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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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 Post subject: My 6AS7 SE Project
PostPosted: 12 Jan 2014, 15:56 
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Joined: 15 Nov 2013, 10:16
Posts: 234
Location: MB, Canada
So, this will be my first Hifi power amp project. I've built a number of guitar tube amps in the past, and although I don't have a great knowledge of the theory behind what I've built, they have been great successes.

I recently came into a bunch of 6080 and 6AS7 tubes, and so began hunting online for a low powered 6AS7 based SET amp. I finally found the project that mwhouston documented on here. viewtopic.php?f=9&t=2243

That got my interest piqued, and after some conversation with mwhouston and Suncalc I've got a schematic that should fit my purposes quite well. Suncalc was so kind to come up with a specific schematic for me similar to the one that mwhouston posted, but slightly different.

Here is the schematic I'll be basing this build on:
Signal: http://i.imgur.com/SYO6jW5.jpg
PS: http://i.imgur.com/KY8SCM3.png

Now I ordered all of the parts I need to build this, but amp still waiting on the Edcor output transformers. I'm told they should be shipping in about a week.

So I'm trying to figure out what a good physical layout for this amp will be. I ordered a Hammond chassis, and it's bigger than I had pictured.. Somehow the dimensions didn't really click until I saw the chassis. That being said, I think it'll be nice to work with a bunch of room in the chassis, and the size will still work for my environment.

I used some spare transformers to take the place of the OT's that I don't have yet, just to try and figure out a good layout.
How does this look?
http://i.imgur.com/kynOtzW.jpg?1

I'm very open to adjusting the layout, but this seemed like it might be a good layout to keep the amp quiet.

I look forward to any feedback anyone can provide. I'll keep this thread updated as I build the amp.


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 Post subject: Re: My 6AS7 SE Project
PostPosted: 12 Jan 2014, 17:32 
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Joined: 24 Feb 2009, 03:24
Posts: 562
Location: USA
I'd like to advice You, the iron of 10 w OT is very small for good sounding for current of 6AS7 or 6080 about 60-80 ma....more iron - better sound. I tried, output tube 6AS7 sound better with current 60 ma and +U about 160V. PS transformer must provide about 2 X 120-130 V AC.
And 12AX7 You can change to 12AT7 for really better sound /change parameters of resistors of first stage for current 5-6- ma/, and change res. 10 kohm in grid of 6AS7 to 1 kohm. /For prestage I use in my SE 6AS7 ECC88, that is better every 12A...7/.
And increase electrolitic caps in cathodes of all tubes.
For better bass You can increase interstage cap to 0.27 - 0.33 uF. 0.047 uF is for sharp guitar sound and increase el. caps 100 uF in PSU.
Sorry, IMO...I don't like Your schematic and modes.... :down:

_________________
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/


Last edited by azazello on 12 Jan 2014, 17:48, edited 2 times in total.

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 Post subject: Re: My 6AS7 SE Project
PostPosted: 12 Jan 2014, 17:41 
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Joined: 15 Nov 2013, 10:16
Posts: 234
Location: MB, Canada
I am actually using the 15w Edcor output. I'm using the driver section as Matt calculated. Here are some comments on the design he provided.

Quote:
Some things to keep in mind. The power stage is biased to 75v. This means that the driver needs to be able to swing a full 150v (+/- 75v) to drive it. The driver is based on a 250v B+. This is very close to the maximum swing that you can get out of the driver with this B+ and a normal resistive load. I would not substitute any component values in the driver; the 3.09kΩ (a standard value for 1% resistors) and the 250kΩ load need to be what they are to get the full drive out of the amp. I have placed the bias point so that when the input stage is overdriven it goes deep into compression before grid conduction starts. Even though, I have included a modestly sized grid stopper on the input to control bias excursions. You may also notice that the interstage coupling capacitor is atypically low. This is due to the size of the grid resistor on the power stage. I used 1MΩ (the maximum 6AS7 grid circuit resistance in cathode bias operation) to control the AC load line rotation in the driver. I recommend making both the 6AS7 grid resistor and the grid stopper 1% resistors to meet the requirements of the 6AS7 grid circuit resistance. Finally, This design assumes the 6AS7 having a grid to cathode voltage of ≈165v. This means that the B+ needs to be about 240v (75v bias + 165v plate voltage) plus whatever drop there is in the output transformer primary winding. Check the primary DCR before setting the final B+. The 6AS7 plate is dissipating 12.3W in this design (13W is the design maximum for the 6AS7). Be careful that you don't exceed the plate dissipation in your final build.


If you do a calculation of that for low cutoff freq with that rc combination, (.047 & 1M) you'll see that it's a cutoff of 3.2hz.. I don't think you can base frequency cutoff of a circuit to just that capacitor value.

I'm not sure I want to change things with this design as I feel like there is a lot of thought that has gone into this design already. I'm hoping maybe Matt can chime in on his recommendation of this driver with this particular amp.


Last edited by thehoj on 12 Jan 2014, 17:54, edited 2 times in total.

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 Post subject: Re: My 6AS7 SE Project
PostPosted: 12 Jan 2014, 17:45 
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Joined: 15 Nov 2013, 10:16
Posts: 234
Location: MB, Canada
Double post


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 Post subject: Re: My 6AS7 SE Project
PostPosted: 12 Jan 2014, 17:52 
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Joined: 24 Feb 2009, 03:24
Posts: 562
Location: USA
I changed my post above about PS transformer....Your schematic are correct modes, but will not provide, IMO, good sound, /in more cases good parameters doesn't mean good sound/. I don't like it....sorry.../I can send my schematic in PM/,

_________________
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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 Post subject: Re: My 6AS7 SE Project
PostPosted: 12 Jan 2014, 18:02 
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Joined: 24 Feb 2009, 03:24
Posts: 562
Location: USA
I see, maybe You started calculate Your amplifier from PS transformer, that is wrong start...First You must decide what kind of tubes and modes You will use and then buy PS Transformer, OTs, caps, res-s....I never will use 6AS7 with current 15 ma....But the choice of amplifier is Yours.

_________________
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/


Last edited by azazello on 12 Jan 2014, 18:10, edited 1 time in total.

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 Post subject: Re: My 6AS7 SE Project
PostPosted: 12 Jan 2014, 18:09 
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Joined: 15 Nov 2013, 10:16
Posts: 234
Location: MB, Canada
azazello wrote:
I see, maybe You started calculate Your amplifier from PS transformer, that is wrong start...First You must decide what kind of tubes and modes You will use and then buy PS Transformer, OTs, caps, res-s....


What is wrong with the B+?.. With cathode bias of 75v and b+ of 240 the 6AS7 see's 165V. Is that not ideal for the 6AS7?


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 Post subject: Re: My 6AS7 SE Project
PostPosted: 12 Jan 2014, 18:18 
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Joined: 24 Feb 2009, 03:24
Posts: 562
Location: USA
Your mode, IMO, is not ideal for good sound. Good mode doesn"t mean good sound. My prefer mode for 6AS7 is 160 v / 60-70 ma with grid bias not 75 v, but about, sorry I don't remember....30v...I must measure, I made my SE more than 1 year ago. I liked it sound and I made OTL - 6AS7G with about the same modes.

_________________
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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 Post subject: Re: My 6AS7 SE Project
PostPosted: 12 Jan 2014, 18:23 
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Joined: 15 Nov 2013, 10:16
Posts: 234
Location: MB, Canada
azazello wrote:
Your mode, IMO, is not ideal for good sound. Good mode doesn"t mean good sound. My prefer mode for 6AS7 is 160 v / 60-70 ma with grid bias not 75 v, but about, sorry I don't remember....30v...I must measure, I made my SE more than 1 year ago. I liked it sound and I made OTL - 6AS7G with about the same modes.


This will be 165V / 74.5mA... Other than this design using cathode bias vs. your recommendation for grid bias, what is different?


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 Post subject: Re: My 6AS7 SE Project
PostPosted: 12 Jan 2014, 18:29 
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Joined: 24 Feb 2009, 03:24
Posts: 562
Location: USA
I use fixed bias, but with auto bias Your PS channel must provide about 190-200 v DC and PS transformer must have 2 X 150-160 V AC.

_________________
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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