• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

New DHT heater

Time for round two:

glödtrafos.jpg
 
  • Like
Reactions: 1 users
I don't have any idea what type circuit is on the filament boards that Tent Labs sells. But My amp went from a mild hum to dead silent with 106 spl speakers by switching to them. You can't tell its turned on without sticking your head inside the speakers. I am using D3a driver tubes and they have AC on the heaters but are silent as well.
 
  • Like
Reactions: 1 users
I have an existing pair of 811A monoblocks which I want to add the Rod Coleman v9 heaters to the DHT 811A tubes. I have the parts, specced for 4A with a raw DC supply and just received some 100VA transformers to power the heater circuit. It's just that I want to make sure I'm doing everything right.

The amps are inspired by the MJ magazine Cathode feedback design and executed by Belgian firm Aquablue. They have three stages EF86-KT66-811A into Lundahl LL9202 OPTs.

If we look at the transmitter tube, this is the schematic. Now the part I highlighted is actually a 100 Ohm pot (WX14-11) to adjust the bias.

DHT heater.jpg


In the Coleman Reg application notes, I read the following:
  1. Rk is always connected to the negative filament position(F-).
  2. If the V9 regulator is replacing an older Coleman Regulator, that connects the Positive (F+) to Rk: the anode/plate current must be checked, and adjusted, to give the same current as before.
  3. Rk is the Cathode bias resistor in this circuit. Connect the Cathode-bypass capacitor directly across Rk.
  4. If you are using Fixed Bias, Rk is a 1Ω resistor used to measure the anode/plate-current. In this case, no bypass capacitor is needed.
  5. This connexion to the filament does not follow the traditional pattern of creating a centre-tap (eg from two resistors) and connecting the cathode resistor (Autobias Amplifier) or 1-10Ω current sense resistor (Fixed-Bias Amplifier) to this centre-tap. This method can be used if desired, but the sound will almost always improve with the wiring method shown in the diagram.
  6. Explanation: A ‘centre-tap’ of resistors is used in most DHT amplifiers, because it gives good cancellation of noise conducted by the filament heating-current. With ordinary dc heating, the centre-tap provides an important reduction in noise. But with a purpose-designed current-driven Filament Regulator, the noise is so low that this noise-rejection is not necessary. The self-noise of the Coleman Regulator is below 56nV/rtHz at 100Hz and above. Line-harmonic spot-noise at 100/120Hz will be around 5µV, for 300B when the Raw DC is built to these instructions, and can be reduced further, with heavier filtering (CRC or LCRC). This noise level is not perceptible even for preamp stages, and so the extra noise-rejection is not required.
Now am I correct in that I leave the pot in the circuit but disconnect one end of it (positive filament)?
Do I need to add the Rk of 1 Ohm additionally?
 
Last edited: