Coupling transformers, a characteristic feature of extremely old circuits, will not distort when a magnetic material without magnetic hysteresis is found. Till now, all core materials exhibit magnetic hysteresis and this is NOT linear. All transformers require a magnetising current component whose waveform is anything, but sinusoidal.
IIRC, one of the older US brands UTC published -3dB at 20Hz, 30, or 40Hz to 20kHz. The higher cutoffs were on their mini sizes and full bandwidth on their medium to large core ones. They guaranteed those -3dB points at their rated levels before saturation onset. It would not surprise me if there were English made ones (Partridge, Savage, Woden, etc.) that met or exceeded in the same way. Years back I watched sweeps on various UTCs that clocked 10hz-50kHz before rolloffs. (Driven at milliwatt levels though.) Squarewave response superb as well. This is not uncommon from high-fidelity transformers 50 years ago. Some of them had price tags in the week's pay realm too.
(edit: If I dig I think I archived a rec.audio.tubes thread on this from USENET days, identical content.)
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santitrucco,
Your post # 19,
I know you did a simplified schematic. But:
You need bias (like self bias for example) for the input tube/driver tube.
You need a load resistor, choke, or current source for the input tube/driver tube plate.
You need a grid resistor from the 300B grid to ground.
You need bias for the 300B.
You need a plate load for the 300B, connecting the output transformer from B+ to ground is not a plate load for the 300B.
I just hope nobody sees how simple this circuit is, and tries to build it like in your schematic.
Your post # 19,
I know you did a simplified schematic. But:
You need bias (like self bias for example) for the input tube/driver tube.
You need a load resistor, choke, or current source for the input tube/driver tube plate.
You need a grid resistor from the 300B grid to ground.
You need bias for the 300B.
You need a plate load for the 300B, connecting the output transformer from B+ to ground is not a plate load for the 300B.
I just hope nobody sees how simple this circuit is, and tries to build it like in your schematic.
For a #45 driving a 300B, voltage gain will be a big issue. You get a mu of 3.5 with a #45, so under ideal circumstances you would need 14V RMS fed to the input of such an amplifier. That's not realistic.
You can use a 1:5 input transformer and a 1:2 interstage transformer to make something like that workable.
You can use a 1:5 input transformer and a 1:2 interstage transformer to make something like that workable.
I have used Interstage transformers and RC coupling for single ended Hi Fi amplifiers.
I liked them both.
I have used Interstage transformers, center tapped autotransformers, and RC coupling for Hi Fi push pull amplifiers.
I liked them all.
But I do have preferences.
Generalizations:
Interstage and Auto transformers:
Cost.
Takes up space.
Magnetic device, so picks up hum.
Steel Chassis is a no-no for Interstage transformers.
Weight.
Not as clean of a square wave (depends on the circuit, loading, and transformer; many resonate outside of the audio band, so no problem if you do not use negative feedback.
Push pull to push pull can be good; and single ended to single ended can be good (But single ended to push pull is the worst).
Can saturate.
Can have distortion.
Step Up interstage transformers often have more problems than 1:1.
RC coupling:
Can have wide frequency range.
Can have good square wave.
Can have low distortion.
Takes up less space.
Less Weight.
Can be less expensive for the performance you get versus interstage / auto transformer.
I do not use interstage transformers on my Hi Fi amplifiers any more.
I use RC coupling on all my single ended, and all my push pull amplifiers (all Hi Fi).
Your Mileage May Vary.
If you want to build a guitar amplifier, that might be another matter.
Someone else can give his opinion.
I liked them both.
I have used Interstage transformers, center tapped autotransformers, and RC coupling for Hi Fi push pull amplifiers.
I liked them all.
But I do have preferences.
Generalizations:
Interstage and Auto transformers:
Cost.
Takes up space.
Magnetic device, so picks up hum.
Steel Chassis is a no-no for Interstage transformers.
Weight.
Not as clean of a square wave (depends on the circuit, loading, and transformer; many resonate outside of the audio band, so no problem if you do not use negative feedback.
Push pull to push pull can be good; and single ended to single ended can be good (But single ended to push pull is the worst).
Can saturate.
Can have distortion.
Step Up interstage transformers often have more problems than 1:1.
RC coupling:
Can have wide frequency range.
Can have good square wave.
Can have low distortion.
Takes up less space.
Less Weight.
Can be less expensive for the performance you get versus interstage / auto transformer.
I do not use interstage transformers on my Hi Fi amplifiers any more.
I use RC coupling on all my single ended, and all my push pull amplifiers (all Hi Fi).
Your Mileage May Vary.
If you want to build a guitar amplifier, that might be another matter.
Someone else can give his opinion.
In this case #45 is unnecessary.
Trioded D3a has enough gain to drive 300B.
If you want to use A2 region, cathode or source follower is practicable.
Use the first tube amplification, would be possible use the DHT 45 driving the 300B.
The problem with D3A->45->300B is that the D3A is more capable of driving a 300B than the 45 is, and the extra gain from the 45 isn't helpful.
Too much gain, added noise from the #45, worse frequency response from the #45 driving the 300B instead of the D3A, more complication in the design to have the extra filament supply for the #45, more heat in the chassis for #45 cathode bias resistor, more current drawn through the filter components in the HV power supply.
I'm sure there are more.
I'm sure there are more.
Thank you audiowize, the D3A is enough , then i prefer this configuration.
D3A 300B.
Not to mention that any added complexity in the audio path usually does more harm than good to the signal integrity itself.
Im not sure if anyone has mentioned it, but there are some rareish tubes out there where RC coupling doesn't actually work out all that well.
The type 50 is a perfect example. It has a maximum grid leak resistance of 10K. There are plenty of ways to deal with that, but an interstage transformer is a popular solution.
The type 50 is a perfect example. It has a maximum grid leak resistance of 10K. There are plenty of ways to deal with that, but an interstage transformer is a popular solution.
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