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…

A 26 DHT pre-amp from Albert

Albert sent me recently some great pictures and feedback of his 26 pre-amplifier using gyrator as the anode load similar to my design on the 26/01a.

Here are Albert’s notes on his 26 preamp:

“I have my 326 with separated power supply, 5U4G – 8uf PIO – 20H – 50 ASC – 30H – 50 ASC + 18uf pp feeding the Salas’ HV Shunt V2 Regulator, then to 2 ( left and right ) cascoded DN2540 gyrators. Rod’s filament supply coupled with filament bias @ 850mA with anode @135v. I don’t have anything to check the distortion, but this is the most good sound preamp that I have finished to date. It pleases my ears so much that I have the intention to turn it on once I’m in the house. It sounds gorgerous with very good image seperation. Sound stages are wide, deep and absolute quiet. The highs are sparkling smooth with lots of details. Mids are lush and warm with admirable vocals. I’ve found it even better at the lows if compare with 2x 156c. It goes deeper with clear notes.”

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.

4P1L with 126C OT load stage

Andy Evans built a pre-amp with the 4P1L and was delighted with the sound of it albeit the 4P1L was running below its optimal operating point: 15mA given the limitations of the 126C interstage transformer.

I went to my workshop to test this configuration and looked at biasing 4P1L with fixed bias and driving it with 1Vrms or more to see what the results were:

So here is the first test at Vg=-4V, Va=74V and Ia=15mA

(all tests were done with the 100k input impedance of the Pete Millett Sound Card interface as the secondary load of the OT. 4P1L had both filaments in parallel and If=600mA)

You can see a richer harmonic profile with the OT and distortion is around 0.13% when driven with a 1Vrms providing an expected Vo close to mu (Vo=8Vrms)

The distortion gets very high when output voltage is higher than 9Vrms:

Now if we bias the valve at a more convenient operating point:

We get a slight improvement in THD down to 0.11%. However the distortion above 9Vrms is still high:

So what if we compare the performance of the OT against the CCS?

As we can see from above the distortion is halved. Now if we look at how well this valve could perform if biased in a better operating point, we can see that distortion can be reduce down to 0.03%

4P1L CCS load THD 6 — 4P1L THD 6 (CCS) minimum distortion figure

Minimum distortion from a CCS (or gyrator) doesn’t mean that it will sound better. Clearly the OT doubles the THD of the CCS equivalent circuit. Gain here is nearly same on both as OT is in 1:1. Only way of judging both is to do a listening test….

(which is what I’m planning to do next)

45 SE Amplifier upgrade

Replacing driver for 7193 valve

Well, after nearly 12 months of playing relentlessly my 45 SE amplifier, had an unexpected failure in the power supply (passive regulator) which forced me to do maintenance to the amplifier. It was a great opportunity to remove the 6J5 driver and do a quick swap for the greatly respected 7193 (i.e. military version of the 2c22)

45SE Amplifier upgraded with the 7193 drivers

Bias point remains unchanged: 7mA and 260V for maximum swing and minimum distortion. Need to look at my notes, but I remember that I was something around 0.30% at 100Vpp driving the 45 (which is not an easy load for anyone). Driver configuration was not changed, so had the 7193 now loaded with same DN2540 single transistor gyrator and mu-follower output for lower impedance. The valve was biased at about 8V with an LED array.

As a test, played the fantastic Symphony No. 3 from Henryk Gorecki….

26 THD analysis

As I’m proud of my 26 DHT pre-amp and also looking to use this valve as the first stage of my 4-65a SE amplifier shortly, I looked at how linear this valve is.

Unfortunately I don’t have an extensive set of valves of same brands, so albeit I have about 40 valves if this type there is a big mix of different brands and many of them are used ones. Having said that, I think this may be the case of many of you out there, so I think that probably the results of this test may be relevant to you (if you are still reading this post).

So in summary, this is the sample set I tested:

39 valves
12 brands
ST (34 valves) and Globe (5 valves)
Type: 26, 226 and 326
State: used (80%) and NOS (20%)

All DUT were tested with the same test set and operating point:

Operating point: Ia=5.5mA and Vg=-10V
Anode load: Cascoded DN2540 CCS set to Ia
Test signal: 1kHz, Adjusted to obtain Vo=10Vrms (+22.22dBu)
THD analysis: Audiotester via Pete Millett’s interface:
- 32768 sample FFT (2.96Hz resolution)
- 256 averaged windows
- Van Hann window
- THD for H2+H3+H4

So in summary the results showed that you should expect a 26 to have around 0.08% (Std Dev = 0.00047). Here is an histogram showing a summary of the tests done:

Looking at brands, the following average THD ranking was produced:

26 THD by Manufacturer/Brand

Interesting to find National Union leading the chart. I found RCA and Sylvania to be my preferred ones in terms of sound. THD shown above are average of sample sets of 4 valves or more. Although I tested 12 brands in total, some of them were just a pair so they are not a representative subset.

Finally, here is a sample THD of a very nice 26 valve:

I wish I could have a bigger collection of 26 to improve the accuracy of this statistical analysis. Either way you can get a view of what you should expect from this great valve…

CCS load for THD measurements

Here is a simple point to point soldered cascoded MOSFET CCS using the classic DN2540. A very simple design: carbon grid 1K resistors and two potentiometers I had at hand: 2K (coarse) + 100Ω (fine). I can set the operating point of the valve under test from 3mA to 100mA. The anode output is directly coupled to a BNC connector which is hooked to the Pete Millett’s interface. No capacitor used as the interface has a DC blocking capacitor.

I used an old aluminium box and build this takes less than 30min!

CX301a DHT pre-amplifier

CX301a DHT preamp

Here is my latest incarnation of the DHT pre-amplifier:

Many claim that the 26 is the best sounding DHT valve for a pre-amp. I will agree to a certain extent, however I personally found the thoriated-tungsten filament sound a bit more rewarding to my ears. A more clear and defined treble in my opinion.

Since I plugged in my CX301a incarnation of my breadboarded preamp, I just left it there as I loved its sound. Certainly there are things to be improved to enhance the dampening of microphony, albeit I have to confess it hasn’t been a problem to me. Have heard some valves to howl, and this is not one of those. Clearly suspending the valve socket or adding the rubber dampers to the valve holding plate or socket will help massively.

Filament bias is a must in my DHT designs. Since discovered it, can’t avoid not removing most capacitors that I can from the signal path. In this case the filament resistor R9 will increase anode resistance by R9 times (μ+1). This will also impact the stage gain, but here all this is not a problem. You may find this is way too much gain in your system. Rod Coleman’s filament DC regulators are crucial to provide a hum-free stage. Attempting AC or other DC regulator is likely to bring frustration to your design. Believe me, I’ve been there before…

Now turning our attention to the anode load I will not open a debate here (or a can of worms!). You can make your choice of using a superior quality output transformer (and by superior means a lot of money!) or you can look at various options. A choke is a great idea, but special care needs to be taken to ensure choke is not picking up any hum from the remaining parts of the circuit – specially the supply transformers, etc. I have experimented for some time various types of CCS or gyrators as sandy loads for the valves with excellent results. If you are one of those that feels that sand is a sacrilege, then I suggest you stop reading this post now.

Gyrators are superb. They can simulate the AC response of an inductor of 300H (but without storing energy as a real inductor) or above very easily at 1/100 of its cost. You can easily adjust the valve operating point ensuring this is maintained despite the ageing impact of the valve or the eventual replacement of it. The anode voltage will be fixed by the gyrator, the current not. Cascoded MOSFET gyrators provide better supply ripple rejection and isolation. Using Q3 as a CCS instead of a high resistance potentiometer to set the anode voltage is better as it helps providing a better frequency response as impedance on this node is increased. A higher value of R10 will help reducing the size of the gyrator capacitor and the smaller the better it will sound in my experience.

M1 and M2 can be your depletion FET of choice. M1 should be a 250V rated one at least. Depending where you live, you will be inclined for using BSP129, LND150 or DN2540.

Previously I mentioned in some other posts that the mu-follower setup of the gyrator here provides a better output impedance and improves the performance of this valve significantly given its high anode resistance compared to other more suitable DHTs for this purpose such as 4P1L, 46 or 71a.

I’m not going to cover the HT supply here, but using a shunt regulator such as Salas, is one of the best choices here.

With Russian PIO capacitors you will get a fantastic result here, no need to start burning serious money on the capacitors until you are happy with the end to end build and you can then start looking at how to improve the sound of it by replacing some bits with better (or preferred) quality components

With an operating point of Ia=3mA you can get THD=0.08% at Vo=10Vpp. This will be subject of the quality of your CX301a. Some older globe 01a’s have a great sound, but they are not that linear. Hard to pick and chose your precious ladies here without testing them for linearity.