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 ProjectPosted: 24 Feb 2014, 20:19

Joined: 15 Nov 2013, 10:16
Posts: 234
Suncalc wrote:
thehoj wrote:
For me, because the tube is not drawing full current until the heaters are warmed up, the voltage drop's through my B+ line are low at the start, and then as current draw through the power supply increases, my voltage drops increase over the chokes and resistor and then my B+ settles in at what I want after about a minute. I wouldn't be concerned about that either except for that maximum voltage rating on the plates of the 6AS7 of 250V.
Stop! Now... breathe... breathe...

Your amp is perfectly fine as is. First, the maximum plate voltage listed in data sheets is the "maximum operating plate to cathode voltage at specified maximum current". Vacuum tubes are generally designed to handle around twice this voltage at zero current. This is why the maximum plate to cathode voltage on the 12AX7 is rated at 300v (Va) but the maximum B+ supply voltage is 550v (Vao). The 6AS7/6080 coincidentally rated at Va=250V and Vao=550v. Here is the cut out of the 1960 Phillips data sheet.
Attachment:
Screen shot 2014-02-24 at 5.04.07 PM.png

Second, the 6AS7 is, first and foremost, a voltage regulator and boost scanning tube. As such is it built "hell for stout" as they say. The peak inverse plate voltage is 1700v. The ability of the tube to stand up to much more than 550V at startup is virtually assured.

The behavior you describe is typical of most amplifiers. It is normal and will do NO damage to your tubes. They are designed for this. I strongly recommend you turn on your amp in the background and start planning your next project.

Okay. I kind of got thinking about plate to cathode voltage only being 170V.. That makes sense. I wasn't aware of the voltage at zero current, but that's definitely a re-assuring rating.

Thanks for the info!

P.S.. My next project will be something based on the 6336 .. So I'm definitely interested in that 6336 concept you told me you were working on before.

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 Post subject: Re: My 6AS7 SE ProjectPosted: 25 Feb 2014, 00:18

Joined: 15 Nov 2013, 10:16
Posts: 234
Oh also, I just wanted to mention the power indicator light I used in these, just because it looks sooo soo good, although it's hard to capture how it looks with a camera.

I used this lamp indicator assembly http://www.tubesandmore.com/products/P-L124-P
This jewel http://www.tubesandmore.com/products/P-L123
And this 6.3V bulb http://www.tubesandmore.com/products/P-47

Except I'm running the bulb at 3.15V, or one leg of the 6.3V winding to the center tap. It's just the perfect amount of brightness when running at half voltage like that. Love it. I was finding it a bit on the bright side, especially when the lighting in the room was dimmed down.

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 Post subject: Re: My 6AS7 SE ProjectPosted: 27 Feb 2014, 20:42

Joined: 15 Nov 2013, 10:16
Posts: 234
Here's a video of the amp in action with the 4S preamp I built, along with a Boozhound phono stage I put together recently, an old turntable I have, and those Fostex 206e (bob brines design) towers I got from ojaw.

Obviously, it doesn't sound great on my iPhone recording, but it gives you an idea of how it sounds.

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 Post subject: Re: My 6AS7 SE ProjectPosted: 10 Oct 2017, 16:33

Joined: 07 Feb 2017, 19:06
Posts: 48
thehoj wrote:
Transformers have arrived! Let the building begin.

Which of the Hammond transformer is equivalent to this EDCOR?
Thanks!

Alex Q

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 Post subject: Re: My 6AS7 SE ProjectPosted: 10 Oct 2017, 18:19

Joined: 30 Aug 2017, 09:49
Posts: 37
The 1627Se that is 2500 ohm. But they are not as good as the Decor and are almost \$200.00 US. Wich is crazy! Besides the 6AS7 is not an audio tube. It has a lot of distortion. It was used as a voltage reg in Textronics scopes.

_________________
Alpha Wolf

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 Post subject: Re: My 6AS7 SE ProjectPosted: 10 Oct 2017, 18:27

Joined: 07 Feb 2017, 19:06
Posts: 48
Alpha Wolf wrote:
The 1627Se that is 2500 ohm. But they are not as good as the Decor and are almost \$200.00 US. Wich is crazy! Besides the 6AS7 is not an audio tube. It has a lot of distortion. It was used as a voltage reg in Textronics scopes.

WOW!I did not expect that,THANKS!

Alex Q

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 Post subject: Re: My 6AS7 SE ProjectPosted: 10 Oct 2017, 22:48

Joined: 08 Aug 2009, 03:11
Posts: 2229
Location: Chilliwack, BC
Alpha Wolf wrote:
Besides the 6AS7 is not an audio tube. It has a lot of distortion. It was used as a voltage reg in Textronics scopes.

A lot of great tubes weren't originally audio tubes. Video amplifiers, vertical deflection amplifiers, radar triggers, etc. Even the 300B wasn't a HiFi tube, it amplified audio signals, sure, but 300Hz-3KHz in and was a pass tube in telephone gear.

The 6AS7 is kinda like the 6C33C in this regard. It's linear *if* mondo amounts of current are going through it. Then THD is low. Also, some sort of NFB (local loop cathode FB is good). That's why most designers avoid using transformers with these tubes (popular in OTL stages).

You can use an Edcor, but you need a CXPP and run her at 90-100mA. Most CXPP under 8K primary can handle up to 200mA (that's AC+DC !). You'll cook that little GXSE, which IIRC is rated 80mA AC+DC. But double check with Brian Weston.

Cheers!

_________________
-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design

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 Post subject: Re: My 6AS7 SE ProjectPosted: 15 Nov 2017, 09:59

Joined: 15 Nov 2013, 10:16
Posts: 234
Geek wrote:
Alpha Wolf wrote:
Besides the 6AS7 is not an audio tube. It has a lot of distortion. It was used as a voltage reg in Textronics scopes.

A lot of great tubes weren't originally audio tubes. Video amplifiers, vertical deflection amplifiers, radar triggers, etc. Even the 300B wasn't a HiFi tube, it amplified audio signals, sure, but 300Hz-3KHz in and was a pass tube in telephone gear.

The 6AS7 is kinda like the 6C33C in this regard. It's linear *if* mondo amounts of current are going through it. Then THD is low. Also, some sort of NFB (local loop cathode FB is good). That's why most designers avoid using transformers with these tubes (popular in OTL stages).

You can use an Edcor, but you need a CXPP and run her at 90-100mA. Most CXPP under 8K primary can handle up to 200mA (that's AC+DC !). You'll cook that little GXSE, which IIRC is rated 80mA AC+DC. But double check with Brian Weston.

Cheers!

The Edcor site shows 130mA max on the GXSE15.. Continuous current in an application like this though I would tend to agree and be inclined not to run with a tube biased much past 80mA.
I had the 6AS7 triodes biased at 77mA, and the GXSE15's I was using seemed to work just fine.

Alpha Wolf wrote:
Besides the 6AS7 is not an audio tube. It has a lot of distortion. It was used as a voltage reg in Textronics scopes.

I don't have this amp any more, but I do feel the need to respond to this.. As Geek pointed out, many many audio amps that people build and enjoy in this DIY audio community are based on tubes that were not originally designed for audio use.. Besides stuff like the 6AS7 and 6C33C and 6336, there are guys doing stuff with sweep tubes with great success.

This amp in particular sounded really really good as I built it. Perhaps it was a little high on the distortion per watt, but I think it was discussed earlier in this thread that it was primarily 2nd order harmonic content, which ends up supporting the fundamental frequency, and so ends up being very pleasing to the ear. I think Matt did an article on this at one point, or maybe he just explained it to me. I can't remember.

Alex Q wrote:
thehoj wrote:
Transformers have arrived! Let the building begin.

Which of the Hammond transformer is equivalent to this EDCOR?
Thanks!

Alex Q

Is there a reason you wouldn't want to go with Edcor?

I had considered a few other options for output transformer when building this amp.
One was to go with the One Electron 1600 ohm primary SE output transformer fo \$99USD https://www.tubesandmore.com/products/transformer-one-electron-output-1600-1684
The other was to go with Transcendar http://www.transcendar.com/single-ended-transformer/15-watt-se/

I just ended up going Edcor on this amp because it was cheaper. But on some future builds I did use Transcendar and they're really great transformers as well. I use their chokes in quite a few things I've built. Excellent value in the Transcendar products. I don't know if his pricing has changed in the past year or two, but the stuff was always very reasonably priced whenever I ordered in the past.

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 Post subject: Re: My 6AS7 SE ProjectPosted: 20 Nov 2017, 17:24

Joined: 07 Feb 2017, 19:06
Posts: 48
It's the Waiting time/cost of the Transformer...
I thought about building this design because i got some of these tubes but then I realized since i have the Bottlehead S.E.X. i decided not to bother doing it.

Thanks again!
Alex

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 Post subject: Re: My 6AS7 SE ProjectPosted: 21 Nov 2017, 11:55
 Project Author

Joined: 06 Apr 2009, 10:08
Posts: 1520
Location: US Pacific Northwest
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.

_________________
Matt

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