Driving the 45 DHT in A2

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:

The 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?

9 Replies to “Driving the 45 DHT in A2”

1. 45 says:

Hi Ale,
in my experience that quiescent point is not very good in terms of musical performance. You better to go for at least 250V/34 mA or 275V/36mA and use 5-6K in place of the typical 3-4K.

1. Ale Moglia says:

I thought so but I was looking at maximising output power. I will redo the operating point analysis….
Thanks

2. 45 says:

2.5 W is a good result as well for 8.5W plate dissipation (i.e. 250V/34 mA). It is about 30%.
At 275V/36mA you should get similar efficiency (i.e. about 3W). This way you don’t put to much stress on the driver as well because the power drive is milder.

1. Ale Moglia says:

Thanks for the input. I will rerun at 6K and the operating points you suggested. Cheers!
Ale

3. 45 says:

P.S.
I am looking now at your picture. Is your 45 of mesh plate type? In that case forget power drive. Mesh plate valves don’t like to be driven into positive grid….

1. Ale Moglia says:

Yes, it’s a mesh full music 45/n. Have a good stock of used 45 various brands though.
Why mesh types are not good for A2?

1. 45 says:

I don’t know precisely the physical phenomenon I have only seen that with these valves you get a steep increase in distortion when you enter positive grid. The valve was a true mesh plate ( 300B AVVT). Maybe with Full Music it doesn’t happen because it is not a real mesh plate (i.e. made with true wires).

Cheers

1. Ale Moglia says:

Interesting, mine is not a true mesh, it’s rather a perforated anode, which is refereed as mesh anyway.
Will have to test both and measure THD…