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!

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!

4P1L PSE tests

Testing parallel DHTs

After listening to a great incarnation of the 4P1L PSE in filament bias output stage from Andy Evans, I decided to have a look at the impact of unmatched pairs of triodes from a distortion point of view. Main reason was that when listening to Andy’s amplifier I noticed a bit of an uncomfortable treble with some strings. Perhaps the increase of odd harmonics, but wanted at least to see what was all about.

4P1L are very easy to match. you can easily get a pair with equal mu. Just randomly I picked from my collection a pair of valves with a difference of 0.5 in mu.:

THD is about 0.03% mainly driven by H2. It happened that one 4P1L from the pair had 0.02% where the other had nearly 0.04% distortion. The difference between H3 and H2 is about 8dB.

Then looked at a more closely matched pair (0.03 mu difference). The distortion wasn’t surprisingly different:

4P1L PSE 60mA matching — 4P1L PSE matched pair

Again, nearly 0.03% and difference between H2 and H3 is down to 7.5dB.

Looking at the individual performance of the 4P1L, now biased at 30mA and similar anode voltage, we can see that despite having a lower THD, the difference between harmonics is just 5dB. This is the THD of the other 4P1L from the pair:

Well, how rthis compares to a 2a3/6C4C? The latter valves are two triodes physically connected in parallel inside the same envelope. So, no matching can be done:

The previous was a low distortion 6C4C I have. Distortion is higher than 4P1L PSE, but not that much. H3 – H2 difference is about 12dB.

My early thoughts:

4P1L are very easy to match
4P1L PSE performs really well. Distortion of the pair is lower than a 6C4C performing at same level.
H3 component is higher in PSE and this could be the reason why is more noticeable when listening to strings – as I proved in practice.

A new 4P1L Siberian from Martin

Martin kindly sent me a couple of pictures of his latest DHT preamp incarnation of the 4P1L Siberian:

Here are some notes from Martin about his pre-amp:
“A bit more about the circuit, I have built the power supply around the nice AZ1 mesh rectifier. First cap after the rectifier is an obligato 6,8 UF oil followed by a 10H choke and a 100 UF ASC followed by the SSHV2 that I purchased via DiyAudio group buy. On your suggestion I have used the CCS from your 301A preamp. If I refer to my previous preamp with the EF6 penthode in triode connection in parafeed configuration I can say that the 4P1L preamp sounds more detailed and with more air around the instruments. Finally I will do some tests with different output caps to find out if this makes a difference. At the moment I use the Vitamin Q.

Best regards,
Martin”

26 preamp (gen2) article

For those who are keen on the 26 preamp in filament bias and OT, here is my latest incarnation of this great pre-amp.

4П1Л/4P1L triode curves 2

Previously I publish my 4P1L/4П1Л triode-strapped curves using my curve tracer. A forum friend who helped me in building my tracer as he was building his own, suggested using two tools for capturing curves (“Graph Grabber“) and plotting curves, loadlines, etc. (“Graph“).

Here is an example of the 4P1L/4П1Л triode-strapped curves using both applications to produce the safe operating area based on 7.5W anode dissipation. This can be extended to 9W if we consider the screen dissipation as well.

New modular preamp 26/01a and 4P1L

Today made some progress in building the modular pre-amp using my latest design on the 26. I’m planning to make this preamp a modular one to enable quick changes and replace pair of valves and OT. This will enable to use 01a and 4P1L as well…

I used a floor plank I found around as the base. The front is a simple aluminium piece that holds the stepped attenuator. Both 4-pin sockets are standing with the rubber suspensors that DHTRob kindly sent me. The two resistor arrays for filament bias are placed between the valves and the OPTs.