Hi everybody,
I am in the progress of instaling isolation transformers inside my little ACDC tube amps, and in the hope of saving some money I would like to use 2 stepdown transformers linked back to back at secondary, I can get them pretty cheap.
In a tutorial at instructable web site I read that without a full wave rectifier, using a 50VA transformer will not be enough because the rectifier half wave (35W4) loading the transformer could bring some heat in the core, so a 100VA is suggested in that case.
Since I don't want to use any diode bridge and neither make too much modification in the amps, I bought some transformers and did some test, the first set I used were 100VA, and everything was OK, after one hour of playing, no heat at all , so I think that 100VA transformers are pretty good.
The second set of transformer I tried, which I dunno the VA rating, were smaller than the first set, the only thnig I know about them is that they are rated at 2 amps at secondary, if I do the math, with 24 VAC at secondary and 2 amps rating, the result would be 48VA, so I did the same test, played for one hour, the result is that the first transformer (the one which the AC line is connected to), is getting just a little warm, but very very very veeeeery little, so little that I can put my nose on it
I mesured the current flowing at secondary (where the two transformers are linked), and I got a little more than 1 amp, so I bet that its correct... but how can I be sure, is there some maths so I can be safe of what I do 🙂 By taking 2 X 50VA linked back to back, do I get 100VA? How does it works?
I could use the 100VA set and no worries, but they are pretty heavy compared to the second set, which are smaller, lighter and cheaper 😀
Any comment or suggestion will be appreciated!
I am in the progress of instaling isolation transformers inside my little ACDC tube amps, and in the hope of saving some money I would like to use 2 stepdown transformers linked back to back at secondary, I can get them pretty cheap.
In a tutorial at instructable web site I read that without a full wave rectifier, using a 50VA transformer will not be enough because the rectifier half wave (35W4) loading the transformer could bring some heat in the core, so a 100VA is suggested in that case.
Since I don't want to use any diode bridge and neither make too much modification in the amps, I bought some transformers and did some test, the first set I used were 100VA, and everything was OK, after one hour of playing, no heat at all , so I think that 100VA transformers are pretty good.
The second set of transformer I tried, which I dunno the VA rating, were smaller than the first set, the only thnig I know about them is that they are rated at 2 amps at secondary, if I do the math, with 24 VAC at secondary and 2 amps rating, the result would be 48VA, so I did the same test, played for one hour, the result is that the first transformer (the one which the AC line is connected to), is getting just a little warm, but very very very veeeeery little, so little that I can put my nose on it
I mesured the current flowing at secondary (where the two transformers are linked), and I got a little more than 1 amp, so I bet that its correct... but how can I be sure, is there some maths so I can be safe of what I do 🙂 By taking 2 X 50VA linked back to back, do I get 100VA? How does it works?
I could use the 100VA set and no worries, but they are pretty heavy compared to the second set, which are smaller, lighter and cheaper 😀
Any comment or suggestion will be appreciated!
are you using back to back transformers ?
not sure about the AC line voltage in Canada is it 117 ? there are many things you can try, first you can try a 117 to 117 transformer I know they exists for this purpose and you'll save one transformer, also you can still use the AC line for the heaters making sure none of the heater valves are connected to ground (I'm guessing you have 35w4, 50C5 and 12AU6 or 12Av6 in pairs), then with only 50va transformer will be enough.
Also you can pump the power a bit by using a foreign adaptor 117 to 220/40 and adjusting the voltage to 155v at the plate with a resistor of 1k5 (5 watts) between the 35w4 cathode (pin 7) and the capacitor and resistor that provides the screen and preamp plate voltage. I did this mod on Harmony and Lectrolab amps.
not sure about the AC line voltage in Canada is it 117 ? there are many things you can try, first you can try a 117 to 117 transformer I know they exists for this purpose and you'll save one transformer, also you can still use the AC line for the heaters making sure none of the heater valves are connected to ground (I'm guessing you have 35w4, 50C5 and 12AU6 or 12Av6 in pairs), then with only 50va transformer will be enough.
Also you can pump the power a bit by using a foreign adaptor 117 to 220/40 and adjusting the voltage to 155v at the plate with a resistor of 1k5 (5 watts) between the 35w4 cathode (pin 7) and the capacitor and resistor that provides the screen and preamp plate voltage. I did this mod on Harmony and Lectrolab amps.
Since only one transformer is plugged into the wall, then you will only have 50 VA available. Just because you use two back to back does not give you 100VA. If both transformers were plugged into the wall (parallel), you will then have 100VA.
You should be okay with 50VA though since you're only drawing roughly one amp from it. And if you can put your nose to it, then I wouldn't sweat it. Transformers will be fine.
You should be okay with 50VA though since you're only drawing roughly one amp from it. And if you can put your nose to it, then I wouldn't sweat it. Transformers will be fine.
Yes I am using back to back transformers. The AC line voltage in Canada is actually 120VAC. The cheapest 1:1 iso transformer by here is 75$ (hammond), since I have 7 amps to upgrade I'm gonna stick with my 3.99$ transformers linked back to back 🙂
What's the effect of grounding the heaters? By using the AC line for connecting them, did you mean like this:
Thank you very much for the foreign adapter tip!
Thanks! That's what I was wondering!
The diagram shown in post #4 almost completely defeats the purpose of adding the isolation transformer, leakage across the heater to cathode insulation is not unlikely, and a short between heater and cathode is not impossible.
The entire amplifier including the filaments MUST be powered off of the isolated secondary for safety.
The chassis should be grounded using a 3 wire power cord.
The entire amplifier including the filaments MUST be powered off of the isolated secondary for safety.
The chassis should be grounded using a 3 wire power cord.
See how much the Triad N-68X costs in CA. It's $15 in the US and is a 50VA isolation transformer. I have used it in a Lectrolab guitar amp, and several of my own design.
As Kevin says DO NOT connect the tube heaters directly across the line. Heater to cathode shorts are not uncommon especially in 60 year old tubes. There have been documented cases of guitar players being killed or severely shocked by touching a hot guitar and a grounded mic while on stage.
If the transformers you are using do not get hot after running for several hours, then they are probably OK. Just turn on the amp for a few hours, you don't have to play it. All these amps are class A and consume nearly the same power at idle as at max output. Most actually draw less power when driven out of class A with a distortion pedal.
As Kevin says DO NOT connect the tube heaters directly across the line. Heater to cathode shorts are not uncommon especially in 60 year old tubes. There have been documented cases of guitar players being killed or severely shocked by touching a hot guitar and a grounded mic while on stage.
If the transformers you are using do not get hot after running for several hours, then they are probably OK. Just turn on the amp for a few hours, you don't have to play it. All these amps are class A and consume nearly the same power at idle as at max output. Most actually draw less power when driven out of class A with a distortion pedal.
Just my own 2 cents
I doubt you will find a cheap source for the Triad in Canada. I never have. Shipping from the US just kills you for total cost; USPS no longer offers shipping by ground to Canada.
You seem to be thinking about back to back transformers needing to be matched. Exactly the opposite is true: best results would be achieved if the one connected to the mains were of a higher rating so it could better supply the magnetizing current demand of the 2nd transformer. For example, a 2A 120V:12V transformer feeding a reversed 1A 120V:12V transformer would be better.
I doubt you will find a cheap source for the Triad in Canada. I never have. Shipping from the US just kills you for total cost; USPS no longer offers shipping by ground to Canada.
You seem to be thinking about back to back transformers needing to be matched. Exactly the opposite is true: best results would be achieved if the one connected to the mains were of a higher rating so it could better supply the magnetizing current demand of the 2nd transformer. For example, a 2A 120V:12V transformer feeding a reversed 1A 120V:12V transformer would be better.
Hey thanks for pointing out the missing ground! I quickly did this diagram to show the heaters connected to the AC line, as suggested by VMUNIX, and I completely forgot the ground... very important! Here's the complete diagram including the ground to chassis and the heaters wired at where (I think) is the correct place.
Leakage across the heater to cathode insulation, I didn't knew that... Thanks for this good advise!
No Triad available for cheap in Canada 😡 This is the first one I looked for because the guy at instructables used this one... 🙂
You are correct, no Triad by here..
Hey thanks for this advise, I don't know the wheres or whys but can we assume the first transformer would be running less hot like this?
One change I would be is to reverse the order of the filament wiring putting the 12AU6 end of the filament string on the grounded end - this should make things much less hummy.. 😀
The larger transformer on the step down side will help.. Make sure you use big enough transformers overall. (hint, the filaments draw quite a lot of current - 150mA directly off of the 117V supply.)
Perhaps Hammond makes a small isolation transformer or you could hunt for something on eBay.ca?
The larger transformer on the step down side will help.. Make sure you use big enough transformers overall. (hint, the filaments draw quite a lot of current - 150mA directly off of the 117V supply.)
Perhaps Hammond makes a small isolation transformer or you could hunt for something on eBay.ca?
Heh! indeed, in the amplifiers the heaters are wired like you say: 35W4-50C5-12AXX.. dunno why I drew it like that lol 😀
Okay I did some test with those little 48VA linked back to back, and mesured the AC voltage at the isolated side. When I power on the amp, the voltage is dropping from 118VAC to 97VAC. As the amp warm up the voltage slowly raise to become stable at 114VAC.
So I did the same test but this time by using the bigger 100VA transformers, and I see exactly the same voltage drop/raise, but instead ending at 116VAC stable voltage.
Seems good?.. Is this what you are talking about when you say that the filaments draw quite a lot of current... 🙂 If for example I take 200VA transformers, is this "stable voltage" would be higher?
Thanks!
Okay I did some test with those little 48VA linked back to back, and mesured the AC voltage at the isolated side. When I power on the amp, the voltage is dropping from 118VAC to 97VAC. As the amp warm up the voltage slowly raise to become stable at 114VAC.
So I did the same test but this time by using the bigger 100VA transformers, and I see exactly the same voltage drop/raise, but instead ending at 116VAC stable voltage.
Seems good?.. Is this what you are talking about when you say that the filaments draw quite a lot of current... 🙂 If for example I take 200VA transformers, is this "stable voltage" would be higher?
Thanks!
185E230 80 VA $28 at partsconnexion
185D230 43 VA $21
The amp should consume 30 to 35 watts, but the power factor will be pretty bad making the 43 VA unit marginal.
I would go with the first of the two, the power factor could be improved significantly by full wave bridge rectification ahead of the 35W4.. The higher ripple frequency should result in more effective filtering.. The 35W4 should still drop substantial voltage across it, tune voltage with some additional series resistance if necessary - perhaps ahead of the bridge. (Do not add a cap right after the bridge!)
The only one (affordable) I knew is the 169RS (60VA), I can get it in a store close to my home, if I remember at $75, but since I have many of these little amps I don't want to spend to much money on this.
I saw the advise on the instructables web site, about the 35W4 half wave causing saturation in the transformer core... I think Im gonna give a try with those 48VA linked back to back and then using a full wave diodes bridge, I didn't though about dropping the voltage at the 35W4 plate by using a series resistor between the bridge and the 35W4 plate, good idea! ( is it where I shouldn't put a capacitor 😛 )! Still Im gonna feed the heaters from the AC (at the isolated side)... I think that way everything will be good and safe.
Okay I have everything to start the job, many thanks to all of you, and Merry Christmas! 😀 Hummmm one last thing... some of my amps are old radios rebuilt into guitar amp (completely rewired from schematic I did, all the radio stuff removed, only the frame and the woodbox was kept, finally they are just like the others but inside a radio box), so in some of them I will have to bolt the transformers on the frame because of the small room available, but if for example there's a distance between the 2 transformers, let's say 8 inches, should I twist the wires (the wires connecting the 2 transformers back to back), in order to avoid possible hum? Should I keep those wires away from particular part(s) inside the amp?
I saw the advise on the instructables web site, about the 35W4 half wave causing saturation in the transformer core... I think Im gonna give a try with those 48VA linked back to back and then using a full wave diodes bridge, I didn't though about dropping the voltage at the 35W4 plate by using a series resistor between the bridge and the 35W4 plate, good idea! ( is it where I shouldn't put a capacitor 😛 )! Still Im gonna feed the heaters from the AC (at the isolated side)... I think that way everything will be good and safe.
Okay I have everything to start the job, many thanks to all of you, and Merry Christmas! 😀 Hummmm one last thing... some of my amps are old radios rebuilt into guitar amp (completely rewired from schematic I did, all the radio stuff removed, only the frame and the woodbox was kept, finally they are just like the others but inside a radio box), so in some of them I will have to bolt the transformers on the frame because of the small room available, but if for example there's a distance between the 2 transformers, let's say 8 inches, should I twist the wires (the wires connecting the 2 transformers back to back), in order to avoid possible hum? Should I keep those wires away from particular part(s) inside the amp?
This is where a True RMS meter is needed. The RMS current can be several times the Average read by most inexpensive meters.
And no, the VA does not add when transformers are in series. Would you expect two fuses in series to add?
Heh! Nothing more logical, please forgive my novice level. This is almost the same as with series resistors versus paralleled... with the invaluable assistance of the people by here I can now figure out this thing 😀
For sure, a True RMS meter is on the purchase list.. If I could get my hand on something like an old fluke 8060A at a good price and in good condition, I would be very happy! But I'm not limited to the oldies, I'll keep my eyes open!
By the way, I'm gonna do some test very soon by using the bridge, let me come back with the result, or.. more questions! 🙄
For sure, a True RMS meter is on the purchase list.. If I could get my hand on something like an old fluke 8060A at a good price and in good condition, I would be very happy! But I'm not limited to the oldies, I'll keep my eyes open!
By the way, I'm gonna do some test very soon by using the bridge, let me come back with the result, or.. more questions! 🙄
You can get a KillaWatt from Amazon or Newegg for under $20. I use one of these on every amp and computer I build. It will measure the Watts, VA and power factor drawn from the line.
P3 P4400 Kill A Watt Electricity Usage Monitor - Low Temperature Alarms - Amazon.com
Yep I knew this thing, a very good tool to have in the shop to keep a look at the overall voltage/current. I actually have a couple of dmm (not true rms though) so it's not in the priority but I will get one as soon as I can!
Okay I did the test with the bridge but I tried with the easy way, I mean like this:
http://cdn.instructables.com/F6J/01RD/G1BBBZC0/F6J01RDG1BBBZC0.MEDIUM.jpg
Iso transformers and the bridge stay cool, the DC voltage (at the bridge +) fall a little low (from 127 VDC to 97 VDC) with the amp powered on, however the sound is different, less punchy I must say. Also, powered on DC I have a new background noise, kind of a "DZZZZZZ", not that loud but... I don't hear it when the amp is powered by AC, without any bridge.
Here's the bridge I used:
http://www.stationaryengineparts.com/images/P/Square_Bridge_Rectifier.jpg
I didn't tried with the heaters connected to AC (as my diagram at post #8) because I doubt that all the required re-wiring will worth. So I came back with no bridge, I mean, the iso transformers direct to the amp, like this
http://cdn.instructables.com/FRG/QZ6D/G145SWLD/FRGQZ6DG145SWLD.MEDIUM.jpg
but this time I monitored the transformers temperature (using my dmm temperature probe sticked on the first (back to back) transformer housing with some masking tape), and I'm pretty satisfied with the result. After 2h30 idle, the max temp I saw is 36 celcius.
As a conclusion, I really think those little transformers will do the job, without any bridge, they are able to handle the heat caused by the half wave for longer than my needs, maybe more, and my ears are telling me that powered by AC, those amps sound better, a little more punchy, and they are almost noise free, just as I like it. 🙂
By the way here they are:
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I should have mentioned clearly not to power the filaments from the ripply DC of the bridge, that is part of the reason it did not work that well; however if performance is adequate without it all the better. Voltages do seem a little low overall.
Yes I knew that it was not the best way.. I was just curious to do as the guy on the instructables tutorial. Maybe that's why the DC voltage is dropping that low...
The amp I am working on has a kind of "L" shape chassis, not much room for the transformers, I had to screw them on the side of the cab, so to avoid having too many cables...
http://leadbox.servehttp.com/diyaudio/garnet_iso1.jpg
http://leadbox.servehttp.com/diyaudio/garnet_iso2.jpg
However I have other amps that have more space on the chassis, and for the sake of learning I'm gonna try the correct DC way, with the heaters feeded AC, as shown in my diagram at post #8. I'll be back to share the result.. 🙂
The amp I am working on has a kind of "L" shape chassis, not much room for the transformers, I had to screw them on the side of the cab, so to avoid having too many cables...
http://leadbox.servehttp.com/diyaudio/garnet_iso1.jpg
http://leadbox.servehttp.com/diyaudio/garnet_iso2.jpg
However I have other amps that have more space on the chassis, and for the sake of learning I'm gonna try the correct DC way, with the heaters feeded AC, as shown in my diagram at post #8. I'll be back to share the result.. 🙂
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