Fuse blown

[sclinchy]

I built a Hoffman Princeton Reverb a couple of years back. It’s been working fine, except that the tremolo doesn’t do a thing. I read that adjusting the bias might help, so I went from -35 to about -18. Tested the amp with the chassis out and it worked fine. Put the chassis back in the cabinet and turned it on. Jewel light came on, but no sound. Turned it off. Turned it back on again and no light, no sound. Checked the fuse and it’s blown.


What should I look at before I replace the fuse and turn it back on?

[shooter]

probably everything, focus on the filament string, PS, and small scraps of wire sticking out from solder connections


how did you adjust the bias?  were the tubes in?
If not, did they go back in correctly or did one kick n scream going back in?, maybe mis-aligned?

[sluckey]

It’s been working fine, except that the tremolo doesn’t do a thing. I read that adjusting the bias might help, so I went from -35 to about -18.

-18v is too low for 6V6 tubes. Set it back to about -30 to -35.

[sclinchy]

Could the low bias cause the fuse to blow?

[sluckey]

Could the low bias cause the fuse to blow?

yes

[mresistor]

Sclinchy the bias vary trem in the Hoffman Princeton Reverb works very well and isn't susceptible to not working well at higher bias levels, its with 6L6's where it doesn't work well.  You might have a wiring error or something else awry inside.  If you can post up some higher resolution pictures of the innards so the helpful people here can have a look see and maybe determine what is wrong. You could also post up the voltage readings for all the pins of the tubes.

[sclinchy]

I’m always amazed at the generosity of the folks on this forum. Thanks for any help. Photos attached.

[PRR]

...I went from -35 to about -18....

WHY did you cut the grid voltage in half??

Look at 6V6 curves. At 250V on the high electrodes, -35V gives like 2mA plate current, -18V gives like 22mA, TEN TIMES more.

At typical fender plate/screen voltages, both currents will be higher, but the -18V will always be MUCH higher.

My guess is that the fuse took the HIGH current strain, once. And if you wrap it in heavy tin-foil, something else will smoke instead. (OT? PT?)

If a stick won't fit in the hole, don't just cut half of it off. Shave it and try, shave it and try.... 33, 32, 31, 30....  easy in an amplifier with a bias trimmer.

[sclinchy]

Bad advice that I didn’t think through. As you may have guessed, I’m strictly an amateur. My vibrato wasn’t working and I read something that said reducing the bias to -16 to -18 might make it work.


I’m learning. New fuses will arrive tomorrow. I’ve turned the bias up to what should be safe (can’t tell for sure until I get a fuse and can measure it) and I’ll go from there.


Thanks for the advice.

[mresistor]

 Once you get the bias set correctly for 65-70% of max plate dissipation with the volume control turned to zero, I think I'd start by examining all the ground connections on the ground bus in the area of the tremolo circuit. Then I'd verify each and every connection in the tremolo circuit that they are per the layout and the solder connection is good and solid. I'd also look at each component and make sure it was the correct value.  Then please post up the voltages on pins 2,3 and 4 of V4. Also 4,5 and 9.  I can't remember if you need a shorting connector or footswitch to enable the trem on the HPR.  I need to pull my amp out and check that.  Nope  it works without a footswitch. 
Also have you tried a different tube in V4?

What brand are the .1 uf film caps you have in there?

[sclinchy]

Uh oh. Got new fuses in today. I adjusted the bias pot almost all the way up (where it was before, when I had -35), plugged my handy Hoffman bias kit in and turned the amp on.  Jewel light comes on (i wasn’t with the blown fuse, obviously), but no current registering on the meter. Turned it off and checked to see if there was any residual voltage on the filter caps - nope. Powered up again and tested for voltage on the filter caps, nothing. Tubes aren’t lighting up.


Sounds like i fried my power transformer?

[pdf64]

Did you fit a time delay / antisurge / SloBlo type fuse?

[sluckey]

Jewel light comes on... Tubes aren’t lighting up.

Isn't the light powered by the filament winding?

[sclinchy]

Yes, its a slo-blo fuse. And yes, the jewel light is powered by the 6.3v secondary. Correct me if I’m wrong, but I think it’s possible that the 6.3v volt AC secondary could be working and the 320v DC could not be. I need to look into how to test the PT. Any suggestions would be appreciated.

[sluckey]

If the jewel is lit but the tubes are not, then look for a broken wire, poor solder joint, etc between the jewel and the tubes. You can measure the actual voltage by setting your meter to AC volts and connect one probe to a 6V6 socket pin 2 and the other probe to pin 7. Should have 6.3VAC

Next connect one probe to the rectifier socket pin 2 and the other probe to pin 8. Should have 5VAC.

Now connect the black probe to chassis and connect the red probe to the rectifier socket to pin 4. Should be about 300VAC. Move the probe to pin 6. Should be about 300VAC.

[sclinchy]

Thanks so much, this is just what I was looking for!


Pin 7 on my rectifier isn’t connected. I get 6.3v AC across the jewel light and 5v AC between pins 2 and 8.


Switching to DC, i get 0v between the chassis and pin 6.


That doesn’t sound good.

[sluckey]

FOLLOW THE INSTRUCTIONS I GAVE YOU

[sclinchy]

I apologize if I’m being dense. On my rectifier socket, there is no tab on pin 7 (5u4), so I tested across the two connections to the lamp itself. Was this wrong?


I tested DC voltage at both pin 4 and pin 6 (left pin 4 out, sorry).


Again, sorry if I’ve misunderstood.

[sluckey]

READ, UNDERSTAND, AND FOLLOW MY INSTRUCTIONS.

[sclinchy]

I’m an idiot. I read your post three times. 6V6!  Not the rectifier! 


I’ve had two beers, so I’ll check this out tomorrow.


Appreciate your patience.

[sclinchy]

OK.  Sober, rested and glasses cleaned.


Between pins 2 and 7 of the 6V6, I’ve got 6.3v AC. 


Between pins 2 and 8, I’ve got 2.9.


No DC voltage on either pin 4 or 6 of the rectifier.

[sluckey]

Next connect one probe to the rectifier socket pin 2 and the other probe to pin 8. Should have 5VAC.

Now connect the black probe to chassis and connect the red probe to the rectifier socket to pin 4. Should be about 300VAC. Move the probe to pin 6. Should be about 300VAC.

[Willabe]

Your mixing up AC and DC voltage measurements.

You need to pay more attention to;

1. Tube

2. Tube pin(s)

3. ACV or DCV measurement. 

Between pins 2 and 7 of the 6V6, I’ve got 6.3v AC.

This is right, pins 2 and 7, acv. 

Between pins 2 and 8, I’ve got 2.9.

Is that AC or DC? You want to measure for DCV across pins 2 and 8 on the rectifier tube. See below:

Next connect one probe to the rectifier socket pin 2 and the other probe to pin 8. Should have 5VAC.

No DC voltage on either pin 4 or 6 of the rectifier.

This is wrong. You need to measure for ACV on pin 4 and then pin 6 of the rectifier tube. See below;

Now connect the black probe to chassis and connect the red probe to the rectifier socket to pin 4. Should be about 300VAC. Move the probe to pin 6. Should be about 300VAC.

[sluckey]

Is that AC or DC? You want to measure for DCV across pins 2 and 8 on the rectifier tube.

He doesn't need any more confusion.

[sluckey]

OK.  Sober, rested and glasses cleaned.


Between pins 2 and 7 of the 6V6, I’ve got 6.3v AC. 


Between pins 2 and 8, I’ve got 2.9.


No DC voltage on either pin 4 or 6 of the rectifier.

Deja vu. We covered all this back in 2017. I think you would benefit by reading that discussion again.

[sclinchy]

OK, sorry once more, and once more I appreciate your patience.  I guess I'm a little stressed about this, I'll try to slow down.


Getting 340v AC on both pin 4 and pin 6 on the rectifier.


This seems like good news?

[sluckey]

OK, you're through checking the PT secondary AC voltages. PT is probably OK.

***SET YOUR METER TO READ DC VOLTS***

1. Plug in all tubes. Do the filaments light up for every tube? You may have to turn the room lights off to see the little tubes glowing.

2. If #1 is OK, then measure DC VOLTS at rectifier tube pin 8. Black probe will be connected to chassis ground. What have you?

3. If #2 is OK, measure DC VOLTS on the four caps in the can. What have you?

Report your findings.

[sclinchy]

1. All tubes glowing except rectifier.


2. Pin 8 is -.5v DC.

[sluckey]

1. All tubes glowing except rectifier.

Change tube and repeat 1, 2, and 3 above.

[sclinchy]

OK, thanks. I don’t have one on hand, I’ll have to order one


This will give you a well deserved break from me.

[sluckey]

Do you remember saying this???

We may have a winner.  I pulled the recto and the center post was broken.  Threw a 5Y3 that I had sitting around and now I've got 437vDC on pin 8.  Similar voltages on the filter caps!

[sclinchy]

I do now. You must have some memory!


My soldering chops have improved quite a bit since then. I’m actually planning on rewiring most of this amp.

[sclinchy]

New rectifier received and installed, still no DC voltage on pins 8 or 2 of the rectifier. DC voltage on cap can pins is .5v.


This doesn’t seem like good news…

[acheld]

still no DC voltage on pins 8 or 2 of the rectifier.

How about AC voltage on pins 4 and 6?  I know you had this in the past. 

Is the new tube bad?  Is the socket wired and soldered correctly?   

[sclinchy]

Yes, sorry, still 340v AC on each of those, so no charge after installing the new recto. No voltage (AC or DC) at the first filter cap. I guess the new tube could be bad, but that does seem like quite a coincidence.


Checking wiring, but i think we’re operating on the assumption that my reducing the bias was the original cause of the problem.

[sluckey]

Turn the amp off and measure resistance between the rectifier socket pin 8 and chassis. What have you?

[sclinchy]

No connection between 8 and chassis.

[dwinstonwood]

No connection between 8 and chassis.

Do you mean OL? No Ohm's number displayed on your meter?

[sclinchy]

Correct.

[sclinchy]

Please see attached.

[dwinstonwood]

Pin 8 is the Cathode, the source of your B+ rectified voltage. If it's shorted to ground that explains why you have no voltage at the first cap.

[sclinchy]

But it’s not. I’m showing infinite resistance between pin 8 and ground.


So, if I’m understanding this, I’m getting AC going from the PT to the rectifier, but no DC (B+) from the recto to the first filter cap?


Which sounds like a bad rectifier, but it’s hard to believe that I got a tube exhibiting the same problem as the old one. (I know, stranger things have happened) Could I have a problem that’s killing my rectifier tubes?

[dwinstonwood]

I might have misread your test. There should be no continuity between pin 8 and ground.

[sclinchy]

Yes, that’s what I’ve got.

[shooter]

Could I have a problem that’s killing my rectifier tubes?

I didn't pull up a schematic so;


Pin 8 IS pin 8, verified?
does Pin 8 feed the 1st filter cap?
If so, ohm pin 8 to cap, short?

[sclinchy]

Just double checked, yes, it’s pin 8 (yellow wire from PT also attached to that pin).  There is continuity between pin 8 and the first filter cap. I reflowed solder on pin 8, just in case, but no change.

[pdf64]

How about is the 5Y3 previously fitted is tried, as reported in post #31?

[sclinchy]

That’s from five years ago. I’m sorry, but I’m not sure what you’re asking.

[Stuff4bikes]

He is asking you if it works with the 5y3 now....

Where are your filter caps grounded....?

[sclinchy]

I don’t think I still have the 5Y3 anymore, this amp uses a GU4.  Filter caps are a cap can grounded to the chassis.