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In Reply to: Re: Impression on Resistor loaded parafeed 2A3 posted by RayP on March 27, 2007 at 06:36:04:
Hi ray,
Very intriguing picture like the three faces looking at amp :-), it look like a pushpull or paralleled single? Change of the brightenss means the current throught the load is not constant I think. You know there's a transistor version of light bulb load by Nelson Pass. Either way I am curious of the sound.
Follow Ups:
It's a single ended parafeed design like yours with cheap Radio Shack line transformers wired as autoformers. The input tubes are 6C45PIs and the out put tubes are 829Bs wired as triodes.I taken it to a few of the meets that we hold a few times a year in the Philadelphia / Washington area and it usually draws good comments. It's easily the best sounding thing I have built and it is the clarity which is a step above any other amps I have tried to build. There may be other causes of this since this was my first amp with extensive use of motor run caps in the power supply and of course umpteen other variables.
I had read the Nelson Pass article and had thought about trying to build it, but I was heavily into tubes by then. But I must admit his article was the inspiration. I had also read Stve Bench's stuff on the resistor load and that was part of the inspiration as well.
I've thought about it for quite a while now and I still have no idea why resistor loading might be a good idea.
ray
Hi ray,Today I replaced the parafeed cap and the soundis getting better. Yes this is also the best sounding amp I have ever built. The clarity shines across the audio band and it makes me stick to the listening position.
If possible could you tell me more on the line transformer that use for autoformer? I want to try this circuit with 300B and am at a loss what to use. I look at the other pictures of the library gathering and it must have been a pleasant meeting I guess.
Radio Shack was selling these 70 volt 10 watt line transformers until about a year ago. They were well under $10 each.The input side of the transformer has taps for 10, 5, 2.5, 1.25 and .625 watts. There is also a tap marked C.
The output (speaker) side has taps for 16, 8, 4 ohms and a tap marked C.
Used as an ordinary parafeed transformer and depending on the turns ratio required, I would connect the output capacitor to the .625 tap and the C tap to ground. Then the speaker to the appropriate taps on the output side.
As an autoformer, instead of connecting the C tap on the input side to ground, connect it to 16, 8, or 4 tap that you are using to connect to the speaker. The C tap on the output side is connected to ground.
Of course for safety reasons, it is essential that the output capacitor be situated between B+ and the transfomer. Thse are cheap transformers and I would not trust them with the cap between the transformer and ground.
This autoformer thing is easy to try. You may like it or maybe not.
Our group has had a wonderful time with our meetings. It really has encouraged attendees to start building, share ideas and be prepared to experiment with different ideas. There has been some wild stuff appear over time. Even the failures are interesting. If you don't belong to such a group, I would encourage you to form one.
Thank you for the info. I do not have any place to source those line tranformers but have one local winder who can do anything for me, though the result is all my responsibility. What I know now is that the coil gauge should be somewhat close to the secondary of normal OPT, resulting in a big size like yours.I'd love to join your group if possible. Tube DIYers in my country are normally obsessed with Marantz 7 ciruit or something that is already 'proven'.
Maybe resistor loaded autoformer parafeed will benefit more with DC coupled circuit, which is in my mind at the moment.
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