Tag: 45 in A2

Sweating the 45… (Part4)

Last week I looked at optimising  the 45 loadline in A2.  Clearly we shouldn’t be attempting to get more than 2W from this valve without a significant level of distortion. However, having about of 3W would be attractive for the transient response of this amp.

So how will this circuit perform in a simulation? Let’s see what the spice results are:

45 SE A2 amp version 02The THD is significantly better due to the harmonic cancellation between the two stages. The driver distortion is  0.3% at full tilt (150vpp) and this could be improved. I guess the 6e5p could do better, but interesting to see how the cancellation of harmonics may play around. The new operating point and the stacked supplies will demand different MOSFET parts of 1kV for sure…

 

Driving the 45 DHT in A2

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Having had a great exchange of comments with “45” in a previous post, I thought it was easier to post this after doing some simulations with the 45 DHT in A2 operation.

I’m a great fan of the 45 valve. I think is probably the best sounding DHT out there. I listened to 300B, 2A3 and even 4P1L as an output valve, but nothing compares to me to the warm sound of this valve.

Later specifications of the 45 show that you can push it to 10W of anode dissipation. I’m currently using it at Ia=34mA, Vak=300V with an 2.5KΩ OT. You could get 2W out, but at the moment I’m squeezing 1.5W at maximum drive. There is a way of getting more out of the valve which is obviously by driving it in A2 (i.e. positive grid current). My current project (4-65A SE in class A2) uses a gyrator-loaded driver and stacked supplies which work brilliantly in A2. The driver provides sufficient grid current at low impedance even when the input impedance of the output valve drastically changes when grid current kicks in closer to 0V.

I have a pair of LL1623/60mA which I’m planning to use in the future to try 4P1L PSE or 6C4C output stages. This OT can be configured for 5.6KΩ, 3KΩ and 1.6KΩ anode loads.

Here is a first simulation of the 45 operating in class A2. The bias point was changed now down to 210V/47mA as the OT is configured for 3KΩ load:

45 loadline in A2 version 2The anode AC power is then:

P{}_{a}=\frac{1}{2}\cdot i{}_{ap}^2\cdot Z{}_{a}

So roughly for Iap=46.5mA and Za=3KΩ, then Pa=3.2W. This is about 32% efficiency. More than double of the current juice I’m getting out of this valve, but at the expense of pushing the grid to +32V and anode peak current of 93-94mA. Grid current should be around 3-5mA from what the AB2 data looks like.

Question is here, is it worth trying this? Complexity of the amp is on the stacked power supplies. The driver will need to swing easily 120Vpp, so a well designed 4P1L in filament bias can do this with minimum distortion.

Thoughts?