New driver for SR-45

[2wo]

I Love the sound of my SR-45 but compared to most of my stuff, it has rather low gain. I know that low gain is not a subject that usually comes around here and I thought rather than pad everything else down I might try to boost this amp up.

 I most often don

[Paul Joppa]

The 6BN7 is fairly close, and has a mu of 28 - don't know if that is enough though. Any idea how much more gain you need? You might get away with something in the 30-40 range, in which case there are many more options among the single as well as dual triodes - as long as you are willing to add a socket. Maybe a 5842 if you are feeling sporty... anyhow, one of the frame-grid VHF triodes might be good.

Of course, the 12AT7 operating point IS well established for this application... :^)

The shunt reg can be segregated to a single tube as well, possibly an EL-84 or 6V6/6AQ5, triode wired. Those pentodes aren't rated for enough voltage, but a zener between plate and screen will make them triodes with better longevity.

[2wo]

Thanks Paul, I hadn

[Paul Birkeland]

I think PJ is maybe being a little bit generous about the flexibility of the design.  The need for two tubes in place of one will leave you a little stressed in terms of heater current on the Paramour power transformer, and you will likely need to adjust both R1 resistors on the driver boards to bring things back into equilibrium.

I would suggest either placing a SUT or a preamp between your sources and the SR45.  I think your squeezebox probably has low enough impedance to drive a nice pair of 600:600CT Altec/Peerless bridging transformers.  Drive the end of one winding to the CT with the source for each channel, then take your output off the 600 ohm tap.  If you really like the sound and functionality of this, you could add octal sockets in your SR45's to accommodate the transformers in the amplifiers.

If the output impedance of the SB is too high to make that work, there are plenty of 10k alternatives that should work quite well.

-PB

[Paul Joppa]

The Bottlehead PT-2 can supply 1.2A RMS of heater current at 6.3v.

I agree, any change of the driver and shunt reg tubes will call for adjusting the currents through these tubes, and the driver bias - in other words, re-designing the operating points.

[2wo]

So I finally got around to changing the driver for my SR-45. I ended up with a 6201, a 12AT7 variant. I went with this because with the mu of 60, I thought I could get the gain I wanted and I thought it would work well at the 3.6ma that the 6CM7 was running at.

Well nothing is ever as easy as it looks. My SR-45 was a quote, unquote, finished amp and was built into a found, Ikea enclosure. No popping the top plate off to make changes and I didn

[Paul Joppa]

Very cool - thanks for posting the steps on this adventure!

Here's another idea which I didn't think of before - it is possible to drive a 45 with a 6SL7. Run it with an HLMP-6000 LED for the cathode, and about 2.0mA plate current. The reason such a wimpy tube can drive a 45 is the very low Miller capacitance of the 45's grid, about half that of a 2A3 or 300B. Gain is 70, a bit more than the 6201, and the tube is very linear.

I don't guarantee you'll like it, but it has great potential and is quite different from the 6201 - so whatever happens, you will have tried more variables and you will have learned more.