AC15H1TV 100 hz Hum

[bor64]

The amp hums and on both channels the volume controls have no influence on the loudness of the hum.
On the TB channel the bass controller has influence on the loudness and the sound-colour of the hum, when I set it on full the amp hums so loud the amp vibrate....
When I pull out the V4 phase shifter ECC83(the most right one) the hum stops.
When I touch the V4 valve the hum get louder and got a kind interference with my hand contact , (difficult to describe)
When I pull out the V3 ECC83 (the middle one)the hum is almost gone.

After 10 or so re-tubes and between every try-out a pause of at least 12 hours to let the currency go out of the amp...I try a desperate move...
While the amp was on I pulled out the V4 and hums stops, I put it back and it's back ( I know it's tricky with the voltage in the amp , but I was really careful).

I exchanged valves multiple times...completely and in parts to filter out the bad valve if it is a bad valve....but it isn't because I used a complete "platoon" of valves....for every position in the amp...

3x EEC83
1x EF86
1x EZ81
2x EL84

NOS Mullard
NOS Philips
NOS Ultron
NOS Telefunken
NOS Siemens
NOS Brimar
Grove Tubes
JJ Tesla
Electro Harmonix
Military grade Sovtec
Also a some valves with a British broad arrow on it so I guess they are military grade also...but brand less...
So I suppose I can rule out a bad valve....

Sure I can empty the TB channel and never use it again, but I want the amp work completely.

The hum stays, so advice needed please.

Cheers Rob

[RayL]

There are lots of other components that can cause hum apart from valves.

From your description it sounds as if a smoothing capacitor has gone faulty.

If you are not experienced in amplifier repair, please take it to a qualified repairer.

[JimN]

Agreed.

Any hum which is a multiple of the mains frequency points straight at smoothing caps as the first port of call.

It isn't a big job, but you need to know what you're doing.

[Olsson]

Hi
I hade that problem, and I pot in another cap 50uF450V with a 15 ohm resistor paralell to C1. That solv the problem. It ended up absolute quiet. Right or wrong, I don´t know?
/Bertil

[bor64]

Thanks guys,

I'm not a experienced amp tech, so how long will take it that the currency is out of the amp elco's....so it's safe to exchange a cap and a resistor like Bertil did?

Bertil did you twist the legs together of the cap and resistor?
Did it changed or influenced the Vox sound in any way?

Cheers Rob

[Patrice]

Hi Rob

If I can help you ?
You says :

1) When I pull out the V4 phase shifter ECC83(the most right one) the hum stops.
OK !

2) When I pull out the V3 ECC83 (the middle one) the hum is almost gone.
This tube V3 is the one which works with the Bass and Treble controls.
So a question now:

If the Brillant (Top Boost) channel volume is setted à « 0 » (electrically just before the V3 input) does the hum, that you hear : decreases or not ?
May be if yes ?! it should be « logic and normal" that if you pull off now : V2 the hum disappears.
And if the hum disappears totally when V2 is pulled off, the « problem » is (obligatory) coming from this tube.
And may be this hum is coming, and due to a wrong switch to the ground, of the input female jacks hot points.
But !
It's just an idea...

Regards
Patrice

[bor64]

Hi Patrice,

Thanks for your reply.
The volume on the TB and the EF86 channel have no influence on the hum what so ever.....
I've pulled out the V2 and V3 ECC83 valves and the hum is gone...
But I like the idea the amp works on both channels ...although I will not use the TB channel a lot.
To be honest I don't use that TB channel on my Matchless DC30 either....only on my Vox AC30 I use that channel.

Cheers Rob

[Olsson]

I took the cap 50uF450V and solder a 15 ohm resistor i serie. The 15 ohm resistor is a protection for the EZ81. Then I put the cap an resistor paralelle to C1, a 22uF cap. I did not note any any influence of the sound.
/Bertil

[TubeToneTim]

Bor64.....you say you are not an experienced amp technician.....please take some good advice from me.....I am
an electronics engineer as well as a musician...take your amp to someone who knows what they are doing....if you can't clear the fault with a simple valve change then as far as you are concerned that is it...
Messing with high voltage circuits is not for you.....
killing yourself is quite easy to achieve.....TTT

[Patrice]

Hi Rob,

Excuse me to insist but
My question was:
If you take off V2 BUT you let in place V3, and in the same time the Brillant volume is setted to "0", does the hum disappears ?
If yes it's an obligation that the "problem" comes "before" V3, and so from V2.

Sure that at this moment the breakdown is not resolved yet !
But you know where the problem is coming...
And this "problem" may (just !) be coming from a wrong ground solder or a simple wrong connection ?

Cheers
Patrice