• 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.

#26 pre amp

I use the modular alu extrusions from Penn Fabrication.

Penn Elcom : MSB - Frame Extrusions

I use the 2U versions - they come in different widths. Then I put two wooden pieces on each side and screw them into the holes on the extrusions. I make everything like this, except when I use Rittal Rackmount subrack parts, which are similar but have better facilities.

Horizontal rail
42 HP RP 3684.560
84 HP RP 3684.562
Threaded insert, M2.5
42 HP RP 3684.608
84 HP RP 9901.816

Rittal RP 9901.816 Threaded insert M 2.5 84 HP (5X2)

I use a pair of horizontal rails for the front panel, top and bottom, with a piece of nice metal in between, and screw the top plates and bottom plate in with M2.5 screws which go into the threaded insert. That way I can use any combination of top plates. My top plates are 275mm front to back, which I find is excellent.

Love playing with this stuff - it's like building meccano.

andy
 
Here are some photos of my 26 preamp in 3 boxes - PSU is AZ1 mesh, the filament supply for filament bias has two large chokes, and you can see the large 10 ohm resistors (4 in series parallel per side) on top of the preamp!
 

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I want to use my 6.3 output but I need to reduce it to 4VAC for AZ1, how can calculate the resistor value because Ducan PSUD2 can't have AZ1 mixed with diodes as hybrid rectifier?

Ley de Ohm: Ley de Ohm - Wikipedia, la enciclopedia libre
The AZ1 has a 4 V filament rated at 1.1 A
6.3 - 4 = 2.3 Vdrop
R = V/I thus R = 2.3/1.1 then R = 2.09 Ohm
The resistor should dissipate P = I2 x R thus 1.21 x 2.09 then P = 2.53 W
I would use 2-2.2 Ohm/10 W minimum, or, better, split the resistor and use 2 x 1-1.1 Ohm/5 W, one resistor on each cathode.

Quite easy ;)

Edit: Ohm?v zákon ve stejnosm?rném obvodu - ? Ing. René Vápeník 2009

Can I use this Tx rated 4.5VAC 1.334A instead 4VAC 1.5A

PRO POWER|CTFC6-4.5|TRANSFORMADOR, 6VA 2X 4.5V | Farnell España?

To drop 0.5V I will connect 0.275R in each cathode, it's OK? will be enough power the Tx?
 
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I am bit puzzled and need little clarification from all you experts. I mentioned before in this forum about the speaker rumble I get when using my 26 pre. Lately I found that the bias voltage changes are proportionate to this speaker noise. Increasing bias voltage decreases the noise and vice versa. Somewhere I read that in fixed gird bias topology, bias voltage increase is reducing the current flow through plates hence assume the high plate current would have introduced this speaker noise. Final result is that for 145 plate voltage and 11.5 bias voltage gives the lowest noise and best sound overall. If I use CCS for plate loads, controlling the noise is easier where as plate chokes makes it bit difficult because of higher bias voltage requires. My doubt is whether I am doing the right thing here or not by increasing the bias voltage. Not sure whether my tubes are over-driven. I couldn't find any related info about how to correctly check plate current of 26 pre. What is not clear for me is the plate resistance to use to calculate the current. Is it 15400 Ohms for both tubes or 7700ohms for a single tube? I am getting totally different results if use these figures separately with v/r=i (145-11.5/7700=0.017 - 145-11.5/15000=8.4). still the figures seems quite high for 26 tube. Am I missing anything here? Greatly appreciate if someone could educate me about this calculation and get this plate current correctly for gird bias . Or may be a generic way to use.
 
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http://frank.yueksel.org/sheets/127/2/26.pdf

There are the curves for the 26. 145v and 11.5v bias looks like 4.5mA. That's fine - well within the comfort zone since the 26 can tolerate up to 6mA. As you see from the curves, increasing the bias voltage does decrease the current at the same plate voltage. 13v bias would take you down to around 3ma. Whether these theoretical values from the datasheet work in practice is another matter, and might be affected by the condition of the tube.

Andy
 
Kanishka, maybe you have some gas in your 26 DHTs. This will certainly give low-frequency noise - and often it is worse as the grid voltage gets nearer to zero (ie grid voltage near to cathode voltage).

Gas is expected with old tubes. You can sometimes drive it into the getter by placing the tube in an oven at 100 to 120 deg C for 12hours or more [if it is an all-glass tube]; doing this with a 26 is too risky, as the moulded base can't stand that much heat.

It is easiest to compare a different 26, and see if it behaves differently. If there is no change, please check the bias voltage for noise. If the bias is a battery, try a different brand.

You can also measure for gas. Insert a 10K resistor in the grid lead. measure the voltage across the resistor, using mV scale of your DVM. Don't be surprised if the value is unstable. Up to about 8 to 10mV is fairly normal for an old tube - much more than 10mV and you have found your trouble!

If you do have gas, the anode current will be unstable, too. To measure the anode current, just add 10 ohms in the cathode-GND lead. You get 100mV for every 10mA that flows.

Please report your measurements, and we can try to help.