DIY Audio Projects Forum

300B SET Design Project
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Author:  mwhouston [ 19 Nov 2010, 02:01 ]
Post subject:  Re: 300B SET Design Project

Could we also choke the filament PS.

Author:  Suncalc [ 19 Nov 2010, 11:45 ]
Post subject:  Re: 300B SET Design Project

Putting chokes in the filter is one option. The second order pole will definitely make the target ripple easier to achieve. However, the chokes are BIG. I was thinking a Hammond 193U 0.2H (1.7 Ohms series resistance) followed by a 10,000uF cap. (alpha =788). This would probably give us the filtering required.

Author:  Suncalc [ 19 Nov 2010, 20:18 ]
Post subject:  Re: 300B SET Design Project

Could we also choke the filament PS.

Here is a preliminary design that should give the filtering required to get down to 2.2mV. It still uses the diode bridge but all else is simple passive elements. Very simple. There are a couple of potential drawbacks. First the cost is up there a little bit. I priced the parts at about $80 (USD) per channel. I'm not sure how that equates to your side of the world at current exchange rates. Secondly, the chokes are fairly large. Almost as large as the 193M 10H choke used in the HV supply.
Schematic Filament Passive.png

If you'd like to go this way, let me know and I'll run the numbers and post a final design with specs.

Author:  Gio [ 22 Nov 2010, 19:48 ]
Post subject:  Re: 300B SET Design Project

My :2c: I would use regulators for the heater supply. The chokes will add so much bulk and weight. Plus the money you save is better invested in the amp circuit or HT supply caps.


Author:  mwhouston [ 03 Dec 2010, 03:19 ]
Post subject:  Re: 300B SET Design Project

Just orderd the regs - $AU48.00.

Matt: Physically how big are the HS for the Regs?

Will any suitable rec. bridge do for the filaments?

Author:  Suncalc [ 03 Dec 2010, 11:42 ]
Post subject:  Re: 300B SET Design Project

The heat sink I reference is 38.1mm (1.5") tall, 41.91mm (1.65") wide, and 25.4mm (1.0") thick. Here is the complete mechanical drawing from the manufacturer.

Just about any bridge rectifier should work so long as the Vf is less then about 1.1v and it can thermally dissipate the heat. I just chose one that was available, fairly cheap, and had a hole for heat sink mounting.

Author:  sampleaccurate [ 03 Dec 2010, 14:26 ]
Post subject:  Re: 300B SET Design Project

Suncalc wrote:
Just about any bridge rectifier should work so long as the Vf is less then about 1.1v and it can thermally dissipate the heat.

Wouldn't "slow" standard rectifier diodes create more EMI than fast recovery diodes, even in a heater supply? If the heater supply is in a separate case I suppose it would make no difference. Just curious, since so many people insist on nothing but fast recovery rectifier diodes in all power supplies.

Author:  sampleaccurate [ 03 Dec 2010, 14:43 ]
Post subject:  Re: 300B SET Design Project

I read a website (can't find it right now) that made big claims about the impedance of the filament supply for directly heated cathodes. Basically this guy was saying that the ends of the heater (cathode) filament in DHTs should NOT be AC bypassed with a normal DC voltage supply or capacitors. In other words, even for grounded cathodes, the heater supply should provide power (in this case DC) to the filament while at the same time looking as close to an open circuit as possilbe on BOTH sides of the filament supply to any AC signal.

A CCS as a heater supply would do no good (in his opinion) since only the end of the heater with the CCS would "look" like a high impedance to the signal. The other end would offer a low impedance to ground. He came up with a circuit using a CCS on one side and some circuitry on the other side that effectively allowed DC to pass and blocked AC, without chokes and very inexpensively. He gave some reasons why this was desireable but I can't remember what they were. It was something to do with the signal potential that develops across the cathode and that shorting it (so the cathode has no AC voltage across it whatsoever - a "unipotential cathode" DHT?) causes a decline in sound quality for some reason, AGAIN that I can't remember.

If his "theory" is correct it would indicate the best place for chokes would be on the heater side of the caps with no caps directly across the heater to short the AC. I wish I could find the article - I'm only giving sketchy details. He may be full of it but I found it interesting.

Thinking about it, it makes some sense. If the cathode is powered by DC, one end will hog a little more signal current than the other. NEGLECTING the DC completely, it seems to me there would be a slight imbalance in the signal voltage that is seen at each end of the cathode. A cap or low impedance DC supply across this will create a small circulating current composed of the signal being amplified, would it not? How this translates into the sound of the amp I have NO IDEA. I just found it interesting.

Author:  KrammeAcoustics [ 05 Dec 2010, 08:21 ]
Post subject:  Re: 300B SET Design Project

mwhouston wrote:
I was just zapped by the primary of the Hannond OPT. I use an analogue multi-meter. While measuring the primary DC resistance of the Hammond OPT I was holding the meter probes on with my fingers. Of course as I let go the back EMF got me. It was a fair belt. This has to be the first time I have got a shock from something not even plugged in!! Ouch..

Tried that to, many years ago, when measuring DCR of a secondary coil, while i was in touch with the primary coil ( SHOCK ). :o Even the low voltage from the multimeter was transformed to high voltage of the opposite coil. I also broke a METEX multimeter by doing the same, i've got multimeters on both the primary and secondary site ( silly me ). My calculations after this experience were, that i've send a spike of 10 - 15000 V. I guess this stupid act, could had made a sparkplug ignited. I was lucky, it was possible to repair the METEX multimeter for about 10$, with a new circuit, mounted in a DIL socket. :mrgreen:

I'll go to 91lieb's site about safety "DIY Audio Information for Beginners" and add this info, to bring a warning about safety of this issue. :xfingers:

Author:  mwhouston [ 07 Dec 2010, 01:25 ]
Post subject:  Re: 300B SET Design Project

I've started a new thread: A Tube Compressor for the common man.

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