I know this has been discussed before but I can't find it. The issue is figuring out the impedance ratio of a 70V distribution transformer for use as PP OPT. This guy tries to explain it but it is not making sense to me. Sounds like he is getting per phase rather than P-P. Also not sure how he can get pri Z only from rated voltage with no reference to secondary Z.
Mini-Tube Audio Amplifier - Part 2
mike
Mini-Tube Audio Amplifier - Part 2
mike
what output power are you looking at?
say you have a power amp that can output 20 watts at 8 ohms.
now this is 12.6 volts....
so then turns ratio is 70/12.6 or 5.55,
impedance ratio is then 5.55^2 = 30.8
say you have a power amp that can output 20 watts at 8 ohms.
now this is 12.6 volts....
so then turns ratio is 70/12.6 or 5.55,
impedance ratio is then 5.55^2 = 30.8
you only need to know the principles behind it in order to get around....
i design and build trafos for my projects...
for distribution, you also need to know your load and
what distances are involved...
100v, 200v, etc can be designed and built...
i design and build trafos for my projects...
for distribution, you also need to know your load and
what distances are involved...
100v, 200v, etc can be designed and built...
It's pretty simple: For a 70V matching xfmr with nominal secondary load, primary Z is simply 5000 (70.7 squared) divided by the power rating of the primary tap in question. Ferinstance, the 1W primary tap puts a 5K load on the distribution line. For 100V xfmrs, primary Z is 10000 divided by watts.
The Robert Weaver article uses a 'cheap' line transformer. Some of these readily available transformers have a primary tapping arrangement that allows a 'centre' tap to have an equal number of turns to one end, and to the other end, of the primary winding - and so can be used in a push-pull output stage.
They can make pretty useful guitar amp output transformers - and there is quite a following for them - for example at
Cheap Output Transformers and search within
http://www.guitargear.net.au/discussion/index.php#c4
They can make pretty useful guitar amp output transformers - and there is quite a following for them - for example at
Cheap Output Transformers and search within
http://www.guitargear.net.au/discussion/index.php#c4
Here's another project using the yet mentioned Altronics M-1115 transformer: EL36: Applications in Audio
This is a 100V transformer rated 10W.
You can even use it for UL config.
They sell them for 5€, not bad for a 10W PP.
Mona
You can even use it for UL config.
They sell them for 5€, not bad for a 10W PP.
Mona
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Has anyone tried to use a 100V transformer as low power single ended OPT? I have two OEP D29A100 transformers rated 30W with 4-8-16 ohm secondary taps. The weight is 1 Kg each, significantly more than a similarly priced SE transformer. I measured the primary:secondary ratio at 50 Hz, and the actual primary impedence between common and 1W taps is about 11K with a 8ohm load on the 8ohm secondary tap. It should be fine for 6AK6. This tube only needs 20mA anode current. Will this be still enough to saturate the ungapped core, or decent performance may be expected? Unfortunately there is no useful center tap on this transformer, so PP operation is ruled out.
Not an OT, but parafeed-coupled impedance matching in OTL amps. I used them. For an OT they have pretty low inductance.
Link to Grant Wills testing a variety of transformers for SE output stage use:
Background to a new SE amplifier
In Australia, many power transformers for 12V 50W halogen downlighting applications are being replaced by LED lighting, and get binned or sold off cheaply. The 240V primary format, especially with 0-120-240 taps, are a boon for guitar amp re-use as output transformers.
If your willing to use CCS biasing you can get great results with a Triad VPT12 series toroidal.
I've built an amp with 6P1P with an B+ of 300V, and 56ma set by an LM317 using cathode coupling and big bypass caps using the VPT12-2080 25VA coil. The primary measures at about 150H, and when I connect 6 ohm speakers to 6V I get about 8k plate to plate. I can 1/4 that by using the 12V set up instead. The bass is tight, the freq response goes to about 0.5hz, and the highs are clean.
I've built an amp with 6P1P with an B+ of 300V, and 56ma set by an LM317 using cathode coupling and big bypass caps using the VPT12-2080 25VA coil. The primary measures at about 150H, and when I connect 6 ohm speakers to 6V I get about 8k plate to plate. I can 1/4 that by using the 12V set up instead. The bass is tight, the freq response goes to about 0.5hz, and the highs are clean.
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For modelling purposes has anyone measured the primary L of a 70V or 100V tranny? Estimate?
mike
mike
In my experiments the not all 0-120-240 transformers are actually centre tapped. The last one I tested for this purpose was 30 ohm 0-120 and 50 ohm 120-240 creating a nasty misbalance. I suggest specifically looking for ones with dual primary.
The primary L will vary depending on the load. Easiest thing to do is give it a known voltage on the primary, measure the voltage on the secondary, divide the primary voltage by the secondary voltage and square it.
EG
230V:6V = a winding ratio of 38.3:1
38.3^2 = an impedance ratio of 1466:1, roughly 12k:8r
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