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DHT heater voltage...how critical is it to be spot on? AC or DC?

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Working on my little 2A3 SET Fluxion amp and going thru all the voltages and checking overall wiring etc and noticed the heater voltage on the 2A3s is down, measuring 2.15 VAC. Line voltage is 118VAC. Everything else is spot on textbook, 250VDC on the plate, -45VDC bias. Is this something that should be corrected or is that okay? I thought perhaps full wave bridge to get it to 2.98 then drop it with a diode forward drop or series R to 2.5VDC. I know there are issues with DHT and DC heaters, the gradient across the cathode etc. The 6SN7 driving it, has heater voltage of 6.2VDC.

Your thoughts?

Cheers
 
kilohertz,

You said:
1. "I know there are issues with DHT and DC heaters, the gradient across the cathode etc."
That is true, but it is not much of a problem for a 2.5V filament.
Where it is more of a problem is with 5V, 7.5V, and 10V filaments on a DHT.

2. But there is a problem with using AC filaments.

A. You can null out the hum at 1 X the mains frequencies. You can adjust the null so there is no hum when there is no music at the amp input. So that part is OK.

B. But, you can not null out the intermodulation that is at 2 X the mains line frequencies (100Hz for 50Hz mains; 120Hz for 60Hz mains).
That means for every musical note, Intermodulation products will appear.
Example, for 60Hz power mains: A pure flute note at 440Hz will have intermodulation products of (440Hz - 120Hz) and (440Hz + 120 Hz).
Now you have 320Hz, 440Hz, and 560Hz.
Yes, the tones at 320Hz and 560Hz are smaller than the 440Hz flute note, but they are there. You only want the 440 tone.
As you increase the 440Hz signal level by 10dB, the 320Hz and 560Hz increase by 20dB.

And all the other music notes will also have intermodulation tones at 120Hz below the note, and 120Hz above the note.
All the notes from the band or orchestra will have these undesired intermodulation tones.
They may be small, but they are not nulled out.
. . . You can see why my next 2A3 Amplifier will have DC filaments, not AC filaments.

Yes, if you post a schematic, it might be helpful to solve the low DC voltage.
 
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Thanks everyone,

I decided to park the amp on the side of the bench and decide what to do. The mfr is useless for help about technical issues with his product, he won't answer my questions directly, so I think I am going to add a small double winding xfmr under the chassis to get a nice clean 2.5VDC on the filaments. That will relieve the power xfmr of the filament load and get everything back in spec. There is a lot of room under the chassis so it will be an easy mod. Looking forward to hearing it all done, it already sounds amazing in it's stock form.

Thanks again.
 

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If you use another transformer for the 2A3 filaments:

1. You need separate secondaries for each 2A3.
The schematic shows a feedback winding on the output transformer.

2. Locate the new filament transformer away from under the output transformers.
Otherwise the magnetic field will couple to the output transformers.

Happy modifying and happy listening.
 
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