AC15H1TV 100 hz Hum

[bor64]

Bertil, thanks for the clarification, maybe that's the way to go....

Tim thanks for the warning...I know current/voltage can be hold inside the amp by the electric capacitors for some time.
But isn't there safe time after switching off the amp, lets say two or tree days or so ....the current is gone ?
Because this fix Bertil suggest is a 10 min job and easily done....
Last time this amp was with a tech he listen and said ok that's a 100 Hz hum, come back next week and it's ready.
The only thing he did ,he replaced V3 with a new EEC83 and keep the amp running 24/7 for a week...without hum.
The bill was £80,- ex valve.....
The kicker the amp runs with hum, when switch off and within a minute or two switched on again and then sometimes it runs without hum.....but that happens only 1 in 10 times.....
So I reckon it was the case with the amp tech....because when I was home and tried the amp again it's started humming again.
I phoned the bloke and his answer was "well you heard the amp in my shop and it was silent....maybe you knocked the poor thing while in transport"....
Needless to say I'm not going back to that tech again...
So where can I measure with my multimeter if there is a current still in the amp after a few days, and do the 10 minutes job myself?

Cheers Rob

[bor64]

Patrice, I'll check it and come back to you about it.

Cheers Rob

[SJB]

HI- Bora,

I have been following the situation for a few days now. TubeToneTim has given some good advice. Voltages are lethal - not only that - scraping you off the ceiling is no fun for the family.

Anyway lets try and help:-
During the 1970's I did quite a lot of repairs to Vox AC30's and 50's. Looking at the circuit diagram for the AC15 I see possible points of fault that were prevalent on the 30 and 50's.

In almost all cases capacitors C5 and / or C6 became electrically leaky. This normally showed up with distortion at low playing levels - but in severe cases overpowering hum. I always replaced these capacitors - but used the 600 Volt rated ones.
If either of these become electrically leaky - the output valves are biased ON - depending on the severity one side could have an EL84 hard on - I think the anode used to glow in severe cases. The effect was to overload the power supply and introduce 100 hz hum into the early stages of the amp - would be picked up in V3

A long shot:-
As you refer to the TB bass pot adjustment affecting the tone of the hum --- I would also suspect C15. I would therefore be suspecting C15 being leaky - but would expect C5 or C6 to also be leaky. In this case the voltage at the grid of V4 would be pulled down towards Ov causing that side of V4 to turn off. This allows the voltage at the anode to raise - which if C5 were leaky will turn on V6.


These items are cheap - so its worth a try.

However a mulitmeter and oscilloscope are required to properly diagnose this one.

Its a pity your not local to me.

Good Luck

SJB

[bor64]

Patrice,

I tried what you ask...and the result was, the hum is gone when I turn both volume pots on the separate channels to 0 ....

Stuart, if it's so "dangerous" to dig in a tube amp, how on earth you amp techies do that?....I supposed your from this earth as the rest of us and not shock/bulletproof....

Cheers Rob

[SJB]

Rob

Lets see where we are now:-

In your first post you said:-
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....

In your last post - after changing the capacitors you say:-

I tried what you ask...and the result was, the hum is gone when I turn both volume pots on the separate channels to 0 ....

So am I correct is saying - "Now C5 / C6 and C16 are replaced we have changed the nature of the fault?

What happens when you alter the Bass Control - when with the hum present (I assume one of the volume controls should be turned up a bit.)


I have not actually found a drawing for the AC15T1TV - but from your valve lineup I think the AC15HTVH drawing is close enough. So if you have a drawing could you please forward to me.


Your comment about Techies working on Amps. Well back in the 60's we were taught how to approach working with high voltage and were very aware where the dangers were. However techies did get shocks - me included. Some were not so fortunate.

finally - how old is your amp?

[bor64]

Stuart,

That first part of my reply was aimed toward Patrice....
When I removed only V3 and leave in V2 the amp reacted that way.
With all valves in the amp I can't hear a difference when both volume knobs are turned ...only with the V3 in his socket the bass has the most influence.
I didn't changed the caps, because you told me not to do it because of the danger involved...
My amp is build in 2008/9 but only in use for about 1.5 years I bought it NOS.

Stuart when you Google Vox ac15h1tv scematic you can download a pdf with the exact scematic of my amp.

Cheers Rob

[SJB]

Rob,

Thanks for clarifying you did not change the caps and the age of the amp.
I note your directions to get the schematic - I seemed to end up with the manual for the HTVH1 on my searches - but its ok.

I have looked around the web a bit - I still think its capacitor deterioration. I note that there are several comments related to poor components.

Going back to your first post:-

"When I touch the V4 valve the hum get louder and got a kind interference with my hand contact , (difficult to describe) "
Answer ---- This could be grease (finger) or some dirt of some sort conducting electricity into your hand up the outside of the glass"
Polish the valves (Glass) with a dry cloth". This helps anyway as it reduces dust and therefore allows the valves to run cooler.

"When I pull out the V3 ECC83 (the middle one)the hum is almost gone."
Then later to Patrice ---"I've pulled out the V2 and V3 ECC83 valves and the hum is gone..."

The main capacitors C1 and C21 do not remove the 100 Htz from the supply. Capacitors C2 / C3 / C17 / C18 reduce the 100 Htz to a minimal amount in the pre-amp stages. This small amount of 100Htz that remains from C1 and C21 does not affect V4 and the EL34's as the 100 htz goes up and down on each side of the output so there is never a difference across the transformer - both EL84's are pushing and pulling at the same time - for an output one needs to push and the other pull.

So the problem is coming in from V3.

Interestingly here is an entry from
"http://music-electronics-forum.com/t16359/"

I had the same fault with an AC15TVH a few weeks ago.
Change C1, C2, C3, C17 and C18.
I used 33uF 450V axials which I had in stock for all of them, problem solved.
The caps they used for these nice amps were not nice!

The person posting missed C21. If you are going to change C1 then change C21 also.


As channel two volume does not change the Hum level we will assume the problem is now in or around V3.
I don't think you need to change all of the capacitors mentioned above - Just C17.

The main safety issue is voltages retained in C1 and C21 when the amp is switched off. So using your multimeter set on a high range to measure at least 400 volts DC - measure across C1 and C21. Make sure the voltage is very low - zero volts is best. On measuring if the reading low - then remove the probes - change the multimeter to a lower setting until you see the volts are very low - should be almost zero.

Now measure across C17. Likewise this voltage should be close to zero. Wait until it drops. If its faulty it will drop very quickly provided C1 and C21 are discharged first.

Replace C17 - make sure the polarity is correct.

Your soldering iron should be earthed - the amp should be unplugged from the mains.

Good Luck.

[bor64]

Stuart,

Thanks for your reply and advice.....

If type or copy it in/to Google this exact line you get the service manual for the VOX AC15H1TV....

Vox ac15h1tv schematic

On the site blueguitar.org with this above line, you get directly to see the pdf and you can save it on your disk.

Cheers Rob