Amp blowing fuses even with rectifier and power tubes removed

[72Blazer]

This is a Vintage Sound Amps 22 Watt Deluxe Reverb Clone
Using my variac, amp is still blowing fuses without the rectifier and pwer tubes installed.  Fuse blows at about 50 Volts from the variac.  I do not see any crispy critters.  Not sure what to do next-any suggestions?

Jim

[pdf64]

Disconnect and isolate all secondary wires.
Use a light bulb limiter, rather than the ‘variac and popped fuse’ method described.
The cost of your blown fuses will probably exceed the build cost of an LBL.

[jjasilli]

I recommend using flowcharts for repair work.  Tubenit posted some on this Forum.  "How to Service Your Own Tube Amp", Tom Mitchell has useful flowcharts.  Also geofex.com tube amp debug page.


Typical culprits:  Shorted Rectifier; Shorted Filter Cap; Bad PT; Bad OT; Loose connection shorting to chassis; Bad Tube; Arc on tube socket. A repair flowchart brings you through a logical progression to find the culprit.


Per Doug's help pages: besides a light bulb limiter, another good investment: circuit breakers in typical ratings.  Use temporarily in lieu of the fuse for amps under test.  Beats going through a box of fuses.

[72Blazer]

Thanks for the tips.  I am in the process of building a LBL.
R/
J.

[jim]

As pdf64 said...If you unsolder and isolate all secondary leads  from your power transformer and it still blows a fuse you have a shorted winding on the power transformer which would need to be replaced.  Jim

[72Blazer]

Ok,

Used the LBL as recommended.

Disconnected all secondary leads and still light the bulb. 

Based on theses results the PT is shot correct? 

[shooter]

how bright?did it blow the main fuse?
what voltage AC do you measure on the secondary wires?

[72Blazer]

Bulb very bright steady but did not blow fuse using LBL.

Using LBL w/150 Watt Bub and Variac dialed at 120V the voltages are:

Red pair:  3.7v
Green pair:  0.35v
Yellow pair:  0.28v

[shooter]

#'s say it's probably bad
what's the black Ecap between the PT and chassis (lower left on image)?
It's not connected to the PT is it?

[72Blazer]

Yes it's connected to the red wire with white stripe on the PT.  Part of negative bias.  It was disconnected when I did the voltage tests above.

[sluckey]

Sure sounds like the PT is bad. To be absolutely sure, pull the PT and put it on the bench. Hook up a power cord to the primary wires and repeat your LBL with variac tests. Also measure the secondary voltages with this setup.

[72Blazer]

Not good results.

Using LBL w/150 Watt Bub and Variac dialed at 120V the voltages are:

Primary: 40V
Red pair:  3.5v
Green pair:  3.5v
Yellow pair:  3.5v

Without LBL transformer hummed, had smell, and got hot: 
Primary 119v
Red: 46v
Green: 35v
Yellow: 35v

[shooter]

IF you're gonna replace and get it working, i would ASSUME the "cause" is still in the amp
tubes, fixed bias circuit, Ecaps............., go slow.  IF you have a schematic, could you post it

[72Blazer]

Yes.  I just ordered an exact model PT.  No schematic.  Amp custom built by Rick at Vintage Sound Amps AB763 circuit and he included these mods for me:
1.  Ability to take the tremelo out of the circuit (tremolo intensity pot has a on/off switch capability)
2.  Normal channel voiced to sound more like a Marshall.

That's all I know.

I'll get back on here when the new tranny arrives/I've got time to figure this mess out.

I want to thank all thus far.  The LBL was a good investment and for a novice like me-sure to save me money on future projects like the 18 watt mojo kit I am currently building.
respectfully,
Jim

[shooter]

AB763 circuit

No schematic.

I've got time to figure

...


..Out IF this amp is laid out like an AB763

[Colas LeGrippa]

What type of fuse are blown ?
Double check the rating.
On power up a bad qualitu fuse or underrated one will blow even with the secondary disconnected
 For example a  .3 A instead of a 3 A

[tubeswell]

What type of fuse are blown ?
Double check the rating.
On power up a bad qualitu fuse or underrated one will blow even with the secondary disconnected
 For example a  .3 A instead of a 3 A

The OP has disconnected the PT sencondaries and the VAC measurements are way off, so it’s not the fuse size

[72Blazer]

Got the new PT installed.

The amp still draws excessive current only when power tubes installed (one or both).  When I remove them the issue goes away even away.  I currently left the rectifier tube in as well as the pre amp tubes and no excessive current.

Again, the issue is only present when power tubes are installed.

I checked screen and grid resistors and they appear/test with in spec.  I do not see any visual damage to other components or burn spots. 

according to the flow chart I am looking at it makes reference to maybe a bad filter cap?

Any suggestions out there?

[mresistor]

Pull the cover off the doghouse (where the big filter caps are)  inspect them for leakage, bulging, etc.  I would also be turning the bias voltage max negative while troubleshooting, and adjust bias properly after the amp is working properly again.   Can you take a picture and post of the filter cap board?  You can check the big filter caps if you have a tool to check capacitance/ESR. I guess you could also disconnect each power supply node one at a time and see if the exessive current returns to more normal, that would help to isolate where the problem is..
You could also have a bad power tube. 

Ok punting to more knowlegable folks.. 

[72Blazer]

forgot to mention I did try known good power tubes with same results.

Here is a pic of the filter caps.  I see no bulges/leaks.  Resistors look good and no loose connections on cap board or inside the amp.

[72Blazer]

another pic

[sluckey]

Again, the issue is only present when power tubes are installed.

Well then, it ain't filter caps. Put the tubes in and measure voltages on output tubes pins 3, 4, 5, and 8. What have you?

[72Blazer]

pin 3:  300v
4:  300v
5:  -18v
8:  0v

[sluckey]

Pin 5 is way too low for a deluxe reverb clone running 6V6s. -18v will cause the tubes to draw excessive current and probably blow fuses. Definitely will damage 6V6s. Needs to be more like -35V.

[72Blazer]

 

Set at 35v.  Amp seams to be stable now. 

Any ideas why a 12 year old Mercury Magnetic PT would just go bad?  All the original tubes are back in the amp.

[PRR]

...Any ideas why a 12 year old Mercury Magnetic PT would just go bad? .....

Maybe because the bias was set too hot?

And how did that happen?

[72Blazer]

When I gave Sluckey the the tube readings I still had it on the LBL.  so the readings were a little off I think.  Other than that I did not adjust the bias pot recently if ever.  I did have it in the shop last month with the original builder of the amp for him to look at and resolve some noise issues.  It is a little quieter but not as quiet as it used to be. 

[mresistor]

Glad to hear it was just bias. 


Is it me or do the Atom filter caps look like they've had the positive leads resoldered back on or something..??  where the leads meet the cap body terminals looks weird to me.  Are they made that way?


Guess if they're working should leave them alone.

[Willabe]

Is it me or do the Atom filter caps look like they've had the positive leads resoldered back on or something..??  where the leads meet the cap body terminals looks weird to me. 

They sure do look like they've been re soldered. 

[acheld]

I was thinking the same thing about the Atoms.   Also, that middle cap's positive lead is really close to that screw, at least it looks so in the pic.   Clearly not causing a problem now, and maybe it's not so close in reality, but that did catch my eye.

[PRR]

> do the Atom filter caps look like they've had the positive leads resoldered back on or something..?? 

They have been replaced. Not by taking the old ones out of the board eyelets, but by snipping the leads to remove the body, and soldering the new caps' leads to the stubs. Saves labor and board removal. Is quite appropriate for tube TV repair. Is a little cheesy for a Classic Guitar Amplifier but it happens.