Driving the 45 in A2 (Part3)

What about driving it at full tilt in A2?:

The driving signal has to be about 160Vpp to provide maximum swing, so power is:

${P}_{out}=\frac{\left( Vmax - Vmin\right)\cdot \left(Imax-Imin \right)}{8}$

${P}_{out}=3.36W$

H2 distortion:

${D}_{H2}=\frac{1}{2}\frac{\left(Imax + Imin \right) -2 \cdot Ip}{\left(Imax-Imin \right)}$

Which yields a whooping 6%!

Ok, the previous operating point wasn’t optimal from a musicality perspective. As suggested by 45, I reworked the load line for 36mA/275V and anode load of 5.6KΩ (which is what I could get with my OT):

The driver should now provide $V{}_{gk (pp)} = 2 \cdot (+20V + 55V) = 150Vpp$ .

The output power will be around:

$P{}_{a}=\frac{1}{2}\cdot i{}_{ap}^2 \cdot Z{}_{a} = \frac{1}{2} \cdot (36mA)^2 \cdot 5.6K\Omega \cong 3.6W$

Efficiency would be around 36% in theory. Happy if around 3W can be obtained from this valve here with a reasonable distortion…

Driving the 45 in A2 (part 2)

On the previous post I explored how more power could be extracted from the 45 in class A2. Here is a first draft circuit using the 4P1L as the driver stage with a mu-follower gyrator load to provide current with low output impedance:

3.5W can be obtained with 13Vpp input signal. Clearly a pre-amp is required, but that is the output level I’m currently getting from my preamp so should be fine. The LND150 CCS reference can be optimised and derived from the stable 280V.

Not a bad idea, just an additional power supply to avoid any nasty cap in the path.

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:

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?

4P1L pentode driver (continued)

After the initial tests done with the 4P1L in pentode mode and filament bias, I thought a bit about how this driver could be implemented in practice. I may try this configuration in my 4-65a SE amp, but am not urged by this at all.

The screen supply is formed by a gas valve (SG3S) which provides a very stable reference when feed by a CCS. In this case the cascoded pair M3 and M4 will provide in conjunction with R4 and U2 a very low noise screen current to U1. R9 has to be adjusted on test to set a current of about 15mA on the CCS. The 4P1L will draw 1.8mA at 81V as screen current, so R4 may also need adjustment to set the right operating point.

The driver should provide about a gain of 150. Driving easily a transmiting vale (or why not a 300B 🙂 ) in class A2 with this configuration. My tests showed a maximum THD of about 0.27% at 200Vpp.

Gain could be reduced if needed by tweaking the RL. And if stacked supplies are not used, then a single +400V supply could be used with additional dissipation across the reference currents (M1 and M3/M4).

Keep thinking…

a C3g driver…

Sunday morning and my brain already started with a lot of activity early today. Should be the other way round. I spent all week working and weekends should be about relaxation, but can’t help it 🙂 Before going up to the workshop and continue with the 4-65a SE amplifier build, I put into work some of these ideas that are flying around.

The 46 filament bias driver with its hefty supply will be “the driver” for the 4-65a, yes, no doubt. I may want to try some other combinations such as 4P1L pentode in filament bias, or why not some other drivers as 6e5p, C3g and D3a. All these are brilliant candidates. But final judge should be my ears. We all know that what looks really good in paper not necessarily translates into a great sounding driver, but at least is the best start.

C3g (as well as D3a) a really linear and revered valves in triode mode. Huge gain, current capability and transconductance. Perfectly engineered valves. C3g can easily provide a great performance as a driver with a gyrator load. I found playing around that with a simple LED bias, a bias of -2V, 190V and 30mA provides an outstanding 0.15% @ 200Vpp at least on the simulations..

I may try this in my 4P1L/6C4C project as well…. (so much to do)

Ale

Edit

Hi Ale,
A few observations.
190V with 31mA provide dissipation of 5.9 W !! It is excessive.
Use Telefunken data. Pa max is 3,5W.
Also, look data : “G2 and G3 connected to A

Just as a long-time user C3g as driver

Rajko

You’re right, interesting oversight which reflects that I shouldn’t be doing this early on a Sunday morning! Looking at the datasheet, we could get out 4.9W maximum if using Pa=3.5W + Pg2=0.7W + Pg3=0.7W. Well, a bit too much, perhaps safer running it below 4.5W.

So let’s see how it performs if we dial down the anode current. I ended up changing the LED to minimise distortion:

The THD is 0.26%@ 200Vpp which is similar to my two stage 4P1L-46. I wonder how much distortion the 4P1L will give at 200Vpp?

4P1L Driver Tests

4P1L is a sublime DHT. As shown before it’s one of the most linear valves in triode mode. I built a breadboard in filament bias to test 4P1L as a driver using a MOSFET gyrator in mu-follower mode:

My test set can only drive the 4P1L output to 30Vrms and the distortion is only 0.027%!

I was intrigued by the performance of this driver in pentode so did a quick modification to provide a screen fixed voltage instead via a source follower and adjusted the bias voltage to minimise distortion. I found that a bias of about 120V was the best. This setup wasn’t the ideal one as in filament bias the frequency response is really poor as there is no cathode resistor bypass. The gain is about 200 with the gyrator used:

A distortion of 0.58% @ 200V peak-to-peak is really good. The filament bias is forcing the pentode to operate with low anode current so I guess that with a lower bias point performance will improve. I will have to test this.

The measured THD was:

0.125% @ Vo=100Vpp
0.34% @ Vo=150Vpp
0.58% @ Vo=200Vpp

Interesting to see the increase of H3 and H5 as a result of the pentode operation.

The breadboard for pentode can be improved for sure. I will look next at reducing the bias voltage as a first step. Interesting results which show that 4P1L is a great driver both in triode and in pentode modes.

4P1L PSE or 6C4C

Andy Evans and myself have been experimenting with the 4P1L extensively. No one will argue that the 4P1L is a cracking valve. It’s very linear, has low anode resistance and its filament is not demanding, so it can be easily used in filament bias. Similarly, pairs are very easy to match, is still cheap and a pair of them can performa as well as a 2A3 at a fraction of its cost.

In one of the listening tests we did at Andy’s I found that a 100% 4P1L system (i.e. pre-amp, driver and output valve) had something missing in its tone. Perhaps is the clarity and neutrality of the 4P1L, but definitely I’d add a warmer valve somewhere in the signal path.

I’ve been toying with the idea of upgrading my 45 at some point or at least provide some way of testing several combinations of output valves. To me, I’d rather implement filament bias but in most of the cases is not possible due to the filament demands. 4P1L in PSE can be easily achieved in filament bias. I did some tests recently, with outstanding results. Ultra-path could be an option, but of course will demand more attention to the HT supply filtering. Fixed bias on the other hand, proved to be very effective, but yet requires a low noise bias. Rod Coleman suggested recently to use his filament regulator boards to provide a very low noise and stable bias by means of providing a stable and low noise current through a bias resistor. A very simple and neat concept which is shown in my early design for this SE or PSE amplifier:

4P1L as a driver is a great choice. Firstly, it can be easily implemented with filament bias, so no nasty capacitors are required. I’ve got a pair of LL1671/20mA which can provide a great 1:1 coupling whilst reducing the HT requirements of the driver stage. 4P1L running at Ia=20mA, Va=243V, Vgk=-17V can drive easily the 6C4C into clipping. R5 performs as the bias resistor for the output valve. The CCS3 regulator is set to about 0.95mA to develop about -44V across the resistor. As there will be no DC current flowing across the secondary there will be no voltage drop and the valve will be biased accordingly. The current meter will allow us to track any bias drift and re-adjust when needed. The 6C4C will be biased at Ia=60mA and Vak=250V. The output transformer is the LL1623/60mA which can be configured for multiple impedance requirements: 5K6, 3K and 1K6. This will allow me some flexibility in the output stage when testing other combinations.

The same circuit can be easily adjusted to fit the 4P1L PSE output stage:

A pair of 4P1L can replace the 6C4C biased at Vak=250V, Ia=30mA and Vgk=-20V. The bias resistor (R7) has to be changed to 20K to allow the 20V bias voltage requirement. No further changes to the circuit are needed with the exception of the CCS2 that needs to be adjusted to fit the 1.3A (2x650mA) required by the filaments.

Merry Christmas!

Category: SE designs