Author: Ale Moglia

"A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp)

814 SE A2 Amplifier

Goodbye 4-65a SE, at least for now

IMG_1401After enjoying the 4-65a SE amplifier for many months, I couldn’t resist myself from upgrading the output stage to the 814s.  I just needed changing sockets and filament raw supply transformers to fit the requirements of this lovely transmitting valve. Needless to say, my recent tests on 814s were very encouraging. The 814 seemed to perform much better than the 4-65a in delivering 10W of class A2 sound at half the distortion levels. This to me, was only worth trying.

Continue reading “814 SE A2 Amplifier”

6ж52п / 6Z52P in triode mode

I have only one 6ж52п which measures within specs:

Operating point in pentode:

  1. Ia=42.1mA @ Va=150V/Vs=150V
  2. gm=4.47mA/V
  3. Is=4.4mA

Triode curves:

6z52p triode

 

Triode SPICE model:

6z52p triode SPICE model-published

 

And resulting SPICE model:

**** 6Z52P TRIODE ******************************************
* Created on 11/17/2013 19:25 using paint_kit.jar Version 2.4 Beta. May 2013 
* Curve traced and model by Ale Moglia 2013 valves@bartola.co.uk
* Curves image file: 6Z52P TRIODE
* Data source link: www.bartola.co.uk/valves 
*----------------------------------------------------------------------------------
.SUBCKT TRIODE_6Z52P 1 2 3 ; Plate Grid Cathode
+ PARAMS: CCG=13.5P CGP=0.5P CCP=2P RGI=2000
+ MU=76.5 KG1=34.7 KP=252 KVB=1272 VCT=-0.11 EX=1.39 
* Vp_MAX=200 Ip_MAX=200 Vg_step=0.5 Vg_start=0 Vg_count=8
* Rp=4000 Vg_ac=55 P_max=11.2 Vg_qui=-48
* X_MIN=64 Y_MIN=48 X_SIZE=421 Y_SIZE=530 FSZ_X=995 FSZ_Y=675 XYGrid=false
*----------------------------------------------------------------------------------
E1 7 0 VALUE={V(1,3)/KP*LOG(1+EXP(KP*(1/MU+(VCT+V(2,3))/SQRT(KVB+V(1,3)*V(1,3)))))} 
RE1 7 0 1G ; TO AVOID FLOATING NODES
G1 1 3 VALUE={(PWR(V(7),EX)+PWRS(V(7),EX))/KG1} 
RCP 1 3 1G ; TO AVOID FLOATING NODES
C1 2 3 {CCG} ; CATHODE-GRID 
C2 2 1 {CGP} ; GRID=PLATE 
C3 1 3 {CCP} ; CATHODE-PLATE 
D3 5 3 DX ; POSITIVE GRID CURRENT 
R1 2 5 {RGI} ; POSITIVE GRID CURRENT 
.MODEL DX D(IS=1N RS=1 CJO=10PF TT=1N) 
.ENDS 
*$

814 SE A2 Amplifier (Part 1)

20131117-133216.jpg
816 in triode mode

It’s time for the leap of faith. Having tested the 814 in triode mode, I will proceed now to upgrade my 4-65a SE amplifier and replace output valve for the 814. To ensure it can withstand the 540V in the anode, the remaining grids are all tied together through a resistor to the anode. All grids and anode are fitted with ferrite beads as well. A pair of UF4007 in series are placed to protect the Output Transformer in case load is accidentally disconnected.

20131117-133236.jpg
UX5 socket prepared

I added to the UX-5 socket a small bar to place two turrets to provide the anode (top connector), the strapped grid connections through the wire-wound resistor and the pair of UF4007 diodes.

Given that the 814 will run @ 540V / 100mA, I will only need to adjust the Rod Coleman regulators to set current down to 3.25A after replacing the raw filament transformers, as the 814 are 10V instead of 6V filaments of the 4-65a.

Minor DC adjustment will be required on the driver circuit via the gyrator load, so can easily implement this new amplifier.

Stay tuned…

20131117-133251.jpg
814 ready to go

4-65a Triode Curves

Tracing the transmitter valve curves

20131102-171745.jpgI posted several times about my 4-65a SE Amplifier and also traced in the past the curves using my analogue curve tracer to get a view of the loadlines of this fantastic DHT in triode mode.

Now that I have the uTracer I traced again the curves including grid current and A2 anode curves which are very handy for this type of transmitter valve.

My tests were conducted with the following heating and bias setup:

  • DC heater using Rod Coleman regulator @ 6V and 3.5A
  • Cathode connection to the negative filament terminal of the regulator and valve.

Continue reading “4-65a Triode Curves”

A collection of latest images

After upgrading my 26 preamp, turntable and adjusted the system to a great sounding state. Looking forward to play many records now…

HT power supplies
Latest System
26 preamp and 4-65a SE Amplifier
DHT filament supplies (and many of them)
26 and bigger brother 46
Reddish 4-65a
26 preamp Gen3
Lenco turntable
Setting up to play in the evening

 

26 Pre-amplifier Gen3

 An updated DHT preamp

It’s been a while since I played with preamps. Here is my third iteration of the 26 preamp. I love the sound of this valve as many of you out there. I found my previous build a bit inflexible to modify bias points and play around so decided to go back to the workshop and update my preamp, again. The design is very simple as you can see in the figure below. I’m reusing an existing supply but you can get away with a 200V HT and probably a 15V raw DC for the filament circuit. Key characteristics are:

  1. Filament bias as I’m a fan of it, no output or input caps sir!
  2. Added a Salas SSHV2 regulator to provide HT voltage flexibility and vary the operating point of the 26. The SSHV2 is extremely quiet so will give a clean HT supply to the 26. Yes, the lovely colour of the glow valves are lost in this version 🙁
  3. The Kelvin capacitor C1 (ala Morgan Jones) will help keeping Salas noise input low, specially HF.
  4. R1 is a simple resistor to measure the anode current. It is located in a handy place to allow easy measurements
  5. T1 is the Lundahl LL2745 in Alt R mode for 5.6:1 ratio. This provides the lowest gain and the lowest output impedance. In my new system, this preamp is more a line stage as don’t need further gain.
  6. SW1 is helpful in case ground loops are to be broken
  7. The Rod Coleman regulators are set to 800mA to starve the DHT and reduce microphonics and distortion. I found 800mA to be better sounding than my previous 760mA.
  8. P1 is my stepped attenuator and R2 will help providing grid bias when breaking before making action of the attenuator.
  9. RF is 2 pairs of 10Ω/20W parallel/series wirewound.

26 Preamp Gen3

The preamp is extremely quiet. The below measurement is with a noisy Sylvania ST valve which picks up significant 50Hz hum. Notice that 100Hz harmonic is very very low (can’t be seen at -100dB). Distortion at 5Vpp input (8Vpp output) is lower than 0.03% which is what you would expect from a 26. With a good selection of valves you can get this down to 0.02% for sure:

26 preamp gen3 THD test1

The most important part of the build work is grounding and avoiding ground loops. A combination of star grounding is recommended. I do the following star ground combination:

  1. Input stage
  2. Filament bias return
  3. Output stage

You definitely want to avoid the filament current ground introducing hum in the output stage so you want to keep the three star ground points separate.

26 DHT Gen3 ready to be tested
26 DHT Gen3 ready to be tested

Listening to the 26 is a fantastic experience. If you haven’t built a DHT preamplifier then do it. The detail and colour of this stage is unique. I found ST valves being more detailed on the top end, a richer treble compared to the Globe ones. Globe ones are sweeter and mellow. Definitely prefer globes for classical and ST for rock and Jazz.

26 Preamp connected to the 4-65a SE Amplifier
26 Preamp connected to the 4-65a SE Amplifier

The preamp is extremely quiet. Best build so far, I can’t hear hum on my high-efficiency FE167E full-range speakers and that is a real challenge.

I will try this new combination of 26 preamp and 4-65a SE for a while and report further impressions at a later stage.

Just need to fix my Lenco turntable and will be a happy man again 🙂

Hope you enjoy this post and encourage you to build one of this.

Ale