Preamp/linestage, switchable between 01A and 26 type tubes

side view

This is an all DHT preamplifier/ line stage.  I has one stage and is built with separate enclosure power supply. This a very flexible solutions preamp. One can choose from:

  • 01A or 26 type tube
  • line output transformer or capacitor coupled output
  • two level of gain on transformer output

The power supply unit uses Hammond 270Ex for main HV in a quasi choke input 0.68uF(MKP)-20H(100mA)-47uF(Obbligato Film-Oil)-20H(100mA)-47uF(Obbligato Film-Oil).

Here is the PSUD2 result:


in reality the rectifier used was 6BY5GA so the above results are slightly different. HV is supplied with a low ripple which is perfectly adequate considering that SSHV will add at least 80dB PSSR bringing the ripple at extremely low levels ( I could not measure with my equipment).  The raw power supply for the filament bias are using Hammond167L12, 1N5822 and 10mF-2×0.33ohms-10mF-0.1uF-220pF. PSU is supplying all voltages (HV and LV) through a 7 wire screened umbilical cord, which is permanently attached on the main preamp side and can be unplugged on the PSU side.

The two raw filament supplies are placed in the PSU enclosure and are providing the following :
– PSU raw filament supply for 26: biased at 900mA. Output 15V, more than 10.3V supplied after Coleman that is 9V on 10ohmsRAwFil 1

Same PSU when switched to 01A ( biased at 230mA), will provide at least 12.5V after Coleman or 7.1 V on 30 ohms bias resistorRAwFil 2-01AThe main preamp incorporates the two SSHVs and four Coleman filament regulators ( one pair for 01A and one pair for 26). In order to switch between 01A and 26 tubes, there are 2 sets of DTDP high quality Carling switches. Three of the poles are used to switch between the Coleman regulators ( each channel has one Coleman filament regulator for the 01A @ 230mA and one Coleman filament regulator for the 26 @ 900mA) and the fourth pole is switching the bias resistors placed between one of the filament pins and signal ground ( a permanently connected 30 ohms Mills MRA12 is used to bias the 01A and when switching to the 26 tube there are 2 more 20 ohms Mills MRA12 added in parallel to the 30 ohms( equivalent bias resistor for the 26 will consequently be 7.5 ohms)).

The other feature of this preamp was to switch between transformer output and capacitor coupled output. An additional DPDT Carling switch will do the job. One section (pole) of the switch will be connected to the anode of the tube and connect the anode to either the primary of the line output transformer (LL2745) or the Gyrator ( Ale Moglia’s Gen 2 ).  The secondary of the LL2745 is using either Alt Q (2.8:1)or Alt R (5.6:1) providing two fixed gain alternatives. A 6 pole two position rotary switch is facilitating this function. There are two sets of RCA Neutrik jacks, and in order to use either transformer or capacitor outputs one needs to physically move the interconnection cables to the appropriate jack.

The preamp is provided with volume control. I installed a stereo stepped attenuator by Gold Point ( a very good price quality solution).


The following measurements are only to give you a sense  of the approximate performance. I used a computer software and interface and this comes with some artifacts and noise that are not present when the amp is connected in a audio path. One, if owns high quality audio analyzer, can take more precise measurements

  • Gain when transformer output T out) : Alt R: 3-4dB(x1.5) and Alt Q: 9-10dB(x3)
  • Gain when capacitor output (C out): 18-20dB(x8)
  • Input impedance: 80kohms, unbalanced
  • Power consumption: max 85VA

“Shiny Eyes” switchable 01A-26  Line stage Measurements

  • 26 type vacuum tube
  • Transformer output
  • LL2745 Alt R  6:1   
  • THD at 3.03V output (1.92Vrms input) Gain 3.58dB, 2nd @ -66dB bellow fundamental, 3rd @ -68dB
  • THD-T-R-26
  • FR 14Hz @ -1.5dB, 21kHz @-1.5dB,  8Hz-29 kHz @ -3dB


  • LL2745 Alt Q 8:1       
  • THD at 5.93V output (1.92Vrms input) Gain 9.8dB, 2nd @ -66.3 dB bellow fundamental, 3rd @ -67.4 dB


  • FR 6Hz @ -1dB, 37kHz @+1 dB,  one dip at 11kHz @-1.35 dB


  • Gyrator load and capacitor coupled output
  • THD at 16.83V output (1.92Vrms input) Gain 18.8dB, 2nd @ -66.3 dB bellow fundamental, 3rd @ -67.4 dB


  • FR 12Hz – 41kHz @-0.25 dB


  1. CX-301A type vacuum tube
  • Transformer output
  • LL2745 Alt R  6:1     
  • THD at 3.1V output (1.92Vrms input) Gain 4.16dB, 2nd @ -66dB bellow fundamental, 3rd @ -68dB


  • FR 20Hz @ -0.7 dB, 20kHz @-1.7dB,  7Hz-25.5 kHz @ -3dB


  • LL2745 Alt Q 8:1       
  • THD at 6.2V output (1.92Vrms input) Gain 10dB, 2nd @ -66 dB bellow fundamental, 3rd @ -69 dB


  • FR 6Hz @ -1dB, 37kHz @+1 dB,  one dip at 11kHz @-1.35 dB


  • Gyrator load and capacitor coupled output
  • THD at 17.6V output (1.92Vrms input) Gain 19.2dB, 2nd @ -66.1 dB bellow fundamental, 3rd @ -98 dB


  • FR 8Hz – 43kHz @-0.5 dB


  • Output impedance measurements
Output type/tube 26 tube CX-301A tube
C out 260 ohms 457 ohms
T out 4dB 413 ohms 462 ohms
T out 10dB 1,656 ohms 1,886 ohms

And here are some more pictures:

top view topplate psu Main-with copperview Mainbottom psucloseup psubottom psutop topplateMain



3 thoughts on “Preamp/linestage, switchable between 01A and 26 type tubes

  1. Hi, I’m considering buying the lundahl 2745 o.t. for a 26 preamp build.
    Did it work well for you and did you have a preference between the alt Q and alt R?

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