• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

12B4 Preamp

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#81
All our tubes oscillated under 3k3. Three indications. 1. Tube will not stabilize bias point completely, it drifts. 2. Weird noises. 3. More microphonic than normal. It will not settle to be a super built noval, but it will go significantly more silent regarding microphony from when oscillating. Under 20mA is more silent in general combined with 3k3 or over grid stoppers.
 
#83
grid stoppers

Regarding wattage rating of grid stoppers it is essential to be soldered directly on both pins ,any rating will do ,just use good quality resistors.
Regarding microphony I found great improvement redusing heater voltage at around 12.2 v.
Hope this helps.
 
#85
Peter Roodzant said:
Hauke: You mention the 26 tube in your last post. Have you ever compared the 26 with the 12B4? Some people claim that the 26 is the Holy Grail and that all indirect heated tubes are not in the same league. What is your opinion?

Peter
Click to expand...


Hi Peter,
yes, indeed, the 26 tube is a very nice sounding tube. But i cannot manage to make them quite; battery heaters, etc...

KR

Hauke
 
#90
vgeorge said:
Well, Salas got me the bug to build one!
As I start collecting bits, I would like advise as to which transformer to buy for this pre. As I understand, I need one with 300V, 5v and 6v secondaries for the heaters. Which one is the best choice soundwise?
It is going to be tube rectified and shunt regulated with a mosfet CCS.
Any advise is welcome.

P.S I already ordered some tubes to get motivated!
Click to expand...

310V-0-310V 0.3A, 5V 3A, 12.6V 1A (120VA custom Toroid, best is custom R Core)

GZ34 47u 450V 1K 5W 220u 450V to HV Shunt

12.6V to bridge to 10.000u to 317 on sink 12.2V set with trimmer under reg to 12B4A heaters in series.
 
#93
vgeorge said:
Thanks Salas,

as I was in Athens yesterday,I ordered one with 2x300V 200mA EI core, not so expensive.
After I finish the project and like it, maybe get an R-core one.
Do you know how makes R-core in Greece? Or I have to order from aboard?

Thanks,
George
Click to expand...

From abroad. But forget for now. Use a separate PSU box as you normally do. Good luck with construction.
 
#96
nicoch46 said:
HI Salas have a Link for R-core supply?
Click to expand...

Custom Magnetics in USA, and some pop up on ebay, like ECU Tamura.
Double C core are good too. Good value is Serbian Trafomatic. They do custom stuff, so I would ask for custom secondary values mains double C since they do valve output double C.
 
#97
vgeorge said:
Thanks Salas,

I will built in a saparate box.
I also need to find the 2n6520 for your HV shunt. No one seems to cary them!
Click to expand...

Use MPSA94. If you go to a good shop and ask for PNP -400V TO92 they usually come up with something. I have found some NEC that way. Just give em a number like MPSA94 and they look up the equivalents guide and look for what they may have in their inventory.
 
#99
Hi,

i revised my preamp to that one:

attachment.php


For Pin 2+7 i used 3K9 grid-stoppers.
@cathode 1K2 and 470uF
@anode CCs from Kevin @ K&K Audio:

http://www.kandkaudio.com/accessories.html

I tuned the CCs to 15mA, this means ~160V at anode and ~18V across cathode resistor measured to earth.

I tried 4 differend 12b4a from different brands; all of them were pretty microphonic. Just tip the tube and you hear a "ping".

This is not what it should be!?

KR

Hauke
 
#100
If you hit the tube with fingernail it will ping without reverberation. Or if you knock the box. If it is stable on bias voltage, does not produce self noises and is quiet for hiss, then you have a normally working 12B4 inventory.
 
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