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: Tube Buffer AmpPosted: 17 Nov 2012, 02:58

Joined: 19 Mar 2012, 19:22
Posts: 168
Location: Wales, UK
Hi Gregg,

Thanks for posting the link. I will take a look at that tonight.

Phil

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 Post subject: Re: Tube Buffer AmpPosted: 17 Nov 2012, 20:37
 Project Author

Joined: 06 Apr 2009, 10:08
Posts: 1620
Location: US Pacific Northwest
I just performed a detailed analysis of the 6SN7 cathode follower stage at the request of a forum member. I thought I would post it for everyone's benefit.
Quote:
I've calculated the operational parameters of the circuit. The bias point of the tube is approximately 128v at 1.35mA with a cathode bias of about -5.8v (at 250v B+). This isn't a bad solution although I might have gone for a slightly lower bias voltage and a somewhat higher current to get a lower plate resistance value. This would have allowed for a lower load resistor and somewhat better distortion numbers. Here are the specifics:

Rin = 928kΩ
Cin = 4.2µµf
Ro = 828Ω
Av = 0.93
D = 0.4%
Vin-max = 84v peak

As for capacitors, the input is easy. Since the input impedance of the stage is almost a megohm, the cap won't have to be very big at all. This impedance will dominate the output impedance of anything driving the amp so with a very low driving impedance, the input coupling cap low frequency rolloff is at 1Hz with a 0.17µf (171nf) coupling cap. The numbers indicated in the schematic are reasonable. The tricker part is going to be the output capacitor.

First, dump the 1kΩ series resistor; it does nothing. Now the low frequency rolloff due to the output coupling capacitor is...

f-lo = 1.0 / (2π * Cc * Req)

where: Req = Ro + 100kΩ || Rin-next_stage

This means the the low end rolloff is going to be highly dependent on the input impedance of whatever you are driving. High impedance loads will not require a very big cap, lower impedance loads will. Here are some examples of the capacitor size required to get a 2Hz rolloff frequency on the output verses some various load impedances.

Code:
-----------------------------
500kΩ     0.95µf
250kΩ     1.1µf
100kΩ     1.6µf
50kΩ     2.3µf
10kΩ     8.0µf

So you'll have to make some decisions based on how you'll be using the buffer.

As to the level control, you have two options. The first is to simply hang a normal 100kΩ to 500kΩ volume control on the front of the stage BEFORE the input coupling cap. This will dominate the input impedance of the stage but if you don't mind the somewhat lower input impedance, this is ok. The other option is to convert the 100kΩ output resistor to a volume control like on the 4S preamps. However, this is going to drive your peak output impedance to approximately 25kΩ depending on control setting. This may also be ok dependent on how you intend to use the stage. However, this will significantly complicate the calculation of the output capacitor. Personally, I would put the control on the input.

I hope people find this useful.

_________________
Matt

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 Post subject: Re: Tube Buffer AmpPosted: 17 Nov 2012, 20:54

Joined: 14 Feb 2010, 13:13
Posts: 663
Location: Lisbon, Portugal

There´s always so much to learn!

Thank you for sharing your knowedge with us, my friend!

Miguel

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 Post subject: Re: Tube Buffer AmpPosted: 17 Nov 2012, 23:26

Joined: 08 Aug 2009, 03:11
Posts: 2229
Location: Chilliwack, BC
Suncalc wrote:
First, dump the 1kΩ series resistor; it does nothing.

Very incorrect.

Some high transconductance (especially Russians) CF's will become a power oscillator with capacitive loads (like long runs of "audiophile" cables). The 1K series prevents this from happening.

I have a set of 6H8C (6SN7) that will squeal into a headphone amp without them.

Every part in my designs has a purpose and I will happily explain them to anyone who asks.

Cheers!

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

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 Post subject: Re: Tube Buffer AmpPosted: 18 Nov 2012, 00:47
 Project Author

Joined: 06 Apr 2009, 10:08
Posts: 1620
Location: US Pacific Northwest
Geek wrote:
Some high transconductance (especially Russians) CF's will become a power oscillator with capacitive loads (like long runs of "audiophile" cables). The 1K series prevents this from happening.

I have a set of 6H8C (6SN7) that will squeal into a headphone amp without them.
I am assuming this is with very low impedance loading (or even matched loading). With a 10kΩ load I calculated more than ample phase margin to prevent oscillation. And I never considered 2600 µMhos a particularly high transconductance tube. Do the Russian tubes really show that much variation?

_________________
Matt

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 Post subject: Re: Tube Buffer AmpPosted: 18 Nov 2012, 02:18

Joined: 08 Aug 2009, 03:11
Posts: 2229
Location: Chilliwack, BC
Oh yes, load it down and there's no problem!

The devices I did have a problem had 1Meg input pots.

No, the transconductance doesn't vary a lot, they are just... odd. Seems the diamond-logo-OTK-stamped ones heavily sought after are the ones that can be the most finicky. Got me as to why

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

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 Post subject: Re: Tube Buffer AmpPosted: 18 Nov 2012, 05:04
 Project Author

Joined: 06 Jun 2008, 18:23
Posts: 5188
Location: Australia
@Matt & Geek: I intended to keep the 1K resistor and my simplistic way of looking at it went two ways: 1 - It is only a 1K resistor (for me that cost 1/8th of \$0.46) and 2: if a short occurred at the output (and pigs grew wings) there would be a load of sorts and not a short.

I'm real glad we have both of you guys around to guide the less knowledgeable of us (OK just me). Keep it up.

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 Post subject: Re: Tube Buffer AmpPosted: 18 Nov 2012, 05:33

Joined: 08 Aug 2009, 03:11
Posts: 2229
Location: Chilliwack, BC
Cheers!

What I love about Gio's board is there's an incredible amount of experience and in different areas

I know I learn stuff all the time!

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

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 Post subject: Re: Tube Buffer AmpPosted: 18 Nov 2012, 07:27
 Project Author

Joined: 06 Jun 2008, 18:23
Posts: 5188
Location: Australia
I am intending to build this 6SN7 cathode follower buffer. Since I received a few 6N7 metal tubes a few weeks back I have been working with Matt to come up with an amp or preamp for these strange looking tubes. Matt has designed a schematic around the tubes in the form of a preamp. At this point I'm going to build the preamp based on the 6N7 and later come back and build a 6SN7 buffer.

Follow the 6N7 preamp build here:http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=4151

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 Post subject: Re: Tube Buffer AmpPosted: 19 Nov 2012, 09:12

Joined: 19 Mar 2012, 19:22
Posts: 168
Location: Wales, UK
Thank you guys for your much appreciated help

I can now build a 6sn7 buffer amp as I have just got my hands on some current production and nos items

Thanks again

Phil

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