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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: Tube Buffer Amp
PostPosted: 15 Nov 2012, 09:13 
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Joined: 08 Aug 2009, 03:11
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Hi Phil,

philip davies wrote:
Thank you very much for your help. Is there an equation that I can use to work out the heater bias?


HT is right for a general rule of thumb.

I use this:
Image

I try and make the cathodes at B+/2, so a calc is more accurate

Cheers!

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 Post subject: Re: Tube Buffer Amp
PostPosted: 15 Nov 2012, 15:18 
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Joined: 19 Mar 2012, 19:22
Posts: 168
Location: Wales, UK
Hi,

Thank you both for your explination on what to do with biasing the heater voltage.

I have read that with srpp stages or cathode follower stages that it is important to bias the heater voltage so that the heater to cathode voltage is not exceeded. Is it just as important to do the same with an amp/preamp that has a standby switch for the HT and heaters running on AC voltage?

Thanks again

Phil


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 Post subject: Re: Tube Buffer Amp
PostPosted: 15 Nov 2012, 18:14 
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Standby switch doesn't matter.... when in standby, there is no Vh-k potential to worry about.

Cheers!

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


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 Post subject: Re: Tube Buffer Amp
PostPosted: 15 Nov 2012, 19:06 
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Joined: 19 Mar 2012, 19:22
Posts: 168
Location: Wales, UK
Hi,

So am I right in saying because I have a standby function on my amp that there is no need for me to worry about the hearer to cathode voltage being exceeded? Please let me know if I have this completely wrong.

P.S the centre tap on my heater winding has been connected to ground inorder to reduce hum

Thanks again for your advice

Phil


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 Post subject: Re: Tube Buffer Amp
PostPosted: 15 Nov 2012, 23:14 
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Oh, I thought you were talking about the Vh-k while being in standby.

Lift your heater supply at the centre tap.

But if you have other tubes hooked up that aren't in such a cathode follower mode, you have to compromise, or you'll exceed their Vh-k. Maybe even just put a cap on the CT to ground to bleed the noise and call it a day :D

Cheers!

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


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 Post subject: Re: Tube Buffer Amp
PostPosted: 16 Nov 2012, 02:57 
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Joined: 19 Mar 2012, 19:22
Posts: 168
Location: Wales, UK
Hi,

Thank you very much for your help. It is very much appreciated :up:

I'm going to give what you have sugested for the heater supply a try and also gather all the parts needed to build the buffer amp.

Many thanks

Phil


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 Post subject: Re: Tube Buffer Amp
PostPosted: 16 Nov 2012, 07:04 
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Joined: 06 Jun 2008, 18:23
Posts: 5143
Location: Australia
Geek wrote:
Here's the one I use. Unconditionally stable and one tube for stereo! Good for miniature 6FQ7/6CG7 too.
Attachment:
6SN7_CF.png

For 6SL7 or miniature 5751, use 820 ohms instead of 4.3K on the cathode.

I was going to suggest the best buffer is generally a cathode or emitter follower. Good current source and low impedance out. You do lose some signal - about 25%. I like this cct. but no good for my low gain tube amps. My current high gain Gainclone would be a great match for this. I may just build one. I am half way through construction of a 250V PS and I'm not 100% happy with the tube preamp I just built for the Gainclone. To get back the 25% of signal make it a two stage tube preamp, gain up front and drive from the rear. http://www.diyaudioprojects.com/Tubes/12AX7_Preamp/

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 Post subject: Re: Tube Buffer Amp
PostPosted: 16 Nov 2012, 14:38 
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Joined: 19 Mar 2012, 19:22
Posts: 168
Location: Wales, UK
Hi,

Thanks for the link. I will take a closer look at that :D

Thanks

Phil


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 Post subject: Re: Tube Buffer Amp
PostPosted: 16 Nov 2012, 17:47 
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Joined: 06 Jun 2008, 18:23
Posts: 5143
Location: Australia
mwhouston wrote:
Geek wrote:
Here's the one I use. Unconditionally stable and one tube for stereo! Good for miniature 6FQ7/6CG7 too.
Attachment:
6SN7_CF.png

For 6SL7 or miniature 5751, use 820 ohms instead of 4.3K on the cathode.

I was going to suggest the best buffer is generally a cathode or emitter follower. Good current source and low impedance out. You do lose some signal - about 25%. I like this cct. but no good for my low gain tube amps. My current high gain Gainclone would be a great match for this. I may just build one. I am half way through construction of a 250V PS and I'm not 100% happy with the tube preamp I just built for the Gainclone. To get back the 25% of signal make it a two stage tube preamp, gain up front and drive from the rear. http://www.diyaudioprojects.com/Tubes/12AX7_Preamp/

I said a 25% loss in signal level - I see in an earlier post it is only 5%. Even better.

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Projects:"retro2308" - chip based headphone amp | ”Calibre 834” - tube phono MM preamp | ”najah” - Raw 180W Tripath Class D power amp | "Icon" - Shuguang CV-181Z preamp


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 Post subject: Re: Tube Buffer Amp
PostPosted: 16 Nov 2012, 21:45 
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Joined: 08 Aug 2009, 03:11
Posts: 2229
Location: Chilliwack, BC
It depends on load, too. The heavier the load, the higher the loss. Why high transconductance tubes are great for this service.

A buddy on my forum just linked a book that deals with cathode followers, you may be interested:
http://w140.com/tube_circuits_in_tek.pdf

Cheers!

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


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