Actually, I have that preamp I was just wondering if one could be built using SRPP to get more gain.
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.
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.
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?
Gain still limited by mu of the lower triode. But drives the capaciance of a transmission line well,
low impedance for both +ve & -ve going edges on a square wave,👍
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...
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.
I made an error, that should read gain depends on the mu of the top tube of the WCF.😳
The bottom tube actively charges & discharges the transmission line capacitance.
As for any CF, gain is always less than one but can get close.
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.
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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No Fair! Showing just one pic of that greatest the world has ever seen amp. Please maybe just a touch of info to get for us staring blindly at this beauty.
I bought that amp on eBay. I couldn’t resist the looks. It is a Magnavox 8601-20 Tube Stereo Power Amp that was made fancy. It sounds great for what it is.
The endbells on the OPTs are jelly tubs from Denny’s restaurant.
The endbells on the OPTs are jelly tubs from Denny’s restaurant.
If I don’t need that much increase from the preamp, maybe I’ll built this and see how it sounds.
https://diyaudioprojects.com/Tubes/Universal-Tube-Preamplifier/
https://diyaudioprojects.com/Tubes/Universal-Tube-Preamplifier/
Seems to me a good way to go. Build what you need. I like overbuilding other aspects of sturdiness for example and even power supply needs, but the actual circuit doesn't sound better with 3X the gain, right?
I have all the bits I need to build little guy above. I can breadboard it and see how it sounds.