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In Reply to: RE: Filaments and filament bias posted by andy evans on April 15, 2016 at 00:27:06
So if I understand correctly the I can put whatever voltage bias I want on the filament as long as the other end has a 5v differential?Does this mean I can use a DHT as a cathode follower if I use a DC filament supply and just keep the filament at a 5vDC differential?
How do I maintain this offset?Also, which end counts as the differential to the plate voltage?
If I have 50v on one end of the filament and 45v on the other, which end counts as the cathode to plate voltage? How does it know the difference?How bout a white cathode follower? Clearly the tube cannot output enough current to run the other filament, but how much filament current is transfered to the plate? Can I just a source follower on the "cathode" of the top triode in a WCF to power the filament and then use the bottom triode to operate the push pull?
In an IDHT the current goes straight from the cathode to the plate, but since the filament has its own current how does it work?
Edits: 04/15/16 04/15/16 04/15/16Follow Ups:
"How bout a white cathode follower?"
IMHO designing WCF with DHTs is too complicated, solvable, but not practical.
Lower output impedance is an advantage, but with other solutions (other tubes, DHT/IDHT mix etc.) give better results.
As you can see the upper tube has DC filament (current or voltage is equal in this respect), the lower use filament bias. The anode current flow trough filament bias resistor, but it is negligible beside the filament current.
Hmmmmm. I thoroughly enjoy my current WCF output stage and I figured since DHTs are so much more linear than the 6as7/6080s I'm using that it simply must sound better. Am I right in assuming this?
In any case I would go for an OTL solution. The filaments complicate it a bit but it won't be too hard to null the offset of the filaments and still get the 1 amp current.
Although I'm still a bit confused on what the cathode to plate voltage is when there are two voltages on either end of the filament.
I'm curious what you meant when you talked about DHT/IDHT mixing and other tubes.
"I'm curious what you meant when you talked about DHT/IDHT mixing and other tubes."
I regurarly use (sand or tube based) CCS loaded DHT tubes, sometimes CCS built with IDHT tube (for example E180F/E280F/D3a, 6N6P etc.).
Where did you get that DHT model for LTspice?
"If I have 50v on one end of the filament and 45v on the other, which end counts as the cathode to plate voltage? How does it know the difference?"
As you can see at the VT25 CF schematic, this simple practice centered the filament.
The CF use negative supply (practicaly direct drive to output tube) and "LND150 CCS bias" to adjust via CF tube the output tube bias.
Distortion a bit high, but if you use appropriate CCS insted of R4, the distortion will decrease one order of magnitude.
Bah, I'm still confused. Are you saying the "cathode" voltage of the cathode to plate differential is the center voltage between both filament terminals?
Also why are you feeding the grid with a current source? Grids don't consume current unless they are positive right?
"Also why are you feeding the grid with a current source?"
This simple bias circuit use CCS (its current largely independent of changing of the PSU voltage) to pruduce bias voltage on the R103 resistor.
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