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Old 27th June 2011, 04:50 PM   #1051
CFT is offline CFT  United States
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That's a good looking set Andy : )
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Old 27th June 2011, 04:54 PM   #1052
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BTW, has anyone tried the Ba tube?
My friend recently built a preamp using it with a 45 tube as output (coupled to an output tranny), and he likes it very much!

Last edited by CFT; 27th June 2011 at 05:03 PM.
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Old 30th June 2011, 04:59 PM   #1053
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Quote:
Originally Posted by merlin el mago View Post
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?
Quote:
Originally Posted by massimo View Post
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 Espańa?

To drop 0.5V I will connect 0.275R in each cathode, it's OK? will be enough power the Tx?

Last edited by merlin el mago; 30th June 2011 at 05:18 PM.
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Old 30th June 2011, 05:45 PM   #1054
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I try and use a transformer of double the VA. So for 4.5v I'd prefer 9VA. It's worth having a margin of comfort in a transformer.

andy
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Old 30th June 2011, 05:56 PM   #1055
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Only get 6VA for 4.5VAC so I will go 2 x 6VAC 12VA

MULTICOMP|MCF/N3506F|TRANSFORMADOR, 12VA, 2 X 6V | Farnell Espańa

To drop 2V I will connect 1R 5W in each cathode, it's OK?

Last edited by merlin el mago; 30th June 2011 at 06:00 PM.
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Old 30th June 2011, 06:59 PM   #1056
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Looks about right.
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Old 30th June 2011, 07:12 PM   #1057
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Thank you Andy.

Cheers,
Felipe
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Old 1st July 2011, 09:57 AM   #1058
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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.

Last edited by coolzero; 1st July 2011 at 10:04 AM.
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Old 1st July 2011, 10:22 AM   #1059
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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
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Old 1st July 2011, 10:22 AM   #1060
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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.
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