Parallel feed (choke load) in output stage

[rcavictim]

Originally posted by Shoog
"Should I load the secondary winding of my existing output transformer of my 45 SE amp with a dummy 8 ohms resistor if i were to wire up another parafeed optx in the amp? Is it harmful to just leave the secondary open? "

Yes - voltages in the secondary of an unloaded transformer can spike very high - infact to damaging levels, which could destroy your transformers. However loading your transformer with a 8R resistor will halve your availble power output. Think about a 22R load.

Shoog

I have to disagree with Shoog's advice here. What he says is absolutely true of a conventional audio output xfmr used in the final output stage and run without a secondary load, however if the audio output xfmer is being employed as a output stage plate choke then I feel there is no need or good reason to place a resistive load on any secondary winding that will represent a stepped down voltage rating to the high Z primary winding that is being used. There will NOT be a wild uncontrolled rise in EMF across this transformer being used as a plate choke provided the parafeed output transformer that is capacitor coupled to the output tube anode and loaded to a loudspeaker or dummy load is present in the circuit.

If you operate the parafeed amp with no loudspeaker or output load then the same risks are present and in this case both the plate choke iron and the DC blocked output iron could see damaging overvoltage and insulation breakdown.

If you are employing a gapped core SE audio output xfmr as a plate choke and unconnected wires really bug you, you can get really creative and wire the secondary (speaker side) in series with the primary winding. In phase this will increase the available L of this hunk of iron which might help LF response of the amp as long as it doesn't saturate (which this will make it more susceptible to do at lower current) or in anti phase to lower the L but increase current handling before saturation.

[Shoog]

It wasn't clear that the existing transformer was going to be used as a plate choke. If so then you are quite correct. I would be interested to see if this arrangement actually works. I have a few doubts since otherwise I would imagine more parafeed people trying this as a cheap alternative to the cost of parafeed chokes which tend to be very expensive.

Shoog

[rcavictim]

I am finding many types of power supply chokes and other transformers can serve adequately as output stage plate chokes. The lower the Plate Z the bigger your list of acceptable alternatives.

My breadboarded parallel SET 6080 x four bottles (8 triodes) amplifier in ultra-parafeed requires a plate choke that can handle 400-500 mA. I am using a HV winding of a completely unmodified microwave oven xfmer as the plate choke. It works great!

My latest experimental amp, a highly successful 1819 in triode driver stage, interstage xfmer, 809 power triode output with ultra-parafeed output coupling to P-P Hammond output iron employs a gapped core, late 1940's RCA television set vertical output xfmer as the plate choke.
It yields 22 henries and handles 50 mA plate current with ease. There is nothing in the hammond catalogue that could do this job in the same space and using just one core.

It is important to use the outside of the winding of your transformer or choke at the plate/anode end. This side has lower self shunt capacitance to the grounded core of the device. Hook the innermost end of the winding to the B+ .

[pedroskova]

I'm using the primary of a line output transformer to plate load my 12B4A preamp. It was originally a parafeed setup using a power toroid as OPT, but it's now just cap-coupled...which was an improvement. I'm also using plate to grid feedback ala Mr.Broskie.

The "choke" loading sounds better than a setup with an IXYS CCS loading the plate...don't know why, and don't care at this point.

[Shoog]

Thats all very interesting. When I started out on parafeed I was going to use some monster power supply chokes as the parafeed chokes but was advised that they wouldn't be suitable. As always the adviser couldn't say why exactly. In the end I built KT88 CCS which gave me a quasi SEPP. Sounds very good and i'am using it now with toroidals as output transformers.
However I have been toying with the idea of using some PL36's to make a cheap little amp. The possability of using microwave oven transformers (which I can get loads of) as chokes sounds very appealing. Do you get any heat issues, do they respond well below 50hz

Shoog

[rcavictim]

Originally posted by Shoog
The possability of using microwave oven transformers (which I can get loads of) as chokes sounds very appealing. Do you get any heat issues, do they respond well below 50hz

Shoog

No. Yes.

[Shoog]

Great, i'll store that away in my useful info section.

Shoog

[j45yip]

Thanks a lot for the contributions, guys.

ok, let's see if i get it right. in short, it doesn't matter if the standard se optx is working by itself or whether it's wired to work as a plate choke with a parafeed optx connected, there's no danger of shorting out due to high voltage as long as one of the secondary winding from either optx is connected to the speaker load.

will it help if both the secondary winding are grounded?