4-65a SE Amp: HV3 supply

Building the HV3 (+330V Supply)

I could easily say that by now I’m tired of building power supplies. Yes, I’m and fundamentally can’t see the day when I get to fire up this amplifier.

Filament supplies and three stacked power supplies is the price I’m paying to get a completely cap-less and DC coupled A2 amplifier. I guess that my analysis once completed will be made with full perspective of every single implication of this amp: iron, heat and weight. Yes sir, this is a heavy-weight challenger.

I guess that this specific HT power supply design is quite flexible as could be easily reused in many of the projects I have in mind, which unfortunately keep growing.

Good thing is though, I can make these supplies pretty quickly, but don’t do this at home ok? There is serious HT involved. I don’t have pets or kids (but do have a wife) and these supplies should be hidden and away from any poking curious finger.

The final design is similar as the one used before. Could be adapted to choke input, but with the components I had at hand, this supply is very well filtered. It provides only 15mV ripple noise which at 330V is a lovely noise floor around -87dB:

The mains transformers are Weiss (excellent quality) which has screened windings. Instead of having a full Graetz valve rectifier and waste more heat (and use more damper valves), the rectifier is hybrid using a pair of UF4007. Capacitors are oil and polyester ones for the output HF decoupling on each rail. Each channel will feed a Salas SSHV2 shunt regulator which will provide the stable DC reference for the 46 driver stage and bias point for the output 4-65a stage.

Bad news is that, there is one supply left to be built before I can test at least one channel!

Loud

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:

The 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 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%!

Driving the 45 in A2 (Part 3)

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?

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 valves@bartola.co.uk
*————————————————–
.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: