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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 Posted: 21 Feb 2017, 21:21

Joined: 09 Feb 2017, 20:50
Posts: 3
Location: USA
In a post on the 4S forum I read a design iteration involving modifying the gain close to unity. I would like to adopt this into the cathode follower version of the preamp. I am considering using a stepped attenuator for volume control. I found on ebay two options for this. A company called valab has a 50k and a 20k attenuator. I plan to use the 20k Attenuator with a 200k resistor on the input. I assume I drop the 10k resistor after the pot shown in the cathode follower circuit. Is this correct?

I would like to understand the math behind how the input resistance changes the gain. It may be more complicated then a single post but what is the equation used to calculate the output? I had a little trouble following the example used. I think using the 10 to 1 example will work well for my SS amps 1.1 v input sensitivity. By using a circuit with unity gain, am I eliminating my preamps usefulness with common tube amps? I will probrably keep it simple and build to match my SS needs but I am now intrigued by the idea of adding switches to give the preamp matching characteristics for tube and SS. Matt has given great information on how to match output coupling capacitors to meet the needs of both types of amps. Should I worry about limiting my gain if I were to use a tube amp in the future? If so, could I use a switch to adjust the input resistance to achieve multiple gain options? Would adding these extra switches just muck things up? For that matter are all switches created equal?

Forgive me for starting to ramble. My first question about the attenuator and resistor choices is really what I need reassurance on to move forward with purchasing my remaining components.

Thanks for any input.
Jesse

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 Posted: 31 May 2018, 02:59
 Project Author

Joined: 06 Jun 2008, 18:23
Posts: 5188
Location: Australia
I'm building this for another who wants to drive into an SS amp but wanted an all tube preamp. I'm assuming all 1/2W resistors is ok. Also I think I have this right, the preamp tube and the driverbuffer tube e.g 12AU7 can be different. So the preamp tube has left and right channel passing through and the driver tube the same.

I think this is great because you can select different tubes for different jobs. I would always use a 12AU7 for the driver but may want to use a T7 or X7 for the preamplification.

Am I right above.

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 Posted: 31 May 2018, 17:40
 Project Author

Joined: 04 Jun 2008, 20:59
Posts: 4097
Location: Arizona, USA
Hi, I'm a bit of a late comer on this thread....but I have a few observations. If jesseday2 does not need more gain, why not just use a cathode follower? A 12AT7 will have a gain of about 0.95 and a 12AU7 about 0.90. With the stepped control it would seem that is what is needed. I really hate to cut signals down to boost them back up as in a 10 to 1 input reduction feeding a gain stage of about 10. That increases the noise level at the output. I hate noise.

Hi Mark, 1/2 watt ones ought to be fine. Yes I would use the same tube for both first stages and the same tube for both followers. You could do the other way, but as you noted you can't do tube rolling if you do that. While all the various tubes with 12A?7 bases will work in the circuit, without doing some math I am not sure all will be in the best operational range. Funny thing is that 12AU7s and 12AX7s can often use the same cathode resistors and to a certain extent the anode ones but the 12AT7s bias differently and are unlikely to be in the most linear region of operation with those values. For small signals it may not matter much, but when you get past about a volt or so it can lead to distortion. I use a program to check out possible values and then run them on the distortion analyzer to verify them. PM me if you want regarding the process.

Edit: I see from the chart that the 12AT7 is IMO not a good choice. Anytime a preamp stage exceeds 1-1.5 % distortion I figure that a different approach is needed. Even then that is about 5 times what I like to see in a preamp stage.

Good listening
Bruce

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 Posted: 31 May 2018, 17:55
 Project Author

Joined: 06 Jun 2008, 18:23
Posts: 5188
Location: Australia
Bruce, thanks for the info. I'll stay away from T7s on your advice and maybe two U7s or an X7 preamp with U7 driver. Hope to have the PS done today. I'm using turrent brds. I've had for years. Mounted some big 0.47uf PIOs on top, no room underneath.

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 Posted: 11 Jun 2018, 07:25
 Project Author

Joined: 06 Jun 2008, 18:23
Posts: 5188
Location: Australia
I had a matched pair of Genalex Gold Lions gold pin 12AX7s. One is now the preamp for the two stage tube preamp. Finished the build today. Had a horrible buzz until I connected the DC filament voltage and HT voltage (negatives that is) together and then to mains earth. Now dead quiet.

Had a quick listen, very quiet and sounded good. Big listen tomorrow.

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 Posted: 21 Sep 2018, 14:19

Joined: 20 Apr 2015, 01:30
Posts: 86
Hi Matt.
I built the 4S Universal some time ago, and have used it in front of a variety of DIY tube amps with great results. I recently picked up a lovely SS amplifier, an Audio Zone Amp-1 built by Peter Daniel, that has an input impedance of 10K Ohm, and I am in the process of adding a cathode follower to my 4S in order to use it with this amp, and with a Nelson Pass Amp Camp Amp with a similar input impedance. I will build with the switch that allows changing the output capacitor value to suit both solid state and tube amps. I am told that the Amp-1 doesn't have any protection from a source that puts out DC, and that it will amplify DC as readily as AC. It would appear that the capacitor at the output will block any DC from appearing at the outputs of both versions of the 4S Universal. Is this correct? Also, are there any situations, such as a passive component failure, in which DC might be passed through to the outputs? I really like my speakers, and just want to confirm this prior to moving forward. Any advice or insight would be much appreciated.

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 Posted: 21 Sep 2018, 15:11
 Project Author

Joined: 06 Jun 2008, 18:23
Posts: 5188
Location: Australia
I feel you should be save as the output cap blocks DC. But Matt will have a better idea.

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 Posted: 21 Sep 2018, 15:41

Joined: 23 Feb 2017, 02:02
Posts: 618
Caps block dc and allows ac(music), theoretically speaking it's impossible to see dc after you put things in series with capacitor

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 Posted: 22 Sep 2018, 17:34
 Project Author

Joined: 06 Apr 2009, 10:08
Posts: 1620
Location: US Pacific Northwest
tizman wrote:
It would appear that the capacitor at the output will block any DC from appearing at the outputs of both versions of the 4S Universal. Is this correct?
Yes. However... It is possible for the output of the cathode follower to see a few volts of DC at power on, which should quickly tend toward zero in 10 to 15 seconds as things stabilize. If you have the tow attached, I would recommend turning on the 4S-CF first, then in about 30s turn on the SS amp. This should ensure that you never put a DC pulse into your amp.

Do you have a schematic of the Audio Zone Amp-1?

_________________
Matt

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 Posted: 22 Sep 2018, 19:08

Joined: 23 Feb 2017, 02:02
Posts: 618
A solution could be to have the pot attached to output of the pre amp instead of input.

By making sure your volume is at zero ever time you power on will enuse you not see any pulses.

12au7 with low imedance output shouldn't have anyplorbems with driving pots on outputs.

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