The tubelab SE amp design has the 10Ω bias shunt resistor in the anode circuit.
I'm using a Rod Coleman DHT regulator and looking to use a bias servo.
Just wondering why the bias shunt resistor is in the anode circuit and whether there's any problem moving it to the cathode where fixed bias resistors usually go.
I'm using a Rod Coleman DHT regulator and looking to use a bias servo.
Just wondering why the bias shunt resistor is in the anode circuit and whether there's any problem moving it to the cathode where fixed bias resistors usually go.
As designed the filaments of the two tubes are wired in parallel making independent current measurements of each channel impossible. Separating them even with independent filament regulators would require a separate transformer winding for each tube. This limits the choices of off the shelf transformers for use in the TSE or would require two transformers.
If you have two completely separate filament sources, then it would be possible to put the sense resistor in the cathode circuit. If there is any connection between the two filament circuits an interaction and possible channel separation issue will occur.
If you have two completely separate filament sources, then it would be possible to put the sense resistor in the cathode circuit. If there is any connection between the two filament circuits an interaction and possible channel separation issue will occur.
Thanks George. Each filament has its own DHT regulator although the 2 regulators do share a common DC source. I'll check for channel separation issues during testing.
Where will you place the sense resistor in that scenario since the path from the regulator to the tube carries at least an amp of filament current? The usual place for the sense resistor in a DC heater system is in the ground path of the DC regulator with everything else left floating. Having a common DC source prevents a single floating power source for each filament leaving no isolated path from "cathode" to ground for the sense resistor.
I have used a circuit in the plate path of the DHT to sense the current for monitoring by a micro controller or even a digital meter. You need a high side current sense chip like the LTC6101 to do it though. The circuit is on page 18 of this data sheet.
I have used a circuit in the plate path of the DHT to sense the current for monitoring by a micro controller or even a digital meter. You need a high side current sense chip like the LTC6101 to do it though. The circuit is on page 18 of this data sheet.
Thanks George.
I'm planning on using a SMPS in this amp. It has 2 independent filament supplies. All good.
Sometime over the last year I decided to add filament regulators to the design. Since the filament supplies in the SMPS max out at 6.5V which is too low to drive the filament regulators, I thought, no problem, I'll put the supplies in series and drive both regulators from that.
If I hadn't posted my question I would have quite perplexed when the bias servo didn't work! Obvious now.
I'll build the amp initially without the filament regulators and bias servo.
Using a high side current sense chip like the LTC6101 to implement a bias servo sounds like a good project.
The SMPS may prove unreliable. Fixing and refining it (perhaps including raising the filament supplies to the 8V required by the regulators) could be another good project.
I'm planning on using a SMPS in this amp. It has 2 independent filament supplies. All good.
Sometime over the last year I decided to add filament regulators to the design. Since the filament supplies in the SMPS max out at 6.5V which is too low to drive the filament regulators, I thought, no problem, I'll put the supplies in series and drive both regulators from that.
If I hadn't posted my question I would have quite perplexed when the bias servo didn't work! Obvious now.
I'll build the amp initially without the filament regulators and bias servo.
Using a high side current sense chip like the LTC6101 to implement a bias servo sounds like a good project.
The SMPS may prove unreliable. Fixing and refining it (perhaps including raising the filament supplies to the 8V required by the regulators) could be another good project.
I have been using SMPS's for heater supplies in tube amps for some time. They often need to be oversized to handle start up current without shutting down, but they work. I have not tried one in a DHT amp, but you are probably OK with a regulator between the SMPS and the tube. The Microchip regulator that I use in the TSE-II can work down to considerably less than a volt of dropout voltage, so that is one possibility. The other is a 7.5 volt SMPS.
Not sure about the Aliexpress thing, but I have had good luck with MeanWell units. Not sure where to find them in your part of the world though.
https://www.jameco.com/c/Power-Supp...ml#/filter:ss_attr_output_voltage:7.5$2520VDC
Not sure about the Aliexpress thing, but I have had good luck with MeanWell units. Not sure where to find them in your part of the world though.
https://www.jameco.com/c/Power-Supp...ml#/filter:ss_attr_output_voltage:7.5$2520VDC