46 DHT driver final tests

Having built the 4P1L filament bias driver stage in a breadboard, I now have the sufficient voltage swing to drive the 46 to maximum sweep. In my 4-65a SE amp, a maximum of 200Vpp is required to drive the amp into class A2.

The following tests conditions were used:

4P1L first stage:
- DN2540 gyrator in mu follower output
- 220nF/450V Capacitor coupled into 46 driver
- Filament bias: 15 ohms, Vgk=-10V
- Vsupply=355V and Va0=210V
- Output set to about 30-32Vpp to drive 46 at 200Vpp
46 driver stage:
- IXYS 01N100 gyrator in mu follower output
- Load impedance is 100K (Pete Millett’s interface)
- Filament bias: 10 ohm / 100W Vgk=-17V
- Vsupply=355V and Va0=204-208V
- Output set to 200Vpp

I tested 28 valves. Just a few of my lot are NOS. The average THD was about 0.4-0.5% but a good selection of 8 valves (mainly Sylvania NOS) provided a consistent 0.18% THD:

Happy now with the initial tests and selection of 46 pairs for the amplifier, I can now continue with the build…

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.

New Year’s curves

New Year’s afternoon spent in tracing some curves whilst testing pentode drivers (4P1L, 47 and others).

Here are some new curves:

Cheers!

4-65a SE Amp: refining the 46 driver

I did some tests today and looked at minimising distortion of this 46 driver in filament bias and found that Va=230V (instead of 184V) to provide best performance:Filament bias resistor array is now laid out horizontally to improve the dissipation of heat.

Here is the performance (0.05% at 17Vrms) at maximum drive input from my audio test set:

4-65a SE Amp: testing the 46 driver

46 driver breadboarded. The mu-follower gyrator, the filament bias resistor array and the nice teflon UX5 socket from Jakeband. The filament bias resistor array is formed by 4 10Ω 20W dale wirewound resistors. These get very hot so probably need to think an alternative layout or further resistors in parallel:The performance is very good. I just picked up a random 46 from my stock and biased it at 204V (which is the operating point in my design) achieving less than 0.05% at 10Vrms. Need to re-run this test to see how will perform at 70Vrms:

Using my spice model created from a good 46 valve, THD should be around 0.15% at 200Vpp with a 100K load and performance is great to loads down to 100 ohms. Clearly the load in A2 will change from high impedance to some kΩ so this driver should maintain outstanding linearity all the way through:

4-65a SE Amp: filament regulators

Finished the 4-65a filament regulators. Filament noise is only 1.5 mVrms at full load (3.5A):

4-65a SE Amp: 46 Driver Gyrator

A day of PCB etching

After a lot of work today in designing many PCBs, I finally got a pair of mu-follower MOSFET gyrators for the 46 driver stage. The driver has to provide very low impedance to operate the 4-65a output valve in class A2. The gyrator in mu-follower configuration will enable the right bias point as the amplifier is DC coupled as well as maximum signal (and current in A2) with minimum distortion.

Many don’t like sand at all in their amplifiers. I have a lot of experience with gyrators and CCS loads in pre-amps and drivers as well. I have to say that with MOSFETs gyrators the sound is really nice. For an A2 driver, not many options are available and the gyrator is a great choice for this job.

I built two PCBs (one per channel) and the circuit is the classic depletion-mode MOSFET gyrator based on the high-voltage IXTP01N100D. I guess that a DN2540 should work as well here but I’ve been saving the IXYS for this occasion. The reference voltage for the anode bias point is provided by the CCS formed by M1 (LND150) which provides a higher impedance in AC improving the frequency response of the gyrator.

The 46 is operating in triode-mode and filament bias with a Rod Coleman filament regulator. R6 is approximately 1/gm and output voltage is set by P1 to achieve the 4-65a bias point as the amplifier has stacked power supplies given coupling is DC, so no capacitors in the path to the grid.

Next: some tests on these gyrators and the filament boards…

4-65a SE Amp: 46 Driver Raw Supply

One more filament raw supply completed today: the 46 driver in filament bias. This driver stage requires 26V @ 1.7A due to the filament bias requirements. Yes, nearly 45W in the filament but will provide a fantastic driver stage with the 46 triode-strapped and filament bias to avoid any nasty capacitor in the signal path.

The power supply design is very simple and follows Rod Coleman’s recommendations for the DHT filament regulators. One drawback in this version, compared to the output stage raw supply, is that this will be pure capacitor filtering with no help of a choke to reduce the input current pulses.

The split-bobbin 150VA transformer provides sufficient current for the capacitor input filtering stage. The DSB10I45 (Schottky 45V/10A) bridge is also mounted on a “L” shape aluminium piece.

The capacitor arrays are soldered to a thick bare wire which provides structure and simplifies connections between components:

I was initially concerned that without shielding the high-current pulses may introduce some noise in the output as F2 fuse is mounted on the transformer frame so the wire is routed back and forward to that point. Reality is that the hum level is very low. I measured 16.4mV peak-to-peak at full load.

4-65a filament supply mono block

Today made more progress as managed to build one of the 4-65a raw filament supplies. They weigh a ton so decided to build monoblocks for all filament power supplies as well as the main amp.

The power supply is built on a piece of wood. This is a fast method for breadboarding the amp which provides fantastic results. I’m using PTFE sleeved wire for the first time. This is rated 600V and 10A, sufficient for the requirements here.

The common-mode choke is built under the capacitor arrays to reduce wire length and optimise space use.

The choke and 200VA transformers are bolted directly to the board.

The Schottky rectifier bridge is mounted on an 4mm alumminium angle which is primarily used as heat sink and also the solid structure helps to mount the first capacitor array very close to the bridge.

Tested the supply with a dummy load of 3R3 50W. Ripple is only 53mVpp which will be dramatically reduced by the Rod Coleman filament regulator.

Edit (28th December 2012)

Just made a minor adjustment of the output voltage by the addition of R1 and R2 and fuse F1 was increased to 2A to avoid blowing it at full load when switched on:

4-65a filament supply

One of the good things about Christmas is that I always managed to get some proper time for working on my projects. This year is devoted to my belated project: 4-65A SE amplifier. We bid farewell to our family guests so had this evening a bit of time to start preparing the filament raw supply for the 4-65A.

This will be a heavyweight amplifier. Too much iron, but 100% DHT and no capacitor in the signal path, a promising design.

A couple of hours were sufficient to prepare some of the key components: a pair of common-mode chokes winded on ferrite rings, capacitor arrays and a Schottky bridge mounted on an aluminium sink:

The raw filament supply circuit follows the standard design recommended by Rod Coleman to use the filament regulator boards. I managed to get a nice pair of 10mH@3.5A chokes made by JMS transformers which is the same company that provides the custom-made split bobbin transformers:

The 10mH choke will help reducing the current pulses and the input capacitance, which is less than 3mF. This circuit will provide 12V @ 3.5A with a ripple current of 31mV peak to peak maximum.