Blues Jr Conversion start up

[Goldpeak]

I have completed my BJ conversion and have attached the speaker, no tubes, using a variac. Slowly bring variac up and at 90 vac R47 just past the bridge rectifier begins to get very hot. See attachment. Any ideas? TIA

[mresistor]

You've probably miswired something on the HV rail..  take a couple of pictures of the whole board and wiring.
and a schematic would be helpful to others to help you


https://el34world.com/Hoffman/files/Hoffman_BluesJunior.pdf




and there are under board connections too.  Do you have a lamp limiter? It would be very useful.  You need to find out which power supply node contains the problem. I might disconnect one end of R49 and then R48 to isolate the problem  But first you need that light bulb limiter   no offense but  I'm not sure its best for you to start trying to hook up a current meter on the ps rail. Is there a current meter on the Variac?

[mresistor]

You probably wouldn't need to use a variac if you used a light bulb limiter at first turn on.   https://www.instructables.com/Light-Bulb-Current-Limiter/

[shooter]

did you verify the resistors value with a meter before installing

[Goldpeak]

did you verify the resistors value with a meter before installing

Yes

[Willabe]

Your solid state (SS) bridge rectifier is wired wrong in the picture you posted.

[SEL49]

The red wire should be connected directly to the FWB and NOTHING ELSE. The only thing that should connected to turret Z is the filter cap (and the underboard jumper).

Hopefully the FWB has not been damaged.

[Goldpeak]

You've probably miswired something on the HV rail..  take a couple of pictures of the whole board and wiring.
and a schematic would be helpful to others to help you


https://el34world.com/Hoffman/files/Hoffman_BluesJunior.pdf




and there are under board connections too.  Do you have a lamp limiter? It would be very useful.  You need to find out which power supply node contains the problem. I might disconnect one end of R49 and then R48 to isolate the problem  But first you need that light bulb limiter   no offense but  I'm not sure its best for you to start trying to hook up a current meter on the ps rail. Is there a current meter on the Variac?

[Goldpeak]

The red wire should be connected directly to the FWB and NOTHING ELSE. The only thing that should connected to turret Z is the filter cap (and the underboard jumper).

Hopefully the FWB has not been damaged.
[/Please, FWB ? quote]

[Willabe]

The black 4 wire solid state rectifier.

It's a Full Wave Bridge = FWB.

Look at where SEL49 hi-lited the picture you posted. Yellow circle with 3 question marks.

There's markings on the other side of the FWB package that show the hook up info. Look at the Hoffman schematic, it shows those wiring marks.

[Willabe]

You probably wouldn't need to use a variac if you used a light bulb limiter at first turn on.   https://www.instructables.com/Light-Bulb-Current-Limiter/

That LBL will work, but it's way too big.

Use the same double steel outlet wall box but with rounded corners, use the deep box version so you have room to fit the outlets/switch. Get the 1 that takes the exposed work metal cover, screw holes for metal cover are in a different place. Don't get the metal cover that extends past the box, that's a wall plate cover. Go to a big box store and make sure the cover fits the box.

Then put in a single on/off switch and a duplex (double) outlet, both in the same box. Then use a single light bulb plug in socket for the light bulb. (Like in the drawing below.) Much smaller package. Make sure to use a 3 wire power cord and ground the 3rd green wire to the metal box. And use a metal box clamp for the power cord.

Remember to break the tabs off both sides of the duplex (double) outlet. 

Some guys like to add the switch, so you can switch between current limit and regular ac wall voltage without unplugging the amp from the LBL box.

You want a 60w bulb for small amps, 15w to 20w, and 100w bulb for larger amps. I use a 40w bulb for little 5w SE Champ amp types. Don't use a 200w or larger bulb. The lower the wattage the more protection the bulb will give. Has to be a real steel filament. Can be a halogen.

And you don't use the variac with the LBL. Once you find there's no shorts with the LBL, you don't use it anymore. And you can't take any voltage measurements while the plugged into the LBL, they will be wrong. 

I can't seem to find pictures on line that have the right file extensions to post here. 

You don't need the on/off switch, or the IEC power cord socket that are in the drawing below.

[Goldpeak]

The red wire should be connected directly to the FWB and NOTHING ELSE. The only thing that should connected to turret Z is the filter cap (and the underboard jumper).

Hopefully the FWB has not been damaged.

Thank you very much! The amp works great!

Edit; Fixed quote, Willabe.

[mresistor]

Here's my LBL and I don't care what people think about it or how it's constructed as it's worked fine for me for years even with the same light bulb wattage no matter what amp it was connected to.   I use an extenstion cord to plug it into that has an on/off switch on it so I didn't feel the need to be all fancy with and if I use it in a dfferent outlet I'm not so lazy that I can't unplug it.  Also the amp under test has an on/off switch to use when the light goes bright.   


The one in the instructable was just an example it would be just fine to build it and use it if you want to. 


Good catch on the bridge wiring.   

Robinette seems to think it's ok to use a variac with a light bulb limiter and I have done it as well, there is no harm,  just set it up correctly,  https://robrobinette.com/Tube_Amp_Startup.htm#:~:text=If%20you%20use%20both%20a,voltage%20is%20below%2030%20volts.

[Willabe]

It's a little confusing on Rob Rob's web site, but,

Robinette says he uses the variac to form the new electrolytic caps in the amp. He uses the LBL to protect from shorts.

Which if their new(er) caps you don't need to do that. And the LBL does slow down the surge dcv current so they don't get with full dcv if the amp has a SS rectifier. He says to use a 15w bulb to slow down the dcv to help slowly form the e-caps without a variac.

And e-cap makers form the e-caps at the factory before they ship them. And e-caps are always re-forming when voltage is applied. 

To me, I don't want to use both a variac and a LBL at the same time for 1st power up. I wouldn't want a direct short to be hidden by slowly bringing up the ac wall voltage until it's high enough to get the LBL to light brightly. Yes, the LBL will be limiting the dc current, but it's still a direct short having dcv applied to the amp longer than if it was just the LBL. No thank you.   

Just use the LBL by itself, 1st with no tubes, them if that passes, put in the tubes and test again.

And his LBL picture shows the type of box/cover + the plug in light bulb socket adapter I was talking about. He just doesn't have the optional limit/full power switch. And he didn't use the deep box.