Military projector amp - last edited 9/26/22 - layout help needed

[yello]

It's possible.. you know what to do next now

Yep! I assume I need to replace the broken resistor, then check voltages while I wait for my new oscilloscope to show up.

[yello]

Parts arrived - after I replace the broken resistor, I'm ready to check voltages.

Do I take voltage readings with all tubes in? Power tubes out? With speaker load?

[Bieworm]

All tubes in. Speaker attached
The fuse doesn't blow I presume?

[yello]

Just replaced the cracked resistor and...fuse blew.

Not currently using my light bulb limiter to start it up as I can't find a non-LED light bulb in Seattle.

I only have one 8/10amp 125v fuse left...

What is a good next step to figure out why I am blowing fuses? I do have two more sets of rectifier tubes I can try out (amp used dual 6x4). Just not enough fuses, gotta find some to that spec to order today.

[Bieworm]

Fuses blow mostly due to shorts. Shorts can be measured .. look for it

[yello]

I can't visually see any components or wires shorting yet. How do I measure to find them?

[Bieworm]

It's likely the resistor was cracked due to a fault in that part of the system. Start there...

Or..

Process of elimination. Pull all tubes. Take your lightbulb current limiter and look if it's not in the B+ line. If the lamp is lighting up strongly without tubes there's a big chance your power transformer is shorted. It's also likely the filter caps are kaput. If it's tube rectified then that also is one of the suspects.

OTOH, if the lightbulb limiter doesn't light up, then there could be a short in the preamp or power amp. Put tubes in, one by one until the limiter lights up. That will narrow your problem area down

[yello]

It's likely the resistor was cracked due to a fault in that part of the system. Start there...

Or..

Process of elimination. Pull all tubes. Take your lightbulb current limiter and look if it's not in the B+ line. If the lamp is lighting up strongly without tubes there's a big chance your power transformer is shorted. It's also likely the filter caps are kaput. If it's tube rectified then that also is one of the suspects.

OTOH, if the lightbulb limiter doesn't light up, then there could be a short in the preamp or power amp. Put tubes in, one by one until the limiter lights up. That will narrow your problem area down

Cracked resistor was a preamp tube negative feedback resistor, I'll being taking a look there.

Thank you for all the suggestions. Thankfully I was able to source a 100w light bulb for my limiter.

Quick questions in regards to your suggestions:

1) Does a new fuse need to be installed to try amp without tubes while using the light bulb limiter?
2) The schematic calls for 8/10 amp 125v fuses. Looks like the modern equivalent is 1 amp 125v fuse?

Anecdotally, the amp was working 10 days ago, albeit with an annoying crackling on all notes (unsure what the problem was, maybe the cracked or failing resistor). I changed out one very microphonic resistor, and cleaned the tube sockets with Deoxit D5, after that is when I started blowing fuses. And each time the amp was turned on, the pilot light would light up right before fuse blew. The amp is running new electrolytic filter caps. I do have a correct spec and tested set of the plug in filter caps that I could try if needed to isolate my new caps as a potential source of the problem if needed, though I tested the new ones before install and they were correct.

[Bieworm]

Yes you need a fuse to get continuity. If you would bridge the fuse when using a lightbulb limiter it would be ok, but there's always the chance you forget to fuse it afterwards.
A 1A fuse would be ok too.
I would also closely examine the rectifier and your transformer's high voltages. Maybe disconnect the B+ after the 1st filter cap and measure the DC from the rectifier.
Also check if there is no short in the heater circuit. Those 2 checks come to mind when you mention crackling noises.
You did check the tubes I presume?

[yello]

Yes, prior to blowing fuses I had tried different tubes in every socket in an attempt to see if my noise issue was tubes. To no avail, noise had remained.

[zaphod_phil]

what kind of noise are you talking about? How does it sound?

[yello]

https://youtube.com/shorts/n6e3QpzRGJA?feature=share

I posted this video on page 2. It is the sound I was hearing before the amp started blowing fuses.

[yello]

Also check if there is no short in the heater circuit. Those 2 checks come to mind when you mention crackling noises.

After looking closely at the whole amp with a light and magnifying glass, I see nothing out of place.

Given your mention of the heater circuit, I am wondering if I have a cold solder joint at the 6.3v PT tabs where the pilot light is soldered on. I did unsolder those wires at one point, move them, then resolder them onto the tabs. Potential there for both the crackling noise and a short?

If so, I could remove them and test, or remove and resolder them and test.

I'm going to do the testing tomorrow with a fuse in place and using my light bulb limiter.

[yello]

I've got the day set aside today to work on the amp. I'll post my different findings as they happen, so will update often.

I started out by re-doing the heater wire connections at the PT.

Here are a few initial measurements, happy to take more if there are any helpful to take (voltages later once amp is working):

Resistance between hot and neutral on my IEC with switch on and unplugged: 5.2 ohms
Resistance between 6.3v tabs on PT: .3 ohms
Resistance between HV wires that go to rectifiers: 131.9 ohms

Next I will use my light bulb limiter:

No tubes: light build stays dim
Rectifier tubes installed (dual 6x4): dim
v1 12ax7: dim
v2 12ax7: dim
v3: 6aq5: starts dim then goes bright. tried another 6aq5 to rule out the tube, bright. Tried two other 6aq5s, bright.

So it looks like the problem area is around the first power tube.

You did check the tubes I presume?

I inspected the power tube pins, no visible shorts. I traced the wires and see no shorting or burnt/cracked components, save R228 and R240, 350 ohm 3w ohmite resistors. They test at 350 ohms, but the edge looks cracked with some black.

So I replaced R228 and R240. I also broke a resistor next to R240 when replacing it, R226 at 10k, so I replaced that.

I then went back to testing with the light bulb limiter to see if replacing the resistors fixed the problem. Rectifiers in, v3 in. Turn it on, Bright still....

What is the next step? Do I need to be taking voltages with my light build limiter and with just the rectifiers in?

I also tried swapping in a set of old filter caps that test well (the type in this amp all plug into tube sockets) to see if it was the new caps I installed, and it glowed bright with them as well.

[JMPGuitars]

If the short is only occurring with V3 inserted, then the issue is most likely something in that section of the circuit. It could very well be a wiring issue, or something else shorting more discreetly.

I would suggest, with the amp OFF, and disconnected from power, pull V3 and test all the connections on the tube socket. Then test the circuit in that section.

[yello]

test all the connections on the tube socket. Then test the circuit in that section.

What tests do I do on the socket, and on the circuit? Am I measuring things, pulling components to measure, inspecting, replacing, ?

[JMPGuitars]

test all the connections on the tube socket. Then test the circuit in that section.

What tests do I do on the socket, and on the circuit? Am I measuring things, pulling components to measure, inspecting, replacing, ?

Start by verifying the wiring with the schematic.

Then check continuity (again, no power cable connected), and see if anything is grounding out that shouldn't.

Then check components for shorts/continuity (for example, a failed capacitor could turn into a jumper wire).

[yello]

test all the connections on the tube socket. Then test the circuit in that section.

What tests do I do on the socket, and on the circuit? Am I measuring things, pulling components to measure, inspecting, replacing, ?

Start by verifying the wiring with the schematic.

Then check continuity (again, no power cable connected), and see if anything is grounding out that shouldn't.

Then check components for shorts/continuity (for example, a failed capacitor could turn into a jumper wire).

Wiring verified as stock and correct. All wiring around the power tubes looks correct, components tested.

I so far can't figure out anything being grounded that shouldn't, or that has shorted.

I do get a beep with my multimeter between the heaters/CT and ground, whether on the sockets or at the PT. It beeps once then reads 223 ohms.

I'm at a loss, I've been in this thing for hours checking, re-checking, re-checking and I can't figure it out.

Could there be a problem with the OT? Not sure how to check or verify that, or if it is relevant given the problem seems to be at v3 power tube.

Is it the tube socket? I have checked for continuity, normal except for the above heaters/ct beep. Every wire is going to the right place, no terminals touching, Components seem ok.

Any ideas or advice?

[JMPGuitars]

Wiring verified as stock and correct. All wiring around the power tubes looks correct, components tested.

I so far can't figure out anything being grounded that shouldn't, or that has shorted.

I do get a beep with my multimeter between the heaters/CT and ground, whether on the sockets or at the PT. It beeps once then reads 223 ohms.

I'm at a loss, I've been in this thing for hours checking, re-checking, re-checking and I can't figure it out.

Could there be a problem with the OT? Not sure how to check or verify that, or if it is relevant given the problem seems to be at v3 power tube.

Is it the tube socket? I have checked for continuity, normal except for the above heaters/ct beep. Every wire is going to the right place, no terminals touching, Components seem ok.

Any ideas or advice?

The heaters CT is a direct connection to ground. With the amp off, you'll get continuity from the heaters to ground. If you disconnect the CT, then you shouldn't get continuity to ground. As mentioned, never test continuity on a live/powered amp.

A bad tube socket is generally more likely than a bad OT, especially since the issue doesn't occur until V3 is inserted.

Did you check the caps for continuity? Press the probes on either side of your caps in that part of the circuit and see if anything beeps.

Post a clear picture of the schematic, especially make sure that V3 section is clear. Also post some high res photos of your build, again, especially around V3.

Thanks,
Josh

[yello]

The heaters CT is a direct connection to ground. With the amp off, you'll get continuity from the heaters to ground. If you disconnect the CT, then you shouldn't get continuity to ground. As mentioned, never test continuity on a live/powered amp.

A bad tube socket is generally more likely than a bad OT, especially since the issue doesn't occur until V3 is inserted.

Did you check the caps for continuity? Press the probes on either side of your caps in that part of the circuit and see if anything beeps.

Post a clear picture of the schematic, especially make sure that V3 section is clear. Also post some high res photos of your build, again, especially around V3.

Thanks for the continued safety advice, I am following it carefully. I have not lifted the CT to check for continuity, but can if that is helpful.

I do not get continuity on the caps in that section. I tested the originals, no continuity, I also replaced them all per the suggested values in the earlier part of this thread for better bass response, and the new ones no continuity as well.

I tested the filter caps, and nothing grounding/shorting, all test within spec.

I will now post several pics of the schematic, amp, v3 area, etc., in a followup thread.