Ashdown Peacemaker 20 with suspected blown output transformer

[J M Fahey]

This video is going to cause a lot of heartache to novice amp repairers and it should be taken down.

 

Agree, this video is HORRIBLE and is going to GET YOU KILLED.

At 2:04 he calls the GREEN wire "COMMON" (it's GROUND)

At 2:12 he calls the same hole "COMMON/GROUND" .
Ok decide on ONE, what will you call it?
Not the same !!!!!!!!!

At 2:19 he says that the WHITE wire goes to the COMMON, and points to a DIFFERENT socket hole.

This alone makes it necessary to take this dangerous video down.

At around 3:20 he claims that the bulb will avoid damage to the device undert test.

Well, that depends on bulb size/power.

His suggested 250/300W bulb will protect ... the house wiring, and will happily FRY an amp 

25W is right for small SS stuff, 75/100W for ~100W`SS amps or up to 30/50W tube ones.

I do have a 150W bulb and I use it very few times, when I have a 100W tube amp, I *already*  consider it repaired and want to do "just a final check" before plugging it straight into the wall.

At 5:02 he recommends "the highest wattage bulb available" which is the same as recommending "the highest sized FUSE you can find".

[DrGonz78]

At 5:02 he recommends "the highest wattage bulb available" which is the same as recommending "the highest sized FUSE you can find".

Here is a fuse guide to find highest sized fuses.

I remember seeing that on youtube.com and always wondered about it. I always have just stuck with bulbs from 40-60-100 watt lights. I have seen his other videos and there are nuggets of good information in those. This one sure needs to be taken down and someone should politely inform him as to why. I mean people are thanking him in the comment section on that youtube page saying it helped them. I guess I could say something...?

[phatt]

Love it,, 

The bullet would certainly be interesting 
Phil

[Roly]

You have a stronger stomach than I have JM.   

I guess I could say something...?

Well somebody should.


The fuse guide reminds me of a couple of things.

When I was doing a lot of off-grid solar servicing I found a brass screw in a fuse holder.  When I asked the owner what it was doing in there he said (with a dead straight face) that he used brass because he understood it conducted better than steel.  He was convinced that the rest of his homebrew installation was state-of-the-art, and not a real horror show that will one day burn his house down.

I recall a report in Electronics Australia of a tech finding a live .22 round in a car 3AG-type holder, just above the drivers right leg.

[DrGonz78]

Roly you inspired me to send a message to Uncle Doug. I linked the light bulb limiter thread to the message and gave a decent explanation for the groups concerns here.

[Roly]

Thanks Doc.   

[DrGonz78]

Please note that I noticed a change to the Uncle Doug video in regards to building a light bulb limiter. He now has an important notice to use lower wattage bulbs 100w or lower for solid state type amps. Personally I am still going to use all sorts of 40-60-100 watt bulbs testing my tube amps too. The highest bulb I have ever used is 120 watt bulb in the case of a powerful tube amp. Personally I always power up devices I am working on with a nice variac with ammeter connected. At least there is the warning on the video now, so that is at least something positive. 

Edit: Also in the first line of the text he added to the video it says... "Bulb wattage MUST be tailored to the current draw of the device".  I hope that helps in the long run.

[Roly]

Better than nothing, and a better result than I had hoped for.

Thanks Doc.   

[txflood]

I'm just now getting back to this forum and it seems that I have caused quite a stir. 

I read all of the posts last evening and thought about what has been said here. It does make sense to try to match bulb wattage with the device under test and use low wattage when investigating a known faulty circuit. I then thought I would run a few experiments and measure some voltages to try to better understand all this.

First I went back to my known bad mains power supply transformer. This transformer is electrically isolated and disconnected from the amp circuitry. To recap, whenever normal 120V is applied it hums loudly and heats up very quickly and I've only ever tried powering it on for a few seconds at a time. It's bad so I thought a few experiments aren't going to hurt anything.

So I tried using a 40W, 100W, and 250W bulb one at a time to see what happens. In each case, the bulbs turned on as expected and the transformer never made any audible hum and it never seemed to heat up for that matter. I measured somewhere around 35V for what is actually making it to the transformer for the case of the 250W bulb and smaller voltages for the other 2 bulbs.

For US power I would calculate that the fully-heated on resistance of the 250W bulb to be somewhere around 58 ohms. If I am seeing a power drop of 120V to 35V across the bulb then I believe that means we are looking at a current of almost 1.5 amps flowing through the circuit. (All of these are crude numbers but I think they are in the ballpark). So that definitely looks like more power than one would want when investigating a faulty circuit.

The second thing that I tried was then plugging in a good working Peavey TKO solid state bass amp that I happened to have on hand. I believe this amp is rated for a maximum power draw of 200W.

With the 40W bulb plugged in, I measured around 95V being supplied to the amp. With a 100W bulb plugged in, I measured around 108V. With the 250W bulb I was seeing very little voltage drop at all from the original 120V.

So I can see that with an assortment of bulbs this can be used as a "poor man's" variac. It also looks to me that the larger bulbs aren't totally useless either. If you have a piece of equipment and it has been repaired and after applying higher and higher voltages with assorted smaller watt bulbs, wouldn't it still be useful to use a large wattage bulb as your final test where you are supplying as close a value to wall voltage while still having some (although admittedly not great) power protection?

[Roly]

it seems that I have caused quite a stir.

Well you and another have drawn our attention to some really bad advice.  I thank you for that, and Doc for doing something about it.  You have helped in saving some poor sods from blowing up their amp repairs by following bad advice.

This transformer is electrically isolated and disconnected from the amp circuitry. To recap, whenever normal 120V is applied it hums loudly and heats up very quickly

This is a classic example of a transformer with a shorted turn.   

It also looks to me that the larger bulbs aren't totally useless either.

That's quite true, and as I said, if I were testing, say, a large air conditioner with a 1HP motor then such a large globe would be justified.

But we can say that even the largest guitar amp is a lighter load than a 1 horse compressor, and that it is hard to justify using a globe larger than about 100 watts.

There isn't a lot of point in using a limiting lamp when testing a repaired amp at full power.  By this time you should have already verified correct operation (specifically that the half-rail actually is) at reduced power, and if you have done it right then there won't be any difference between a 100 watt globe and direct on mains.

This might be justified if you were using a substitute transistor type and you were unsure about its ability to handle the duty, but this would be a bit of an iffy situation anyway; any substitute should have rating equal or greater.

So the situation is that by the time you get up to a 100 watt globe, in almost all cases you are already well out of the woods, even using a 100 watt globe is being quite conservative and cautious.  If a repair comes up okay on a 40 watt globe at about half voltages, and the half-rail is where it should be, within a few 100mV of ground, than that really should be all the proof you need.

Put another way, the primary role of the limiting lamp is in the first few tens of watts, to show you the repair isn't okay without smoke, without blowing up a newly fitted set of transistors.


Again, you may never see it, but by bringing this serious mistake to attention you will have saved a lot of people a lot of heartache (and cost some transistor suppliers multiple sales).   

[J M Fahey]

Congratulations to Dr Gonz, Texas Flood, and , of course, to Uncle Doc who obviously takes care about what he publishes.        

[txflood]

So getting back to the amp, I was going to try to order a replacement mains transformer but I had a general design question. When considering the current needed for the high voltage section of the power supply, I'm a bit confused as to what the two EL84's will be drawing.

The transformer schematic specifies 270-0-270 VAC at 150 mA. You guys have already recommended trying to find something with a lower voltage if possible, but I'm not sure what we expect the current draw to actually be.

[Roly]

A small reduction in voltage, say 270-0-270V down to 250-0-250V is pretty marginal and won't reduce the current much.

I would be disinclined to go any lower than 150mA.

[txflood]

Actually I was wondering if the original 150mA was a good design point in the first place and if it should be increased to say 200mA. I didn't know if there were some good rules of thumb for the worst case draw for these output tubes.

I also got some more info from Ashdown as they have provided the DC voltages in the power supply:

CT   355 v   + /- 10v.
G2   345v    +/-  10v.
HT1   240v   +/-  10v.
HT2   210v   +/-   5v.
HT3   200v   +/-   5v.
A1     215v   +/-   5v.
TP1  ( BIAS )   10v   +/-  0.3v.

[phatt]

You won't get a secret sauce by adding more salt to a perfect recipe

Go read the Valve books,, EL84 - shoot for 300 Volts supply +/- 10%

Go faster secrets is for racing cars,, Yes you can get to the corner shop in under 10 Seconds
but you have to rebuild the motor when you get home. An expensive bottle of milk.

This is a valve amp and all the tricks are well known.

If there is any secret then that would be stick to the recommended design figures and enjoy a hassle free build that will last a long time.
I recall reading on the Aussie valve forum that 6V6's work best at around 350 volts Yet most fancy name 6V6 amps seem to rum closer to 400Volts, partly the reason they sound so harsh.

I run a small 6GW8 powered valve amp (similar to EL84) and tiss very sweet at 250VDC. 

The only reason they run higher voltage supply is to get an extra couple of watts. The kids will buy whatever is the biggest number ,, as bigger must be better
So yes if you belt the crap out of EL84 you can pull maybe close to 20 Watts but it won't last long and likely sound brittle and harsh.

My 2 cents worth.