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What preamp should I build?
What preamp should I build?
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Old 15th January 2009, 03:40 AM   #151
Poindexter is offline Poindexter  United States
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Join Date: Oct 2004
Location: Maui, Hawai'i, USA
Milen, I can't imagine where all that hiss is coming from; 6T4 is not a noisy tube (something we worry about when using these teevee utility tubes that are not optimized for audio).  My circuit is dead silent, between 20 and 30µV.   Two possibilities come to mind.  The first is that regulated supply - don't forget to put a high quality cap to ground between it and the circuit, fractional µF should be big enough if it's hiss.  The other is that funny volume control.  Test the input to the grid to see if that noise is already there.



Edit: you'd think I could spell by now.
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Old 15th January 2009, 04:00 AM   #152
milen007 is offline milen007  Indonesia
diyAudio Member
Join Date: Dec 2007
Hi Poinz

thanks for the tips.

i do not use regulated psu. i am using tube rectifier EZ81 (or maybe this tube that hiss) i might change the tube then. and filament ac of 5.8vac (too low.. maybe not pure enough tranformer winding=guessing)

the LDR pot i am using suppose to be dead silent. as i am using the same thing on my solid state pre and its dead silent. but i will double check.

i am using sikorel 125 (no idea hows the quality, its NOS, but i think sikorel shouldnt be too bad though) parallel few caps to get the required capacitance.

or do you suggest regulated psu for heater to solve the hissing?

do you found 6t4 tubes to be varies in quality? i got 5 RCA and 5 GE. test them all and cant get the voltage to balance between left and right channel. they also have different load that cause the B+ to varies between tubes (or they need to run in 1st to stabilized ?). i am using RCA to the B+ and GE to the ground.

one more thing, anyway to lower the gain? as i suspect my system gain is too much for driving my 100w amp, which later will be replace by smaller watt amp/ Class A.

PS: great stuff Poinz, maloha for the schematic. really nice preamp. very open sound and details. soundstage is scarily huge. very natural. best that i heard till now.

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Old 17th January 2009, 07:17 AM   #153
milen007 is offline milen007  Indonesia
diyAudio Member
Join Date: Dec 2007
hi poinz

has been reading about the voltage devider.

will i be able to lower the gain by reducing the resistor to the ground at the output? for instance to 9K? not sure if this work.. but voltage devider always work with 2 resistor.

or add 1K resistor in series at output?

thanks in adv.

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Old 21st January 2009, 08:11 AM   #154
milen007 is offline milen007  Indonesia
diyAudio Member
Join Date: Dec 2007
hi guys

i change the tube to balance out the right and left channel.. i got

B+ 136vdc

Heater floats at approx 23.5V above ground
Top cathode: approx 103V
Bottom plate: approx 101
Bottom cathode: approx 0.9
Heater voltage: 5.9VAC

the circuit only draw 22mA both channel... what do you guys have?

as before the dropping resistor is 174v, after 1.5K the voltage become 140vdc

suggestion? but it still sound good tho but hiss louder.

i got other tube that have the top cathode 63v. forget to measure the current. which is better?

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Old 21st January 2009, 12:07 PM   #155
milen007 is offline milen007  Indonesia
diyAudio Member
Join Date: Dec 2007
edit. The above setup with 22mA current sound bad

Fit other tubes with current draw of 33mA. Cathode voltage ard 81v. Superb

Poinz, what's the objective here? 75v cathode voltage, or 140v B+?

What's the ideal current draw for double channel?
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