Blown C5 Cap

[77seriesIII]

Thanks Ed, it is a Hammond 156R.

I guess what I find as odd is the extremely loud hum, screaming from the speakers regardless of where i put a good 300B. My thinking is which circuit is common/shared and it is the PSU circuit. the c5 is not in the PSU circuit but I wonder if something in the PSU circuit went south and cascaded through the system. When I inserted a single 300B in the circuit, hum was heard from that channel. Remove the 300b, no hum as the circuit is not completed to the speakers. I dont think it is OPTs because a Hum would have been heard regardless of the 300Bs. Even though the C5 blew all resistors diodes appear to have tested out OK in the ac rectifier circuit. Could what caused the c5 to blow have originated from the PSU circuit?

The resistors w/in the PSU (the entire board) tested out ok, cold and no power applied. Would the application of power with the increase of heat cause a resistor that tested good cold but fail under a load be possible? I am failry certain the answer is an unequivocal yes but has anyone seen it?

thanks, still trying to sort this through.

/e

[TomMcNally]

Are both of the 300B's good ?

This whole things makes me thing one of the 300B's
has an internal cathode to plate short, making the
hum ...

[77seriesIII]

tom,

both 300Bs are new and good (well hopefully good right out of the box but not unthinkable)

The tubes that were in the amp during the meltdown are standing patiently in line to be tested at a local tube store as I have no way of testing them.

Other than tubes what are some other potential problems.

/e

[TomMcNally]

Well the beauty of a stereo amp is that it has a lot of
troubleshooting help built in.

The first thing you do it rule out the common things ...

1) do you have B+ ... (yep not much common)

Then check your filament voltage on each channel with
the tubes out of the sockets. Then plug the tubes in
and check under load. If things don't check out -
compare !

Now the easy and fun part ... if one channel works and
the other doesn't ... you measure every single voltage
and look for "what's different" then examine the parts
associated with the "difference"

Keep in mind the variance in a tube circuit can be like
10 or 20% ... things aren't critical ... digital voltmeters
make us think things need to be precise, but often in
a tube amp it's "go or no-go"

Do you still get the hummm with the 300B's in and
the driver tube removed ?

[77seriesIII]

Do you still get the hummm with the 300B's in and
the driver tube removed ?

Tom,

Just did this test and no hum in both sides with one tube in checked both tubes and and then did it with both tubes simultaneously, not even a hint of hum.


To be fair this is not ear next to speaker and I think I hear hum. the hum I am getting is the deafening room vibrating version, with the standard reaction of HOLY SH...What did I DO?

/e

[TomMcNally]

Just did this test and no hum in both sides with one tube in checked both tubes and and then did it with both tubes simultaneously, not even a hint of hum.

I'm not sure I understand your reply ... that sentence
isn't clear. Is the hum gone ?

[77seriesIII]

sorry about that .

All tests were conducted with the 6SL7GT removed.

1st test powered up the amp with no 300Bs installed, no hum
2nd test, powered up the amp with a solitary 300B in the left channel, no hum
3rd test repeat of the 2nd but in right channel, no hum.
Tests 4&5 were repeats of 2&3 but with the other 300B, same results no hum
Test 6 both 300Bs installed no hum

Test 7 installed and tested 3 different 6SL7GT and both 300Bs installed with immediate build in HUM...aka TURN it OFF TURN IT OFF panic.

Sorry just got off work when I did the test and wrote the results in that unique form of babble...

[TomMcNally]

OK - much clearer ...

NOW -

did you have a preamp connected to the inputs,
or preferable have the inputs (RCA jacks) shorted
out for this test ?

You don't really want to run a tube power amplifier
with floating inputs.

If you DID have an input source, like a preamp,
try shorting the inputs to the amp with some RCA plugs that
are shorted to ground, with everything else isolated.

If you don't have any hum with the inputs shorted,
you have a nasty ground loop problem, maybe a bad
cable ... it's hard to say.

If you STILL have hum with the inputs shorted, carefully
check everything associated with the driver tube, from
the input jacks on through.

Some pictures of your input wiring would be nice.

[77seriesIII]

Tom,

I have a star ground. The amp is stuffed into a 6 bottle wine box that I thought was cool. The lid is also wood. I have attempted to follow a good star ground system and even went to and read the star ground link that someone posted here. I think, think being key, I am ok. On the board there is only one post to the ground and it is J6 point 2. I checked my grounds across the board and they test out ok, show a load of .1 to 0.0, I have continuity and no open loads. All transformer grounds and the above J6 point are connected to the plug ground on the AEC via a 12g solid core copper wire. Is there other areas I should check or connect to the star ground? This system was working until the C5 cap went, keep that in mind. I dont know why it went but I think a ground loop hum should have shown up before then and should not have caused the cap to release its magic.

I tested the system with the RCA connections connected to my laptop. Computer was plugged into the same outlet strip as the amp. the volume on the computer was set to zero (no volume control on the amp). What is the best way to short out the RCA inputs? Again with the "think" word, I think the cable is ok, was working when I left with the amp to hembrooks House before I blew up the cap, and I didnt take it with me.

Ahh pictures of the input wiring....because I chose the box that did, I had to make the wires long enough to flip the lid, not ideal I have learned. So as neat as I try to make it, it resembles a minor rats nest. AND because I am learning cool doesnt always make good I have not screwed the OPTs or Power tranny down yet. I know I should but the box has all sides top and bottom. Like I said it sounded good when I chose the box hasnt worked out to be the best in the end but at this point I am working with it. I should have knocked the bottom out before putting everything in, would have made it easier but me more nervous about getting my hands anywhere near it as the box is 7 inches deep.

Having said that I will have the wife take some photos of it this weekend..not too much razing please

/e

[TomMcNally]

Take two new RCA plugs, and solder a wire across
the tip and shield, effectively shorting them out.
If you don't have any plugs, take a cable and
sacrifice it by cutting it about an inch from the
plugs, stripping the wires and shorting them
together.

Plug them into the inputs of your amp.

Is it quiet ?

A computer is the worst source of hum to
feed an amp. When testing for noise, you
need to isolate the amp, otherwise you are
feeding unknown signal into it.

Do you have a battery operated device like an
iPod to feed into the amp ?

Any hum ?

... tom

[ecir38]

Do you have a battery operated device like an
iPod to feed into the amp ?
... tom

or a battery for your laptop? I would suspect the ps for the laptop may be the problem or at least contributing.

[Shannon Parks]

I have a star ground. The amp is stuffed into a 6 bottle wine box that I thought was cool. The lid is also wood. I have attempted to follow a good star ground system and even went to and read the star ground link that someone posted here. I think, think being key, I am ok. ...

Having said that I will have the wife take some photos of it this weekend..not too much razing please

77, some pix will be awesome. You've got me lost when discussing the grounding scheme, as the board <is> the grounding scheme. We'll get this, and thank you for the patience. This hobby can be frustrating as hell sometimes, but the satisfaction when things are good is hard to beat.

[77seriesIII]

ok I've run it with a ipod and laptop on battery, same results...TURN IT OFF TURN IT OFF. You have to yell it because it is that loud.

From that I am beginning to suspect some of the components around the 6sl7. Tom is this what you were trying to isolate?

/e

[TomMcNally]

I would really like to know how it acts when it is not
connected to ANYTHING ... with the inputs shorted.

You could even simply diconnect your RCA jacks
from the terminal strips, and short them with
short jumper wires.

[77seriesIII]

ok I'll be a monkey's freakin uncle...DED silent with shorted out rca connects. I did exactly what you suggested Tom, I disconnected the RCA's and ran a short wire across the jumpers. I tested one tube, in both channels then tested the other tube in both channels, no hum. Both 300Bs in, no Droning HUM.

So with that experiment out of the way...I am guessing I have a nasty ground loop problem.

Since I am using a wooden box this is what I have grounded to the AEC plug ground point through a bus wire.

-pole 3 on the on/off switch
-point 2 on J6
-the ground from the Power tranny from the Hv
-I have also taken a ground wire to each housing for the OPTs/choke as they would have been mounted to a metal chassis.

At this point I think I need to go through my bus and probably re-wire it...IF Tom/(or anyone else for that matter) concurs.

Amp has been running now with shorted RCA connects for 20 minutes not a peep, no pffts/smoke, just glowing tubes.

I am going to turn it off now and wait for a response. At this point I have gone into dumb apprentice mode and will sweep sheepishly with my broom while awaiting orders.

/e...dont mind me, just sweeping