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JohnBlue TL66 Monoblocks hum problem. Help please

jascha

Member
2015-05-08 10:49 pm
Hi everyone,

I have recently purchased a pair of beautiful monoblocks that are SE and use just a 6AU6 and an EL34 or equivalent output valve. When I bought them I luckily opened them up to have a look inside and to my amazement noticed that there was no mains earth connected to chassis ground. I straight away fixed the problem however since then I have introduced hum.
I have read a great thread of a similar nature on this forum which is very helpful and informative however I would be most grateful if you guys can share your thoughts to help me fix my amps and get rid of this hum without getting rid of mains earth which is very dangerous!

I have tried all sorts until I realised today that it was the mains earth I introduced that caused the hum. These include twisting the heater leads to the 6AU6 as they are rather long but were kept straight (no difference) and trying out different valves (hum varied).

I figured out that if I unplug the input jack (signal from my equippment), the hum goes away as well, but that's of course not an option. I have tried desoldering the earth shield of the coaxial cable that feeds the grid of the input valve and that had a positive effect. What did no difference was to earth the grid of the input valve and even worse earthing the grid of the Output valve!

I have finally tested the situation by connecting between the star ground and mains earth a 44R and a .1uf cap (in parallel to each other) and the hum levels reduced, I say close to as having no mains earth present. I have not gone down the route of trying DC heaters as I felt it might not be necessary at this stage until I have some feedback from you guys. I also added extra capacitance for further smoothing but again that made no difference.

Just FYI, the Mains transformer sec windings are not CT, neither the two separate windings to the heaters nor the HT (which feeds a bridge rectifier).

I attach a circuit I made of this amp and some jpegs I took. Please note that i have disabled the potentiometer (volume control) and have wired the phono socket signal straight to the grid of the 6AU6.

Many thanks
 

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jascha

Member
2015-05-08 10:49 pm
Hi,
I am fairly sure that all ground points are finally terminated at one point before then going to chassis ground. Originally this was not the case but have adjusted the "error". All the grounds that are connected to a common earth bar are then connected to the chassis at just one point before running the earth to the mains earth point on the iec connector. I have not yet included any diodes or anything. What should I use exactly please as regards to diodes types and value of a resistor?
Also as the heaters (AC) have no CT windings, for both valves on each monoblock, one leg of the heater is also connected to earth. Does this affect the situation?
For the time being I have reconnected the ground but have disconnected the earth to the input jack until I await further instructions.

Thanks alot :)