Is it just me or is this totally unsafe?

[PStevenson]

someone on another forum was asking about this kit so I quickly perused it only to find that the seller is providing perfboard to power a 400v valve amplifier. I use stripboard a lot but I don't know that much about it's general specs but I would have thought that using it in something that has that much voltage and dare I say quite a bit of current is a bit of a stupid thing to do.

I wanted to contact the seller to let him/her know that it could potentially kill someone but they've disabled questions. 

http://www.ebay.com/itm/MTA-1-Tube-Amp-Kit-Basic-Model-NEW-original-low-cost-tube-amp-guitar-/180794432989?pt=Vintage_Electronics_R2&hash=item2a18301ddd#ht_1854wt_1185

why can't people stick to transistors !!

[Psi]

i wouldn't say its totally unsafe. It's got a metal case which i imagine is earthed, so it should be safe to touch no matter what the inside is like.

It's definitely not legal for a new product but old valve stuff was often worse than that.
Lots of uninsulated wires soldered everywhere carrying 100s of volts.

We don't know just how the perfboard is wired, he might have cut tracks out and kept all the 400V a good distance away from other tracks. Or it might have 400V on tracks next to each other.

I would say it's potentially a fire hazard more than a safety hazard.

It could definitely do with a grill on top to cover the transformer and valve. There could be exposed wire on the transformer and there will definitely be 400v on some of the valve pins.

[8086]

i wouldn't say its totally unsafe. It's got a metal case which i imagine is earthed, so it should be safe to touch no matter what the inside is like.

Nope. The kit only provides the board and components, so if you were operating it from a build just using the kit, you have 400V without a case.

[IanB]

I don't get it. Where do you think the danger lies? Everything is nicely enclosed inside a metal case which will be earthed for safety. There are no visible holes to poke or prod anything through and touch live parts. OK the valve and the output matching transformer are on the outside, but that is normal with such designs.

Nothing is wrong with perfboard construction, in fact it may even be preferable for higher voltages. It provides more clearance, better insulation and less chance of arc tracking across PCB traces.

[Psi]

Nope. The kit only provides the board and components, so if you were operating it from a build just using the kit, you have 400V without a case.

Then yeah, it not safe to build unless the person is knowledgeable about high voltages.

But if the kit is intended for people who know what they're doing then i don't see an issue.
He should really have a warning about that.

[amspire]

It is just you. 

Without seeing his instructions, you can not know if it is safe or not, just like every other mains powered project in existence that you do not know about.

It is very probably that only low voltage parts are mounted on the stripboard. There is a good chance that the HV wires are limited to that terminal strip, the valve socket and the wires to the output transformer. If there is any HV on the board, the usual procedure is to strip off track segments to get a safe clearance.

Richard

[PStevenson]

You must provide:
1. A chassis box
2. An 8-Ohm speaker
3. An ordinary power cord
4. Hookup wire and solder
5. Mounting hardware (#6-32 screws & #4-40 screws, washers, and nuts)

there is no way to vet who buys this, sure someone like us could safely build that, but on the other forum I was on the people building stuff had only ever really built things using 9v batteries.
I just don't think something like this should be available unless it is a bit more better put together than "here's some perfboard, have fun with the mains" job off eBay

[IanB]

OK, I get that you have to provide your own case.

But doing a quick search, it does appear that comprehensive build instructions are provided and advice is available from the designer.

I don't think it is fair to say it shouldn't be sold. It's an entirely reasonable kit for someone to want to build. The world would be a very boring place if we banned the sale of every dangerous item. Let's ban guns and knives before we ban hobby electronics 

[PStevenson]

It is just you. 

Without seeing his instructions, you can not know if it is safe or not, just like every other mains powered project in existence that you do not know about.

It is very probably that only low voltage parts are mounted on the stripboard. There is a good chance that the HV wires are limited to that terminal strip, the valve socket and the wires to the output transformer. If there is any HV on the board, the usual procedure is to strip off track segments to get a safe clearance.

Richard

you have to provide the wire and it says the perfboard is for the power supply,maybe you're right, I don't know, I'm not for banning high voltage projects, I love them personally I just think that if I were doing something like that I would at least use turret boards or something more substantial than perf, it's cheap enough to get a PCB manufactured these days

[PStevenson]

Let's ban guns and knives before we ban hobby electronics 

they have here

[slateraptor]

Let's ban guns and knives before we ban hobby electronics 

Move to Cali. Why ruin it for the rest of us.

[IanB]

they have here

Yes, I know (I'm a Brit far away from home)   

[mobbarley]

Is it actually built on the perfboard or is it mostly point-to-point? Is there an example pic of the built pcb?

[mobbarley]

Not to mention that if it is back-to-back transformers (haven't seen a schematic) then the output is pretty safe - no mains reference and probably reasonably high impedance once it goes through the anode resistors... Wiring the mains to the transformer has to be the biggest hazard.

[IanB]

It's odd. I'm looking for a review and test report and I can't seem to find one. This is where Google fails, since I'm sure someone somewhere on a forum like this has bought one, assembled and tested it, and reported on it.

[vk6zgo]

Guys,aren't we being a tiny bit precious,here?

From what I can see,it is a very low power one valve/tube amplifier,similar to many built by people over the years.
I very much doubt it has 400v HT,although you may have access to information I don't!
Generally,looking at it,I would suggest  the transformer would be about 225v at around 40mA--pretty much a standard figure for old style mantel radios.---It may be even lower.
225v RMS will probably give a similar DC HT voltage with a bridge rectifier.

If the perfboard is the type without copper tracks,it is a very good insulator.---many years ago I built a voltage multiplier from 300v to 2kV using plain perfboard--they used to call it matrix board.

The outside mounted power transformer does not constitute a risk,as it has insulated leads (certainly rated to many times the operating voltage),with the only connections being inside the chassis.
I guess,if you really tried hard,you could poke a pin into the anode or screen holes in the valve socket,& come into contact with a few hundred volts,but you could do that easier with a mains power socket.
A cover on the bottom of the chassis would be a good idea.

Not everything in this world is double insulated,there are still quite a few devices which rely on an earthed external case to provide protection---this is just one of them.

My feeling,is it is just a waste of time to build such an amp,& builders are in for a disappointment.
It would make more sense to build a solid state amplifier,which could offer equal or better performance.
Of course,it wouldn't have the magic"tube sound".

VK6ZGO

PS: I just went back for another look,& they quote "3 transformers" in the parts list.
I was puzzled,but it looks like one is a filament transformer--fils were included on the HT transformer way back when.
They refer to the tube as a triode/pentode---it's probably something like a 6BM8,so they will be battling to get more than about 5 watts out of the thing.

Another thing is,that it is only a valid design for  people in 120v power supply zones.
If someone in a 240v area bought the kit & built it up,they would let the smoke out as soon as they turned it on!

They do stress that people who build it should have experience of high voltage circuits,be over 18,etc,(right at the bottom of the page)

[amspire]

Well I think you can make some pretty good guesses as to how it would work.  The anode resistor for the class A output stage is the 5W resistor, and the transformer will be capacitive coupled. So the amplifier is probably 1W out - perhaps 2 watts out.

Distortion will be about 1% (if you are lucky - probably more likely 5%). You need an efficient speaker to get a decent volume out, so you wouldn't use it with a  modern 4" speaker. You would want at least an 8" speaker, and if your home made speaker box falls of at 200Hz, you will not hear the mains hum from the valves. There is every chance the transformer will not be great at 20,000Hz - it may be OK to 10,000Hz.

I think all the talk of extremely low noise is nonsense - it is hard to see how a valve amplifier will have lower noise then a well designed transistor or IC power amp.

Nothing special - just a basic two stage amplifier.

On the positive side, the ebay buyers for this seller do give genuine praise including the documentation, and for someone who has never used valves and will probably never use them again, this project would be a bit of fun.

There really is a difference in working with valves to working with modern circuits. It is an interesting experience.

[vk6zgo]

I Googled, & it turns out that the output is quoted variously,as 1.5 watts & 2.5 watts,the tube is a 6EB8,not a 6BM8 as I suggested.

I think you will find that the output stage load will be the transformer primary,with no capacitive coupling involved.
The 5watt resistor is probably part of an RC filter for the HT line--these were sometimes used to save the cost of a filter choke.

Mains hum from the valves?---Indirectly heated valves did not contribute any significant noise from this source,unless they were faulty.
A more likely source of noise will be from the "El Cheapo" RC HT filter--in this case,it will be at twice mains frequency.
Valve equipment had highly satisfactory noise performance,but as you say,modern solid state equipment will give superior results.

VK6ZGO

[amspire]

I Googled, & it turns out that the output is quoted variously,as 1.5 watts & 2.5 watts,the tube is a 6EB8,not a 6BM8 as I suggested.

I think you will find that the output stage load will be the transformer primary,with no capacitive coupling involved.
The 5watt resistor is probably part of an RC filter for the HT line--these were sometimes used to save the cost of a filter choke.

I was trying to decide if that transformer was able to take the DC anode current of about 25mA without saturating. Perhaps it can. It is an inefficient use of a transformer. The typical operating conditions for the pentode is 200V at 25mA, and I think over 1W out, the distortion will be climbing.

Mains hum from the valves?---Indirectly heated valves did not contribute any significant noise from this source,unless they were faulty.
A more likely source of noise will be from the "El Cheapo" RC HT filter--in this case,it will be at twice mains frequency.
Valve equipment had highly satisfactory noise performance,but as you say,modern solid state equipment will give superior results.

VK6ZGO

My valve experience is extremely limited. All I know is that every valve amplifier I ever remember using had audible rectified mains hum, but then again I probably never used much high quality valve designs. I seem to remember seeing regulated DC being used for valve heaters when very low noise was required.

[vk6zgo]

I Googled, & it turns out that the output is quoted variously,as 1.5 watts & 2.5 watts,the tube is a 6EB8,not a 6BM8 as I suggested.

I think you will find that the output stage load will be the transformer primary,with no capacitive coupling involved.
The 5watt resistor is probably part of an RC filter for the HT line--these were sometimes used to save the cost of a filter choke.

I was trying to decide if that transformer was able to take the DC anode current of about 25mA without saturating. Perhaps it can. It is an inefficient use of a transformer. The typical operating conditions for the pentode is 200V at 25mA, and I think over 1W out, the distortion will be climbing.

Mains hum from the valves?---Indirectly heated valves did not contribute any significant noise from this source,unless they were faulty.
A more likely source of noise will be from the "El Cheapo" RC HT filter--in this case,it will be at twice mains frequency.
Valve equipment had highly satisfactory noise performance,but as you say,modern solid state equipment will give superior results.

VK6ZGO

My valve experience is extremely limited. All I know is that every valve amplifier I ever remember using had audible rectified mains hum, but then again I probably never used much high quality valve designs. I seem to remember seeing regulated DC being used for valve heaters when very low noise was required.

It is the standard circuit in use for valve output circuits.
The current through the primary has a large ac component.
The primary impedance of around 5000 to 7000 ohms constitutes the load for the ac component,so the average dc current is considerably less than in the dc only case.
Output transformers will not saturate in normal circuit configurations,even without audio applied to the amplifier.

An audio amplifier with audible hum is a faulty amplifier,valve,solid state,or whatever!

Millions of valve amplifiers in all sorts of applications were used before,& well into the solid state era,with noise (including hum) figures
of better than-60dB below reference level.(commonly +16 dBm for line amplifiers,& usually a specified power output for amplifiers driving speakers).
Whole racks full of equipment commonly reached these specs(overall).

A "smart trick"which was used in designing  push-pull valve amplifiers,was to have extra filtering for the low level stages,& a lesser degree of filtering for the high level (push-pull) stages.
The hum would be cancelled in the output stage,so overall all would be well.
If one of the output valves was faulty,the push-pull stage would lose balance,& hum would no longer be cancelled,so the thing would produce around 10w of hum-----lovely!!
As the valve era,faded away,valve standards deteriorated,until the unbalance problem which might have turned up once every 10 years became far more common.

Test instruments often used dc filaments to obtain the very lowest levels of hum,as did some Radio & TV transmitters which used directly heated cathodes in their PA stages.
A more common method with Broadcast Transmitter PAs was to use centre tapped filament transformers,so that cathode current flowed equally both sides of the centre tap,thus cancelling any hum modulation.

VK6ZGO
 

[G7PSK]

Valves in TV's of the 60's and 70's were on a pcb and so was the eht for the crt and that ran into the tens of kilo volts for larger color tv's the danger as shown in the picture is from the exposed transformer terminals and the valve itself, and I cannot se the point of having a low mu valve without a can for it.

[saturation]

When you build from a kit, you are on your own.  Anything that works over  30V AC or DC has a shock potential.  The kit maker does have a disclaimer, because anything that requires line voltage has all the risks regardless of whether its basic design is sound or not.  There is no guarantee its assembler will do it correctly or the parts used have flaws and will cause an malfunction as soon as its powered up.  You have to know about testing parts before assembly and partial assembly, and not find out by powering it up and then get magic smoke.

[G7PSK]

I have some old Mullard valve data books dating from the 50's & 60's. A triode output pentode such as they refer to in the spec on ebay would be somthing like a UCL82 UCL83 the Va would be between 170 and 200 volts to give up to 7 watts output you could operate the circuit described as low as 80V

[vk6zgo]

Valves in TV's of the 60's and 70's were on a pcb and so was the eht for the crt and that ran into the tens of kilo volts for larger color tv's the danger as shown in the picture is from the exposed transformer terminals and the valve itself, and I cannot se the point of having a low mu valve without a can for it.

I never was fond of valves on PCBs ,the old Marconi TV Transmitter had them,& had trouble with tracks burning.

There are no exposed terminals on the power transformer,the insulation is anchored inside the winding,& as I said,insulated for a lot more than the operating voltage.
The TV manufacturers never used a can,although in this use,it might be necessary.
It all depends on the sensitivity of the triode stage in this application.

VK6ZGO

[Psi]

Dad has one of these from an old airport transmitter.
http://www.tubecollector.org/450tl.htm

4kV  and a  7.5V/12A filament!


Now that's a value