The 300B amplifier journey continues as planned. A bit lower than expected, however small steps being made. Recently I mentioned about the 46 driver.
I made some progress on the LL7903 step-up transformer which is wired on 1:8.
With 1:8 wiring, don’t expect same performance level as in 1:2. However, I carefully trimmed the Zobel network in the secondary to reduce the resonant peak. The suggested network obviously isn’t fit for purpose in this mode and even tuning the resistor with the suggested 400pF isn’t good enough to tame the peak. I resourced my box of Russian NOS mica capacitors and pulled out a 4,700pF one. By playing with the frequency response test y tuned it manually with a potentiometer and found that 4K7 was right resistor:
You will see on the picture below, the driver will have 2 boards. One mounted on top of the other. The lower PCB holds the LL7903 pair and there is further room for a couple of fixed bias regulators. When I get to experiment again with pentode drivers, I will use this space to place the screen regulator:
Here is the frequency response for the LL7903 in 1:8 with the zobel network. The secondary is unloaded as it goes straight into the 46 grid. The source impedance in this case is 50Ω. Shouldn’t be massive difference in response with the DAC’s impedance which is lower. I guess you can’t expect the 70kHz response of the 1:2 wiring and the LF pole will be dominated by the 46 hybrid mu-follower. The 27kHz is good enough for me:
The driver circuit remains the same. I will play with the operating point so Rk may change. As am running the 300B from a single supply, I may run the 46 cooler somewhere between 20 and 30mA depending on the current balance needed with the output stage:
With a mu of 5.6 and 1:8 SUT we should get a gain close to 44 (32.8dB). With high efficiency speakers it will do fine with a single stage for the 300B. Promising. As said before, my previous 46 driver incarnation was a delight and sounded fantastic. Yet, I will never implement the crazy idea again of filament bias with these bias voltage and current levels needed!
Hi Ale. I am Doing an amplifier with 46 RC coupling to 46, then Lundahl interestage 1:2.25 connection to 300B, and Lundahl OPT transformer 3K:8ohms.. Do you think is enough gain to drive the 300B? Never use interstage transformer, so first try one .. If it goes well, I can put another one between the two 46 tubes .. I always have the option 6s45, D3a or 6e6p,to drive the 300B if i not get a good result with the 46.
Hi Santiago, no. Assume you have a 2V source, that will put about 25V on the 300B grid. It will give you a 1W maximum output power. If you have SPL close to 100dB, a small room and if you plan at low volume you can get away with it. Otherwise is a waste of the 300B power in my view.
Ok, 46 46 300B , is a waste of the 300B.
I trust in you.
I thought that having 5u gain in the first 46, multiplied by 5u gain in the second 46, multiplied by 2 with interstage 1:2 transformer before 300B reached.
Maybe use a 6e5p, a 46, and a 300B.
Would the attempt be worth it?
Or I go alone by the 6E5P Interestage transformer Lundhal 1:2.25 and 300B
Sorry, misread that it was 46 into 46. I think if you’re going 3 stages with DHTs then better to use first stage with a mu of 8 like 26, 10Y, 01a. It will give you a gain of 80 overall with the step up IT.
Hi Ale,
How are you wiring the second grid? just connecting both grids together or are you connecting G2 to plate? Assuming the G2 to P.
The 46 tube documentation notes this:
As a Class A1 Amp: Grid No.2 connected to plate at socket
As a class B Amp: Grid 1&2 connected together at socket.