So I'm looking for a company that will do REALLY cheap transformers. I'm building an amp for a friend with a $120 budget (!!). I know that the transformers you see in kits like the SEX amp and the K-12 kits are cheap. I'll likely need power transformers, opt's and chokes and I would also get a bunch of them like 10 each. Were should I be looking? I was on the Triad Magnetics website and they seem a likely candidate. Any suggestions?
OK, here is my power formula for cheap amps:
Allied 6K56VG power transformer, a real tube type power transformer made by Hammond. Can power small amps with a B+ of about 320 volts. Can use a tube rectifier. $41.33
Power formula for ultra cheap amps:
Triad N68-X isolation transformer wired in reverse to generate 230 VAC. Use a silicon diode bridge to get about 285 VDC. $11.20 from Mouser
Xicon filament transformer 41FD030 from Mouser $7 each.
Total power transformer cost $18.20. I don't think you can beat this for new (Chinese) parts!
Output transformers:
The original S.E.X. amp used surplus TV vertical output transformers. I used to make guitar amps from these back in the 60's. Hard to find today.
It is possible to use surplus power transformers and some toroidal transformers for P-P OPT's but serious experimentation is needed to find something that works. The OPT is the most critical device in the amplifier so use an Edcor OPT's, Available in SE or P-P flavors. $20 each.
Tubes:
6EM7, or 6DN7 for a 2 WPC SE amp, $5 each, one tube per channel.
A triode or UL wired 6LU8 or 6LR8 makes 3.5 watts in triode and 6 or 7 watts in UL. $6 each, one tube per channel.
Use a 12AT7 and two 6AQ5's in each channel for a 10 WPC P-P amp. These tubes can often be found surplus for cheap.
Two 6LU8's or 6LR8's (per channel) can be connected together to make a 15 WPC P-P amp.
There are schematics and ideas in this thread:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=60346&highlight=6lu8
The Allied transformer I mentioned is a drop in for Jan's 6LU8 amp. With the Edcor OPT's you are at $81.33 for the iron, $12 for the tubes. Can you scrounge up the other parts and make a cheap chassis for $25? If not use the ultra cheap formula and settle for a little less power.
Allied 6K56VG power transformer, a real tube type power transformer made by Hammond. Can power small amps with a B+ of about 320 volts. Can use a tube rectifier. $41.33
Power formula for ultra cheap amps:
Triad N68-X isolation transformer wired in reverse to generate 230 VAC. Use a silicon diode bridge to get about 285 VDC. $11.20 from Mouser
Xicon filament transformer 41FD030 from Mouser $7 each.
Total power transformer cost $18.20. I don't think you can beat this for new (Chinese) parts!
Output transformers:
The original S.E.X. amp used surplus TV vertical output transformers. I used to make guitar amps from these back in the 60's. Hard to find today.
It is possible to use surplus power transformers and some toroidal transformers for P-P OPT's but serious experimentation is needed to find something that works. The OPT is the most critical device in the amplifier so use an Edcor OPT's, Available in SE or P-P flavors. $20 each.
Tubes:
6EM7, or 6DN7 for a 2 WPC SE amp, $5 each, one tube per channel.
A triode or UL wired 6LU8 or 6LR8 makes 3.5 watts in triode and 6 or 7 watts in UL. $6 each, one tube per channel.
Use a 12AT7 and two 6AQ5's in each channel for a 10 WPC P-P amp. These tubes can often be found surplus for cheap.
Two 6LU8's or 6LR8's (per channel) can be connected together to make a 15 WPC P-P amp.
There are schematics and ideas in this thread:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=60346&highlight=6lu8
The Allied transformer I mentioned is a drop in for Jan's 6LU8 amp. With the Edcor OPT's you are at $81.33 for the iron, $12 for the tubes. Can you scrounge up the other parts and make a cheap chassis for $25? If not use the ultra cheap formula and settle for a little less power.
Toroidals are essential power EI's will sound rubbish.
I do not think you will hear a lot of difference between the brands, the only major issue to look out for is to try connecting them in both directions as you may get ringing in one direction and not in the other. Ringing is always a potential issue and a scope is a real useful tool to get the best from them. You should look to go for one with a VA rating of at least 4x the expected output wattage. I have used ones which are 20X the wattage without issue. You much eliminate DC imbalance if going PP and the easiest way to achieve this is seperate CCS in the tails of the output valves.
Low rp valves will work best as Inductance may be down on a EI.
If you can make them work they will definately sound cleaner than any cheap EI transformer but they won't automatically be any cheaper.
Shoog
The second statement is generally true. Most EI (conventional) power transformers have poor high frequency response, and lower efficiency than a real OPT. I used power transformers as OPT's for guitar amps routinely in the 60's and early 70's. They were essentially free at the time due to the easy availability of dead TV sets. I did find a few that could be used for what I considered to be HiFi at the time. I have an amp around here somewhere that was one of the prototypes for a possible SimpleP-P. I tested several ultra cheap OPT possibilities. I did find some surplus PC board mounted EI transformers that made decent OPT's at the 10 watt level. Frequency response was 30 Hz to 18 KHz at 3db. In general the smaller EI's have the best chance of working. They are more tolerant of mismatched tubes and current imbalance than toroids are. The chances of finding an EI power transformer useful for an SE OPT are slim due to core saturation because of the lack of a gap. It is possible to restack the core to create a gap, but it is probably not worth the effort.
I have experimented with power toroids also. Many have the frequency response capability for HiFi audio. The problems are saturation related. They are not gapped and many can not handle any current imbalance. Lack of primary inductance has already been mentioned. The primary was designed for 240 volts center tapped at 50 (60) Hz. They can generally handle about half of that at 25 Hz. This restricts operation to low power or low Rp tubes. A cathode follower output stage is useful here.
Most users try to find a power transformer with a 240 volt CT primary and then choose a secondary voltage to get the desired impedance ratio. This limits the available primary inductance. I offer another possibility that works better. Use a 240 to 240 volt isolation transformer. One that has four 120 volt windings is advantageous, as is 50 Hz capability. Wire the primaries and secondaries in series to create a single 480 volt winding. This is the primary it has at least twice the inductance of a 240 volt winding. You then wind a new secondary on the transformer. It doesn't take many turns and should be relatively easy to do. Some experimentation is needed to find the ideal number of turns. You can experiment with the order in which the 4 windings are connected to optimize the high frequency response.
Any toroid will be far more succeptable to saturation effects than an EI transformer of the same power rating. This is largely because a toroid has far less iron in it. This applies to toroids that were intended for OPT's as well. I have a pair of very large Plitron OPT's that are rated for 400 watts at 20 Hz. they are as large as a 1.5 kilowatt power toroid, yet they show noticible low frequency saturation effects with current imbalance of 20 mA. Often a method of controlling the imbalance is needed. Sometimes it is sufficient to choose a toroid that is larger than needed, although too large a transformer can lead to loss of high frequency response. A power toroid will not be usable for SE use, unless a parafeed arangement is used.
I got a box full of 200VA toroids with four 120 volt windings that were used in line conditioners. I have one running in a P-P amp using the paralleled sections of two 6080 cathode followers. The amp makes almost 30 watts and the frequency response is 24 Hz to 30 KHz at 20 watts. There is no saturation visible at 24 Hz and 20 watts. There is a balance pot that has been adjusted once since I found a stable 6080.
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