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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: Re: My 6AS7 SE Project
PostPosted: 21 Nov 2017, 14:30 
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Looking forward to that....


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 Post subject: Re: My 6AS7 SE Project
PostPosted: 18 Sep 2018, 06:34 
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Joined: 13 Jan 2018, 21:33
Posts: 80
Location: australia
thehoj wrote:
mwhouston wrote:
Seeing you have so much room how about making this a true integrated amp and add a single stage tube preamp. Plenty of room. Go for it. You can still put PS one side and preampamp the other.


I'm kind of regretting not doing that now.. I already built the 4S in a nice neat great working enclosure.. lol.

I've just noticed that I have to order a few switches from Mouser.. Maybe I'll look at the enclosures.

If one were to integrate the 4S preamp, how would the circuit look?. Would the 4S preamp replace the front end of Hoj's amp? Could the 6sn7 version of the 4S preamp work here? Regards Woodo

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 Post subject: Re: My 6AS7 SE Project
PostPosted: 18 Sep 2018, 18:16 
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Joined: 06 Apr 2009, 10:08
Posts: 1467
Location: US Pacific Northwest
Woodo wrote:
If one were to integrate the 4S preamp, how would the circuit look?
Hmmm... ok, let's talk about the amp first. The power stage of this amp is cathode biased at ≈75v producing around 3W peak output power. So, to drive this stage we need a driver that first, can swing to +/- 75v peak and second, does not require an exorbitant amount of drive voltage to get there. So when I designed the amp I chose a high µ tube, used large load and coupling resistances, and biased it specifically to meet the swing requirement without undue distortion. The result was a stage with a gain of about 71v/v and very even swing characteristics. This means that this amplifier only requires about 75/71 or about 1.06v peak or ≈750mV RMS to produce full power. This design represents a total driver gain of 37dB.

So to integrate a 4S preamp your going to have to make some choices. Obviously, a 4S in front of the existing gain stage would result in severe overdrive in virtually all conditions. If you simply replaced the driver stage with one of the 4S preamps, you would likely not have the gain required to meet the drive requirements of the power stage. I think what I would recommend would be a modified approach.

I think I would replace the driver used here with a high bias and low gain stage to meet the swing requirements of the power stage. They I would put one of the 4S preamps in front of this with the volume control between the two signal stages. Then the 4S provides the desired tonal shaping, and the volume control moderates the resultant drive but still allows the power stage to go to full output. I have high bias driver designs for both 12AU7 and 6AS7 but they might need to be slightly tweaked for the lower B+ voltage. But overall this shouldn't be too difficult a task.

Does this make sense?

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 Post subject: Re: My 6AS7 SE Project
PostPosted: 19 Sep 2018, 02:16 
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Joined: 13 Jan 2018, 21:33
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Location: australia
thanks Matt it does. It sounds a worthwhile project to advance. Reading TheHoj and Marks comments on their experience with this valve is motivating.

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 Post subject: Re: My 6AS7 SE Project
PostPosted: 24 Sep 2018, 16:46 
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Posts: 80
Location: australia
Suncalc wrote:
Well I just ordered the iron for a different version of this amp. I've always wanted to bias this tube higher with a larger load to help reduce distortion and help the power. Now that I have a driver that will easily drive highly biased low sensitivity triodes, I've moved the design point for the 6AS7 up to Vp=176.5v and Ip=66.5mA (Eco ≈ -80v). I've also increased the plate load to 2.5kΩ. This should be a nice warm amp which should excel at the classical, jazz, and easy listening genres.

I'll report back after I've done some prototyping early next year. :sing:

Hi Matt- have you reported back on this research in another thread?

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 Post subject: Re: My 6AS7 SE Project
PostPosted: 24 Sep 2018, 18:15 
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Joined: 06 Apr 2009, 10:08
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Location: US Pacific Northwest
Woodo wrote:
Hi Matt- have you reported back on this research in another thread?
OH NO!!!!

I just realized that I used the 2.5kΩ outputs I ordered for a different amp without ever doing the prototyping of this amp. :blush: Now I need to order another pair of the 2.5kΩ outputs. Still have the intended power transformer and nice Edcor 7H choke, but no outputs.

Well, 6 more weeks of waiting. :bawling:

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