For those who are keen on the 26 preamp in filament bias and OT, here is my latest incarnation of this great pre-amp.
Tag: DHT preamplifier
Testing a CX226 pair
A nice pair of CX226 in the new preamp. They are in very good shape, very low distortion measured in the workshop time ago and no hum induced, so great for this preamp.
Sound is definitely warmer, without loosing any of the bass and bright treble from this preamp. A great preamp so far, happy like a bunny!
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26 pre-amp build near complete!
Today made significant progress on the new version of the 26 pre-amp. Only bits outstanding are the CCS modules part of the HT supply on board. Tested glow valves, output transformers and filament circuits…
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DHT dilemma
26 DHT preamp with LL2745 OPT
Today I managed to finally test the Lundahl LL2745/8mA specially designed by Lundahl for Thomas Mayer for a 26/01a preamp / line stage.
The circuit tested is here.
Initially did some tests with fixed bias, normal DC heater supply from my workbench and HT from a passive capacitor multiplier also available in my workbench. Circuit breadboarded has long cables and we shouldn’t expect good 50/100Hz noise levels as have many transformers and things around 🙂
Test 1: fixed bias
- Vgk=-6,8V
- Ia=6mA, Va= 119V
- THD=0.033% @ Vo=1.4Vrms
;
Test 2: fixed bias with Rod Coleman regulator
- Vgk=-6,8V
- Ia=6.5mA, Va= 119V
- THD=0.031% @ Vo=1.4Vrms
- Vf=1.4V (minor starvation)
Test 3: Filament bias with Rod Coleman regulator
- Vgk=-6,3V
- Ia=6.5mA, Va= 119V
- THD=0.031% @ Vo=1.4Vrms
- Rfilament=5Ω, Vf=1.4V (minor starvation)
As we shown earlier in other tests, filament starvation reduces THD slightly as expected. The OPT performs really well. Probably will look at starving a bit more filaments whilst doing a listening test
Will now proceed to rebuild the 26 DHT preamp with LL1660 with this circuit 🙂
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%
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)
A preamp design with the LL2745
Lundahl LL2745 based OT DHT pre-amplifier
Before breadboarding a pre-amp with these nice OT provided by Thomas Mayer, I decided to simulate some options in LT Spice to see what results I got.
CX301a DHT pre-amp
First one is my preferred CX301a thoriated-tungsten DHT. I wired the LL2745 in 5.6:1 step-down configuration. This should provide a low output impedance which is what we want in this configuration where we need to drive the cables to the amplifier with sufficient capability:
Looks very promising. Of course gain will be much lower than a gyrator-based pre-amp, the 01a anode load is optimised providing very low distortion: 0.015% based on my SPICE model with curves taken from real CX301a. Gain is low at 3.1dB, but we don’t want loads of gain in this pre-amp. Sound is what we are after…
26 DHT pre-amp
Now it’s time for the revered 26. I used Dmitry’s model based on the RCA manual curves. I’d like to simulate this again using a model based on starved filament curves with a real 26.
Originally planned to bias the 26 in a different operating point based on feedback from Andy Evans, however after playing a bit with the OP I found that a more linear point was around Ia=5mA and Va=114V @ Vgk=-6.85V
26 looks more interesting in principle as the output impedance with this model is lower than the CX301a thanks to having a lower Ra (7KΩ against 11KΩ) so LF response will be slightly better in a side by side comparison.
Will be trying these two with filament bias and Rod Coleman’s filament regulators. The HT will be provided through a Salas HV shunt regulator.
Stay tuned…
New Output Transformer for the 01a / 26 pre-amps
New Lundahl LL2745 OT pair arrived today in the post. Very excited, Thomas Mayer kindly send me this pair for trial. These are specifically designed for the low current and high anode resistance DHT used in pre-amps (e.g. 26, 01a, CX112, etc.). Wiring and primary details are similar to the LL1689. Hope we can get a final datasheet in September 🙂
These are gapped at 8mA. So the primary inductance is about 200H in theory (160-180H in practice according to Thomas). The labels are incorrect. This is 2×2.8:4×1, providing the option of wiring it as 5.6:1 if both primaries windings are in series and all secondary windings are in parallel.
Will rebuild the 26 or 01a preamps and do some test.
I would like to build a quick breadboard with the 01a to do a comparison against the gyrator loaded preamp I’m using at the moment.
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.
DHT preamp evolution
My 01a DHT preamplifier has performed flawlessly over the last 4 months or so. I do enjoy its warm sound and clear tone. I do prefer it to my 26 OT preamp, despite everyone says the contrary. I personally feel that the thoriated tungsten filament gives some sonic unique mark to the sound here.
I want to do the following test and compare differences in sound:
1) Gyrator load
2) Antitriode load
3) Choke
4) OT
Have my LL1660s which can take out temporarily from the 26 preamp, and also got a couple of chokes to use as well, oil caps, etc?
What do you think?