Phono stage for 2 7F7s (stereo) plus buffer?

[GordonW]

Got a chassis here... only have room for two three octal or loktal sockets for the phono stage.

I'm thinking 7F7, after checking around everywhere I could think, due to the much-lower capacitance (compared to any octal I could find) and good-looking plate curves. With probably a 7F8 or 7N7 as a cathode follower, in the output...

Anyone got any pet circuits that would work? I've already looked at the Hagerman Cornet circuit and Bob Danielak's DC-coupled design (modified RCA, apparently), subbing 7F7s for the 6Sl7s. If I used Bob's circuit, I would add a cathode follower buffer to the output.

Any other designs I should be looking at, that can use a total of three loktal twin-triodes for the entire stage (including buffer)?

Thanks!

Regards,
Gordon.

[GordonW]

Here's the schematic for the Bob D. DC-coupled design.

I would probably spring for diode-generated bias, at least in the first stage, to get rid of that huge cathode cap... it's just under two volts needed...



BTW- it looks like the Hagerman Cornet (from the notes on his drawings) only has 36dB gain?! That's awfully low. Need at least 40-46 dB or so, IME, to work well with most moving-magnet cartridges (i.e, to match the output levels of most CD players, tuners, tape decks, etc... between .5 and 1v line output with 5mv input). Here's a link to the schematic for that:

http://www.hagtech.com/pdf/cornetoctal.pdf

Regards,
Gordon.

[Eli Duttman]

Gordon,

I_B in the high mu/high R_P types is too low for reliable LED operation. A tweaked RCA circuit, using grid leak bias in the 2nd gain block, improves bass extension and eliminates a cathode resistor bypass capacitor. Look here. A 'SL7 and 2X 'SJ7s will yield all the gain you could possibly use. Buffer the 'SJ7s with ZVN0545As.

[GordonW]

Originally posted by Eli Duttman
Gordon,

I_B in the high mu/high R_P types is too low for reliable LED operation.

I was thinking that myself.

Of course, I can probably find a couple of fast-recovery diode models, that I can connect in series, that won't mind the low current...

Regards,
Gordon.

[SY]

Take a look at my measurements in the 12AX7 thread. That was with an LED and 0.8mA with a comparison to the same setup with the LED running 8mA. No real difference.

If it bothers you, you can bump up the LED current by running a resistor from the B+ to the LED-cathode junction sized to give you whatever current you want. That won't affect the tube's operation at all.

[GordonW]

Originally posted by SY
If it bothers you, you can bump up the LED current by running a resistor from the B+ to the LED-cathode junction sized to give you whatever current you want. That won't affect the tube's operation at all.

This was what I was thinking of.

Probably something like a 100K resistor... that would get the total current up into the 4ma range. Should be able to be done with a 2w resistor, if my calculations are correct...

Oh, one LED question- would there be any benefit to bypassing the LED with a good high-quality film cap? Possible high-frequency diode noise shunt?

Thanks...

Regards,
Gordon.

[SY]

No, no real benefit. The dynamic impedance of the LED is on the order of a few ohms. FWIW, I use LEDs to bias each stage of my MC phono preamp with no noise problems.

[GordonW]

Given the low impedance of the LED... SY, have you ever used more than one channel's worth of tubes, on one LED? I'm thinking that the constant-voltage aspect of the diode would make "crosstalk" between the two pretty much nil? This would put more current on the single LED, making it more "stable", in even a worst-case condition...

Sorry for the piecemeal questions... the thoughts are coming a bit at the time this morning...

Regards,
Gordon.