Vox AC30 Top Boost TB 1968

[Travis8]

Hi, my AC30 was dead, I replace broken power fuse and put all tubes news. (rectifier included)

It´s work well yesterday but today the electricity goes out on my home every time I try to switch it on...

Some ideas?

[HotBluePlates]

... I replace broken power fuse ...

... but today the electricity goes out on my home every time I try to switch it on...

Some ideas?

Find the thing that blew the fuse.  Probably some kinda of short-circuit.

You will probably need to build a Lightbulb Limiter to allow troubleshooting without spending $300 on fuses, or burning the house down.

[tubeswell]

The mains fuse blows if too much current is drawn through it. Merely replacing the fuse won’t fix the thing that is causing the excessive current problem. Get the amp to a tech (rather than attempting to repair it yourself).

[Latole]

Bulb lamp limiter is a good idea

You may try another test .

Remove all Power tubes EL84
Put new fuse

Power amp ON.
If fuse don't blow, you may have issue in the Power tubes circuit

If fuse blow, see a good amp tech.

[Travis8]

Bulb lamp limiter is a good idea

You may try another test .

Remove all Power tubes EL84
Put new fuse

Power amp ON.
If fuse don't blow, you may have issue in the Power tubes circuit

If fuse blow, see a good amp tech.

Ok I will try, I have one.

I repair some amps and I´m learning more about it. That´s why I´m asking for ideas to this kind of issue.

Thanks, let me try today!

[Latole]

Learning on a Vox AC30 Top Boost requires perseverance and courage, it is not an easy amp to lear. Bravo

[Travis8]

Bulb lamp limiter is a good idea

You may try another test .

Remove all Power tubes EL84
Put new fuse

Power amp ON.
If fuse don't blow, you may have issue in the Power tubes circuit

If fuse blow, see a good amp tech.

I try to power it on without power tubes and with my lamp limiter and all the house goes out of electricity.

no matter if I use the lamp limiter in series or parallel

Fuse is ok

[Travis8]

Without all tubes and new GZ34 still the same issue...

[Jalmeida]

Does the amp have the original voltage selector? Check that it is correct voltage setting if so. Not sure if this would pop the fuse.

Does it blow the fuse with the rectifier tube out? If so, then I would check wiring from the mains plug(voltage selector included) to the rectifier socket. Hopefully it is just a broken solder joint and not a mains transformer.

[Latole]

Without all tubes and new GZ34 still the same issue...

Issue in Power supply ;
Filter caps ?
Power transformer ?

New fuse with no GZ34 and no EL84s
Fuse blow ; power transformer issue.
                  voltage reading needed to be 100 % sure

[Travis8]

I´m from spain so the voltage selector is on 225 volts.

The fuse is perfect, let me try without GZ34..

Wich voltages I must read after that in the power transformer?

Thanks

[Latole]

I´m from spain so the voltage selector is on 225 volts.

The fuse is perfect, let me try without GZ34..

Wich voltages I must read after that in the power transformer?

Thanks

Schematic show all .

If you don't know what is volts, don't put your hand in a amp

[PRR]

....with my lamp limiter and all the house goes out of electricity.....

This sounds dangerous. The "house" is MUCH bigger than an amplifier and should not "go out" so easily.

What "goes out"? How do you get power back?
The whole-house main fuse/breaker?
The room/circuit fuse/breaker?
In places in Italy (maybe Spain) the wires/transformers are small; you can take a large load for a short time and then it goes out. However several hundred Watts is fine and the amp (if wired right) is less than a hundred Watts.
A "ground fault" "residual current" shock preventer? GFIC, RCB

[Travis8]

I´m from spain so the voltage selector is on 225 volts.

The fuse is perfect, let me try without GZ34..

Wich voltages I must read after that in the power transformer?

Thanks

Schematic show all .

If you don't know what is volts, don't put your hand in a amp

Yes I know it, it just I didnt see the schematic, sorry.

[Latole]

Too many issue in the house and in the amp .
Travis 8 must see a pro(s)

[Travis8]

....with my lamp limiter and all the house goes out of electricity.....

This sounds dangerous. The "house" is MUCH bigger than an amplifier and should not "go out" so easily.

What "goes out"? How do you get power back?
The whole-house main fuse/breaker?
The room/circuit fuse/breaker?
In places in Italy (maybe Spain) the wires/transformers are small; you can take a large load for a short time and then it goes out. However several hundred Watts is fine and the amp (if wired right) is less than a hundred Watts.
A "ground fault" "residual current" shock preventer? GFIC, RCB

The diferential switchvox of the house go down every time I try to turn it on. Even If I remove all tubes etc...

The main fuse is perfect conditions.

It´s was working perfectly, just happens.

[Travis8]

Too many issue in the house and in the amp .
Travis 8 must see a pro(s)

the house is ok, just the amp make it going down...

[Latole]

Yes I know it, it just I didnt see the schematic, sorry.

Reply 9 show a picture of the schematic ,
If you don't see, something wrong with your PC .

Schematic is there ;


https://el34world.com/charts/Schematics/files/Vox/Vox_ac301960.pdf

[Latole]

Too many issue in the house and in the amp .
Travis 8 must see a pro(s)

the house is ok, just the amp make it going down...

If a amp ( with the right fuse) shut off the house electrical circuit, IMO there is a issue with house electrical circuit.
 

[Travis8]

Too many issue in the house and in the amp .
Travis 8 must see a pro(s)

the house is ok, just the amp make it going down...

If a amp ( with the right fuse) shut off the house electrical circuit, IMO there is a issue with house electrical circuit.

Happens first in my little studio and now at home. Is not the house.

I´m doing it safety and I repair some shorted amps, I just wanna learn about this hard situation. Thanks every one for the help...

I check the power on switch, works well 1 to 4 and 2 to 3, also 1 and 2 are conected all the time. Looks good

Im not sure how to check voltages and power transformer If I cant get it on with the lamp limiter

[sluckey]

I check the power on switch, works well 1 to 4 and 2 to 3, also 1 and 2 are conected all the time.

1 and 2 should never be connected! Maybe you are seeing the very low resistance of the PT primary.

[Travis8]

I check the power on switch, works well 1 to 4 and 2 to 3, also 1 and 2 are conected all the time.

1 and 2 should never be connected! Maybe you are seeing the very low resistance of the PT primary.

Yes thats why I said must be ok.

How can I check power transformer?

[sluckey]

I try to power it on without power tubes and with my lamp limiter and all the house goes out of electricity.

no matter if I use the lamp limiter in series or parallel

Lamp limiter must be wired in series with the amp! If it is wired properly there is no way it would kill the house electricity. Even if the amp were a dead short the only thing that would happen is the lamp would just glow full brightness, same as any other lamp in the house.

[Yeatzee]

Learning on a Vox AC30 Top Boost requires perseverance and courage, it is not an easy amp to lear. Bravo

Can confirm lol

[PRR]

Gentlebeings: he has a GROUND FAULT, not an over-current.

(The "differential" is another name for GFI/RCB/etc.)

Something is leaking from Line to Ground/Earth.

A GFI/RCB will trip at 0.030Amps or less. No fuse or lamp will prevent that.

I have a bad feeling that a 60 year old transformer may have an internal breakdown.

This can be VERY dangerous.

[Travis8]

Gentlebeings: he has a GROUND FAULT, not an over-current.

(The "differential" is another name for GFI/RCB/etc.)

Something is leaking from Line to Ground/Earth.

A GFI/RCB will trip at 0.030Amps or less. No fuse or lamp will prevent that.

I have a bad feeling that a 60 year old transformer may have an internal breakdown.

This can be VERY dangerous.

I´m agree, lamp limtier is correctly wired and the issue is with and without it.. also I use it too many times without problem.

Wich main transformer do you recommend me? I wanna try to swap it.. thanks

[Jalmeida]

Gentlebeings: he has a GROUND FAULT, not an over-current.

(The "differential" is another name for GFI/RCB/etc.)

Something is leaking from Line to Ground/Earth.

A GFI/RCB will trip at 0.030Amps or less. No fuse or lamp will prevent that.

I have a bad feeling that a 60 year old transformer may have an internal breakdown.

This can be VERY dangerous.

I´m agree, lamp limtier is correctly wired and the issue is with and without it.. also I use it too many times without problem.

Wich main transformer do you recommend me? I wanna try to swap it.. thanks

100% get a Soursound transformer. His are modeled meticulously after the Woden. If they are not on his site, call and order.

If you need something quicker, the Mojotones are nice.

Avoid the Pacific Power transformers. They use the wrong EI lamination size and require drilling your chassis. The Mojotones and soursounds bolt right in.

[Travis8]

I replace the PT and works fine, but after some hours the amps turn off without any noise or smells.

The fuse look fine...

Some ideas? This is a nightmare

[PRR]

> The fuse look fine...

May need a meter to get a true diagnosis.

[Latole]

> The fuse look fine...

May need a meter to get a true diagnosis.

Right, fuse may look with eyes and may are not good

[Latole]

I'm afraid you haven't found the problem that existed from the beginning

[Travis8]

I'm afraid you haven't found the problem that existed from the beginning

Fuse is ok...

Wich will you test to check the real issue?

[tubeswell]

Dude, get that amp to a tech.


And get a qualified electrical tradesperson to look at your house

[Latole]

Dude, get that amp to a tech.


And get a qualified electrical tradesperson to look at your house

I agree, this amp must see a amps tech.

[Travis8]

Ok, and some good books to learn about repairs?

I repair a lot, but this one hit me

[Greenwichpaul]

It's the RCD that's tripping; don't believe it's a house wiring issue. Some current is returning via earth so perhaps a winding is shorting on the casing or similar.

If it's a 68 original, much better to get the PT rebuilt rather than simply buying a new one.

One of the best rebuilders in Europe is Transformer Equipment Ltd in Sandwich, Kent. You can see their previous rebuilds on Facebook, they have done lot of Vox etc transformers, the fella who repairs eg Johnny Marr Pixies amps Voxes sends his transformers there for rewinds and rebuilds.

I know there will be customs issues due to the dreaded Brexit but if you clearly mark the parcel as 'second hand item for repair and return' my experience is you'll be ok.

http://www.transformerequipment.co.uk/

[Latole]

Ok, and some good books to learn about repairs?

I repair a lot, but this one hit me

You have to learn electricity followed by electronics.

Without this knowledge, you go by trial and error, you succeed with the easy problems but when it becomes complicated you can't get there because you haven't learned the profession.

[pdf64]

On one hand, it’s greatly beneficial for the repair tech to have an accurate mental model of a circuit’s theory of operation, so that an hypothesis of how the symptoms might be caused by a particular root cause can be devised, and then proved or otherwise. So at least a couple of years getting conversant with the boring, hard electrical and EM theory is necessary for that.

On the other hand, the only root cause here can be imperfect insulation in the mains current path.
As the mains transformer has hopefully now been eliminated as the source of the insulation issue, it just leaves the wiring, voltage selector, fuseholder and switch.

[Travis8]

Ok, I learn all of these for years but now Im trying to use it on real life.
I repair some amps and I build simple tweeds..

I found this https://robrobinette.com/Tube_Guitar_Amp_Troubleshooting.htm#Completely_Dead and its help me

My bulb gets bright when power tubes are connected. If I try to turn up the amp without it the amp works and the bulb just dim a little.

[Latole]

If your bulb ( bulb lamp limiter ? ) stay almost full bright with power and with power tubes, you have a short in power tubes circuit.
Tubes are bad/ short or too hot bias
Output transformer short
Ans more

See answer # 3 , 3 week ago

[Travis8]

If your bulb ( bulb lamp limiter ? ) stay almost full bright with power and with power tubes, you have a short in power tubes circuit.
Tubes are bad/ short or too hot bias
Output transformer short
Ans more

See answer # 3 , 3 week ago

Ok, I think tubes are ok, they are new...
 
How can I check correctly the power amp section and the OT?

I mean a good order to proceed and diagnose the fault.

[Latole]

I use it many times with success

Testing Output Transformer and finding Impedance

1- Signal generator set to 1000 hertz AC 0.5 volts

2- Send this signal to OT secondary , check with accurate meter to have 0.5 VAC on OT tap. Or close

3- Read AC voltage at OT primary

4- For exemple if you read 14.35 VAC you divide by 0.5  ( or what you read at 1- )

    14.35 / 0.5 = 28.7

5- Square the result  ; 28.7 X 28.7 = 829.67

6- Multiply by secondary output impedance where you are connected  , say 8 ohms
    829.69 X 8 = 6631

7 - This reading 6631 is the primary input impedance

If you use two 6L6 in push pull , look on Tube Book for Plate Load resistance . You’ll see you that you need 6000 to 3800 ohms  for two 6L6 in PP class AB

The reading we got 6631 is on spec
_______________________________________
If transformer is no good your reading will be very far from what we should get .

By experience these test do not work on Super Reverb Transformer or other with 2 ohms Output Transformer impedance

You can do this test on unknow OT to see how / where you can use it

[Travis8]

Ok, very helpful information

OT check and is ok.

I check the resistors value around the power tubes and they are ok too.

[Travis8]

When I replace the PT the amp works for some hours and then die, then I test it again with bulb limiter and I saw that the bulb lights when power tubes are connected.

OT is ok
Fuse is ok
PT is ok

I replace tubes (with new ones) when I replace the PT and they works when I replace PT, so I assume power tubes are ok.

That´s all the information I have right now

[PRR]

> That´s all the information I have right now

How many watts is the limiter bulb?

[Travis8]

> That´s all the information I have right now

How many watts is the limiter bulb?

60w bulb

[pdf64]

AC30 draw a lot of idle current. A 100W bulb in the limiter may be more appropriate.

[Latole]

AC30 draw a lot of idle current. A 100W bulb in the limiter may be more appropriate.

Yes a 100 watts "old style" bulb is a must.

Travis, you are very brave to dedicate yourself to wanting to repair this amp but you lack too much knowledge in electronics.
And I'm talking about elementary, basic knowledge.

[Travis8]

AC30 draw a lot of idle current. A 100W bulb in the limiter may be more appropriate.

Yes a 100 watts "old style" bulb is a must.

Travis, you are very brave to dedicate yourself to wanting to repair this amp but you lack too much knowledge in electronics.
And I'm talking about elementary, basic knowledge.

I´m justr trying to learn and to practice in real life, there is a big step between solving problems on paper and facing real things.

I haven't realized, 60w is very little value for ac30, I will changue it and continue..

[Latole]

This does not work , you did not see from the beginning of your post ?

You have to study electronic first, too much lack in your knowledge.
And you can't learn with forums.

We don't learn medicine while we operate on the patient.