Bassman trouble, again

[Oddvar]

Hi again.

I tried to use the V1 on the Bassman 5f6a as a lead channele as described elsewhere, with to sperate caps going to ground from V1.  End of that story was that the amp made som terrible hum and then the fuse went.  I put it back to standard and now I can't get any voltreading on th B+'s, only small amounts.  The fuse only blows when using the GZ34 rectifier, the pilotlight lights up.  The filterCaps at c only has  4.90 k, I have changed that three times, the B has 218k and the two others has 84k reading.  I have also change the outputtubes resistors, checket the outputtransformer to be ok, so I am wondering if the powertransformer is blown somehow. No voltagereading on the preamptubes.  Any suggestions on where to start troubleshooting?

[pompeiisneaks]

Schematic, and pictures really help.  Low to no voltages and fuses going out means short to ground to me. 

Phil

[Oddvar]

This is the layout used.

[eleventeen]

Got a 5Y3 or 5V4 rectifier tube (or temporarily, even a 5U4) you can throw in there? (or, another GZ34) You don't want to leave a 5U4 in there too long as it draws 3A heater current instead of the 2A of a GZ34 or 5Y3. OK for a brief test, though.


I did not see in the list of things you changed any rectifier tube swap.


We are hearing of fair numbers of GZ34 failures. You're implying that the amp worked fine before you did the preamp-patch thing, and why that should cause the the GZ34 to fail, I have no answer for that. It shouldn't. Maybe when you turned the amp on with that patch in place maybe one channel's volume control was on "10" and the gain of the amp ramped out of control and it drew excessive current from the rectifier tube. Sounds kind of far fetched, but strange or not, we are hearing of more than minor numbers of these tubes failing.

[Oddvar]

Hi again.  I have tried two JJ Gz34's, same thing.  At one point there was a blue light inside one of them.  I am trying to ohm my way through the board, but can't find anythin irregular.  Only thing might be that I gave grounded the two outputtubes, one tube to the other,  as indecated on the layout, to starground.

[pompeiisneaks]

Maybe check the .047 cap on the standby switch?  If is grounding for some reason that would send the b+ staying to ground pretty quickly.

You could also check that the rectifier socket hasn't arced causing a ground out there.

Phil

[HotBluePlates]

... I tried to use the V1 on the Bassman 5f6a as a lead channele as described elsewhere, with to sperate caps going to ground from V1.  End of that story was that the amp made som terrible hum and then the fuse went.  I put it back to standard and now I can't get any voltreading on th B+'s ...

... Only thing might be that I gave grounded the two outputtubes, one tube to the other,  as indecated on the layout, to starground.

The amp was working, you mention fiddling with preamp tube cathode components, but now wonder about shorting out the output tubes??

Exactly what all has been done to the amp since it was working on the 2nd?

[Oddvar]

I tried this, I might have connecting something wrong here, not sure, but after this it made a loud humming noise and the fuse blew after a few seconds.

[pompeiisneaks]

So you added a separate cathode resistor and cap for the other input?  That shouldn't cause any problems that I can think of like you're discussing, unless you've reversed a cap and blown it, or something like that.  Or you've got a solder blob that's touching ground somehow.  I think those eyelet boards always have a second insulating layer below them, though right?   I have basically a very similar setup on my bassman based on Doug's layout, but I used turret boards instead, and it was pre-built for that layout.  I would give everything a once over to ensure you're not grounding something accidentally due to the changes you made.

~Phil

[HotBluePlates]

I tried this, I might have connecting something wrong here, not sure, but after this it made a loud humming noise and the fuse blew after a few seconds.

You have a wiring error/issue somewhere else then. It's not possible to blow a fuse by hooking up any of those cathode components incorrectly. Worst-case, you may be able to cause the cathode resistor or an associated plate resistor to go up in smoke.

Got a lightbulb limiter? (Or a bag of fuses?)

Pick a spot on the B+ rail where there is a dropping resistor, and unsolder one leg of the resistor. Try powering up the amp using the lightbulb limiter, and check for indications of excessive current draw (the bulb glows with perceptible brightness). By doing this, you are disconnecting the B+ to portions of the amp.

Find what part of the B+ you can leave attached without excessive current; then look at things which directly connect to (or are near) that part of the B+ supply/cap. You may need to unsolder the choke to isolate the 1st filter cap from the rest of the circuit. Figure out if you can leave anything connected to the rectifier without drawing excessive current or popping a fuse.

Continuity or low-resistance between points which shouldn't have a direct connection may help you find the short. You may wish to keep tubes out of the sockets until the integrity of the B+ & bias supply is confirmed. Preamp tubes may be safe to add back to, or keep in, their sockets.

I think there's a good chance some wiring fault or near-fault which existed before is coincidentally manifesting now.

[Oddvar]

There is a board underneath the eyeletboard to prevent grounding.  I can't find any groundin issues, but there might be.  I am a bit uncertain where the grounding on pin 8 on the powertubes should go.  I have tested with or withou, but no change.

[Oddvar]

I'll try to do that...

I tried this, I might have connecting something wrong here, not sure, but after this it made a loud humming noise and the fuse blew after a few seconds.

You have a wiring error/issue somewhere else then. It's not possible to blow a fuse by hooking up any of those cathode components incorrectly. Worst-case, you may be able to cause the cathode resistor or an associated plate resistor to go up in smoke.

Got a lightbulb limiter? (Or a bag of fuses?)

Pick a spot on the B+ rail where there is a dropping resistor, and unsolder one leg of the resistor. Try powering up the amp using the lightbulb limiter, and check for indications of excessive current draw (the bulb glows with perceptible brightness). By doing this, you are disconnecting the B+ to portions of the amp.

Find what part of the B+ you can leave attached without excessive current; then look at things which directly connect to (or are near) that part of the B+ supply/cap. You may need to unsolder the choke to isolate the 1st filter cap from the rest of the circuit. Figure out if you can leave anything connected to the rectifier without drawing excessive current or popping a fuse.

Continuity or low-resistance between points which shouldn't have a direct connection may help you find the short. You may wish to keep tubes out of the sockets until the integrity of the B+ & bias supply is confirmed. Preamp tubes may be safe to add back to, or keep in, their sockets.

I think there's a good chance some wiring fault or near-fault which existed before is coincidentally manifesting now.

[HotBluePlates]

Understood on the insulator board. You might need to look for solder blobs, stray wire strands, or a wire on the wrong turret/eyelet.

Your prior posts mentioned the "Weber OT gave in" and now you're blowing fuses. And while I've seen it exactly once this forum since ~2003, I have yet to have a brand new transformer be bad, shorted, etc. For you to have 2 bad transformers is either horrid luck, Weber's supplier really dropping the ball, or a wiring error.

I can only speak for myself, but I suspect wiring error in this case. While I haven't had any fuses blow due to a bad transformer, I've popped a LOT of fuses by making a bonehead mistake during my builds.

The troubleshooting process I outlined earlier should help you narrow it down, and confirm whether or not the power transformer is bad. If you unhook every secondary on the power transformer, and it still blows fuses while only being hooked to the primary, then the PT is shorted or there's a fault in the primary wiring to the power cord/fuseholder/switch.

[Oddvar]

In the end it was the GZ34 JJ tubes that both were faulty though quite new.

[HotBluePlates]

In the end it was the GZ34 JJ tubes ...

Did you happen to notice of those GZ34's fit in the sockets loosely? I don't have any of them, but other have occasionally posted that the pins of some new-production tubes were a thinner-than-typical diameter, and tube sockets gripped them loosely. As a result, those tubes tended to lose contact and arc in their sockets.