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 Post subject: What Am I Missing?
PostPosted: February 19th, 2024, 3:01 pm 
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Heater supplies. Thought is to use one heater supply. The AC portion would supply a single damper diode at 1A6/6V3. The DC portion, rectified and LDO regulated to ~6VDC, would supply a total of 1A4 load. A portion of that load, 0.66A would be near cathodes close to ground potential. The other portion of the heater load (0.73A) would be near cathodes at ~+150V. So the DC supply would be referenced to ~+70/75V.

Is this workable? That is to ask: must the damper diode supply be from a separate winding, or not? There is no cathode connection to the heater in a damper diode (6BY5GA).

I keep thinking that I'm missing something.....

Stuart


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 Post subject: Re: What Am I Missing?
PostPosted: February 19th, 2024, 4:16 pm 
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As long as the AC side is floating, it shouldn't be a problem. Keep in mind that even though it's the DC side that's at 75vdc reference, the AC side will be elevated too... 6Vdc from a 6.3Vac winding is cutting it pretty close even with an LDO regulator, but that's a whole different issue ;)

Roscoe

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 Post subject: Re: What Am I Missing?
PostPosted: February 19th, 2024, 4:19 pm 
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Thanks Roscoe. Using Schottky diodes gives a little extra cushion for the regs.


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 Post subject: Re: What Am I Missing?
PostPosted: February 19th, 2024, 11:05 pm 
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The voltage breakdown cathode to heater is quite high as in damper service the cathode is elevated and TV sets generally had only one heater supply for all tubes. Furthermore, many were designed for series strings operating directly from the line without an isolating transformer.


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 Post subject: Re: What Am I Missing?
PostPosted: February 20th, 2024, 9:12 am 
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dberning wrote:
The voltage breakdown cathode to heater is quite high as in damper service the cathode is elevated and TV sets generally had only one heater supply for all tubes. Furthermore, many were designed for series strings operating directly from the line without an isolating transformer.


Thanks David. Not an issue here. While the dual damper diode 6BY5GA has a relatively low cathode to heater rating of 475 volts (compared to tubes like the 6AX4), my B+ will only be 250 volts and since I'm referencing the heater supply to ~75 volts, that differential is reduced.

My concerns were whether using a common supply for all those tubes would lead to unwanted signal injection at the most sensitive cathodes.

This is a proposed phono stage. ~50dB gain without SUTs. Both stages are Beta followers with 6DJ8s on top, BC558 as the "Beta" component and a 1st stage gain tube of 6GK5, 2nd stage 12AT7 (subject to change). Passive RIAA in between stages.

I'll post a schematic once it's finalized.

Stuart


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