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Coupling Cap or Lundahl as interstage?

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#9
If using a coil (interstage transformer or driver plate loading choke) to drive the 300B, then make sure you . . . Do not use a magnetic steel chassis, power transformer and B+ choke signals can couple hum to the coil that drives the 300B. Do not use a magnetic steel chassis on a stereo amp, left and right coils signal may couple, reducing separation. Do not use a magnetic steel chassis even on a mono-block amp, the output transformer may couple into the interstage transformer. Even with an aluminum chassis or wood 'chassis' you have to pay attention to the angular relationship and spacing of all the magnetics in the amp (power transformer, filter choke, interstage transformer, driver plate load choke, and output transformer. An input transformer that drives the input/driver stage is even more sensitive to the magnetic fields of the other devices listed above.
 
#10
marantz said:
Coupling Cap or Lundahl as interstage.
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It depends. It looks like the circuit you posted doesn't make more than 6-7W before clipping. I certainly would not call that "reference" amplifier. Rather simplistic. Do not expect anything special....

In general my opinion is that if one has to make a 300B on tight budget a better idea is to forget about 300B.
 
#12
45 said:
In general my opinion is that if one has to make a 300B on tight budget a better idea is to forget about 300B.
Click to expand...
The same is true for audio magnetics: if you can't afford a good transformer then better to use no transformer.

Actually, in most cases no transformer is better anyway - except for those few instances when the particular features of a transformer are needed, such as impedance transformation or ground isolation.

As a general rule, almost any coupling capacitor will have lower distortion and wider bandwidth than almost any transformer. It will also probably pick up less hum and interference.
 
#14
Lundahl transformers are reputed to be excellent quality, and the price reflects this. You can also get suitable transformers from Hammond and possibly Jensen.
FWIW I've had decent performance using 25VA toroidal isolation transformers for interstage coupling (indeed, for 25$ the results were stunning), but I still prefer the simplicity, cost, and size of a capacitor.
 
#15
DF96 said:
The same is true for audio magnetics: if you can't afford a good transformer then better to use no transformer.

Actually, in most cases no transformer is better anyway - except for those few instances when the particular features of a transformer are needed, such as impedance transformation or ground isolation.

As a general rule, almost any coupling capacitor will have lower distortion and wider bandwidth than almost any transformer. It will also probably pick up less hum and interference.
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It's not just the transformer distortion which is fraction of that of the active stage if done right and only where there is no music basically (so in practice quite irrelevant) but how the amplifier behaves overall. There is never a solution better than another by default. Hence the capacitor is not better a priori as is it is true the contrary.

Going back on topic as I have built a RC coupled 300B for a friend that wanted it that way I know that there are a least two or three better solutions despite the fact that amplifier delivered 9W RMS with just 1% THD without any feedback.
 
#16
I'm not countering anyone, please.
I also use transformers and capacitors in my system. I have no exclusive opinion.
But I ask, why 7-8-12-15 meters of some plastic (or waxed paper, or...) would sound better than 30-40-50 meters of copper wire wound on permaloy or supermaloy or... ?
 
#17
Most better quality capacitors have extended foil construction, where the electrical length is no more than the physical length of the capacitor. The plates inside are metal foil, and the dielectric is polypropylene, Mylar, etc.
 
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#18
RajkoM said:
I'm not countering anyone, please.
I also use transformers and capacitors in my system. I have no exclusive opinion.
But I ask, why 7-8-12-15 meters of some plastic (or waxed paper, or...) would sound better than 30-40-50 meters of copper wire wound on permaloy or supermaloy or... ?
Click to expand...

The argument of distortion in reality is a weak argument because the transformer that introduces significant distortion ( below 50Hz and above 10 KHz only anyway) is the one you can't get rid of: the output transformer. As I said earlier this can be quite irrelevant if done right.

A small signal transformer or an interstage, if properly done and used, introduces really tiny amounts of distortion (in the -80 to -100 dB range...at full signal) and can also guarantee bandwidth way better than most RC coupled circuits. The latter has also to do with circuit itself and components. But that's it.
 
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