26 DHT pre-amplifier (Gen2)

DHT addiction

As always opinions are very personal in this subject, however there is a shared view out there that DHT sound is very unique. I do like DHT pre-amps and have tried all sorts of configurations and many valves (e.g. 46, 10Y, 01a, 30, 30sp, 71a, CX112, 26 and 4P1L). After testing many of them I enjoyed most out of the 4P1L (Siberian pre-amp), CX301a and 26. The 26 is very special to me so decided to improve the previous version incarnated around the LL1660/8mA.

This time I decided to use the LL2745 provided by Thomas Mayer. Again, filament bias and Rod Coleman boards were the foundation of this design. I replaced my Salas shunt regulator for a simple stage of Russian glow tubes fed by a CCS (a la Gary Pimm). The test results were very promising, so this became the main HT regulation for this preamp. I added the Morgan Jones’ famous Kelvin Caps as shown below:

Glow tubes in the dark: very cool

R3 has to be adjusted on test to ensure the stack of two glow tubes draw about 10mA and the remaining is supplied to the 26 amp (about 5mA). Glow tubes are bypassed by russian PIO C3 and R5 is fitted to ensure both glow tubes strike as expected.

I’m using balanced connectors now despite the input is unbalanced. A special cable is required to fit this input. Rf is classic well known dale wirewound stackable 10W resistors. I’m currently using a pair only as filament starvation reduces the power dissipation requirements.

The output is balanced and floating. It can be wired to provide unbalanced if needed. I’m planning to build in the future a balanced input PP 6C4C amplifier so wanted to have the preamp ready for this amp.

The 26 is running slightly starved. I found an operating point that minimises distortion, microphony. Filaments glow will be very dim here.

The output transformer is wired in 5.6:2 mode to maximise the gain of this stage. This comes out paying the price of additional output impedance. In this case I’m not driving an TVC or long cables so is not an issue. Otherwise, the OT should be rewired to 5.6:1 which will give a lower output impedance around 300Ω, but lower gain.

The HT and LT raw supplies are unchanged, and are same ones from original design. You can obviously improve these, but I found the pre-amp to sound very nice so will keep them as they are:

Some measurements

Filaments are very quiet using Rod’s DC regulators. The noise at the filament is about 180μV. Originally I added an RC network at the output of the glow tubes which is not ideal as increases the output impedance.
After fitting the Salas SSHV2 regulator the following measurements were taken:

  • Operating point:Va=145V, Ia=4.5mA.
  • The supply ripple measured at the anode was -85dBu (50μV)
  • Filament noise/output noise with filaments only: 440μV/180μV
  • THD: 0.03% @ Vo=3Vrms (9dB gain)
  • THD:0.01% ” Vo=1Vrms
  • HT ripple: -80dBu (100μV)

How does it sound?

Well, what a tricky question. The typical answer from any consultant applies here: “it depends”. You can’t compare this component of the sound reproduction chain in isolation. Yet, I can provide my view based on other preamps used with the same system. The 26 pre-amp sound is very clean and warm. Has a good bass, overall tone and nice treble as well. I’d say I prefer slightly the sound of the CX301a preamp with gyrator load. Still need to listen to this pre-amp a bit more.

As usual my big thanks to Andy Evans, Rod Coleman, Thomas Mayer, DHTRob, Vyacheslav, Dmitry Nizh and Paul Leclerq

26 DHT Gen 1

If you want to check out my first 26 DHT preamp (Gen 1), take a look here.

26 DHT Gen 2 builds

Rui Lourenço built a fantastic version of this preamp. If you’re interested on this, check it out here.

26 DHT Gen 3 build (Added Nov 2013)

A new version of the 26 DHT pre-amplifier can be found here.


86 Replies to “26 DHT pre-amplifier (Gen2)”

    1. Gracias Pedro. Espero que la información sea de utilidad y deja tus comentarios cuando hayas implementado algunas de las ideas aquí presentadas.

  1. Much work has been done. It looks very good! Congratulations!
    look forward to the real test – FFT, ripple rejection 50Hz/100Hz, amplitude-frequency response (-3dB) and distortion 2nd and 3rd harmonics… at the maximum useful signal level .

    1. Hi Vyacheslav,
      Yes, I need to bring the preamp up to the workshop and do the tests.

      I have a faulty temporary cable which I’m using at the moment as my 45SE doesn’t have a balanced input so had to adapt a cable to force the output of the stage to be unbalanced (i.e. I’m not using the centre tap connection pin 1 and taking output from pin 2 and forcing pin 3 to be earthed). as cable is faulty I have a hum which need to get rid off first!


  2. Dear Ale,
    Could you be so kind as to tell me where can I find the 10W Dale resistors?

    Also, what’s Rod Colleman’s output voltage regulation that you use (knowing that input as schematic is 18VDC)?
    Kind regards,
    Rui L.

    1. Hi Rui,
      There are available on eBay from the US.
      Rod Coleman’s regulator is set to 700-750mA and with 10 ohm filament resistor will give you 7-7.5V bias. You have to ask Rod to send you the right resistors for the regulator for using them in filament bias.
      Hope this helps

  3. Hi Ale,
    Can you please tell why on your filament raw supply above, you don’t connect C7 ground to C8 and C9 ground?
    Thanks and regards,

    1. Hi Rui, yes they are all connected to the raw supply ground, it’s a typo in the diagram. The actual ground star is at the input of the pre-amp where the stepped attenuator, the return of the filament bias resistor and the ground bus are all connected.

  4. Dear Ale,
    First of all let me wishe a happy new year.
    Sorry to be bothering you again, but I had some time this part of the year to catch up with my 26 preamp. I now have gathered all the necessary components to start implementation. During some bench tests, mainly of power supplies, a big doubt arose, I have thought using Salas shunt, followed regulator followed by simple stage of Russian glow tubes fed by a CCS (KK audio boards), is this correct? I really like Salas boards, so should I skip the KK audio CCS and maintain the glow tubes (they look wonderfull), or maintain everything?

    By the way, filaments will use Rod Colleman’s boards.
    Thanks for your feedback.
    Warmest regards,
    Rui Lourenço

    1. Hi Rui,
      Happy New Year too!
      Some argue that the VRs are noisy so not beneficial to locate them after the Salas regulator. I will suggest you try both scenarios as you can set the operating voltage with the Salas regulator and you can comment on the noise level/sound

  5. Hi Ale,
    Sorry to be bothering you again, but some doubt occurred today when wiring up the LL2745 (bought from Thomas Mayer), I find, looking at the LL1689 data sheet (which I believe has the same wiring scheme as the 2745), I should go for 5.6:1 (alternative R) for less gain but also better lower output impedance, or 5.6:2 (alternative Q) for higher gain, but also not so good higher output impedance. Until here no doubt, the problem arrises, when I look to your schematic above, and find that the pin numbers you mention and connect, are different from both wiring scheme R and Q on the LL1689 data sheet.

    Could you be so kind as to give me some indications on this?

    Thanks and regards.

    1. Hi Rui,
      The pin numbers in my diagrams are wrong. I wired it in 5.6:2 but if you don’t need the gain I will recommend you to go for the 5.6:1 configuration

      You can wire it and test it on the bench with a signal generator, can you do this?

      1. Thanks Ale,
        I will just that on my bench with the generator.
        As soon as I have something concrete, I’ll let you know.

        1. Hi Rui,
          You should be able to connect the primary to the signal generator directly at low signal levels subject to your generator drive capability. If not you can add some resistance in series to limit the current and ensure it can drive the transformer. It’s a good exercise to get yourself familiarised with the transformer and its windings! Let me know how you get on

          1. Dear Ale,

            Thanks for your message, I have already wired the transformer according to the schematic from LL1689 and it’s working fine.

            Nevertheless I have bigger issues which I am not knowing how to resolve, mainly I have breadboarded one channel of the preamp, first power supply using 0-300VAC/100mA into an AZ11 mesh plate with two U4007 in graetz config, this feeds a CLCLC line, into Salas SSHV2 set for 185DCV and 40mA, connected to KK audio CCS and 2 VR75 type valves (as in your Gen2 schematic) and I get 148VDC.

            Then I have a raw filament supply composed of an EI transformer, rectifier bridge built around ixys diodes with snubers and the an CLCRC line (first cap 15000uf, and last 2 of 10000uf each, all Panasonic as you use), this gives roughly 18VDC, that are feed into Rod’s regulator with around 1,2V into 26 filaments.

            I have 5×50 Ohm dale resistors in parallel to get 10 Ohm, these are connected with Rod’s reg. as per your schematic. 

            So far so good with “thousands” of alligator connectors on the bench. Time to fire up, after all settles I get that nice sinusoid trace on my scope, so I have amplification and all seems to work. I then start to check some voltages, all seem correct, although the voltage for bias seems a bit high, like around 11V, first doubt?

            Then I try to check the current on the filaments with my meter in series with Rod’s regulator and the resistor array, and I don’t get more than 280mA, second doubt?

            Then I measure current at the 1 Ohm series resistor from supply to plate, and I don’t get more than 1,1mA, third doubt?

            I have Salas on board pcb heat-sink quite hot, nothing more is hot, Rod’s heat-sink is very lightly warm, KK heat-sink is dead cold.

            I connected a pair of head-phones to the output of the transformer and I get an amplifying signal from the generator, and almost dead quite hum, that completely disappears when I connect this output to the general output of the equipment. 

            26 tube is very lightly warm.

            I then tried to regulate all different trim pots on the boards, they move voltages and currents, but I can’t seem to get the right currents as on your schematic.

            Finally I tried to connect a 0R15 Ohm resistor in parallel with the resistor array (that lowers to around 1R8), and then I get the current on the 1 Ohm resistor to go to 4,5mA, but only around 2 to 3 volts bias. 

            I really fill all is related to the value of the resistor array, it what and how to solve it.

            Sorry for the long  message, but maybe you can help?

            Thanks in advance.
            Kind regards,

  6. Hi Rui,
    I suggest you follow these steps to do some root cause analysis:
    1) Test filament supply: if your Rod Coleman regulator is working, you should be able to measure 7-8V across the 10 ohm bias resistor array. If voltage is 10V then means you are running the filaments at full spec, which is not bad. You can adjust the trimpot in the regulator to measure the voltage across the resistor. Measuring the voltage across the filament is not helpful as the filaments vary across samples and we are interested in the current, so using the 10 ohm resistor as a reference is best. If your voltage is stable here this is fine.

    2) Test your HT supply using a dummy load. The Salas regulator can be tricky. Follow the instructions to set the shunt current and then do this simple test. Hook up the CCS which needs setting to at least 15mA per channel, the glow tubes and a dummy load 30K 2W resistor. This will emulate the 26 load. If you then can measure 148-150V across this load and the VR tubes are glowing fine, then your HT supply is working ok. Otherwise needs setting probably by adjusting firstly the Salas regulator and then the CCS trimpot. I suggest you build both HT channels (with both dummy loads) so you can properly load the HT supply, otherwise the raw HT will be higher due to the load variance. It will depends on whether you have one or two SSHV2 modules, don’t know your design details.

    3) If test 1 and test 2 are ok, then should be ok to test the 26 with the OT as a load. The filaments should glow (specially if you are running them at 1A) and the voltage in the anode should be somewhere around 145v. If not, something is wrong with either your 26 wiring, the valve itself or the transformer wiring. Check this then.

    Hope this helps

    1. Hi Ale, Thank you very much for your kind answers, so here it goes:
      Point 1)
      – Tested Rod’s boards without B+, both are working fine and I can really fine tune them on the trimpot.
      – First check: Filament voltage (measure on the tube pins…I know not the best reference, but to be shore), I have adjusted on the trimpot to get exactly 1,200VDC (but can easily go up or down).
      – Second check: Bias resistor value (with above 1,200VDC) is 9,180VDC steady and with no variations, meaning if I’m correct 918mA. (Anyway, if I increase the filament voltage on the regulator trimpot, I increase also the voltage across the Bias resistor to say 10VDC, but always very stable).
      – Third check: Adjustment of trimpot on regulator board makes changes in the Bias voltage.

      So, I believe above cheks are OK, correct?

      Going to point 2).
      – I tested the Salas boards (I will use 1 for channel) out of the circuit with a dummy load of around 6K, and managed to set 185VDC (I can of course go higher), and 40mA (also idem).
      – I settled the KK CCS to 15mA (150mV measured on a 10R resistor in series with 9V battery). Please note here one big difference from your CCS, is that I am using KK Audio kit, and the resistor values are completely different: My R1 and R2 are 910R (yours 120R), and my R3 is a trimpot with around 150R (yours is 39R).
      – I had doubts on how to connect the dummy load to the CCS and Glow tubes (can you please clarify?), nevertheless, reconnected all back (as per your schematic VER1,5) the preamp works and amplifies the signal with a nice clean sinusoid on the scope, the fact is that the 26 tube is running cold (you can easily touch it, and keep the end), but no experience with these, and maybe its normal. The filaments inside the tube are lighten but with no big glow (1,200VDC and almost 1A)
      – Glow tubes after the CCS in series to ground (with R5 and C3 in parallel as schematic). And glow right, but also run cold (I think it’s correct because they don’t have filaments).
      – The problem, I think is that I measure 1,1mA on the plate resistor R6, and should measure 4,5 or 5mA, but maybe this is an erroneous measure done with the DVM and all is ok, but if so, how can I be shore of the exact current on the plate?
      – I Also measure 185VDC at CCS (IN), and 142VDC (OUT), so a drop of 43VDC.
      – B+ (142VDC) is then connected to OT (which was wired as LL 1689 type R schematic) to pin 18 (with 21 and 16 connected), and Plate voltage coming from pin 13 is 136,8VDC (so a drop of 5,2VDC on the transformer primary)

      I recon, with the above checks, not everything is bad correct?

      I thank you in advance for your patience, and kindly ask for your advice.
      Kind regards,

      1. Setting the current with 9V on the CCS is not enough volts across the CCS to bias it properly. I don’t know the circuit used but probably better to use at least 50V across it or add a current meter in series to be sure on the current set. You may just have enough current to light up the VR as the output voltage is down in 142V. A drop of 5V across the primary is ok.
        The 26 won’t get hot and the filaments won’t light up too brightly.
        Can you measure the input and output sine signals with your scope?

        If you can’t put a series current meter just measure the voltage drop (current) across the 1 ohm sensing resistor and adjust the CCS until you get the 5mA needed. The VR should maintain the 150V across them. If this doesn’t work, then is the CCS.


        1. Thanks again Ale,
          The 1M dollar question -“What’s the voltage drop on the plate resistor?”, of course! I am so dummy! I was trying to measure the plate current directly on the resistor terminals with my DVM, not even in series with the circuit!!!??? Now it has become clear when I measured the voltage drop (used a 10R resistor to better measure), and with Ohm’s law, it has become clear. :-)))

          Everything is working fine, I had to adjust trimpots on Salas (Salas running 185VDC/50mA) and CCS (current no measured, but fine tuned on the plate resistor to get 45mV drop, so 4,5mA) and bingo! all voltages and currents are correct, I have now 150,2VDC on the OT and VR tubes well lighten and now runnig hot (touchable).

          Thank you so much again.
          Will send you some photos when the work is done.

          PS-I see that you are using Balanced out from the OT, how did you wire it?

          1. Hi Rui,
            Glad you found the root cause. Please send me pictures and keep posted on progress.
            Raw supply filtering and layout are key for a hum-free preamp. Please pay close attention to this

            For unbalanced output you need to connect the one of the secondary to ground and take the output from the other end. Ignore the CT connection. If you don’t do this you will get some nasty ground loops and a lot of hum!


          2. Hi Ale,
            Thanks again, I will start building next week, still waiting for some mechanical parts.

  7. Hi, How would I purchase Filament regulator kits? Plus I am in the process of gathering various parts to build a 26 Pre based on the gen 2 model & some of those parts are a little on the scarce side, so would appreciate anyone’s knowledge.

    1. Hi Barry,
      You can contact Rod directly, I have no commercial relationship on the regs. There is a page on this site about DHT filament DC regulation where you will find Rod’s email at the bottom.

      What is scary about the parts? Anything you need help with?

      1. Hi Ale,

        I will, thanks for your help, I will need mains transformers at some point. No I meant scarce as in not easy to find, at least not for me!.

        Many Thanks

        1. Hey Barry,
          Rod is very helpful and will be able to assist in whatever you need with the regulators.
          Regarding the transformers, well multiple options are available. For the filament TX, I suggest using split bobbins to reduce the noise coupling from the Mains.
          Where do you live? shipping of TX may be very expensive so actual selection of manufacturer will be determined by your location

          1. My previous project was a massive modification to a to Supratek chardonnay using 2c22’s with diode bias & ccs in place of plate resistors, plus using better quality parts & removing many unnecessary ones, the results were well worth it, so this is my first project from scratch.

          2. Brilliant!
            I also live in the UK (London). Sowter is great and pricey. I get the Lundahls from Thomas Mayer (Vinylsavor). Thomas is very helpful and can get the transformers at a very good price. I also bought from Thomas the power transformers which are of excellent quality.

  8. Hi Ale, 2 questions if I may, the first is how would you connect up to make a single ended version as opposed to balanced & would K&K’s CCS be ok to use before regulator supply( I have 2 spare 3 amp kits unbuilt knocking around)??

    1. Hi Barry,
      I’m unsure about the K&K’s CCS circuit, but I guess they are probably based on the DN2540 or similar. They should be rated for 250V and 15mA. So if they are 30mA its fine providing you can trim the current down to 15mA (or use one CCS for two channels).

      The LL2745 pin numbers on my diagram they are not correct. Anyway, you can wire the output connector pin 3 to ground, ignore the CT of the LL2745 and take the output from pin 2.

  9. Hi Ale,
    They are rated from 10V to 1KV, & are adjustable from 2.5mA to 100mA, they are based on 2 mosfets (IXYS IXTP01N100D & IXTP08N100D2 they are adjustable & once set the pot can be changed to a fixed resistor & they work excellently with my 2c22 Pre.
    I am having trouble tracking down the Gary Pimm ones as they now appears to be unavailable.


    1. 01n100d is excellent. It’s VDS max is 1000V. Similar to the DN2540 but with a bit higher capacitance. A bit of an overkill for this application but should work perfect.

      The classic cascoded DN2540 performs beautifully and it’s used widely in many circuits…


  10. Hi Ale, where can I get an sshv2? It appears they aren’t being maded at the moment. I am assuming only one is needed as the circuit splits after that point.


    1. If it doesn’t happen any time soon you may need etching your own pcb. Can you do this? I can’t promise anything but I may need doing some and happy to do an extra pair however this wont happen any time soon….

  11. I don’t have the equipment to do that, but I have added myself to Tebags list. I assume that you are using 2 in your 26 pre?

    Many thanks

    1. I’m currently using one simply because I’m reserving the remaining two boards I have for my current project. You can use one or you could also get away without one as a workaround until you get the boards if you have a well regulated HT supply and you use the VRs with the CCS boards…

    1. Simply because until we don’t reach a certain number of boards the group buy won’t be initiated, so we are on a waiting list 🙁

      1. I have had an email from DIY stating that they will be sorting it out this weekend.
        Ale, On your Filament supply you have 2 chokes, & where can they be sourced & or who makes them also on the Power Supply you have 2 chokes are they 2X 10H giving a total of 20?

        I have ordered most things, but not everything yet, I won’t start it until it’s all here.

        Best Barry.

        1. Hi Barry,
          That is great news!
          HT Chokes are 20H from Hammond. You can source them from Bluebell Audio. Give Philip a ring, he is very nice and helpful. A great pleasure dealing with him. I buy most of my chokes from him.
          The common mode chokes are better these ones:
          I wound mines originally.

          LT supply can be improved and if you can afford a pair of 1H @ 2A chokes you could do a great choke input supply instead. High quality Panasonic caps are recommended.

          1. Ale
            As regards Panasonic Caps, would that be the new EZP 800V ones that are NP? I was thinking Mundorf, Solen non elctrolytic maybe as they are great PS Caps, but the Pans are cheaper.


          2. Electrolytics only for the filament supplies. I wouldn’t use them in the HV. I prefer using oil caps for the HV.
            For the LT supplies, look at the ones with lower ESR, they are more expensive but better 🙂

  12. Those chokes from Farnell are only 15mH, would they be ok? Your drawing quotes 200Mohm is that between windings? I did use chokes in the P/S from Lundahl for my last project & TBH I was dreading the cost, also can the mains transformer be sourced from Bluebell, I see it’s a Weiss, but I can’t find a 10/56 online anywhere.

    1. Yes, Lundahl chokes are too expensive. The Hammonds perform really well.
      The choke is a common-mode and 15mH per winding is ok. You are using it to filter common mode noise so it won’t contribute too much to the ripple filtering. You can use whatever mains Tx you can. The Weiss is from Thomas Mayer, they are more expensive but really good quality.

  13. Hi Ale,
    Sorry to keep bothering you Your parallel Cap array in the filament supply are those values correct, as in the simple supply I have for my other project I’m using 16000uF 16V, whereas you are using 14.5uF only, is this sufficient? Also I’m having trouble contacting Thomas Mayer!!

    1. All values in “mF” which means rthat 14.5mF is 14,500uF formed of 3 x 4,700uF capacitors.

      Thomas is very responsive. Did you email him?

      1. Hi Ale
        Yes, He is sending me some 2745’s & weiss 10/56(100mA) transformers, but He knew nothing about any transformer with centre tap (15V+15V), I’m assuming I will need 2 of these?


        1. Great stuff. Thomas has some filament transformers. You may need to adapt the design if you don’t get a CT one and you will need to use a full wave rectifying bridge instead
          You can get cheap 50VA from Rapid worst case

    1. No in my original design but if you can get a split bobbin EI transformer is the best. No toroid please
      This is for minimum Mains noise transfer. JMS can wind your custom split bobbin 100VA for a low price

  14. Ale
    Is it better to use two transformers for the filament supply & would 50VA be sufficient, ie 2 X 25/30VA.

    Best regards

  15. Ale
    Your drawing states 2 coleman regulators, one on each channel.
    I do hope I’m not getting out of my depth with this!!


  16. Ale,
    Although I have been thinking of building a 26 Pre-amp for a couple of years, I have only just plucked up courage to actually do it, I just want to get everything together before I start, as I know from the mods on other equipment I have altered have been a right pain if you don’t get it sorted in the first place, otherwise it just ends up looking like giant birds nest, LOL.

    Thanks for all your advice it’s much appreciated.


    1. Hi Barry,
      That is a very sensible approach. you will learn more and correct some issues before the final build. I’m breadboarding everything now as can give you a good insight on the circuit. You can test, correct and listen to it before drilling which is then more difficult to modify.
      Let me know how you get on and what else you are struggling with. Good luck!

  17. Hi Ale, I have built the HT Raw Power supply & have tested it briefly with no load, the Primary side or the Weiss 10/56 Transformer is configured for 240V, output is 221V with no load & after CLCLCR it’s 304V DC, is this correct? the Rectifier is Mullard AZ3, Filament Voltage is 4.41V, all voltages are higher than expected, but 304V with no load seems a quite high, is this normal? The issue here is that I would have expected a drop, not a rise, very strange!!

    1. Yes, because you are not loading it and the supply will not sag. You need to add a dummy load to reflect your actual load, then the voltage will drop.

  18. Hi Ale’

    You mentioned 2 pairs Rod Coleman kits are required(4 kits) is that correct? Also you have 2 B+ outputs from PS, is this just in case you are using 2 Salas Boards? I will be using only one, at least to start with!!

    1. Hi Barry,
      One pair only. One board per channel. Both channels need separate LT supplies including Rod Coleman boards

      HT supply can be one and two salas regulators ideally.

  19. Hi Ale,
    Now that i have completed the Power Supply, where/how would I obtain Morgan jones Kelvin capacitor to use in this project? They appear to be like Hens teeth.


    1. Hi Barry,
      They are manufactured by Suppression Devices in the UK. You can avoid them, in fact I didn’t use them in my final version. Check the schematic above.

    1. Hi Barry,
      4 10 ohm resistors used two parallel pairs in series to deliver 10 ohms. This will keep them running cool to keep the noise level down.

      1. Thanks for your reply Ale, I am running tests at the moment, but without Salas Regulator’s, on my filament checks with 15ohm dummy load, I am getting a minimum reading of 1285mA with Coleman Regulators set at their lowest points, is this due to being supplied the wrong resistors? If so what value should I use on the Regulators?

        Also should I connect the 30K dummy loads for the B+ circuits between Plate’s & ground to check that all is ok with Glow Tubes/current draw etc?

        Many thanks

        1. Hi Barry,
          When testing filament boards you need to either add the filament in series with your filament resistor or a 1.5 ohm dummy load to replace the 26 filaments. Not sure whether you asked Rod for the boards to work on filament bias, so can’t help there. The current adjustment should allow you to trim current below 1A.
          Regarding testing the HV with a dummy load is a good choice to ensures is working as expected

  20. My B+ circuit is working well with 4.5mA(133V) /4.61mA(141V) using dummy loads on plate pins, this can be adjusted using KK CCS. But Rods Boards appear to have both packed up using a 15 ohm dummy load across filament pins, that’s why they won’t adjust, the resistors supplied appear to be the correct ones, according to Rod, I have the filament resistors configured as you suggested 2 pairs paralleled together to give 10 ohms, these are connected to ground, voltage at the pins with no load is way too high at around 22 volts, with the filaments loaded there is exactly 18 VDC coming from the Filament Boards, with around 2.6V at filaments.

    Best Barry

    1. If you send me a drawing of your filament setup with measured voltages I may be able to understand better the situation.

          1. Hi Ale,

            I have salas regulators built now, would 30k 7 watt resistors on output be about right for dummy load or should I load it after glow tubes? Also should I set both Salas boards tp 15mA?

          2. Hi Barry, you want to set up the salas regulator to about 200V/50-60mA if using one to support two CCS and VR sets (40mA for output and 20mA to allow the shunt regulator to work effectively) . Each VR will use 10-15mA and 5mA for the 26. So the output dummy load for the SSHV2 should be about 200V/40mA=5K. Get an aluminium clad resistor of about 5K6 and 15W or equivalent combination of resistors depending of what you have at hand. Use a variac to bring the voltage up slowly. Measure the shunt current at the test point (10 ohm resistor) and the output voltage at the same time. If the output voltage is not coming up effectively then stop as the regulator may have a problem and you may run a risk of blowing the sand devices. If ok, then set the current to 50-60mA and then adjust the output voltage to 200V. You can then replace the dummy load resistor for the CCS and VR pairs.
            Hope this helps

    1. So each SSHV should be set to 30-40mA. Id go for 30mA and see how it performs. 25mA won’t allow too much current for the shunt to operate properly. The dummy load should be fine with 10K then.
      Let me know how you get on! Cheers, Ale

      1. Hi Ale, I have sent you some photo’s of my (almost) finished project, although I am still tweaking voltages at the moment, however I have found that if using 2 Salas Regulators with just one power supply, it is not possible to get a drop of 20 volts between input & output, this makes for bad regulation & one or both board subsequently become damaged, but with just one everything is just fine, I get around 207V going in & set the regulator at 185V going into CCS. I can see the way around this would be to use 2 Power Supplies from one larger transformer.
        Thanks for all you help regarding this project.

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