Help needed with debug/accident investigation

[Quatro]

I think I probably found the bad part on this one but I'm not sure what or how it happened...

I built a 5e3 style amp for a guy and when he received it he played it for a few hours - no problem. The next day he puts it on a Radial Headbone head switcher that he bought USED. He just bought it so he has never used it before. This is his first try. I have one of these as well and use it with my amps all the time with no issue. The other amp is Ceriatone of some variety. He starts with the Ceriatone - no issue. Switches to the amp i built  - no sound, blown fuse. Replaces fuse and hooks up to regular set up - no signal only very loud hum.

I get the amp back. Loud hum is not affected by volume knob or by pulling tubes -  except rectifier tube.  I try new known working in tubes in all sockets - no change. I replace power rail (it's kind of a unit in this amp) - filter caps and dropping resistors -  no change. Swap known working OT -  no change. Light bulb limiter is "inconclusive" -  maybe a bit brighter than usual but not the brightness I have seen before with a real short. I'm not really taking voltage measurements because I'm only switching  it on for a second at a time because I'm afraid to run it for longer. AC voltage measured on rectifier pins with rectifier tube pulled seems ok except  pin 8 where I get zero. I try this measurement on a working model and get zero there as well, so i disconnect the the B+/ filter cap and measure again. Result: 100v!


 I haven't done it yet but PT replacement must be the fix.


Any ideas what has happened or how it might have happened?

Thanks in advance for your ideas.

[phsyconoodler]

Quote:"AC voltage measured on rectifier pins with rectifier tube pulled seems ok except  pin 8 where I get zero."

  Ok,you measured each leg of the PT secondary with no rectifier tube and you get like 325v AC from each red (or whatever yours is) and 5v AC or so on each yellow wire?
  If you get no voltage from one leg of the 5v winding then the PT is done.
How it happens is the amp was run with no load from his amp switcher.It would possibly kill the PT because the OT center tap is hooked up to pin 8 where it goes to the board on a 5E3.An HT fuse would have saved your PT most likely.

[plexi50]

Recheck input jack grounding

[Quatro]

Ok,you measured each leg of the PT secondary with no rectifier tube and you get like 325v AC from each red (or whatever yours is) and 5v AC or so on each yellow wire?
  If you get no voltage from one leg of the 5v winding then the PT is done.

yes, I get the expected amount (365v for this PT) on the red wires. With no tube or b+/filter/ot attached to pin 8, pins 2 and 8 show around a 100v. Is this a short of some kind?

[Quatro]

Recheck input jack grounding

Checked that ground and all other grounds. Can't seem to find any issues.

[sluckey]

yes, I get the expected amount (365v for this PT) on the red wires. With no tube or b+/filter/ot attached to pin 8, pins 2 and 8 show around a 100v. Is this a short of some kind?

Are you measuring 100v between pin 8 and chassis with the rectifier tube pulled? If so, don't do that. That's a meaningless check and your meter is lying to you. The only valid voltage check involving pin 8 with the tube pulled would be to check the 5VAC filament voltage BETWEEN pins 2 and 8.

[Quatro]

The only valid voltage check involving pin 8 with the tube pulled would be to check the 5VAC filament voltage BETWEEN pins 2 and 8.

makes sense, of course.

having done so I have around 5.5v. So what gives? PT is good. OT is good. Tubes good. I checked every resistor value and checked the most caps for shorts/opens. Any ideas how every part seems to check out yet the amp will not? Feeling stumped.

[Quatro]

To summarize at this point:

rectifier tube in socket - no other tubes - flip the switch -  instantaneous loud hum from spk - less than 200v making it to the first coupling cap. Third OT tried. Can't guarantee latest OT is good, but it worked the last time it was in an amp. The amp was then parted out.

[sluckey]

Disconnect the OT primary center tap and turn it on. Does the B+ come up to expected voltage (maybe high)? Does the hum go away?

[sluckey]

Are you measuring 100v between pin 8 and chassis with the rectifier tube pulled? If so, don't do that. That's a meaningless check and your meter is lying to you. The only valid voltage check involving pin 8 with the tube pulled would be to check the 5VAC filament voltage BETWEEN pins 2 and 8.

I've thought about this statement and I really need to clear it up. It's perfectly normal to measure some voltage between a rectifier filament pin and chassis for a properly working amp. This is a sneak voltage that has very little, if any, current capacity and can be ignored. The real voltage that you're interested in is the 5vac across the filament.

However, your amp is not working properly. In this case, that 100vac you measured may be the normal sneak voltage reading, but since you may suspect the PT, you can't just rule that reading out. You could have a short from some other winding to the 5VAC winding that's putting 100vac on that winding. So, use your ohm meter to check for any resistance from the 5vac winding to all other windings. There should be no resistance (open, infinity, etc.). If you have some resistance the PT is defective.

[Quatro]

So, use your ohm meter to check for any resistance from the 5vac winding to all other windings. There should be no resistance (open, infinity, etc.). If you have some resistance the PT is defective.

With one end of the meter clipped to pin 2 I get some resistance on all the rest of the secondary taps (very small but measurable resistance). Also, very high but measurable resistance on one of the primaries. Definitive?

[sluckey]

Disconnect any wires from pin 8 that go to the filter caps or STBY switch so that the only wires on pins 2 and 8 are the transformer wires. Then recheck and post the actual resistance numbers.

[Quatro]

Disconnect any wires from pin 8 that go to the filter caps or STBY switch so that the only wires on pins 2 and 8 are the transformer wires. Then recheck and post the actual resistance numbers.

No tube, pin 8 has nothing attached but transformer wire.

Clipped meter probe to pin 8.

All primaries and secondaries are infinite - except there is a  reading between pins 2 and 8  = .2 ohms

[sluckey]

Sounds good to me.

[Quatro]

At this point I'm not sure if that's good news or bad news.

Thanks, Slucky, for going the distance.

If the issue is not the PT, I don't know where to go next.

Here's what I know for sure::

Something caused the mains fuse to blow when hooked to the head switcher.

As is it stands right now there is very little circuit left in this thing. We have PT, swapped out "believed to be good" OT, known to be good rectifier tube, output jacks, known to be good replaced power supply (four filter caps and dropping resistors), all grounds go to a buss wire that terminates at the PT bolt. All grounds seem solidly soldered to this point. Continuity tester shows buss is in contact with chassis.

When previously described is powered up  - instantaneous loud hum. What could be the culprit? There are not that many elements. The whole rest of the circuit is out of the loop.

I guess the next step is to swap the PT are not a lot of options left.

[sluckey]

I guess the next step is to swap the PT are not a lot of options left.

I guess so, but I'm not very confident about that being the fix.

rectifier tube in socket - no other tubes - flip the switch -  instantaneous loud hum from spk

This statement keeps bugging me. Do you mean that even before the rectifier tube warms up, you have loud hum? Have you swapped the rectifier tube?

Disconnect the OT primary center tap and turn it on. Does the B+ come up to expected voltage (maybe high)? Does the hum go away?

Have you done this? If so, what were the results?

[Quatro]

rectifier tube in socket - no other tubes - flip the switch -  instantaneous loud hum from spk

This statement keeps bugging me. Do you mean that even before the rectifier tube warms up, you have loud hum? Have you swapped the rectifier tube?

Maybe instantaneous is an exaggeration but within one half second I would say. Rectifier tube: swapped in my "test set" right away. Repeated with three 5Y3s today to double check. No change with alternate tubes.

Disconnect the OT primary center tap and turn it on. Does the B+ come up to expected voltage (maybe high)?

Voltage at pin 8 with OT disconnected is about 490v. Normal voltage at the plates in working amp is usually 370 - 395 ish.

Does the hum go away?

Hum goes away with center tap disconnected.

[Quatro]

I guessing the ship has sailed as far as someone nailing this one down for me, but I'd like to throw out one more detail/clarification:


Regarding the sound/quality of the hum -the last time I heard a similar sound was with an old gibson amp someone brought to me. The multisection filter cap was suspended from the chassis with  a metal clamp ring bracket thing. The bracket  had a bit of play in it allowing the cap can to move and vibrate a bit. As a result the ground wire for the can flexed enough times that it finally broke. Flip on the power -  giant hum.