Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Tubes and Hybrids => Topic started by: txflood on December 08, 2014, 10:57:13 PM

Title: Ashdown Peacemaker 20 with suspected blown output transformer
Post by: txflood on December 08, 2014, 10:57:13 PM
I was looking for advice on where one could find a suitable output transformer for an Ashdown Peacemaker 20 amp. It has EL-84 output tubes and the schematic can be found online here if needed:

http://music-electronics-forum.com/t34072/

I was wondering how cost effective this repair would be for this particular amp.

Thanks
Title: Re: Ashdown Peacemaker 20 with suspected blown output transformer
Post by: phatt on December 09, 2014, 04:44:15 AM
What makes you suspect the OT?
There are many other things that will cause no output and are a lot cheaper than a transformer to fix.
Phil.
Title: Re: Ashdown Peacemaker 20 with suspected blown output transformer
Post by: txflood on December 18, 2014, 01:42:23 PM
This was a buddy's amp that he had a tech look at a few years ago and was told the "output" transformer needed to be replaced. I now have the amp and took it apart.

The tech had left the wires from the power supply transformer disconnected from the PCB so I started there to see where things really stand with this amp.
The AC inline fuse was blown and I found scorched marks around R76 in the power supply section on inspecting the PC board in detail. There is a wire that runs from the power supply at R76 to the center tap of the primary side of the output transformer. The connector is attached where the board silkscreen reads "FROM OUTPUT TX" next to R76 and the plastic insulator on the connector appears melted. R76 sits between the 2 filter caps and it does measure to be 470 ohms.

I also removed the PCB from the chassis and took a look at the area under R76. It looked like the solder had been worked on that resistor so I am guessing that the original part has been replaced. Close inspection showed that the current resistor has a 6W power rating while the schematic only calls out for 2.5W.

I disconnected all the leads from the mains transformer to the PCB and then took no load AC measurements on the secondary winding wires coming directly from the transformer. I only applied power for short intervals. The transformer buzzed fairly loudly and got warm fairly quickly even with these short tests, so I knew that wasn't a good sign already.

On the high voltage pair I only measured around 10VAC. The yellow pair only came in at 6.6VAC and the red pair at 4.8VAC. I would say this confirmed my worst fears on the power transformer.

So the question remains as to why that resistor failed and if it took the power supply transformer out with it or if that occurred later.

I contacted Ashdown and they don't have replacement power transformers but they did give me the specs on the three secondaries:

270V @ 0.15A
6.3V @ 1.6A
13V - 0 - 13V @ 0.5A

I couldn't find an off-the-shelf part to match all 3, but I found a few candidates to match the first 2:

Hammond 270FX, Weber W025130INT, or maybe Weber WPTGP (that one would be 280V)

My thought is to get one of these transformers and then add another small transformer to the chassis to take care of the 13-0-13V.

I also tried to test the filter caps to make sure there wasn't any obvious problem (visual inspection, low impedance) but could not find anything. The PCB indicates the amp was assembled in 2002 time frame so I'm thinking it would be a good idea to replace them anyway.

I'm still concerned that perhaps there is still a problem that caused the power supply failures in the first place that hasn't been addressed so I was going to proceed carefully when trying to re-energize the circuit after getting replacement parts.

Since I've never actually heard one of these amplifiers, I am curious as to what it really sounds like and how they are regarded in general. I haven't found much info on them at all.
Title: Re: Ashdown Peacemaker 20 with suspected blown output transformer
Post by: Roly on December 19, 2014, 01:11:30 AM
There seems to be some confusion here between the power transformer (PT), mains to rectifier, and the output transformer (OPT), output valves to speaker.

Quote from: txflood
The transformer buzzed fairly loudly and got warm fairly quickly even with these short tests, so I knew that wasn't a good sign already.

On the high voltage pair I only measured around 10VAC.

Those are pretty fair indications that the PT high voltage winding has a shorted turn, however it doesn't follow that this was caused by the OPT failing, in fact that would be an unusual result of a typical OPT failure.

An important early check are the main rectifier diodes, <peacemaker-40-b.pdf> D10-13.

Here's a radical idea - test the OPT.

Fully isolate it and apply around 6.3VAC to the speaker side.  It will have a turns ratio of around 20:1-30:1 so you should get something like 20 * 6.3 = 126VAC to 30 * 6.3 = 189VAC on the anode side.  The exact voltage isn't that critical, but if it's very much lower then it suggests that the OPT has a shorted turn, otherwise it suggests that it's okay.

Quote from: txflood
I also tried to test the filter caps to make sure there wasn't any obvious problem (visual inspection, low impedance) but could not find anything. The PCB indicates the amp was assembled in 2002 time frame so I'm thinking it would be a good idea to replace them anyway.

Don't complicate things.  If the caps hold charge from a Megger or similar high voltage source, leave them alone, at least until you can get some high voltage on them.

Until you are sure all is well you should only power up the amp via a Limiting Lamp (http://www.ssguitar.com/index.php?topic=2093.0).
Title: Re: Ashdown Peacemaker 20 with suspected blown output transformer
Post by: phatt on December 19, 2014, 07:48:23 AM
Go with Roly's advice,, He eats pentodes for breakfast and washes them down with a mathematical smoothie,  ;) :P
Meantime my observations;
The fact that R76 is running very hot suggests that something *After* that Resistor is pulling too much power.

I'd guess, the Screen grids are likely getting belted to death.
As you have 270 volt AC supply then your DC at CT is going to be pushing close to 380VDC yet R61 is only 50 Ohms so unless I'm sadly mistaken the power tubes are running way too hard.

Your power amp (V7,4 and 5) is a simple copy of the classic AC30 and they ran 300~320VDC and they were known to run hot. So R61 would need to be raised in value to compensate for the higher supply voltage.
You can see signs of over dissipation by looking at the plates of the power Valves, those little holes in the side of the plates. If there is any sign of black burn marks around those holes it's a fair bet they have been flogged to death.
 
Obviously the power tranz is dead so maybe look for something lower than 270 VAC as a replacement.
This issue of higher than needed HT voltage is common and from what I understand some transformers were likely meant for valve rectification where there is a lot of voltage drop across the rectifier Valve but with SS diode rectification the drop is only small so the working voltage gets bigger and without a rethink of power valve conditions and rebiasing the power tubes suffer greatly.

Valves are tough and can handle over voltage by a fair amount but NOT over current. So the higher the voltage the more critical the dissipation becomes. The screen grid is the weak link in power Valves and is a common failure mode.

Regarding a new transformer, just take the secondary voltage and multiply it by 1.414 (1.4 is close enough) that will give you a clue as to the DC result.
That voltage will drop when running so if the equation runs out at 320~330 VDC it will be close enough.
Keep at it, if you take time to work through it I see it as a worthwhile fix. :tu:
Phil.
Title: Re: Ashdown Peacemaker 20 with suspected blown output transformer
Post by: txflood on December 19, 2014, 11:12:13 AM
There seems to be some confusion here between the power transformer (PT), mains to rectifier, and the output transformer (OPT), output valves to speaker.

Right I was initially going by what I was told before ever looking at the amp.

An important early check are the main rectifier diodes, <peacemaker-40-b.pdf> D10-13.

Those tested fine using the diode check on my meter.


Here's a radical idea - test the OPT.

Fully isolate it and apply around 6.3VAC to the speaker side.  It will have a turns ratio of around 20:1-30:1 so you should get something like 20 * 6.3 = 126VAC to 30 * 6.3 = 189VAC on the anode side.  The exact voltage isn't that critical, but if it's very much lower then it suggests that the OPT has a shorted turn, otherwise it suggests that it's okay.
Thanks for this tip. I did get the specs from Ashdown now on the OPT and I'm guessing that it is bad as the drawing shows 8K ohms resistance on the primary windings. I am measuring a little less than 600 ohms.

Wouldn't this low primary resistance explain R76 burning up feeding the CT of the OPT?


Until you are sure all is well you should only power up the amp via a Limiting Lamp (http://www.ssguitar.com/index.php?topic=2093.0).
I was already thinking that I should have one of these when I attempt to replace the power transformer and energize the circuit.

Thanks for the info!
Title: Re: Ashdown Peacemaker 20 with suspected blown output transformer
Post by: txflood on December 19, 2014, 11:43:17 AM
I'd guess, the Screen grids are likely getting belted to death.
As you have 270 volt AC supply then your DC at CT is going to be pushing close to 380VDC yet R61 is only 50 Ohms so unless I'm sadly mistaken the power tubes are running way too hard.

Your power amp (V7,4 and 5) is a simple copy of the classic AC30 and they ran 300~320VDC and they were known to run hot. So R61 would need to be raised in value to compensate for the higher supply voltage.
You can see signs of over dissipation by looking at the plates of the power Valves, those little holes in the side of the plates. If there is any sign of black burn marks around those holes it's a fair bet they have been flogged to death.

The amp I have is the 20W version so R61 is 100 Ohms  and V8 and V2 are not fitted if that makes much of a difference.

I took a look at the output tubes and didn't see any black burn marks but maybe I don't know what I am looking at. The guy I got the amp from claimed all the tubes had low hours on them as well.

Obviously the power tranz is dead so maybe look for something lower than 270 VAC as a replacement.
This issue of higher than needed HT voltage is common and from what I understand some transformers were likely meant for valve rectification where there is a lot of voltage drop across the rectifier Valve but with SS diode rectification the drop is only small so the working voltage gets bigger and without a rethink of power valve conditions and rebiasing the power tubes suffer greatly.

Valves are tough and can handle over voltage by a fair amount but NOT over current. So the higher the voltage the more critical the dissipation becomes. The screen grid is the weak link in power Valves and is a common failure mode.
OK thanks for that info. What I am taking away from this is just how well thought out is this amp design and does it need changes to actually make it reliable. I don't know enough about valve amp design to know what corrections are needed here but I am willing to learn.

Regarding a new transformer, just take the secondary voltage and multiply it by 1.414 (1.4 is close enough) that will give you a clue as to the DC result.
That voltage will drop when running so if the equation runs out at 320~330 VDC it will be close enough.
Keep at it, if you take time to work through it I see it as a worthwhile fix. :tu:
Phil.


So you would recommend a replacement power transformer that is more around 220V (if I did my math right) as a good design point?

Well at this point I am guessing that both transformers are going to have to be replaced as my initial cost factor, but this looks like it might be a good learning experience and hopefully I end up with an amp I actually might want to use.  :)
Title: Re: Ashdown Peacemaker 20 with suspected blown output transformer
Post by: g1 on December 19, 2014, 05:08:21 PM
Thanks for this tip. I did get the specs from Ashdown now on the OPT and I'm guessing that it is bad as the drawing shows 8K ohms resistance on the primary windings. I am measuring a little less than 600 ohms.

Wouldn't this low primary resistance explain R76 burning up feeding the CT of the OPT?
The 8K figure is impedance, not resistance.  Impedance is an AC only spec. and we can't measure it with a normal type meter.  We can measure DC resistance but it will give a much lower measurement.  Your 600ohm resistance reading does not seem low at all, some amps OT primaries measure even lower.
R76 does not feed the CT.  Rather it feeds everything after the CT and the fact it is burning up implies the problem could be anything other than the output transformer.



Title: Re: Ashdown Peacemaker 20 with suspected blown output transformer
Post by: phatt on December 21, 2014, 07:48:35 AM
Yes as G1 noted Impedance and Resistance can be confusing.

Impedance is AC so it's in the frequency realm. Example, 8 Ohm speaker may read 6 Ohms DC resistance but the Z (impedance) at 10,000 CPS might be closer to 30 Ohms. :o
A Microphone label might say 600 Ohms Z @ 1,000CPS but a multimeter will read the DCR down around ~150 Ohms.  DCR is nearly always lower than the Z Ohms.

If the Valves look ok then do not worry as the issue might have little to do with High voltage section,, remember you have 3 supplies so the blown primary winding may have been caused by the other secondaries.

15/0/15VAC drives the 12VDC SS supply section through Reg chips (IC 4&5)
That also powers the filaments (Heaters) for the AX7's

And one more AC winding powers the Filaments for the power Valves.
So you have 3 possible failure points to check.

Check the bridge diodes on the 12/0/12 supply and don't forget the fuses,, make sure they are the right value. Some clown may have put in a 10 Amp fuse rather than fix the underlying issue, meanwhile it's melted the primary. I've seen that one a few times ::)

Don't forget Roly's OTr test and build the lamp tester. :tu:

Re the transformer over voltage, anything lower than 270VAC will be a good but if 270 is all you can find then just remember to check the bias when it's up and running again but that is the least of your worries at the mo,,, need to find the fault first.
Phil.
Title: Re: Ashdown Peacemaker 20 with suspected blown output transformer
Post by: txflood on December 21, 2014, 09:58:10 PM
OK, I made a bad assumption that it was DC resistance so that sounds like potentially good news on the OT.

Thanks for the tips and it will probably be after the holidays before I will have a chance to proceed any further on the amp.
Title: Re: Ashdown Peacemaker 20 with suspected blown output transformer
Post by: Roly on December 22, 2014, 01:37:11 AM
The first thing you should do is the OPT test I suggested - if you get 100+ volts between the primary ends (anode connections) you can forget about the OPT being faulty.

The impedance (Z) of a transformer winding is its DC resistance (R) plus its AC reactance (Z = R + XL, where XL = 2 Pi f L) so the impedance will always be higher than the DC resistance, often very much higher.  This is also true for loudspeakers.
Title: Re: Ashdown Peacemaker 20 with suspected blown output transformer
Post by: txflood on January 22, 2015, 09:22:07 PM
OK so I finally had a chance to get back to the amp. I've built a cool current limiter with a pair of 250W bulbs and tried it on my fried power transformer as a test. It lit up beautifully and the transformer never buzzed or heated up at all, so I would say I now have a good limiter for future repair work.

The only AC source that I had handy tonight output 9.9VAC according to my mulitmeter. Since the transformer is supposed to have 8000 ohms primary for the EL84s to see and the speaker is 16 ohms, I believe that gives us an impedance ratio of 500 and a winding ratio of roughly 22.4.

I applied the 9.9VAC to the primary (with the source plugged into my handy dandy new limiter of course) and expected to get about 0.4V on the secondary. I measured 0.3V and I know my meter isn't the best in the world precision wise so I'm thinking that it's in the ballpark.

You guys were suggesting that I also test it the other way by applying the AC to the secondary, but I'm wondering is that the normal practice for testing an output transformer? Or do you only worry about applying voltage to a secondary if you are dealing with an unknown transformer? I've gathered that it can be frowned on as an unsafe practice or am I misinterpreting things?

Title: Re: Ashdown Peacemaker 20 with suspected blown output transformer
Post by: Roly on January 23, 2015, 08:55:00 AM
Quote from: txflood
a pair of 250W bulbs
{twice in one week?  :o }

I hope this is a typo and you mean 25 watts - the idea is to limit the power into the amp to a safe level, and two-hundred-and-fifty watts is not a safe level.


Transformers are (approximately) bi-lateral devices.  If you apply 6.3VAC to the speaker winding the step-up ratio should be, as you correctly calculated, 22.4:1, giving around 22.4 * 6.3 = 141.1VAC (or 22.4 * 9.9 = 221.8VAC) on the valve side.  These voltages are within the range of normal operation ('tho the actual voltage may be a bit less due to transformer losses).

It's also generally easy to isolate the valve side by pulling the OP valve(s), then picking up 6.3VAC from the heater line with a clip lead or two.

You also need to remember that the transformer losses will slightly load your driving transformer, so you really should measure the actual applied voltage for the most accurate results.

There is a significant difference between a good transformer operating unloaded with about 5-10% losses, and a dud transformer with a shorted turn(s) with about 90-95% losses.

It's "unsafe" in the sense that 150 volts will give you a shock, but you are careful not to touch it while the soup is boiling, right?  The reason for doing it step-up rather than step-down is that the results are rather more definitive, say 5 or 10VAC rather than the expected 150-160VAC.  As you found you need to apply a fair bit of voltage to the valve side to get anything measurable on the speaker side.

Whatever, it looks like your OPT is healthy.
Title: Re: Ashdown Peacemaker 20 with suspected blown output transformer
Post by: txflood on January 23, 2015, 10:25:00 AM

I hope this is a typo and you mean 25 watts - the idea is to limit the power into the amp to a safe level, and two-hundred-and-fifty watts is not a safe level.


No actually I meant a 250W bulb. I thought the idea of the device is that if your equipment under test has a short in it, it will still be protected as the light bulb will then turn on and the power will be flowing to it. The large wattage bulbs are required as they have the low resistance needed for this to work in practice.

https://www.youtube.com/watch?v=wRFRwOnLsZI

The best part is about the 7 minute mark when he shorts the circuit with a screwdriver.

 :o

He has a link to his suggested 250W bulb being sold on Amazon.
Title: Re: Ashdown Peacemaker 20 with suspected blown output transformer
Post by: Roly on January 23, 2015, 12:00:48 PM
{There had to be a reason for this "300 watt" co-incidence}


Well this is a video that needs to be resounding and loudly bagged.  "The larger the better" is just plain WRONG, so I'm not going to do this guy the honor of watching more than 60 seconds of his misinformation.


The Limiting Lamp represents a non-linear resistor in series with the Device Under Test (DUT).  When it is cold its resistance is low, but as it heats up its resistance rises rapidly, by a factor of around x10-20.  In this way it acts rather like a self-resetting fuse, and like a fuse it has to have a suitable rating for the load, not just "bigger is better".

In our case the DUT is a guitar amplifier, either valve or solid state, and typically with an output power rating somewhere in the range of 10 to 100 watts.  Since they are almost always Class-AB or Class-B the input power will be of the order of twice the output power, say 20 to 200 watts.

In a valve amp there is heater power to consider and this may be up to about 30-odd watts in a larger amp, while in a s.s. amp we will be looking at an output stage idle current of around 50mA across, say, +/-35V supplies;

P = E * I

2 * 35 * 0.05 = 3.5 watts


The object of a Limiting Lamp is to allow you to liven up a circuit which has a fault so you can do some faultfinding, without doing any more damage.  This will mean a lamp with a starting value of around 25 watts for a s.s. amp, and perhaps 40 watts for a valve amp.  As a rule of thumb you should use a lamp rated at no more than the amp nominal output power.

Another particular need for a Limiting Lamp is where you have repaired a s.s. amp and need to bring it up gently to confirm correct operation, and not blow up all your new transistors if you have missed a fault.  Again you would start with a 25 watt lamp and work up to say 100 watts if all seems okay.

A "limiting lamp" of 300 or more watts simply won't provide a safe power limit into your amp, it will pass an excessive amount of power, and you can expect smoke before it is repaired, and to replace all your expensive new transistors after you have repaired it and missed something - it is simply useless for the task.  {a similar consideration applies to using a Variac which provides a fairly "stiff" supply and no non-linear limiting like a lamp, and requires greater care in use}

For electrical stuff, large motors, heaters, etc, there may be some call for such a high power limit, but using a lamp of 100 watts or higher initially with electronic equipment such as a guitar amp (and particularly lower power ones) is simply asking for serious trouble.

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

 :grr
Title: Re: Ashdown Peacemaker 20 with suspected blown output transformer
Post by: J M Fahey on January 25, 2015, 01:56:43 AM
Quote
This video is going to cause a lot of heartache to novice amp repairers and it should be taken down.

 :grr
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  :loco

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".







Title: Re: Ashdown Peacemaker 20 with suspected blown output transformer
Post by: DrGonz78 on January 25, 2015, 02:47:34 AM
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...?
Title: Re: Ashdown Peacemaker 20 with suspected blown output transformer
Post by: phatt on January 25, 2015, 08:04:54 AM
Love it,,  :lmao: :lmao:

The bullet would certainly be interesting  :duh
Phil
Title: Re: Ashdown Peacemaker 20 with suspected blown output transformer
Post by: Roly on January 25, 2015, 09:20:14 AM
You have a stronger stomach than I have JM.   :dbtu:


Quote from: DrGonz78
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.
Title: Re: Ashdown Peacemaker 20 with suspected blown output transformer
Post by: DrGonz78 on January 26, 2015, 03:49:22 AM
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.
Title: Re: Ashdown Peacemaker 20 with suspected blown output transformer
Post by: Roly on January 26, 2015, 06:35:28 PM
Thanks Doc.   :dbtu:
Title: Re: Ashdown Peacemaker 20 with suspected blown output transformer
Post by: DrGonz78 on January 28, 2015, 01:07:24 AM
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.  :tu:

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.
Title: Re: Ashdown Peacemaker 20 with suspected blown output transformer
Post by: Roly on January 28, 2015, 09:19:25 AM
Better than nothing, and a better result than I had hoped for.

Thanks Doc.   :dbtu:
Title: Re: Ashdown Peacemaker 20 with suspected blown output transformer
Post by: txflood on January 28, 2015, 01:12:17 PM
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?
Title: Re: Ashdown Peacemaker 20 with suspected blown output transformer
Post by: Roly on January 28, 2015, 07:16:44 PM
Quote from: txflood
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.


Quote from: txflood
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.   :dbtu:


Quote from: txflood
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).   :dbtu:
Title: Re: Ashdown Peacemaker 20 with suspected blown output transformer
Post by: J M Fahey on January 29, 2015, 02:52:34 AM
Congratulations to Dr Gonz, Texas Flood, and , of course, to Uncle Doc who obviously takes care about what he publishes.  :dbtu:   :dbtu:   :dbtu:

Title: Re: Ashdown Peacemaker 20 with suspected blown output transformer
Post by: txflood on February 03, 2015, 01:27:48 PM
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.
Title: Re: Ashdown Peacemaker 20 with suspected blown output transformer
Post by: Roly on February 04, 2015, 09:17:55 AM
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.
Title: Re: Ashdown Peacemaker 20 with suspected blown output transformer
Post by: txflood on February 04, 2015, 10:00:53 AM
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.

Title: Re: Ashdown Peacemaker 20 with suspected blown output transformer
Post by: phatt on February 05, 2015, 05:38:00 AM
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. :duh

This is a valve amp and all the tricks are well known. 8|

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.  <3)

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 :-X
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.
Title: Re: Ashdown Peacemaker 20 with suspected blown output transformer
Post by: Roly on February 05, 2015, 08:05:06 AM
I would estimate the power transformer requirement as;

a) the total heater power, plus

b) worst case HT/B+ power required by the output stage.  This will be the rated output power x2, plus a bit for snake bite, say 20-50% to taste.  Transformers are generally available in stack sizes such as 20, 30, 50, 100 VA (volt-amps, read "watts"), so whatever figure you come up with, say 37.6VA, you will go to the next highest available stack, say 50VA.

Using a transformer with a higher rated HT/B+ winding will produce a stiffer supply with less sag, and which will generally run a bit cooler.

As a designer normally all I'm really interested in is using a tranny that has ratings in excess of my expected worst case conditions.

That's the quantitative, but I'd take notice of what Phabb says about the qualitative - no point in having it a bit louder if the cost is having it sound like a chain saw.