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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 29 Oct 2011, 21:06 
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Hi Allen, You can use some high temp epoxy. Otherwise the nylon ties are a good method. Just put the junction point on the bottom of the board so it won't show. An alternative if the pins stick through enough is to bend them 90 degrees to the side. That will secure it pretty well. The transformer isn't heavy and there are 12 pins.

Good listening
Bruce

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PostPosted: 17 Nov 2011, 01:39 
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hello,
I just finished the preamplifiers. I am using tesla ecc802s tubes (with the yellow logo) on the pin 6 i have 205volts. Is this ok? (the schematics says 215v) I am using the lm317 for the heaters supply but because my transformer drops the voltage from 8 to 7,1vac i am getting 5,8volts on the heaters. Is that ok or i will must rewind the transformer? The preamp sounds very good. later i will post some photos.


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PostPosted: 17 Nov 2011, 12:06 
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Hi, Good show. :thumbsup: I believe you will find the preamp quite good. By all means post some photos for all to see. The B+ is fine and while the voltage is a little low on the heaters it should be fine as well. Depending on which version of the schematic you used there is an update that seems to be beneficial. The four cathode resistors are probably 470 ohms. I have found that you can reduce the distortion a small amount by changing them to 820 ohms. :) The change is small (under 0.08% across the board). It will also result in an increase in the B+ of about 30 volts. This is of no consequence and no other changes are needed.

Good Listening
Bruce

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PostPosted: 17 Nov 2011, 16:51 
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hello,
thank you for the reply. I finally decided to rewind the transformer. now i have B+210V and heaters at 6,3V. everything ok. But i have a major problem and i don't know how to deal with. I have connected the preamp with my power amplifier. I also have a mute relay on the preamplifier output whitch connects the output to the power amp. after 10sec delay. When the mute goes off (with the power amp. already on - and it most cases it is already on) i hear a loud pop from the speakers. In that 10 sec the preamplifier output is connected to ground via the 2,2K output resistor. Maybe this happens because the output capacitors have some dc voltage?(my power amp. also has a coupling capacitor at the input) .
I also hear a loud pop everytime i mute the output. The mute relay connect the output to the ground (power amp. input to the ground) Why this happens?
Otherwise the sound is very good. Of course I tried the preamplifier with a cheap speakers. I am posting some photos. the preamp is remote controlled with a sony remote whitch i already have from my home theater amplifier.


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PostPosted: 17 Nov 2011, 16:53 
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two more photos....


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PostPosted: 17 Nov 2011, 21:40 
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Very nice build :thumbsup: , the remote section reminds me of one I have built into one of my Forewatts. On the thump, one or the other capacitor is holding a charge and causing it. The one in the preamp is charged on the tube side to about 1/2 the B+ applied to the tube anodes ( 1/2 of the 220 volts) so I would guess it is the one thumping. It takes a number of seconds for the voltage on that side to drop to a low value. If you extend the delay a bit it may solve the problem. Alternatively I would eliminate the 2.2K resistor and directly ground the output on mute. If you un-mute quickly you might get a thump then. :? Another possibility is to disconnect the output of the preamp when it is muted. This is similar to what we do in the commercial kits. Disconnect the output and let the cap discharge through a resistor. We use a relay to do the switching.

Good listening
Bruce

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PostPosted: 18 Nov 2011, 00:12 
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hi,
Are you saying that i can ground the output capacitor without the 2,2k resistor without having any problem with the tubes? This resistor is not needed? and this only when muting or i can put out the resistor permanently? Now when it is in mute state the output capacitor is grounded via the 2,2k resistor and a 1k resistor( i tried to short the second resistor without any change) and the power amp. input directly grounded. See the photo what i mean.
I dont understand the second solution. you mean that on mute i must ground the power amp. input like now and one another relay puts parallel with the output cap a resistor to discharge it?


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PostPosted: 18 Nov 2011, 15:51 
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hello,
finally after a lot of search on the net and testing i found the circuit that works fine for me. It is on the attached picture. Please tell me if it is ok for that kind of amplifier. the mute relay unmutes 30sec after power on and i dont have any thumps on power up neither on mute-unmute.
My 6,3 volts for the heaters is from a lm317. when the tubes are cold and i power up the amp. the heaters glows alot for a sec and then glows normally. maybe because they are cold the heaters have smaller resistance and draws more current. This can shorten the tube life? What is your sollution for this (if it is a problem). I thought to put a NTC thermistor (like the CL60) in series with the heaters. what do you think? It will help?


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PostPosted: 18 Nov 2011, 20:24 
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Hi, Sorry for the confusion, what you have in the drawing will cause no harm. The question about tube life and the momentary brightness on start up is something that comes up fairly often. Tube heaters have very low resistance when cold and it increases as they warm up. In an ideal world we would slowly increase the voltage to the heaters to prevent the initial inrush of current. Practically, I have not found it to be a problem. Tubes have been doing this forever and they continue to function fine. Some brands and types seem to have greater brightness than others. So while it would seem to be a problem, don't worry about it. BTW I use both the LM317 and LM7806 ICs for heater power.

Good listening
Bruce

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PostPosted: 06 Dec 2011, 09:46 
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Joined: 26 Oct 2011, 19:53
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I can't find the power transformer listed on the schematic - perhaps the model number has changed. I found an Edcor XPWR083A-120/240 on the Edcor website. Looks like the same specs.

When I look at the parts list and prices, I think this might be a good first tube preamp for me. I'm wondering if anyone has thoughts on using it with a SS integrated amp in line mode. Here is my thought:

CD Player (PS1) or iPhone (universal dock) ---> ForeWatt ---> Pioneer Elite A-35R (line mode)


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