• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

6080 tube in SRPP tube amp???

Replace it with a more conventional preamp style tube like 6SN7/6BL7/6BX7 to get more gain and add a loading resistors to EL34 cathode to sink all that 50mA current similar to Alan Kimmel's "mu-stage" circuit. This way you have the least number of parts to change and keeping the same number of sockets for stereo. It's easily reversible so if you don't like it you can change it back. I never understand the appeal of using power tube for preamp gain stage. Buffer or cathode follower, yes, they are great for that. Even EL34 is overkill. I'd prefer to use 6V6 as buffer. Look at Salas' 6V6 thread for inspiration.
 
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6080 tube in SRPP tube amp????​

Yes - it can be used interestingly as a power tube for an OTL SEPP output stage :

LUvYJb-P1100328.jpg


T
 
Actually, I have that preamp I was just wondering if one could be built using SRPP to get more gain.
An SRPP will not give you more gain than the same triode in a fully bypassed common cathode config.
The most gain you can get out of the triode is using a mu-follower config. I suppose you could stack the two triodes in a 6080 as a mu-follower. Or use a big pentode as the plate load, as in the audioXpress article mastouffer shared.

PMillettFigure1Schematic[1].jpg


Or actually, you could use the two triodes to make a cascode. That would give you more gain than as a mu-follower, but with much higher Zout and more distortion.

How much gain do you need?
 
Better result with WCF. Personal experience :)
Didn't the OP ask for voltage gain? WCF has gain just slightly less than 1.

That said, if one adds a voltage amplifier stage before the WCF and then uses NFB from WCF output to the grid of the input stage, the Zout can get really low and the THD as well. I bought some big surplus 245uF film caps to use for a headphone amp built on that topology. Haven't gotten around to that yet, though...
 
An SRPP will not give you more gain than the same triode in a fully bypassed common cathode config.
The most gain you can get out of the triode is using a mu-follower config. I suppose you could stack the two triodes in a 6080 as a mu-follower. Or use a big pentode as the plate load, as in the audioXpress article mastouffer shared.

View attachment 1202711

Or actually, you could use the two triodes to make a cascode. That would give you more gain than as a mu-follower, but with much higher Zout and more distortion.

How much gain do you need?

That is the preamp I built for my main system, and I love it. I work at home and have these speakers and this Magnavox 8601 driving them on my desk, but I need a little more umph going into the amp. So, to answer your question, I don't know how much I need, just enough to ease the amp as I need to turn it up too much to get the listening level I want.

20160620_111555.jpg


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I don't know how much I need, just enough to ease the amp as I need to turn it up too much to get the listening level I want.

If you use the aforementioned common cathode stage DC coupled to White Cathode Follower stage, you can use negative feedback to set the gain as desired.

A while ago, I went on a headphone amp simulation jag and PRR and artosalo suggested a couple of too-good-to-be-true designs based on the above topology.
  • Basically, the input stage is a common cathode triode like a 12AU7 or 6FQ7 which is DC coupled to a White Cathode Follower output stage.
  • The input stage is configured as an 'anode follower', with negative feedback taken from the output of the WCF to a voltage divider before the grid of the input stage. The voltage divider sets the gain of the whole amplifier.
  • The more NFB you add, the lower the gain and the lower the output impedance (Zout).
  • Taking a 12AU7's gain down from about 15x to 3x (5x reduction) reduces THD by 5x and reduces the Zout of the cathode follower by 5x.
You shouldn't need to sink 20mA from your preamp into the load. I'd think 5mA Ip would be enough to drive a normal cable run and the input of your power amp.

Good results can be achieved using a 12AU7, 6SN7, 6FQ7, 6DJ8, 6N6P, etc etc etc as an anode follower into a high voltage MOSFET source follower. Choose a MOSFET with low Crss (IRF710 or better). That's simpler than a WCF.

If you simply must go all-tube, you can use one twin-triode per channel (12AU7, 6SN7, 6FQ7, 6DJ8, 6N6P, etc etc etc) as an anode follower with cathode follower output stage.

I built a test version of the above, using a single 6DJ8 anode follower into an old mono phono preamp chassis, I set it to 2x (6dB) gain, I use it to goose the level from my Bluray player, which seems to fatten up the sound from movie soundtracks. It's just for effect, not for lab-grade accuracy 'high fidelity'.

Many people hate to employ NFB, though. Without NFB you will have to find a triode that has Zout of less than 1k ohms and gain of about 3x. Good luck with that.

PS - I love the 1950s classic auto look of your build. Very nice.
 
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