I have a 100 watt EL34 Push Pull amplifier here that has some strange symptoms. A friend of mine said his amp was dead quiet until the other day when there was a noise though the speakers, even in standby (typical standby that just opens the B+ to the amp but leaves the heaters on).
Ok so I asked him if he ever noticed his power transformer being so noisy and he said he isn't sure. Well to me the noise through the speaker is the same noise that power transformer is making, you can even feel the chassis to the amp vibrating.
So here is where I am at. TUBES ARE REMOVED AND NO HELP
I disconnected the B+ to the standby switch taped it up and left it floating and no help. Next I disconnected the primary taps from the output transformer and that didn't help. Next I measured 500mA of AC current on the primary side of the power transformer. I disconnected the power transformer high voltage secondary taps, taped them up and left them floating. I measured about the same current on the primary of PT and the noise is still present. Next I disconnected the 6.3vac heater taps for the EL34's, taped them up and left them floating, I measured about the same current on primary side of PT and the noise is still present. Next I disconnected the 6.3vac taps for the preamp tubes, taped and left them floating, I measured about 400mA of current on primary of PT (the on bulb is on this circuit) and the noise is still present.
If I relocate the output transformer off the chassis the noise goes away through the speaker.
Ok so with nothing but the primary side of the power transformer connected I see almost a half amp of current. Is that normal? I would think not. Is this a power transformer or output transformer issue? I would think power.
It seems as though the noise is being injected from the power transformer into the output transformer and on to the speakers.
Ok so I asked him if he ever noticed his power transformer being so noisy and he said he isn't sure. Well to me the noise through the speaker is the same noise that power transformer is making, you can even feel the chassis to the amp vibrating.
So here is where I am at. TUBES ARE REMOVED AND NO HELP
I disconnected the B+ to the standby switch taped it up and left it floating and no help. Next I disconnected the primary taps from the output transformer and that didn't help. Next I measured 500mA of AC current on the primary side of the power transformer. I disconnected the power transformer high voltage secondary taps, taped them up and left them floating. I measured about the same current on the primary of PT and the noise is still present. Next I disconnected the 6.3vac heater taps for the EL34's, taped them up and left them floating, I measured about the same current on primary side of PT and the noise is still present. Next I disconnected the 6.3vac taps for the preamp tubes, taped and left them floating, I measured about 400mA of current on primary of PT (the on bulb is on this circuit) and the noise is still present.
If I relocate the output transformer off the chassis the noise goes away through the speaker.
Ok so with nothing but the primary side of the power transformer connected I see almost a half amp of current. Is that normal? I would think not. Is this a power transformer or output transformer issue? I would think power.
It seems as though the noise is being injected from the power transformer into the output transformer and on to the speakers.
Sounds like bad transformer to me!!!
If there is noise coming out of the speaker with all of those lines disconnected then there is possibility that the primary maybe shorted to the core.
BE CAREFUL IT MAY BE LIVE!!!!!!
Especially if the sound goes away by just removing it from the chassis.
500ma of Idle current seems kind of High to me if there is no load on it,I may be wrong as it has been a while since I have messed with a tube power supply transformer.
FWIW
jer
If there is noise coming out of the speaker with all of those lines disconnected then there is possibility that the primary maybe shorted to the core.
BE CAREFUL IT MAY BE LIVE!!!!!!
Especially if the sound goes away by just removing it from the chassis.
500ma of Idle current seems kind of High to me if there is no load on it,I may be wrong as it has been a while since I have messed with a tube power supply transformer.
FWIW
jer
I tried to move the output transformer 90 degrees and no help. But if I move it off the chassis the noise goes away. The layout is good, the power transformer is mounted on the total opposite side of chassis a good foot apart. This is an old amp from the 60's and my buddy has owned it for 6 years or so and he said the noise was never there, and he is total sound nut and would have noticed.
For a 100W amplifier, that could be normal. The key point being that with the secondary disconnected, the current is almost 90 degrees out of phase with the voltage - it's just acting as an inductor rather than as a transformer in this case - and so the power lost in the transformer is very low.
As for the main problem, I am inclined to suspect magnetic coupling. What is the chassis made from?
Almost half amp of idle current without anything connected to secondary seems like 100V primary (maybe some old Japanese transformer?) or overheated and so shorted turns. If you have variac you can slowly turn up voltage and measure input current. If it goes up linear something is shorted. If you will notice some point where it will start to increase rapidly you will find out for which primary voltage was calculated. 0.5A is probably too high even for 2KW unit.
Cheers
Cheers
Thinking some more, and re-reading, you could be right. I missed the bit in the original post about the noise coming from the mains transformer. That does begin to sound like an internal short somewhere.
I would split the chassis from the wall socket safety ground, and measure current chassis (transformer shell) to safety ground. Hammond Clock Co suggested you put a 10k 2 w resistor parallel to a .01 uf cap from chassis to safety ground, and measure AC voltage. More than 4 v they said your organ needed a new power transformer.Use clip leads and one hand at a time, don't touch the chassis while making this measurement.
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The other thing could be,
The transformer is creating a shorted turn via one pin through the laminations and the paint is no longer isolating the end bells from the chassis. ie a shorted turn.
This is assuming that the Tx is not primary or secondary shorted to the laminations (it would be strange but not impossible if the former has melted).
So assuming the power tx is not electrically live, test by isolating the power Tx from chassis and test (AC voltage) with a meter from earth to transformer end bells and laminations. If its dead then the screws through the Tx laminations should be isolated from the end bells with fibre washers or plastic insulators...
Where is the chassis main Earth attached?
As already mentiond take care the Tx laminations or end bells could be live!
Regards
M. Gregg
The transformer is creating a shorted turn via one pin through the laminations and the paint is no longer isolating the end bells from the chassis. ie a shorted turn.
This is assuming that the Tx is not primary or secondary shorted to the laminations (it would be strange but not impossible if the former has melted).
So assuming the power tx is not electrically live, test by isolating the power Tx from chassis and test (AC voltage) with a meter from earth to transformer end bells and laminations. If its dead then the screws through the Tx laminations should be isolated from the end bells with fibre washers or plastic insulators...
Where is the chassis main Earth attached?
As already mentiond take care the Tx laminations or end bells could be live!
Regards
M. Gregg
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Ok so I took a 10k resistor and placed a .02uf cap (i didn't have a .01) in parallel. I then lifted the safety ground from chassis and placed the 10k/.02uf in series with the chassis and the safety ground. With the amp on I placed one test lead on the chassis and my meter will read anywhere from 12VAC-20VAC. When I placed the other test lead to the earth side of the 10k/.02uf I get 30mVAC.
One more thing. If this amplifier has been working before and than started to buzz than probably something kicked the bucket. That something is from your description probably transformer. If this is some diy or vintage, long not working machine, than or transformer isn't for right primary voltage or is destroyed somehow. Again. I suggest test with variac, A and V meter. Could you tell us more about this amp? Brand? Maybe some pictures? Are there any signs of overheating or oxidation? Please tell us more and maybe someone could help you.
Cheers
Cheers
I don't have a variac, but I do have a transformer for an old American Flyer Train, I think it goes from 5-15VAC. If I was pulling half and amp at 120v than maybe there will be too much current for the train transformer to use for testing.
The amp is a Vox AC100 and I didn't add that information for the sake of forum nazi's moving the thread to "musical instruments". No offense to the guys over in the "musical instruments" section but I like this sub forum better. Anyway the amp gets used and hasn't been sitting for a long time.
I can post pictures but not now for my camera is being borrowed and my phone camera kind of stinks.
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15vac won't be enough.
I suspect that idle current of this amp was too high for longer period probably because tubes were worn out. It cause secondary to overheat and than make internal short. Such case make very high transformer idle current. Of course it can be also destroyed by water.....beer.... If you can try to borrow variac.
Cheers
I suspect that idle current of this amp was too high for longer period probably because tubes were worn out. It cause secondary to overheat and than make internal short. Such case make very high transformer idle current. Of course it can be also destroyed by water.....beer.... If you can try to borrow variac.
Cheers
If transformer hums while on chassis but not away from it, complete the test:
1) place a piece of wood on the chassis and sit the transformer over it, so there's, say, from 1/2" to 1" separation.
Hum??
2) now replace wood with something still insulating, but physically thin, such as a glossy magazine cover.
Hum??
3) now with the transformer still floating, touch its metallic body (laminations or cover) with a piece of wire, other end grounded to chassis to a point close to where the original transformer sits.
Be careful, there might be a voltage difference between transformer frame and chassis.
Hum??
4) same as 3) , but now measure AC voltage between frame and chassis, obviously without the grounding wire.
Voltage?
5) disconnect *all* secondaries, still 0.5A primary current?
6) leave it on, still secondaries floating, check temperature for a couple hours.
Warm is fine; unbearably hot is not.
Post results.
1) place a piece of wood on the chassis and sit the transformer over it, so there's, say, from 1/2" to 1" separation.
Hum??
2) now replace wood with something still insulating, but physically thin, such as a glossy magazine cover.
Hum??
3) now with the transformer still floating, touch its metallic body (laminations or cover) with a piece of wire, other end grounded to chassis to a point close to where the original transformer sits.
Be careful, there might be a voltage difference between transformer frame and chassis.
Hum??
4) same as 3) , but now measure AC voltage between frame and chassis, obviously without the grounding wire.
Voltage?
5) disconnect *all* secondaries, still 0.5A primary current?
6) leave it on, still secondaries floating, check temperature for a couple hours.
Warm is fine; unbearably hot is not.
Post results.
"If transformer hums while on chassis but not away from it, complete the test:"
I haven't removed the power transformer yet but as stated earlier lifting the output transformer from the chassis removes the noise.
"5) disconnect *all* secondaries, still 0.5A primary current?"
I have done this and yes there is still .5A primary current.
I will remove the power transformer and report back my findings.
I haven't removed the power transformer yet but as stated earlier lifting the output transformer from the chassis removes the noise.
"5) disconnect *all* secondaries, still 0.5A primary current?"
I have done this and yes there is still .5A primary current.
I will remove the power transformer and report back my findings.
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