troubleshooting M40 Echolette with loud hum

[notea]

Hi Mickey,
I removed the output valves with rectifiers in place, i just have time to measure 0VDC on the first El84 at Pin7, the anode, before hearing a big crack.
So i shut down immediately. I will have more time this w-e to re-check and make sure i had a good contact in the tube socket, but i'm affraid you're right for the power transformer..
This the pic of the output tube sockets connection.
Cheers Nick

[Mickey]

Hi Nick,
If you remove the rectifier valves and no noise is present you should be able to measure about 300volts AC from each anode of these valves (on their sockets). If this is not showing then I would suspect the problem may be the power transformer.
If you want to, you can measure the resistance from each anode of the output valves to the cathode of the rectifier valves with the rectifier valves pulled out and the AC to the amp disconnected, this will indicate if the output transformer is defective. Should be the same resistance from any output valve anode to the rectifier cathodes.
Cheers, Mickey

[Mickey]

Hi Nick,
Just had a thought, is the amp set for 240VAC or 120VAC?
Cheers,
Mickey

[notea]

Hi Mickey,
the amp was set on 240 but i putted back on 220V,
-i measured again with rectifiers and 12AX7 in place and all EL84 removed.
I measured counter clockwise from above at pin7 of each EL84 sockets,
the first try i've got this:
- 0V on tubes 1&2, the closest to the PT
- on tube 3&4 it seems that the DC is slowly coming as the hum increases
-just had time to check the anode of the first 12AX7 and the DC was also increasing

i tried few more times and now i've got 0V everywhere!
i'll wait for the unit to get cooler and i'll try again but i'm affraid i stressed the unit with those on-off-on-off switching, i hope i didn't break the OT..

- the 2 filter caps are supposed to be brand new but i measured 0v and 2v on it,
i'll buy new ones, i wonder also if i could have a defective rectifier tube base that could block the DC from going to the filter caps..
cheers, Nick

[Mickey]

Hi Nick
DON"T BUY ANYTHING !!!!
Back to the beginning:
What happens with ALL the valves removed?
If everything seems OK just replace only the Rectifiers. If the amp begins to overheat remove one of the rectifiers and make a note of the consequences. If OK then replace the other rectifier. If the filter caps were shorted you would most likely have had them explode! IF there is a short in the DC circuit somewhere your rectifier valve anodes would be glowing a bright red. The anode is the dark grey metal sleeve inside the glass tube.
Now if there is a short, we have to find out where! There won't be any voltage on the filter capacitors unless the rectifiers are working properly so I don't think your capacitors are faulty.
If you can locate the wire which goes to the centre tap of the output transformer, remove it at the transformer. Now try the amp gain with only the rectifier valves in place, if there is still a short then the fault is not in the O/P transformer

The power supply of the amplifier (the power transformer, the rectifier valves and the filter capacitors) supply HT to all the stages of the amplifier. The output stage with the four EL84s draws a lot of current, the other stages with the 12AX7/ECC83s don't draw much current so they wouldn't be causing your overload problem.

Now with the amplifier unplugged from the AC and ALL valves removed let me know what the resistance is from each EL84 anode (pin 7) to the cathodes of the rectifier valves ALL VALVES REMOVED ! If these ohm measurements are not the same then you may have a fault in the O/P transformer

You have to ensure that the power supply is working properly and then you begin to look for faults elsewhere. If the fault is in the power supply you then have to repair the fault before looking elsewhere!
Cheers, Mickey

[notea]

Hi Mickey,
we make progress here, first i feel stupid cause i was interverting OT&PT position guessing that the PT was close to the power socket..
But since i did a bit of homework and i hope i finally locate the wires on OT/PT exept for the primary of the PT
which i don't know where is the 20 and the 220V, i guess the 220V is far left?(see the pics attached)
anyway i followed your procedure:
-with all the valve removed=no hum
-with one rectifier at the time=hum
-with both rectifiers=hum
-with all valves removed the resistance at pin7 of the EL84 are: 100ohms,100ohms,98ohms,98ohms
-Now that i think i can locate the wires, i followed the procedure of Oc at #11 , which is to measure the resistance between the center tap and the anode of the EZ81,
(see the attached pic) don't know if i'm correct though but i plug my meter between the center tap and the 2 lugs of the PT secondary that connects to the anodes of the rectifiers
and i measured 72ohms and 20ohms!
so it means that i have a short in the 20 ohms half according to Oc.
-To be sure i followed your procedure which is to disconnect the center tap, play with the gain with the rectifiers in place and no hum!!!
i think we've found one part or maybe the all part of the problem which is a defective PT
What do you think?
I'll make some research to see if we can find a brand new PT that could fit in there or perhaps an old one that works...
Cheers, Nick

[Mickey]

Hi Nick,
Make Haste slowly!
If you look at the schematic you will notice that there is a diode connected via a resistor to a tap on the P/T secondary winding.
Disconnect this and then with the rectifiers removed measure the resistance from the anodes of the rectifiers to ground. Let me know your readings.
Think about this? When you remove the rectifiers there is no hum so that means that the hum is caused by a load on the power transformer. Have you interchanged the rectifier valves when you have only one plugged in, only one may be faulty! Now when you plug in both rectifiers does the hum appear immediately or only as the valves warm up? Did you disconnect the HT lead coming from the rectifier cathodes and if so what was the result?
Your O/P transformer appears to be OK as your readings from the EL84s are all very close.
Cheers, Mickey

[notea]

Hi Mickey,
you're right don't need to rush!
1°) i disconnected the diode and measured the anodes of the rectifiers and we'vegot:
-46 ohms on Pin1 of each tube 10&11
-51 ohms on pin7 of each tube 10&11
2°) i tried with the diode connected and i have the same readings.
3°)i interchanged the rectifiers and the same, the hum increases as the valves warm up
4°) i disconnected the HT lead coming from the rectifier cathodes (pin3) and no hum
that's mean that the PT is producing HT otherwise there would be no hum?
cheers Nick

[Racing]

The M40´s are really really solid builds. Well designed by people that knew what they were doing.

I do work on lots of german made amps and the general holds true.... Suspect anything cap within a german amp of age. To the letter.
Coupling caps by WIMA,ERO and what have you not more so then the e-caps,but be wary of those too.
Ie; if you experience hum like you do make sure that the first hit e-lyte is hooked up as it should and furthermore that it´s working as it should. If you don´t own an ESR meter chances are that your local parts vendor do and you could have it checked there.

The M40´s fail in one minor detail and that is that SOME of them do not apply to "the golden rule". It is imperative that the grounding of the first hit e-lyte hits the exact same spot as the centertap for the HT winding,in the case where a centertap is applicable,or the amp WILL hum. NOTHING else is to be attached to that point. 1mm away..ok,but NOTHING else to that exact spot.
It is not uncommon to run into amps that has seen previous work where this has been altered with.

IOW..locate the HT centertap and make sure that it complies to the above. If not..remedy.

When i get M40´s in the door it is more the rule then exception that i end up replacing anything coupling cap. Might be that some are still in order,but seing these amps age i for one see no real point in keeping a few of them if the rest of them are replaced. Ditto as far as cleaning them up inside. Liberal use of rubbing alcohol and a brush does it,and as these amps are open to free air they tend to end up rather nasty within over the yrs.

That done tho..they just keep running.

A fresh M40 is a rather loud proposition all said and done seing the rather limited power by the 4pcs of EL-84´s. That goes for the entire line of Klemt amps,right up to the later Winstons (the GA/BA and PA-200).

If you intend to use the amp ,mainly for guitarwork it is adviseable to reset the topology for the entry triodes. Just look up a favorite design of yours and copy.
Also be aware that you can "jumper" the channels by extracting signal via the "loop" outlet and head that into one of the other channels.

[notea]

Hi Racing,
thanks for sharing your tips, i'm far from a local parts vendor but i was thinking of buying a cheap ESR meter, cause i'm a beginner and i will use it punctually.
The previous owner replaced the filter caps but the layout could be suspicious if you look at the attached pic.
The center tap and the HT coming from the anodes of the rectifiers seem to hit the same spot, but i'm not sure about the 2 ground wires that are connected straight to the negative leads of the 2 filter caps. When i try to find some pictures on the internet of the filter caps side it seems that those to wires are passing through a hole and are connected together on the other side of the unit,
don't know if one those 3 possibilties could change anything or if the result will be equal ?:
- two ground wires connected to the negative leads of the filter caps (as on the picture)
- or those 2 wires simply hooked together
- or connected to the chassis

Sorry Racing but i'm a beginner and when you say "reset the topology for the entry triodes",
you mean replacing rectifier tubes by a diode bridge for example?

i didn't know for the loop outlet, the output in the front or in the back of the unit?
I might be wrong but that means we could use two different pedal chains? the second one connected between the output and the input 2 activated by an on/off switch for example?
Cheers Nick

[Mickey]

Hi Nick,
When you disconnect the HT leads from the rectifiers what voltage can you measure at the cathodes?
If 300V plus that would be OK and if so I think you should disconnect the lead that goes to the centre tap of the O/P transformer. When doing this see if the hum is there and if not again measure the voltage at the cathodes of the rectifiers.
What I'm trying to ascertain is what part of the amplifier is drawing a lot of current which is causing the hum. If the hum is only present when the centre tap is connected and the output valves removed that would indicate that the O/P transformer is faulty.
A comment when you say there is a loud hum is this hum coming from the speakers or is it coming from the amplifier, either the power or the O/P transformer.
Do you understand my fault finding procedure?
Cheers, Mickey

[notea]

Hi Mickey,
i've got bad news, first this the result of the readings; with the HT disconnected we have 243 V on the cathodes, we've got the same result with the center tap disconnected.
I decided to continue the measurements i couldn't have any readings on the heaters, but i've got 268/271 VAC on the anodes of valve 10 and 273 on valve 11.
But i waited too long during this test and the PT starts to smoke..Now we're sure that there is short in it..
I will do more research this w-e to understand better all the procedure and how works OT and PT, i already watch some videos of "uncle doug" on youtube which are very helpfull, i will continue to study all this, but for the moment i think i'll have to find a brand new PT..
cheers Nick

[Mickey]

Hi Nick,
I don't understand that you have 243V on the cathodes with the centre tap disconnected, the current flows from the transformer through the rectifiers then through the load (the rest of the circuit) and then back to the centre tap of the PT transformer. If you disconnect the centre tap current can not flow unless it is flowing through the OA150 diode. Disconnect this diode and remove all the connections to the HT windings of the PT transformer.
If the transformer has an internal short it will NOW still smoke when connected to the AC, if it doesn't smoke then your problem is not in the transformer (maybe?).
Re a replacement transformer, the original is probably not available and will cost to much. There are other options but lets see if your power transformer is faulty, even though it smoked it may still be useable.
Cheers,
Mickey

[notea]

Hi Mickey,
thanks for your patience,it's getting confusing, i disconnected the wires connected directly to the first leg of the first filter cap which are the HT that coming from the cathodes of EZ81 and the center tap connected to the Output transformer, i've got 243V.Then i disconnected the Diode, measured again i've got 243V.I try to diconnect the AC voltage that arrives at the anodes, and i could only locate for the moment the voltage that arrive at pin1.. i desoldered one at the time the wires at the secondary of the PT and it seems that the voltage that arrive at pin 7 of the EZ81 is coming somewhere from the primary side of the PT.. i will dig in to this this w-e it could be the isolated lug that i couldn't figure it out what it was (see the previous pics)
the good point is the PT didn't smoke during those tests!
Cheers Nick

[Mickey]

Hi Nick, get yourself a pencil and a piece of paper, now draw a transformer with one winding on the primary side. On the secondary side draw one winding for the filaments/heaters of the valves, then draw another winding which has a centre tap, this is the HT winding. DO NOT draw any other taps or windings as I'm trying to make it easy for you to understand.
Now think what happens when you connect AC to the primary, a voltage for the heaters is induced in the heater winding and a voltage is induced in the HT winding. Now if you can transpose this mentally to your M40, can you understand what is happening? When the rectifier valves are connected the current flows through them then through the circuit (the output stage and the preamp stages) then back to the centre tap of the transformer, this completes the circuit.
Now somewhere there is excessive current drain which is causing the power transformer to overheat.
If you remove the rectifier valves you can measure the resistance and the AC voltage by gently touching the appropriate pins on the valve side of the valve socket, saves unsoldering the wires.
Now with all valves removed you should read 243V on all rectifier anodes to ground (to which the centre tap of the HT winding is also connected).
If this is correct then replace the rectifiers one at a time WITH the output from the cathode (the wire that goes to the output transformer and the capacitors etc) all disconnected but with the centre tap connected to ground. Now you should measure at least 250V DC at the cathodes of the rectifiers AND no smoke.
If everything is OK so far then you connect the wiring from C34 etc but not the output transformer. The DC voltage on C34 should now read somewhere about 300V DC.
Let me know what you find.
Cheers, Mickey