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PostPosted: October 4th, 2019, 9:53 am 
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Shashi,

I really recommend NOT building the DRD circuit as your first SET project. I believe a cap coupled SET is much better suited for your application. The DRD is better for a Full range amplifier but you really need aware of alot of things that add to the complexity of this otherwise simple circuit.

1. The B+ power supply needs to be around 600-650 VDC since you need around 350-400 VDC across the 300B and around 150-175V across the driver, plus whatever drop you have through the choke feeding the plate of the driver. This means you need to use series-connected power supply capacitors (with voltage balancing resistors) since electrolytic caps are not available more than 450V rated. You also need to use large value capacitors since you half the capacitance when you stack the caps, and you need really good filtering for low noise, since the design feed AC to the cathode through the ultrapath capacitor from B+. This design has very poor PSRR (power supply rejection ratio). USe of filter chokes are a must.
2. Output transformer needs to have an operating voltage of at least 650VDC. High pot rating at 2000V is a typically based on insulation breakdown, not operating voltage. Just a warning.
3. Need a high voltage ultrapath capacitor between the B+ and the 300B cathode. I have used a 100uF/450VAC PP in oil (motor-run), Jack Elliano started recommending using a high value high voltage electrolytic, but I am not thrilled with that approach. Now days, you can probably use a 800V or 1200V PP DC-Link capacitor. Still, it will not be cheap for the value you need.
4. Balancing the circuit. You cannot rely on the resistor values in the cathode shown on the schematic. You need to balance these to the driver tube and 300B tube that you use. I end up using a tapped (adjustable) wirewound resistor as a cathode resistor. It is very tweeky to both set the 300B operating point and the driver operating point, since both change when you make the adjustment. It has to be redone when you change tubes.

It is SO MUCH EASIER to use a capacitor coupled (or even transformer coupled) circuit. The power supply voltage is much more reasonable using available components. You can use a fixed resistor in the 300B cathode (with a lower voltage rated cathode bypass capacitor, compared with the Ultrapath cap). The driver tube operating point is also a set and forget once you select the appropriate plate and cathode resistors (and cathode bypass capacitor). It is far more likely to work on first turn-on. I already recommended that a cap coupled circuit is better suited for your bandwidth limited application anyway. And the lower voltage is safer to work with, including output transformer rating.

Regarding the output transformers, they should work for your bandwidth limited application, but much cheaper output transformers would work fine as well as a start. If you were considering them for full range at all, the inductance is a little on the low side, and the weight at 2kg each is pretty light. I would not expect these to have very good low frequency response. 35kHz at the top-end seems low if this value is based on -3dB (if based on -1dB, then OK). But otherwise, should work. I would put these under a cover, though, for protection.

David


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PostPosted: October 4th, 2019, 10:06 am 
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One other think I noticed about the transformer. It looks like it is cobbled together using 8 small C-cores. More typically, 4 C-cores are used, each core at full length. Not sure about the impact on performance using two c-cores butted up for length.

David


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PostPosted: October 4th, 2019, 1:06 pm 
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Many in the group had as our first projects a kit by Eico, Heath, Hafler or Dynaco. They aren't around anymore, but Bottlehead is.

As a learning experience, it is much less of a challenge to build a kit, then modify it as you gain knowledge and familiarity.

Just a thought.


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PostPosted: October 4th, 2019, 3:16 pm 
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Good suggestion. Also DecWare, but apparently no longer kits. Steve Deckerts' designs are well documented and supported online. He tends to use triode strapped pentodes (EL84) for low wattage SE designs. A 2W EL84 SE amp is perfect for a horn tweeter. Don't discount a triode strapped KT88 for an SE amp either, it is a pretty fine sounding tube. Nice not to have to worry about a filament supply on the first build.

David


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PostPosted: October 4th, 2019, 3:20 pm 
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David McGown wrote:
Good suggestion. Also DecWare, but apparently no longer kits. Steve Deckerts' designs are well documented and supported online. He tends to use triode strapped pentodes (EL84) for low wattage SE designs. A 2W EL84 SE amp is perfect for a horn tweeter. Don't discount a triode strapped KT88 for an SE amp either, it is a pretty fine sounding tube. Nice not to have to worry about a filament supply on the first build.

David

:character-oldtimer: 6CB5A is another good choice for a triode strapped amp. Cheap too...

https://vinylsavor.blogspot.com/2011/01 ... 6cb5a.html

Roscoe

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PostPosted: October 4th, 2019, 3:40 pm 
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Joined: January 14th, 2015, 11:15 pm
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One of the advantages of building a kit is that you learn by example about placement of parts and other techniques for building a reliable and hum free amp.

Another way is to watch this long series (12) of videos on how to build a KT88 single ended amp. I am not suggesting that you build it, but I think it is well worth watching. The presenter goes into detail about the choices he made and why he made them. I found the series very interesting.

https://www.youtube.com/watch?v=eNBgo9Md8IU&list=PLo7DGF0CxJGnfxQSKWOXtvhF23jLjEpOC

ray


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PostPosted: October 4th, 2019, 10:49 pm 
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Ok, I will look into cap couples amps.

Couple of basic Qs on theory of operations.

1. Since OPT secondaries are not grounded, how does damping work?
2. How to calculate damping factor with OPTs.

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PostPosted: October 5th, 2019, 2:46 am 
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Quote:
Couple of basic Qs on theory of operations.

1. Since OPT secondaries are not grounded, how does damping work?
2. How to calculate damping factor with OPTs.


The OPT secondary is most definitely grounded. The 0 Ohm or G tap on the OPT is ground, and should be tied to circuit ground.

The damping factor for a SET amp is typically low. Damping factor is a function of the output impedance of the amplifier (I think it is the ratio of the speaker+cable impedance over the amplifier output impedance). No feedback SET amps have a high output impedance. Feedback PP amps have a lower output impedance, and solid state amps have very low output impedance. Tube amps, particularily SETs, typically have a worst time with low impedance speakers (4 ohms), since this lowers the damping factor, whereas 16 ohm speakers are a much better match. But this is of little concern if not driving a bass driver (since it is indicative on the control on the driver), and should not be an issue at all with horn loaded midranges.

Do not worry about damping factor for your application and do not overthink the design. If you are concerned about specifications, no-feedback, Class A SET amplifiers suck. No one builds a SET amplifier to achieve high damping factor, low THD, or low S/N ratio. They build them for the sound that a minimalist circuit using very linear output tubes into an output transformer that is operating in the linear portion of the BH curve at very low power can provide.

David


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PostPosted: October 5th, 2019, 8:11 am 
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To follow up later in the morning, just remembering you are using the 300B amp in a midbass horn (100-500 Hz), I believe I had slightly better results using a PP2A3 amp than a set amp on my setup. Nevertheless, it did work fine. Roscoe has run SETs on big JBL/Altec horns with no issue at all. So I would not worry at all.

David


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