is probably in the power transformer.

[Blackdog]

Good stuff guys, really helpful.

If it is indeed a bitch to find leakages, much more so intermittent ones...
I think I nailed it, or so it seems so far... Quite a few turn-ons and 5 hours burn-in without fuse blowing seem to suggest so.

But let me address your points:

-if you think it's that switch, replace it so you won't be suspecting it.

In the end I didn't replace the switch. It was unlikely, and the power switch that failed on me was a DPST one and had a mechanical failure. Not much reason to really suspect the Standby switch...

-do you remember soldering anything stubborn repeatedly mercilessly cooking the part or melting it's insulation like a shielded wire?

No, I have my fair share of experience in electronics. But I agree, this one is a classic.
This is a very old-school build, it doesn't have any shielded wire anywhere.

-look all around inside at the chassis, any burnt spots? post a gut shot.

No burn spots at all. I'm posting a few gut shots anyway. I did a thorough visual inspection of the HV lines and didn't spot any suspicious discolorations that would suggest leakage...

-did you tie off and insulate the center tap for the rectifier tube filaments where it can't short to anything?

The rectifier filament secondary has no center tap.

-does it blow fuses with all the tubes out even rectifier? some kind of wiring mistake or bad PT

Apparently not. The first ever occurrence of the failure was with the rectifier and power tubes in.

-does it blow the fuse with all the tubes out except the rectifier and switched to hi-volts on? something in the B+ power supply. if all the parts are new look for dull cold solder joints, somewhere where the solder is just sitting on top of the connection.

Same as above. Still I re-did most of the solder joints to the tube sockets (recto and power), and to the HV distribution, on the internal and the capacitor boards. I didn't see anything really suspicious, but just to be on the safe side.

-does it blow fuses with the power tubes in but no pre-amp tubes? bad signal coupling cap, shorted cathode resister, shorted pre-amp tube.

This was indeed the case, and if it ever blows a fuse again, leaky coupling caps would be my next guess...

-did you use any NOS coupling caps? could be leaking.

No, everything is new modern production.

- right after it blows a fuse, unplug the amp,drain the filter caps and feel around for a hot part.

This is a good one, could be a good way to find a leaky cap if it happens again. But I guess I would just replace them straight away if it comes to that.

- And if you suspect rectifier arcing, it probably won't hurt anything to install the 5U4 to see if that keeps the fuse from popping. If it does, the 5AR4 was likely bad.

That was going to be next, but I think I found the culprit:

Always suspect wiring error or solder drips, stray wire on your part. Everyone makes the same dumb errors; the experienced guys just assume they made a dumb error, and therefore find it faster. I know I've made plenty of them. I just know to look for mistakes on my part in a new build.

Amen, brother !!
In my passion for tidiness I tie-wrapped all the groups of wires that run together for some distance. I ended up tie-wrapping together (and quite tightly) the bunch of wires that come from the cap-board from the first and second filter stages, including it's ground.
Maybe cloth-covered wires are not that well insulated at 500V.... I freed these wires so that the ground is not tightly pressed against the +B anymore but running free and a few millimeters apart, and it seems that solved the issue.

So now I guess I'll play the amp and see if it holds, but I'm rather optimistic.

Additionally I found that an old 12AX7 in the trem socket instead of the brand new one cured most of the beating sound and produced a much mellower and pleasant tremolo effect.

The promised gut shots in next post.

[Blackdog]

One last thing, mostly out of curiosity:

Touching with a chopstick the components around V4 (specially the wire to pin 7 grid, but also the cable to pin 6 plate or the associated Rk//Ck) has the effect of amplifying the produced mechanical sounds. Very microphonic. However I exchanged the tube with V1 and the microphony doesn't change: it remains with the components at V4. Slightly tapping on the tube itself doesn't have such a dramatic effect, so I guess the "sensor" is not the tube itself. Any experience on this ??

Thanks again for your time.

[bnwitt]

Touching with a chopstick the components around V4 (specially the wire to pin 7 grid, but also the cable to pin 6 plate or the associated Rk//Ck) has the effect of amplifying the produced mechanical sounds. Very microphonic. However I exchanged the tube with V1 and the microphony doesn't change: it remains with the components at V4. Slightly tapping on the tube itself doesn't have such a dramatic effect, so I guess the "sensor" is not the tube itself. Any experience on this ??

resolder all of the wires on those V4 pins and at the other end of their terminations.  I have had this problem with less than perfect solder joints at sockets.  Also tap all of the components connected to that socket and see if the noise gets louder.  I've had coupling caps that were microphonic as well.

[eleventeen]

Did you solve this yet? Does the fuse blow occur with *only* the rectifier tube installed?

If so, sounds like it could be a brand new bad or hinky electrolytic in the B+ filter section. Not that difficult to isolate. You may have to go to the filter cap board and unsolder the several connections that come off the voltage divider (meaning the series resistors) that create the lower and lower levels of B+. It could also be a heater to cathode short in the rectifier, if you're using a 5AR4/GZ34. If you can substitute in a 5Y3 or (for a brief time only (say 10 mins) because of the extra amp of filament current drawn) a 5U4 and the problem goes away, that would scream H to C short in your 5AR4. To me.

As for the microphonic component, that's not all that uncommon. It can be difficult to isolate the bad guy, as your tapping vibrates most of the neighboring components.

I used to have a brown Vibroverb. I sold it, alas, to Austin Pittman himself.

[Blackdog]

Did you solve this yet? Does the fuse blow occur with *only* the rectifier tube installed?

Thanks for the interest.

The first time the fuse blew was with the 5AR4 AND the 6L6s in place, but given the intermittent nature of the problem I can't infer that it would not happen with just the rectifier tube.

Anyway, so far the problem seems to be solved. After I untied the HV wires (as explained in my post above) the problem never appeared again.
It seems that there was some kind of "almost-short-circuit" between the +B wire and ground as they were tied together rather tightly from where they enter the chassis to their final connections at the fiberboard and ground respectively.

After untying I inspected the wires in all their length but couldn't find any discoloration/burning marks that would suggest a "contact point", but the fact remains that with the wires loose it doesn't fail (so far, at least).

I still need to work on the microphonic problem. But as I am expecting a replacement power switch (the original had a mechanical failure after just a few uses) and a 50KL pot with a built-in switch to put in the Tremolo Intensity position, I didn't want to open up the amp until I can do all the mods/repairs in one go.

I will certainly post a follow up when I identify the microphonic component. 
Thanks again.