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PostPosted: 31 Jan 2016, 17:05 
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Joined: 16 Apr 2014, 00:41
Posts: 5
Location: Romania
Hello all,

Need advice what value should have a volume pot for this 6V6 Push-Pull amp ?
Can be the R=470K replaced with a volume pot?
Or the volume pot should be positioned at the input?

Regards,


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PostPosted: 03 Feb 2016, 16:08 
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Joined: 24 Feb 2009, 03:24
Posts: 559
Location: Bulgaria - EU
You can add 20-50 kohm pot. Input goes to the middle point of pot. And change input 10 kohm to 1 kohm, res. 470 kohm remains. If You use CD player, I doubt signal maybe will not enough for driving this schem /CDs have output about 0.5-1 V/. You can use DVD pl, it has output signal about 2 v, acc. it instruction.

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Projects: OTL 6AS7 Gen, Electric, SEs 2A3 RCA, 300B JJ, 6S4S, 4P1L, EL11 Telefunken, 6AS7 RCA, 6S33S, 6S41S, 6S19P, PP 6005 Gen. Ellectric , headphone ampl. OTL Loftin White 6AS7 RCA....SE E84L& E80CC Siemens&Tel-n.
http://azazello-sound.blogspot.com/


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PostPosted: 03 Feb 2016, 21:55 
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Location: Arizona, USA
Hi I agree with the 1K change, but I most likely would use a 100K pot. I find that value matches well with 12SL7s. I use it regularly.

Good listening
Bruce

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PostPosted: 04 Feb 2016, 00:36 
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Joined: 08 Aug 2009, 03:11
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Location: Chilliwack, BC
Yeah, 100K is pretty much the "tube standard".... low enough to swamp noise, high enough to not load the preceeding stage down.

Cheers!

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PostPosted: 04 Feb 2016, 09:07 
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Joined: 24 Feb 2009, 03:24
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Location: Bulgaria - EU
If volume is in min., upper part of pot. 100 kohm is big resistance for small HF input signals /it reflects for small sound details/. I use pot, about 2-10 kohm and sound is brighter and fresh in low volume. My Sansui CD player has very low output res. and 2-10 kohm pot. isn't problem for loading of CD output.
Sound is really better. Today new players have low output res.

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Hi End is back proportional to the number of composite parts!
Projects: OTL 6AS7 Gen, Electric, SEs 2A3 RCA, 300B JJ, 6S4S, 4P1L, EL11 Telefunken, 6AS7 RCA, 6S33S, 6S41S, 6S19P, PP 6005 Gen. Ellectric , headphone ampl. OTL Loftin White 6AS7 RCA....SE E84L& E80CC Siemens&Tel-n.
http://azazello-sound.blogspot.com/


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PostPosted: 04 Feb 2016, 10:22 
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Joined: 11 Oct 2011, 19:39
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I built this amp and used a 6sl7 instead of a 12sl7. I also built a 6sn7 preamp stage to feed it. Sounds pretty good.


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PostPosted: 04 Feb 2016, 12:54 
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Joined: 16 Apr 2014, 00:41
Posts: 5
Location: Romania
Thanks for yours answer.
I plan to use this amp with a CD Player of course , and with a DAC.But also with a Tube Phono Preamplifier.
Also in many tube amp schematics I see an input cap ,usually with values between 0,022uF and 0,1uF as DC blocking.
It is mandatory this input cap?I read that isnt.
So , a 100K pot followed by an 0,022uF cap is OK?

Also it would be very interesting a balance pot.Can anybody re-draw the input stage with a volume and balance pot?

Regards,


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PostPosted: 04 Feb 2016, 13:31 
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Joined: 24 Feb 2009, 03:24
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Location: Bulgaria - EU
Without cap....

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Hi End is back proportional to the number of composite parts!
Projects: OTL 6AS7 Gen, Electric, SEs 2A3 RCA, 300B JJ, 6S4S, 4P1L, EL11 Telefunken, 6AS7 RCA, 6S33S, 6S41S, 6S19P, PP 6005 Gen. Ellectric , headphone ampl. OTL Loftin White 6AS7 RCA....SE E84L& E80CC Siemens&Tel-n.
http://azazello-sound.blogspot.com/


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PostPosted: 26 Sep 2017, 20:00 
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Joined: 24 Nov 2010, 14:39
Posts: 26
Location: California
I built an stereo amp a couple of years ago, using the same basic circuit as the o/p, with a couple of differences. I had a couple of Edcor output transformers on hand, 10K primary / 6-ohm secondary, so I used them. I had components to build a solid-state rectified power supply that provided approx. 250 volts B+, so in they went. I used 12 volt heater versions, 12SL7 and 12V6's, since it allowed me to use NOS tubes at a really cheap price. The amp sounded pretty good, not overly powerful, and a little bright at the top, but in a pleasant sort of way. I decided, after a couple of years, to rebuild it and make the values closer to what was called for in the original design, hoping for a little more output. I upped the B+ to about 305 volts (and about 255 to the12SL7 input stage), and changed the output transformers to Edcor GXPP15-8K's (8K primary, 8 ohm secondary).The amp is now way too bright in the midrange and top end, and produces audible distortion at even at relatively moderate volume levels. Thinking maybe I I had the feedback out of phase, I swapped the connections of the coupling capacitors to the output tubes, and was greeted with a loud squeal as soon as the tubes came up to temperature. I guess that wasn't the problem.... My thought is that it is still an issue of the output transformer change and it's interaction with the negative feedback, but I'm not sure how to proceed at this point. I would appreciate any good suggestions. Sorry for writing a book here on an old thread, but it seemed like an appropriate place.


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PostPosted: 27 Sep 2017, 16:35 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, A common issue is failure to ground the negative end of the output trannie. It will cause behavior like you mentioned. I would also increase the 47uf on the output tube cathodes to 470uf and probably go for 0.22uf coupling caps in place of the 0.1uf ones there.

Good listening
Bruce

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