Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: sclinchy on February 13, 2022, 12:49:20 pm

Title: Fuse blown
Post by: sclinchy on February 13, 2022, 12:49:20 pm
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?
Title: Re: Fuse blown
Post by: shooter on February 13, 2022, 01:16:13 pm
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?
Title: Re: Fuse blown
Post by: sluckey on February 13, 2022, 01:32:08 pm
Quote from: sclinchy on February 13, 2022, 12:49:20 pm
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.
Title: Re: Fuse blown
Post by: sclinchy on February 13, 2022, 01:42:20 pm
Could the low bias cause the fuse to blow?
Title: Re: Fuse blown
Post by: sluckey on February 13, 2022, 01:46:42 pm
Quote from: sclinchy on February 13, 2022, 01:42:20 pm
Could the low bias cause the fuse to blow?
yes
Title: Re: Fuse blown
Post by: mresistor on February 13, 2022, 02:32:18 pm
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.
Title: Re: Fuse blown
Post by: sclinchy on February 14, 2022, 02:09:51 pm
I’m always amazed at the generosity of the folks on this forum. Thanks for any help. Photos attached.
Title: Re: Fuse blown
Post by: PRR on February 14, 2022, 07:36:38 pm
Quote from: sclinchy on February 13, 2022, 12:49:20 pm
...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.
Title: Re: Fuse blown
Post by: sclinchy on February 14, 2022, 09:21:45 pm
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.
Title: Re: Fuse blown
Post by: mresistor on February 15, 2022, 01:39:56 pm
 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?
Title: Re: Fuse blown
Post by: sclinchy on February 15, 2022, 04:34:08 pm
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?
Title: Re: Fuse blown
Post by: pdf64 on February 15, 2022, 04:54:27 pm
Did you fit a time delay / antisurge / SloBlo type fuse?
Title: Re: Fuse blown
Post by: sluckey on February 15, 2022, 04:58:42 pm
Quote from: sclinchy on February 15, 2022, 04:34:08 pm
Jewel light comes on... Tubes aren’t lighting up.
Isn't the light powered by the filament winding?
Title: Re: Fuse blown
Post by: sclinchy on February 15, 2022, 05:15:48 pm
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.
Title: Re: Fuse blown
Post by: sluckey on February 15, 2022, 05:31:05 pm
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.
Title: Re: Fuse blown
Post by: sclinchy on February 15, 2022, 05:51:01 pm
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.
Title: Re: Fuse blown
Post by: sluckey on February 15, 2022, 05:56:58 pm
FOLLOW THE INSTRUCTIONS I GAVE YOU
Title: Re: Fuse blown
Post by: sclinchy on February 15, 2022, 06:29:49 pm
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.
Title: Re: Fuse blown
Post by: sluckey on February 15, 2022, 06:36:23 pm
READ, UNDERSTAND, AND FOLLOW MY INSTRUCTIONS.
Title: Re: Fuse blown
Post by: sclinchy on February 15, 2022, 06:43:59 pm
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.
Title: Re: Fuse blown
Post by: sclinchy on February 16, 2022, 09:23:35 am
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.
Title: Re: Fuse blown
Post by: sluckey on February 16, 2022, 09:39:45 am
Quote from: sluckey on February 15, 2022, 05:31:05 pm
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.
Title: Re: Fuse blown
Post by: Willabe on February 16, 2022, 09:44:38 am
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. 

Quote from: sclinchy on February 16, 2022, 09:23:35 am
Between pins 2 and 7 of the 6V6, I’ve got 6.3v AC.

This is right, pins 2 and 7, acv. 

Quote from: sclinchy on February 16, 2022, 09:23:35 am
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:

Quote from: sluckey on February 15, 2022, 05:31:05 pm
Next connect one probe to the rectifier socket pin 2 and the other probe to pin 8. Should have 5VAC.

Quote from: sclinchy on February 16, 2022, 09:23:35 am
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;

Quote from: sluckey on February 15, 2022, 05:31:05 pm
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.
Title: Re: Fuse blown
Post by: sluckey on February 16, 2022, 10:04:48 am
Quote from: Willabe on February 16, 2022, 09:44:38 am
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.
Title: Re: Fuse blown
Post by: sluckey on February 16, 2022, 10:08:43 am
Quote from: sclinchy on February 16, 2022, 09:23:35 am
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.
Title: Re: Fuse blown
Post by: sclinchy on February 16, 2022, 10:58:23 am
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?
Title: Re: Fuse blown
Post by: sluckey on February 16, 2022, 11:27:05 am
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.
Title: Re: Fuse blown
Post by: sclinchy on February 16, 2022, 11:43:44 am
1. All tubes glowing except rectifier.


2. Pin 8 is -.5v DC.
Title: Re: Fuse blown
Post by: sluckey on February 16, 2022, 12:11:54 pm
Quote from: sclinchy on February 16, 2022, 11:43:44 am
1. All tubes glowing except rectifier.
Change tube and repeat 1, 2, and 3 above.
Title: Re: Fuse blown
Post by: sclinchy on February 16, 2022, 12:15:47 pm
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.
Title: Re: Fuse blown
Post by: sluckey on February 16, 2022, 12:23:00 pm
Do you remember saying this???

Quote from: sclinchy on August 10, 2017, 02:35:39 pm
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!
Title: Re: Fuse blown
Post by: sclinchy on February 16, 2022, 02:05:33 pm
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.
Title: Re: Fuse blown
Post by: sclinchy on February 26, 2022, 09:45:21 am
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…
Title: Re: Fuse blown
Post by: acheld on February 27, 2022, 10:02:01 am
Quote
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?   
Title: Re: Fuse blown
Post by: sclinchy on February 27, 2022, 02:11:06 pm
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.
Title: Re: Fuse blown
Post by: sluckey on February 27, 2022, 02:23:57 pm
Turn the amp off and measure resistance between the rectifier socket pin 8 and chassis. What have you?
Title: Re: Fuse blown
Post by: sclinchy on February 27, 2022, 02:48:49 pm
No connection between 8 and chassis.
Title: Re: Fuse blown
Post by: dwinstonwood on February 27, 2022, 03:35:10 pm
Quote from: sclinchy on February 27, 2022, 02:48:49 pm
No connection between 8 and chassis.

Do you mean OL? No Ohm's number displayed on your meter?
Title: Re: Fuse blown
Post by: sclinchy on February 27, 2022, 03:36:22 pm
Correct.
Title: Re: Fuse blown
Post by: sclinchy on February 27, 2022, 03:47:19 pm
Please see attached.
Title: Re: Fuse blown
Post by: dwinstonwood on February 27, 2022, 04:46:35 pm
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.
Title: Re: Fuse blown
Post by: sclinchy on February 27, 2022, 05:02:46 pm
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?
Title: Re: Fuse blown
Post by: dwinstonwood on February 27, 2022, 05:06:02 pm
I might have misread your test. There should be no continuity between pin 8 and ground.
Title: Re: Fuse blown
Post by: sclinchy on February 27, 2022, 05:10:01 pm
Yes, that’s what I’ve got.
Title: Re: Fuse blown
Post by: shooter on February 27, 2022, 05:35:00 pm
Quote
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?
Title: Re: Fuse blown
Post by: sclinchy on February 27, 2022, 05:58:39 pm
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.
Title: Re: Fuse blown
Post by: pdf64 on February 27, 2022, 05:59:03 pm
How about is the 5Y3 previously fitted is tried, as reported in post #31?
Title: Re: Fuse blown
Post by: sclinchy on February 27, 2022, 06:10:40 pm
That’s from five years ago. I’m sorry, but I’m not sure what you’re asking.
Title: Re: Fuse blown
Post by: Stuff4bikes on February 27, 2022, 06:32:40 pm
He is asking you if it works with the 5y3 now....

Where are your filter caps grounded....?
Title: Re: Fuse blown
Post by: sclinchy on February 27, 2022, 06:44:16 pm
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.
Title: Re: Fuse blown
Post by: shooter on February 28, 2022, 04:14:22 am
usig this PS drawing;


after you disconnect R68 (on the "right")  clip a gator clip on the free'd leg.  connect meter + to the gator clip.
connect the neg meter lead to chassis
powerup and read VDC,  should be 100's of VDC
Title: Re: Fuse blown
Post by: sluckey on February 28, 2022, 04:57:11 am
Show us a ***BIG*** hi-rez pic that shows your rectifier socket, PT, and filter cap can. When I say ***BIG*** I don't mean a little 640x480 pic. Need to be able to see the wiring between PT, rectifier socket, and cap can. Also want to see where the PT center tap (usually red/yellow wire) is connected to chassis. Should be able to see all of this in one hi-rez pic.

Title: Re: Fuse blown
Post by: sclinchy on February 28, 2022, 07:07:49 am
OK, thanks. Did my best to get everything in one picture, but I’ve attached a second picture of the cap can wiring as well. As big as the forum would allow.
Title: Re: Fuse blown
Post by: sclinchy on February 28, 2022, 09:23:48 am
Re: Fuse blown (https://el34world.com/Forum/index.php?topic=28552.msg315175#msg315175)
« Reply #50 on: February 28, 2022, 04:14:22 am »
Like (https://el34world.com/Forum/index.php?action=like;topic=28552;msg=315175)[/r]
Title: Re: Fuse blown
Post by: sluckey on February 28, 2022, 09:36:16 am
Quote from: sclinchy on February 28, 2022, 07:07:49 am
OK, thanks. Did my best to get everything in one picture, but I’ve attached a second picture of the cap can wiring as well. As big as the forum would allow.
I specifically asked for a ***BIG** pic, not a tiny 480x640 EGA size pic. Read the last line in this ***BIG*** pic. Your 135.9K size pic ain't even close to exceeding the forum limit.
Title: Re: Fuse blown
Post by: sclinchy on February 28, 2022, 11:12:33 am
Let me try again. I tried Actual and Large, both were rejected. Maybe if I send one per post.
Title: Re: Fuse blown
Post by: sclinchy on February 28, 2022, 11:14:16 am
One at a time seems to work.
Title: Re: Fuse blown
Post by: sluckey on February 28, 2022, 11:48:52 am
Maybe I'm just seeing a shadow but looks like the tube is plugged in wrong. Look at the pic then look at your amp and please confirm.

If the tube is plugged in correctly you should see the filament glowing.
Title: Re: Fuse blown
Post by: sclinchy on February 28, 2022, 12:10:24 pm
I think you’re right!  I just pulled the tube and broke the base (the part that goes into the middle of the socket). I did the same thing years ago!


I’ll have to order another one (sigh) and carefully plug it in when I get it. I foolishly assumed that the tube wouldn’t fit into the socket unless it was aligned properly.


Thanks once again for the help, let’s hope this does the trick.
Title: Re: Fuse blown
Post by: shooter on February 28, 2022, 12:44:41 pm
Quote
Pin 8 IS pin 8, verified?


 :laugh:
ok, i'll quit using words, pics only from here on out  :icon_biggrin:
Title: Re: Fuse blown
Post by: sluckey on February 28, 2022, 02:15:12 pm
See why we want ***BIG*** pics?

You can still use that tube if you plug it in correctly. Use a paint pin to put a white dot on the chassis next to pin 8. Put another white dot on the tube base next to pin 8. Align the white dots and insert the tube. I have a 6L6 that has a broken key. Dots work fine.

You may even want to buy a pack of these...

     https://www.tubesandmore.com/products/keyway-replacement-piece-octal-tubes
Title: Re: Fuse blown
Post by: mresistor on February 28, 2022, 02:18:07 pm
Also next time try tapping it out gently from inside the amp. Put a soft wad of cloth under the tube for when it falls free.

Title: Re: Fuse blown
Post by: sclinchy on February 28, 2022, 05:35:45 pm
Pro tip:  If you plug the rectifier tube in correctly, it will send DC voltage to your filter caps!


Now measuring 449v DC at the first filter cap. Don’t think I can test any further until I replace the dropping resistor that I broke, but it looks like I’m in good shape.


Many thanks to all of you who stuck with me through this, apologies again for my impatience.
Title: Re: Fuse blown
Post by: RadioComm on March 01, 2022, 01:56:40 pm
Quote from: sluckey on February 28, 2022, 02:15:12 pm
You may even want to buy a pack of these...

     https://www.tubesandmore.com/products/keyway-replacement-piece-octal-tubes

Nice! Never knew about these! And all these years I've been hunching over, squinting my eyes, and trying to align octets with broken keys. I'm ordering some just to have them on hand. ttfn.

Couldn't wait. Just added some to my cart.  :icon_biggrin: