• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

will laziness burn my amp down?!

fuzzball

Member
2005-05-16 7:40 am
home
I'm used to building battery powered guitar pedals, and I've dicided to build a tube amp.
specifically, thefirefly tube amp .
where i live i have 220vac from the wall outlet, and the amp itself needs 207vac, and it seems a little silly to me to add a transformer: 220 to 207...:cannotbe:
can i just connect the amp to the power grid with no transformer?:confused:
is there another way to "get rid" of 13 volts? a risistor perhaps?:eek:
will it burn if i dont get rid of 13 volts?:bawling:

all help is apriciated.
 
Unless you are on a mission to become a 'rock and roll suicide' you will definitely need a transformer. Transformerless ht was often used in old valve radios but this meant that every part of the circuit including the chassis had to be insulated and not 'touchable' by a user. The reason: If the mains polarity gets reversed, the chassis becomes live - in your case, your guitar would become live:dead:
You will need a transformer to supply the heaters anyway so why not find one which does both
 
pinkmouse said:
Guys,

How much is your life worth to you and your family/friends?

More than the 30-40$ it would cost for a mains transformer?

I would use the transformer if I were able to get it. Using two transformers (one to lower the voltage second to raise it again) is OK for small power, but for higher power it costs and weights too much.

Could anyone explain why my idea is wrong? Chassis is grounded, so noone would get hurt if touch the chassis. If I touch chassis and B+ at the same time I would get almost the same shock even if I would use a transformer.
 
"Could anyone explain why my idea is wrong? Chassis is grounded, so noone would get hurt if touch the chassis. If I touch chassis and B+ at the same time I would get almost the same shock even if I would use a transformer."

Your idea is wrong because it's deadly.
A transformer will give you isolation and, most importantly, will limit the amount of current that will surge through your body when you touch the B+ line. Without a transformer, the current is limited only by the rating of your circuit breaker (or fuse).
 
pinkmouse said:
If your amp were plugged into a socket, or had a cable with live and neutral/earth reversed, the whole thing would be at mains, including any instruments plugged into it.

Once again, is your life really worth less than 30-40$? Really?


I don't know how wires are arranged in England, but jhere in Lithuania we have 3 wires for mains: Live, neutral and ground. Live and neutral can ger reversed, but ground stays the same (PC case is grounded too, so what if ground and live gets reversed?) . Even reversing the plug does noit disconnect ground and it stays where it were (live and neutral gets reversed if I reverse the plug).

To kill, current must be around 20mA A.F.A.I.K, and transformer is able to produse more than that (of course if it is not transformer for preamp/phono amp only) and capacitors can give jolt even with disconnected power.
 
OK, I got it.

How about switch mode power supply for tube amplifier?

My thoughts about it: Normal power supply (how is it called?) operates at frequency of 50Hz (or 60Hz in some countries) which, if not properly filtered creates humm in speakers. Switch mode power supply operates at much higher frequency which is outside of hearing range, so even if not properly filtered the high frequency would not be heard and I would need smaller capacitors.

I posted this here so I had not to create a new topic.
 
Hi Pentium100, i will try to explain why it is lethal to not use a
transformer. Well you are right, it needs 20-40ma to kill you...
But, to get this current flow, you need 2 voltages, the one is your
hand touching something in this amplifier, speakerterminal for example.
Ok, now the question: where is the second voltage ?
For the case of mainsvoltage: everywhere ! The floor, the air...
For the case of transformer: the second terminal on your other hand...

A transformer isolates the level of the voltage from ground, lifting
the gnd-level to some "free" level.
The mainsvoltage is always relative to earth.

Normally you can buy 1:1 transformers with quite high power,
but i can't answer your question about the 13volts.

Mike
 
I'm sure hammond makes a cheap power transformer that has about an amp at 6.3 volts (you need .9 amps since each 12A_7 takes .3 amps at 6.3. or .15 amp each at 12.6 volts) and has a high voltage winding good for a coupla hundred volts. you're looking for about 300-350 volts after rectification it seems. you can also use a center tapped transformer. this is the best way to do it, and would only require one transformer. these transformers are MADE to be used with tubes.

the cheap solution would be to use two 12.6 or 6.3 volt transformers (easy to get a hold of) with the low voltage windings connected together. I'd sya get some 2 or 3 amp ones. You could even make the one that connects to the line bigger by an amp and use it to feed the filaments. Look for a pedal called the "real mc tube" (distortion box using one 12AX7) for this approach. you probabally want at least a 3 amp, 6.3 volt tranny for the first one, and a 2 amp or so one for the one that steps the 6.3 volts back up to 230 or whatever.These transforemrs are cheap and available at any electronics place.

NEVER NEVER EVER use a "hot chassis" amplifier, besides the horrible hum, they're the ones that'd keep biting you, and could possibly kill you iof someone cut the ground off of it
 
Sorry, but I can't just memorize "It is because it is" without normal understanding "why".

Apart from the safety considerations (which cannot be emphasised enough), your suggested method would trigger RCD circuit breakers - which should be stndard in any modern installation - and would contravene regulations here in the UK, and probably most other countries, leading to severe penalties if anything went wrong.
Bear in mind too that a fault condition could cause other devices on the same circuit (or even in neighbours' houses) to become live if the physical earth was inefficient.
'Live' chassis connection, whilst not recommended for the reasons already posted, is safer than your proposal - the neutral rail is already grounded somewhere by definition, so why substitute the safety earth?
 
Using neutral would be way too risky because there is no way of telling which wire is live and which is neutral and they can get reversed anytime.

As for the ground, I see only two wires coming to my house, so ground must be grounded in my house. There are no ground protections, you can draw as much current from the ground as long as ground wire does not burn out.
 
Pentium 100, now you are scaring me :hot: In a mains supply ground or earth is a safety device and should only pass current in a serious fault condition. It is there to prevent electric shock and is not an alternative neutral connection.
There are some basics which need to be understood before you start messing with tubes and high voltage circuits - like domestic electricity supplies, wiring and safety. Please get a book and do the learning.
I sound patronising, well I'm sorry but I would rather you were alive and annoyed than dead