Checking current

[The Captain]

There was no resistance to gnd at any stage or any place of checks. MM set for DC of course. All valve pins/ continuity checked numerous times around amp, as is wiring etc etc..

I can't be sure now these caps are defo shagged..? Im no closer knowing wtf is wrong with it.. unless either a 2w resistor is a duffer (checked again- read 4.7k/ 22k) or a crap 0.1 uf (RS) or bad 5Y3GT (TAD) could be the cause.

What voltage is normal at the filter caps with just recto in; am I 100v too high? should these caps all read the same?

[stokes]

Cap'n,with just the rectifier in the voltage at each cap should be the same as there is no current being drawn without tubes in the amp.Whether or not the 500v is too high is not relevent to the problem.As I have said before,any healthy power supply I have ever seen shows no more than 1-2 volt drop down the whole line.The fact that you are seeing a 5volt and 8 volt drop indicates there is current being drawn,the only thing in the circuit to draw current is the caps,and they would not draw current if they were not leaking.If the 500+ volts you are getting out of the PT is too high,it is because the PT is not the correct one.Lets get to the bottom of the overheating PT,then we can put the tubes in and see how high your volts are under load.

[The Captain]

Ok Bruce, so Im pulling the 3 16uf caps + sides up again, powering up both switches closed and measuring V at that 100k junction to ground & at junction of 4.7k and 22k? forgive my ignorance but when you say 'DC resistance'.. Im muddled- resistance in ohms, DC as a voltage..

[Tom Phillips]

If you got something better,by all means please let us know,as this is getting quite frustrating on both sides of the ocean.

OK. Below are my current thoughts. (I stand by my previous posts too)

If you read the prior thread I pointed you to…

Yep, I actually followed that thread as it played out and I see why this has been a frustrating experience for the Captain and those who are trying to help. The Captain is just learning electronics, troubleshooting techniques and the proper use of test equipment. Not an easy thing to do.

… fuse really blackened)- ? Fuse is odd, maybe it just had enough already what with all the onning and offing…

The black in the fuse indicates that it blew because of a heavy current surge as opposed to a long minor overload. It appears that you have an intermittent short and when it does happen it draws lots of current.

Tom, the idea is that the leaky caps are drawing more current than the PT is rated for,causing overheating and eventual PT failure… my theory is that the cap is not shorted,as this would blow the fuse,but leaky enough to slowly overheat the PT and cause failure

We don’t agree on this point. I calculated and reported previously that the “leaking” current is around 1ma. IMHO that is not the cause of the transformer overheating or the fuse blowing. There is a low resistance short somewhere. Hence the blackened fuse glass that Captain reported. It appears that the short is intermittent. That can be difficult to find. Furthermore, if we are to trust the plate current readings that Captain reported, the power tubes are drawing way too much current. This could be because the plate voltage is too high or the cathode bias resistor is off. The high bias current is much more significant than the extra 1ma of “leaking” current. I still suspect that the PT secondary voltage is too high. That’s why I asked for the AC voltage readings of the unloaded secondary windings a while back. I’ve been burned before when I assumed that a part was OK just because it was recently replaced.

Cheers everybody,
Tom

[Enzo]

DC resistance means we are not talking about impedance - which would be "AC resistance." It just means your ohms scale on the meter. We get used to using terms like DC resistance because so many times we have to correct people who wonder why their 8 ohm speaker (impedance) measures only 6 ohms on their meter (resistance), as an example.

I wonder why that 4.7k resistor drops the voltage 7 volts BEFORE the second cap is reconnected. It is only a milliamp, but where does it come from. Or is the wiring from these nodes still attached?

A dead short to ground on the low side of the 22k would be like 537 volts across 26.7k (22k+4.7k). That is roughly 20ma. I don't see that blowing fuses.

What meter are you using?

[The Captain]

Im using a Mastech M-830B DMM perfectly fine it is, not ££s though.

Im now wondering if the replacement PT has correct 2ndary windings! I checked the PT model no. was correct of course; there surely tho must be a simple solution to this prob. Can someone remind me how to check 2ndaries V again.. is it: reading MM for AC, pin 4 of 5y3GT to ground, and rectifier tube in only?

If not this and or caps, I'll have to take whole thing apart, bin power tube Rs caps etc, replace and start again I guess.. I don't think I can face just for another prob to crop up.

[Gee]

Im using a Mastech M-830B DMM perfectly fine it is, not ££s though.

Im now wondering if the replacement PT has correct 2ndary windings! I checked the PT model no. was correct of course; there surely tho must be a simple solution to this prob. Can someone remind me how to check 2ndaries V again.. is it: reading MM for AC, pin 4 of 5y3GT to ground, and rectifier tube in only?

If not this and or caps, I'll have to take whole thing apart, bin power tube Rs caps etc, replace and start again I guess.. I don't think I can face just for another prob to crop up.

Take things one at a time. Whether the PT secondaries are wired correctly is not the immediate issue. You have something shorting out or a failing component that is causing current to flow when there are no tubes.

One way to find out the problem would be to remove all components from the board and add them back in one at a time. Then each time turn on, measure voltages and resistance to ground and you should find the problem when you either add back in the problem component or disturb or fix the short.

[Tom Phillips]

... Can someone remind me how to check 2ndaries V again..

See my first post of 4/8/2007 in this thread.

[Bruce / Mission Amps]

OK... I've been very ill over the last three weeks and haven't been real good about reading all the threads...
I realise now you previously said the main 1a fuse blows with a black film in it... that does mean very heavy and instant current flow.

At +500vdc the 22K, preamp dropping node resistor would limit the current to around 23ma. The 30VA power tranny and primary fuse could suppply this current for hundreds of hours with no serious problems, but the 1/2 watt 22K resistor would burn up.
Using simple Ohm's law...
23ma x 23ma x 22Kohm = 11 Watts!!

What causes a fuse to blow like that is something like a dead shorted filament supply or a miswired socket where the raw high voltage AC (if the rectifier) or the B+ from the rectifier, standby, + ends of the power supply caps, vacuum tube's plates or screens is shorted to ground... maybe even through the filament supply ground reference... instanly or after a few moments when the tubes warm up.. etc.
Another cause is when using a multi tapped primary power tranny.
As used for the combination of North American and Euro consumers, where there is the possibilty of the primary windings being miswired or accidentaly wired out of phase if it is a split primary wind.
If yours is an auto transformer, (where there is not seperate windings) using taps pulled to create the proper turns ratio for 100v, 120v, 220v and 240v mains, then this is probably not the case.
Unless... the unused leads have not been dressed properly and are shorting somewhere together or to the chassis or mistakinly wired wrong.
Because you are a novice builder and the fact that you have had two transformers go bad or over heat and blow fuses suggest to me that, there is a miswired lead or socket somewhere that has simply eluded you right now and it is after the power tranny.
The lead that can cause this has to be one where excessive current can flow and that would be close to the B+ rail circuitry, filament supply or tube socket.
This is where years of trouble shooting experience is so handy.
Many of us, who if having this amp on our benchs in real time, could probably sort it out in a matter of minutes, but being thousands of miles away and having to read text makes it very hard and frustrating for all.
I know this is a pain in the rear end but you've come this far....can you get a camera out and take good, focused pictures of each tube socket, the standby switch (if used), the main filter caps and the wires from the PT... did you say it was a second or replacement WeberVST Deluxe power transformer?
I did much of the BETA testing for Weber and their transformer line a couple years ago and I did real destructive testing on them.
Although inexpensive, Chinese made and not the best made iron in the world,
if the transformers are not defective and the circuit is correctly assembled, their Deluxe power tranny is more then capapble of running a 5E3 amp for years.

[Bruce / Mission Amps]

No, what I'm talking about doing is measuring resistance in ohms to ground from the B+ nodes with the postive end of the filter caps lifted.
Other possible problem areas need to be checked and the amp has to be off and unplugged from the mains with all the B+ drained off.

With your black negative lead of the DVMM grounded, use your red positive probe from the rectifier lug 8, the standby switch lugs, the centertap of the OT, lugs 3 of the power tubes, both sides of the 5K1 resistor, both sides of the 22K resistor and the filament wires on all sockets with the center tap of the filament supply lifted if grounded.
If it using the 100 ohm balancing resistors, then that's OK, you'll see 50 ohms or so unless shorted to ground.

[The Captain]

Much help there Bruce: Yes thought a std continuity test must be what you meant (a few terms for similar things now and then tricky for the learner): so all from pin 8 5y3gt along B+ rail nodes read no resistance to ground.

I do get resistance to ground at all filament nodes/ tube heater pins. I am right in assuming this is correct.. right? cheers capt

[MWJB]

Yes, a little resistance (probably ohms rather than several hundreds of ohms or Kohms) to ground from the filament connections will be normal as there will be relatively little resistance from either leg of the heaters to the grounded centre tap, or if you have used 2x100ohm resistors these of course connect directly from each leg to ground...then there's the pilot bulb bridging the 2 legs to consider...removing this may affect your reading?

[The Captain]

Sh*tbags- I thought I had it solved there!! rereading hoffman excellent heater info I saw those 100ks would go to gnd therfore being resistance there anyway.. one thing I did read there was caps leaking/ noise resulting etc: I did notice some noise when I played it last, like a 'ghosting' distortion from the spkr when strings played increacing on hitting strings harder, even at low vols/ clean sound: an only slightly irritating distortion in background. Just seems worth mentioning this now..

Im waiting on cap replacement so will reprt back rechecks once formed/ all back in etc. Cheers folks, Captain.

[stokes]

Sh*tbags- I thought I had it solved there!! rereading hoffman excellent heater info I saw those 100ks would go to gnd therfore being resistance there anyway.. one thing I did read there was caps leaking/ noise resulting etc: I did notice some noise when I played it last, like a 'ghosting' distortion from the spkr when strings played increacing on hitting strings harder, even at low vols/ clean sound: an only slightly irritating distortion in background. Just seems worth mentioning this now..

Im waiting on cap replacement so will reprt back rechecks once formed/ all back in etc. Cheers folks, Captain.

I thought we covered the resistance to ground earlier,and found no resistance to ground on the B+ rail,no?If you read one of my earlier posts I said the caps are likely the problem if there is no other resistance to ground on the rail.That is why I arrived at the caps being the problem.The ghosting you describe is a symptom of bad filter caps.If you are using 2x 100ohm resistors for a CT on the heater you will see 50 ohms on each leg to ground,if you have a CT on the heater winding you will see close to zero.In this latest thread the fuse blowing was intermittent,I was assuming this was something you were doing wrong.I hate to sound like a broken record here,but if you see no resistance to ground in the B+ rail with no tubes in the amp and there is a voltage drop,the only thing that could be drawing current is the cap.This is of course assuming you wired everything right.Some time ago,Capn,I was going to suggest you ship the amp to me and I would look at it for you,and as somebody pointed out this is a problem that any one of us could find in 5 minutes if we had the amp in front of us.I was really hoping somebody would come up with something you and I arent seeing here Capn,but I still keep coming up with leaky caps.As someone pointed out the amount of volts being dropped with no tubes does seem to be small,my theory is that when the tubes are in the amp and the caps are stressed this seemingly insignificant current draw becomes worse,not to the point of popping the fuse,but enough to slowly burn the PT.You have to remember before the first PT failed you had a 3amp fuse where a 1 amp belonged.It is possible that the problem would have blown a 1amp fuse,but not a 3amp.Hey,I could be wrong here,as all we have to go on is what you are telling us,so keep the ideas coming guys,I'm sure we all want to see this amp working,after all the effort and typing we have been putting into this one.I dont know what the cost of shipping this amp would be,but I would be willing to have a look at it if all else fails.

[The Captain]

Yes I was re-rechecking reisitance as Bruce and others have helped out with ideas/ let them know etc, and as that fuse went around the 22w/ 2nd cap back in.. I take in all your input Mr. Stokes! & thanks so much for the offer of taking a look at it- some kind folk here for sure. Now then, this may put a spanner in the works: I found my old caps (TADs- thought I binned them) so put them in & fired it up= ghosting as before (as above slight but there), hot as before too (ie after 15 mins all warm, after 1.5 hours playing power tubes were fiercely hot/ chassis pretty hot/ PT hot). I'll do those V checks no tubes etc tmrw. I wasn't expecting these findings though. Im eying the .1uf coupling caps now (given to me, perhaps old..)- just a vague idea as mine always are. Cheers Capt.