Autoformer output stage

[Bas Horneman]

Does anyone know of Rainier Zur Linde's autoformer output stage? Apparently it is in one of his books..is it a pre-out? Or a power output stage?

Maybe other schematics with this concept?

Thanks in advance.

[Bas Horneman]

I can get that autoformers for a good price..hence my interest in the circuit..
The taps on the autoformer are VA/CT/VAT/SCH

[Gluca]

of using an autoformer vs transformer at the output?

Gianluca

[rcavictim]

If the autoformer is in the cathode output of a power tube then this would work very well as the speaker output could reference circuit ground. In a conventional plate load circuit you would have system B+ on the speaker wire and that would be hazardous. I guess you could go to a lot of trouble and ground the positive terminal of the B+ supply but then the ground sleeve of the amplifier's RCA input jack would be at several hundred volts minus to ground.

Of course you could parafeed into the autoxfmer from the o/p tube anode with a coupling capacitor and have the loudspeaker lead ground referenced, but now you need a plate choke for the DC supply to the output tube anode.

Rob

[Fuling]

In my experience autoformers works well in output stages.
I´ve prototyped a PL504 SE amp with DIY OPT´s where about 25% of the primary winding is between cathode and ground, and a part of those 25%´s are wound with thicker wire and acts as secondary winding.

[Gluca]

mmm Interesting. Would you post a schem just to make things clear to me?!

I can imagine the use of an autoformer in a preamp out but ... errr ... I can't figure out how in a power amp can be.

g

[dave slagle]

Originally posted by Fuling
I´ve prototyped a PL504 SE amp with DIY OPT´s where about 25% of the primary winding is between cathode and ground, and a part of those 25%´s are wound with thicker wire and acts as secondary winding.

how did you keep the DC out of the speaker?

I did a PP amp this way which took care of the DC, but i have always feared even with a really thick secondary i would still get some DC on the speaker if it tried this with SE. Maybe a ma or 2 dc could be a good thing :-)

dave

[kevinkr]

I suspect the benefit diminishes greatly once the transformation ratio starts to approach that required for a typical plate loaded application, however for cathode followers this could work nicely with the fairly low ratios usually required.

You could for example use a current source to bias something like a 6C33 and capacitively couple to an autoformer having a winding ratio of something like 4:1 so that it presents about a 1K load to the cathode with an 8 ohm speaker. The reduction in windings required should result in tighter coupling and lower leakage inductance than with a conventional transformer - I don't think this relationship holds well at higher ratios and I can't see the point frankly of doing anything much higher than the cited 4:1 - leakage inductances, stray capacitances and winding interleaving requirements would reduce the benefit considerably.

Just my opinion. And hmm makes me think I might want to try the above...

Kevin

[rcavictim]

Originally posted by kevinkr
I suspect the benefit diminishes greatly once the transformation ratio starts to approach that required for a typical plate loaded application, however for cathode followers this could work nicely with the fairly low ratios usually required.

You could for example use a current source to bias something like a 6C33 and capacitively couple to an autoformer having a winding ratio of something like 4:1 so that it presents about a 1K load to the cathode with an 8 ohm speaker. The reduction in windings required should result in tighter coupling and lower leakage inductance than with a conventional transformer - I don't think this relationship holds well at higher ratios and I can't see the point frankly of doing anything much higher than the cited 4:1 - leakage inductances, stray capacitances and winding interleaving requirements would reduce the benefit considerably.

Just my opinion. And hmm makes me think I might want to try the above...

Kevin

Kevin,

My post was more theoretical in nature and in the case of high tranformation ratio as usually required in plate load I must agree with you. You correctly address the nuances of practical output tranformer design reality.

There are a few low Mu triodes, especially when uses in parallell that can be practical exceptions. I'm thinking 6C33, 6AS7/6080, and that expensive other one, the 6528.

Rob

[2wo]

Ray, one of the DC Bottleheads built a 829 amp, using a 70v line transformer wired as an autoformer. Parafeed, but without plate chokes, (he uses light bulbs).

Ray probably watches this forum, so I don’t want to steal his thunder.

So lets just say his amp sounds great…John