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?

4-65a SE Amp: testing the 600V raw supply

Burning the 6D22S damper valves

After a failed initial test which caused the silicon and the damping series resistor of the hybrid bridge to blow up due to a gassy NOS 6D22S, I burned in the valves for 30min and tested the supply with a dummy load.

Some few changes to the supply design included the replacement of the HV diodes with a series pair of 1200V@5A fast diodes. The filaments were referenced to cathode (+B) instead of ground to minimise Vhk and a snubber network formed of a pair of 100nF LCR 1.5kV capacitors across the input choke were added.

Supply under test

This is a hefty supply: massively heavy. The heaviest piece of iron I have built so far. Anyway, great decision to build this amp in a modular fashion, otherwise I would have need some mechanical aid for moving the stuff around and clearly my wife would have kicked me out of the house (she hasn’t seen this yet so the latter is yet to happen).

The load resistor is a pair of 3k3 50W aluminium clad ones bolted to an aluminium piece which at the same time is held by a vice (thanks Rod for the suggestion). This provides sufficient mass and heat dissipation capacity (need about 120W).

Initial tests were great, need to do some further ones.

here is the diagram so far:

Update 9th Feb 2013: Minor updates including rightsizing of the mains fuse and removal of earth switch as this supply will be used in floating mode

01a preamp (revisited 2013)

Looking at improving the CX-301a preamp with cathode follower output I modified the gyrator load by replacing the DN2540 by a LND150 and 2SK170 which have a lower capacitance and will improve the performance. Likewise, the tail CCS now has low-noise audio transistors such as the KSC1845 and KSC3503. Bias point is roughly the same, a bit lower than before. Overall distortion in the simulation is great: THD<0.005% @ Vo=16Vpp and loading 100K and 330pF (cable load representation).

U2 could be 6Z52P, D3a or PC86. The latter will require and adjustment as the filament is 3.8V and not 6.3V.

I think is time for building and listening to a potential great preamp…

3A5 DHT (Continued)

Lars requested the paralleled triode curves. Here they are:

And the SPICE model which is more accurate than my previous version:

* Created on Sat Jan 19 15:50:19 GMT 2013 using tube.model.finder.PaintKIT
* model URL: http://www.bartola.co.uk/valves/
* Created by Ale Moglia 2013 [email protected]
*————————————————–
.SUBCKT TRIODE_3A5-2 SMALL 1 2 3 ; P G K ;
+ PARAMS: CCG=.9P CGP=3.2P CCP=1.0P RGI=2000
+ MU=12.46 EX=1.3857 KG1=1965.0 KP=132.0 KVB=1.875 VCT=-1.6 ; Vp_MAX=200.0 Ip_MAX=0.032 Vg_step=2.0
*————————————————–
E1 7 0 VALUE={V(1,3)/KP*LOG(1+EXP(KP*(1/MU+(VCT+V(2,3))/SQRT(KVB+V(1,3)*V(1,3)))))}
RE1 7 0 1G
G1 1 3 VALUE={(PWR(V(7),EX)+PWRS(V(7),EX))/KG1}
RCP 1 3 1G ; TO AVOID FLOATING NODES
C1 2 3 {CCG} ; CATHODE-GRID
C2 2 1 {CGP} ; GRID=PLATE
C3 1 3 {CCP} ; CATHODE-PLATE
D3 5 3 DX ; FOR GRID CURRENT
R1 2 5 {RGI} ; FOR GRID CURRENT
.MODEL DX D(IS=1N RS=1 CJO=10PF TT=1N)
.ENDS
*$

Looking at distortion, as expected the pair of paralleled triodes performs better than a single triode:

3A5 DHT

The 3A5 DHT is another interesting valve to bear in mind. Many have used them successfully even in a phono stage. Great examples are Dmitry Nizh’s DHT phono stage and Sheldon’s. I will probably use them as well at some point.

Well, many DHT have been dismissed over the years due to their microphonic sensitivity. True, they are more microphonic, but this is manageable. Look at the 4P1L for example. I built a great preamp a couple of years ago and played around with the mechanics of the preamp to quiet it down. And I managed to, succesfully (as many have as well). The 4P1L sounds fantastic, despite so many turning it down because of its natural singing skills 🙂 Now, 4P1L is looked after, many are building DHT preamps and even full DHT amplifiers.

Looking at the specifications of the 3A5, we can see that the heater is attractive to be used with rechargeable batteries. Well, at least 220mA @ 1.4V is manageable. Key specs are:

Mu of 15
Anode resistance (ra) is 8,300Ω.
Anode dissipation (Pa) is 500mW per valve.
Vf=1.4V / If =220mA (paralleled filaments)
Transconductance (gm): 1.8mA/V
Maximum anode voltage: 135V

How linear is this DHT? Let’s look at the curves first:

Continue reading “3A5 DHT”

Can you perceive the difference?

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 Pentode Driver (Test 2)

Improving the driver with a gyrator load

After the early experiments with the 4P1L driver in pentode mode, I decided to look at improving it somehow given advice given. The gyrator load is not a good match for a pentode unless the reflected impedance is low enough to control the gain of the stage. Gary Pimm recommends:

“In the driver experiments the plate resistor was increased to a value larger than in traditional Pentode driver stages to get more gain.A CCS was placed in parallel with the plate resistor to add plate current to compensate for the high value plate resistor. This allows you to have independent controls of the gain and operating current. The resistor is chosen to set the gain and the CCS is used to set the Pentode operating current.
To maximize the circuit performance the resistance in the screen circuit is adjusted for minimum distortion. There are draw backs to this- The circuit has to be tweaked for each tube. As adjusting the screen voltage and resistance also effects the gain of the stage you have to compromise some to have the gain match between 2 channels. This is not a circuit where you can swap tubes around without “calibrating” the stage on the test bench.

Another interesting way of applying the circuit is to place the plate resistor in parallel with the Pentode and have the CCS supply all the current needed by the stage. This allows the Pentode driver stage to have PSRR similar to CCS loaded triode stages. It also makes the signal current loop very small including only the Pentode, cathode, and plate resistors. The noise and capacitor colorations of the power supply are quite effectively removed.”

So I opted for adding a resistor in parallel (RL) to adjust gain, minimise distortion and improve PSRR:

The load resistor is 68K. I optimised the operating point to reduce distortion at maximum swing (i.e. 200V peak to peak). The input impedance of the soundcard interface which is 100K didn’t produce a significant impact on the distortion when measuring from the anode output or in the mu output:

Interesting to see that distortion is now nearly half of previous operating point and 0.27% for 200Vpp is very good.

The screen current is approximately 1.8mA at 81V bias.