60's projector amp - blowing fuses

[dayn]

I have a 1960's era military film projector amp that I am working on and it is blowing fuses. I have already replaced the electrolytic caps. They all tested correctly before install.

This is the amp schematic, page 21:

http://www.acofs.org.au/part_5_files/Be ... 614EVM.pdf

The fuses blow instantly when the amp is powered on. I am using the correct 8/10 125A fuses for the amp.

https://robrobinette.com/Tube_Guitar_Am ... lows_Fuses

Following the above guide on tracing blown fuses, I did the following with my light bulb limiter with a 75 watt bulb:

Amp with no tubes installed - light bulb limiter is DIM.
Amp with both rectifier tubes installed (there are two 6x4), DIM.
Amp with both rectifiers in, and first preamp tube, DIM.
Amp with both rectifiers in, pull first preamp, and second preamp tube in, DIM.
Amp with both rectifiers in, preamps out, first power tube (6aq5), BRIGHT. Tried 5 different 6aq5 tubes to deduce if bad tube, still BRIGHT.

As I then understand it, there is a short in the output tube related section. I have traced every wire, inspected the turret board, inspected the tube sockets, everything that I can think of, and can't figure out where the problem would be. Following advice on another forum I am part of, I replaced components in the power tube circuit (which I was going to mod with slightly better guitar related values anyway), as well as changed both power tube sockets. Problem persists. Possible PT issue? I do have an extra vintage PT from the same type of amp that I could swap in if needed.

I've been on The Amp Garage for many years, but only recently started working on tube amps myself. I've modded maybe 10 vintage tube amps so far all with great results. Fun, but this one has been a challenge given my inexperience.

Any ideas on what to do next?

This morning I did come across these two things from here on The Amp Garage, wondering if there is a good next step outlined in either for me to try?

https://ampgarage.com/forum/viewtopic.php?f=6&t=32210

http://www.geofex.com/ampdbug/fuseblow.htm

I appreciate any advice (or help in Seattle) from anyone who can chime in!

Here is a shot of the full amp circuit:



Here is the PT:



Power section components, some replaced (problem identical before and after replacement):



Power tube sockets (before replacement, problem identical before and after replacement):





This is how filter caps go in, they plug in like tubes into sockets. The missing (film related) components were clipped off and weren't connected to the caps. Those pins aren't used:

[Stevem]

Something is not making sense here!
Those 6X4s are the type that are indirectly heated.
This means that they will not put out the intended full DC voltage until they warm up, think of it like a soft start.

In light of this fact I can’t see how your fuse right off the bat blows once the output tubes are in.

If a output tube socket was shorted / arced over with carbon due to a tube going bad, then I would expect the fuse to blow with the tubes out, but only after a few seconds that the 6X4s need to warm up and not blow out right away.

The only other thing that comes to mind is that all of your 5 output tubes are bad and they are bad by having a short from the heater section in them to either the plate of the tube, or the screen .

You could confirm this with a simple resistance test from tube pin to tube pin.

Please report back on what you find and please also take and post up some gut shots of the power supply and output section of this if you would?

[johnnyreece]

Does it have the same behavior in either socket?

[dayn]

Something is not making sense here!
Those 6X4s are the type that are indirectly heated.
This means that they will not put out the intended full DC voltage until they warm up, think of it like a soft start.

In light of this fact I can’t see how your fuse right off the bat blows once the output tubes are in.

If a output tube socket was shorted / arced over with carbon due to a tube going bad, then I would expect the fuse to blow with the tubes out, but only after a few seconds that the 6X4s need to warm up and not blow out right away.

The only other thing that comes to mind is that all of your 5 output tubes are bad and they are bad by having a short from the heater section in them to either the plate of the tube, or the screen .

You could confirm this with a simple resistance test from tube pin to tube pin.

Please report back on what you find and please also take and post up some gut shots of the power supply and output section of this if you would?

Without my light bulb limiter, the amp would blow fuses right away.

Using the light bulb limiter, it takes about 5-10 seconds for the 6x4's to warm up, THEN the light bulb goes bright signifying a short. Does that match with what you were saying about 6x4's?

You could confirm this with a simple resistance test from tube pin to tube pin.

I will test resistance on the 6aq5 tube pins when I get home this afternoon.

And I will post a few pics in the original post of the amp insides.

Does it have the same behavior in either socket?

Yes, whether a 6aq5 power tube is in v3, or v4, or both, the light bulb limiter will glowly brightly after the rectifiers warm up.

[Stevem]

Yes, the bulb glowing brighter as the two 6X4 warm up is what a would expect.

You have to narrow down on what’s taking place.
Test those output tubes for sorts first.
Pins 3 and 4 are the heater and you should not read any resistance from either of those to pins 5 or 6.

If that checks out then try this.

First disconnect the center tap of the output transformer from the first power supply filter node.


If with that unhooked your still blowing fuses, then Lift the 6X4 side of r228 and see if the fuse still blows.

[dayn]

Yes, the bulb glowing brighter as the two 6X4 warm up is what a would expect.

You have to narrow down on what’s taking place.
Test those output tubes for sorts first.
Pins 3 and 4 are the heater and you should not read any resistance from either of those to pins 5 or 6.

If that checks out then try this.

First disconnect the center tap of the output transformer from the first power supply filter node.


If with that unhooked your still blowing fuses, then Lift the 6X4 side of r228 and see if the fuse still blows.

To be more clear, when I turn the amp on, the light bulb goes on dimly at first, then gets a little brighter as the rectifiers warm, then goes back to same dimness as when first turned on and stays that way. That takes about 10 seconds.

BUT when the power tubes are in, the same happens as above, but after a slight settling of the brightness after the rectifiers are warm, it then begins to glow very noticeably bright as in 10x brighter, which I thought signified a short.

I will test tubes for shorts between pins 3 and 4 to 5 or 6. I assume they are measured outside the amp, or do you measure with them installed and test them at the pins, or both?

I will do the OT and 6x4/r228 tests you suggest above after confirming tubes have no shorts. Are those tests done without the light bulb limiter, as in literally blow the fuse, or do them with a limiter and see if it glows brightly? And are all tubes in for those tests, if not which ones are in?

[sluckey]

Measure the voltage on pin 1,7 of the power tubes. It should be ZERO. If you have a positive voltage present, then the coupling caps C212 and/or C213 are leaking. This will cause the 6AQ5 to draw excessive current and light the dim bulb or blow the fuse.

[Stevem]

You make the short test of the tubes out of circuit.

The other two test you can make while going thru the limiter, since by now you likely know how bright the bulb gets before things are going wrong enough to pop the fuse.

If you want to know how much current is being drawn and you have some clip leads then wire up your voltmeter set for low amps in line with either side of the on/ off switch as might be the easiest place to open up the primary side of the power transformer.

[dayn]

Measure the voltage on pin 1,7 of the power tubes. It should be ZERO. If you have a positive voltage present, then the coupling caps C212 and/or C213 are leaking. This will cause the 6AQ5 to draw excessive current and light the dim bulb or blow the fuse.

Am I measuring pins 1 and 7 of power tubes with amp on? With limiter? With any tubes? And am I measuring between pins 1 and 7 (they are internally connected in the tube right?) or between pin and ground?

FWIW, C212, and C213 are both brand new. The fuse problem was there before and after swapping them, in this instance for a lower value (.1 to .022uf).

[dayn]

You make the short test of the tubes out of circuit.

The other two test you can make while going thru the limiter, since by now you likely know how bright the bulb gets before things are going wrong enough to pop the fuse.

If you want to know how much current is being drawn and you have some clip leads then wire up your voltmeter set for low amps in line with either side of the on/ off switch as might be the easiest place to open up the primary side of the power transformer.

Am I doing those tests with any tubes in? Rectifiers and power tube or tubes I assume?

And for measuring current, I have a variac with an ammeter, could I use that along with my limiter?

[Stevem]

You don’t need to do the test for voltage on pins 1 and 7 since your coupling caps are new, at least in my opinion.

The power supply test I posted about is done with the outputs in the amp if they check out for no internal shorts.

You can’t use your variac since it will also be measuring the current drawn by the light bulb.

[dayn]

I tested the power tubes, and did not find shorts between pins 3 to 5 and 6, or 4 to 5 and 6.

A few historical details on the amp. A few weeks ago after recapping it, it was working, briefly, though with some static noise. Someone suggested I clean tube pins by spraying deoxit 5 in the tube sockets, so I did. I let it dry for about 15 minutes then turned amp on and blew fuse. I also noticed that I blew resistor R210, which I replaced. In hindsight spraying directly in the socket was bad, and possibly still wet and/or conductive in some way and caused problems. Maybe not, but mentioning it in case it helps lead to troubleshooting ideas. That is also what led to the advice I got to change the power tube sockets given the short seemed to be around power tube, but that hasn't fixed the issue. All sockets were sprayed including the two octal sockets that hold the filter caps, C214 ABC and C216 ABC.

First disconnect the center tap of the output transformer from the first power supply filter node. If with that unhooked your still blowing fuses, then Lift the 6X4 side of r228 and see if the fuse still blows.

I will next do the above quoted suggestion this evening and report back. The OT center tap goes directly to pin 7 of one 6x4 rectifier, then a wire goes from there to pin 7 of the next 6x4 rectifier, then a wire goes from there to C216A via the filter cap socket. Do I remove the center tap at the first rectifier tube pin 6, or at the Filter cap which would cut rectifier out from power filtering?

Looking at the schematic - would pulling C216 ABC do the same thing as disconnecting the above, or does removing all three filter caps at once introduce too many variables?

[Stevem]

In thinking about this again I suggest that you unhook the wire from the OT that goes to pin 5 one at a time, if after removing both of those the limiter is still glowing bright then unhook the feed to the center tap of the OT where it lands at the 6X4.

[dayn]

In thinking about this again I suggest that you unhook the wire from the OT that goes to pin 5 one at a time, if after removing both of those the limiter is still glowing bright then unhook the feed to the center tap of the OT where it lands at the 6X4.

Removed OT wire from v3 pin 5, installed both 6aq5's, turned amp on, limiter bulb still BRIGHT.

Removed OT wire from v4 pin 5, still BRIGHT.

Removed OT center tap from pin 7 of v5 6x4, still BRIGHT.

Seems there is still a short somewhere. Do I now reattach all those OT wires, and test something else? Is it disconnect the wire going from R228 to the filter cap, removed at the filter cap end (which is the filter cap tube socket)?

[Stevem]

Sorry, I forgot that has a multi section filter.

Disconnect the end of r228 that does not feed r240.

Also please hook up your meter set to read amps so you can report back with accurate numbers and not just a level of bulb brightness.