Power transformer as Parafeed output?

[TonyTecson]

What about those 100V line distribution transformers. What can be done with them. Can they be used in as parafeed transformers or similar applications?

do you have them on hand? what are the taps available? is that autotransformer type?

we can determine ratios if can give us the data on those 100 volt line transformers.

 

[soundbrigade]

Darn, have no data. Have no LC-meter, have no ...
They have separate pri/sec-windings. Measuring DC-resistans give nothing.
They are marked: 100V 8ohm, and have 5 sec taps 0, 10W, 2.5W, 1,25W.

 

[316a]

Darn, have no data. Have no LC-meter, have no ...
They have separate pri/sec-windings. Measuring DC-resistans give nothing.
They are marked: 100V 8ohm, and have 5 sec taps 0, 10W, 2.5W, 1,25W.

Why bother when a toroid will have better performance ? Those 70/100V transformers don't usually have enough inductance to give any sort of decent bass . In fact why parafeed at all ? A toroid can be used SE if there is a tertiary winding to offset the core DC , then the parafeed cap and anode load choke (or CCS) are redundant . Below is a schematic of a recent amp which used a 30VA 230:9+9 toroid where one of the secondaries is used to (nearly) 100% offset the anode current . -3dB points at 15Hz and 47khz with a 280mH choke (not 30mH as in the schematic) . Finding suitable offset chokes is the main problem , although if there is enough compliance I'm sure a CCS could be used

An externally hosted image should be here but it was not working when we last tested it.

cheers

316A

 

[316a]

So your conclusion is that smaller (to a point) would be better.

Shoog

Smaller units have higher turns per volt and hence more inductance . 30VA came out slightly better than a 50VA unit in my case . The smaller 15VA toroids , which also had potted cores measured worse .

cheers

316a

 

[revintage]

Darn, have no data. Have no LC-meter, have no ...
They have separate pri/sec-windings. Measuring DC-resistans give nothing.
They are marked: 100V 8ohm, and have 5 sec taps 0, 10W, 2.5W, 1,25W.

Hi Magnus,

100V line, 10W, 8ohm is around 1 kohm primary (winding ratio ca 11:1).
It is about time you make something out of those 12B4s, maybe by parallelling a few and parafeed them into the line formers!
See you again at "hifi-forum"!

 

[soundbrigade]

Thanks Lars. See you!

 

[TonyTecson]

They are marked: 100V 8ohm, and have 5 sec taps 0, 10W, 2.5W, 1,25W.

okey, so at tap 1.25W and 8ohms, we have a transformer good for 3.5k to 8ohms....

 

[soundbrigade]

I may consider use the transformers for a headphone amp I need no power at all ... hardly.

 

[revintage]

okey, so at tap 1.25W and 8ohms, we have a transformer good for 3.5k to 8ohms....

Nope,
In my calculations the winding-ratio is 31,6:1 which corresponds to 8k:8ohm impedance-ratio.

 

[TonyTecson]

yes i believe so, the primary will never see 100volts so response should be okey...

 

[TonyTecson]

Nope,
In my calculations the winding-ratio is 31,6:1 which corresponds to 8k:8ohm impedance-ratio.

ooppps, my bad, my finger slipped, yes you are correct.....

 

[yagoolar]

Choke in the anode would be better. Finding the right choke to set the bias would difficult, or you would need supplementary fixed bias to set it right. Also you would need to bypass the choke for AC.

I have got my 807 parafeed setup with Microwave Oven Transformers as the anode choke load. Works well - but not as good as a CCS.

Shoog

If I pass the parafeed choke by a cap, I will short signal to the ground, right or at the worst build a generator?

--Yag

 

[Shoog]

If I pass the parafeed choke by a cap, I will short signal to the ground, right or at the worst build a generator?

--Yag

The original question was about placing a choke in the cathode for bias purposes. I think this would be difficult to set the correct bias point and so I wouldn't recommend it at all. I think you would need a CCS as well to fix the current through the choke, and if you do that you don't need the choke. If you could get that to work you could create a cathode follower by placing the output transformer in parallel with the choke, but with its own parafeed cap. Overall I think this is just a bad wasteful idea.

If using a choke in the anode then bias is set by whatever you place in the cathode.

Shoog

 

[yagoolar]

The original question was about placing a choke in the cathode for bias purposes. I think this would be difficult to set the correct bias point and so I wouldn't recommend it at all. I think you would need a CCS as well to fix the current through the choke, and if you do that you don't need the choke. If you could get that to work you could create a cathode follower by placing the output transformer in parallel with the choke, but with its own parafeed cap. Overall I think this is just a bad wasteful idea.

If using a choke in the anode then bias is set by whatever you place in the cathode.

Shoog

Nope. The original question was about impedance matching. Nothing about biasing. The main reason behind using a choke in the cathode in parafeed xformerless was low voltage at the output. Just in case something goes wrong with a cap.

I actually made it in headphone amp based on 6C45PI as an output tube.

--Yag

 

[Shoog]

I actually made it in headphone amp based on 6C45PI as an output tube.

How did you set your bias point ?

Shoog

 

[yagoolar]

How did you set your bias point ?

Shoog

That is what I have done. See attach.

--Yag

 

[Shoog]

Seems like a good solution for a headphone amp.
How does it sound, and does it sound better than a simple cathode resistor.

Shoog

 

[yagoolar]

Seems like a good solution for a headphone amp.
How does it sound, and does it sound better than a simple cathode resistor.

Shoog

Weeell. The performance was good for Sennheiser SL450 but Philips HP1000 (Zo=32ohm) was not satisfactory. However I think I made a wiring fault or there is a problem with the phones. Have not checked yet. As I remember details are better. Need to do some more work on that.

 

[Pyka]

Choke in the anode would be better. Finding the right choke to set the bias would difficult, or you would need supplementary fixed bias to set it right. Also you would need to bypass the choke for AC.

I have got my 807 parafeed setup with Microwave Oven Transformers as the anode choke load. Works well - but not as good as a CCS.

Shoog

It is very creative to use the transformer of the microwave oven as the anode choke load of the 807. How to connect, use only the primary winding?

 

[Shoog]

I don't think a MOT would make a particularly good plate choke, inadeuate inductance and no attention to parasitics. That comment was from way back when I was messing around with the ideas.

Shoog