Hello, John.
1. Current through the valve 6CG7 will be lower than 4mA, since a lot will
drop across the cathode resistor 39K. Have you considered to reduce the
cathode resistor to, say, 10...15K to get higher transconductance oand hence
lower output impedance on "tape" output?
2. Beware of the cathode-to-heater voltage. It shall be less than 100V. For
that reason also why not reduce the cathode resistor.
3. [Obvious]. Ripple on the 165V supply rail shall be low, since the mu of
this tube is low (about 20) and it will not give good power supply noise
rejection ratio.
4. If your taperecorder connected to "tape" output happens to have
relatively low input impedance (say, 10K) you will lose bass, because your
interstage capacitor 0.47uF will not be sufficient. Why not then increase it
to 2.2uF? (Otherwise make sure to connect only high impedance loads to your
"tape" output.
5. [Warning!] Connect a 100R...1K resistor in series with the grid close to
the socket. It will guarantee against parasitic oscillation (in Kolpitz
mode) on the Ls and Cs of the circuit, particularly when your input
"Balance" pot happens to go to ground.
6. Good thing is that your choice tube has a very common pinout. You can
plug almost any twin triode in there and compare results. I believe, ECC85
is a better option. And if you decide to play with high transconductance
ones, like 6ES8 (to get very low output impedance on "tape"), you will
indeed appreciate advice #5.
Regards,
Alex
"Jon Yaeger" wrote in message
...
My latest project is to complete some modifications to an Eico 2080
integrated amplifier, which uses 7591s in PP configuration, and is similar
in many respects to the ST-70 circuit.
Among my goals we
1. To install a remote volume control, using the very nice kit sold by
Mikkel Simonsen, formerly of R.A.T.
2. Remove the tone control circuitry;
3. Design a new faceplate for the reduced number of controls.
In order to achieve goal #1, I used one of the tone control tube positions
as a cathode follower, to drive the 50K motorized volume control.
Mikkel's
circuitry included a driver for a mute relay.
I am housing his circuit inside of a metal box installed under the
chassis,
to reduce interference as much as possible.
A schematic of one channel the proposed design is posted on:
http://www.yaegeraudio.com/eico_mod.pdf
I welcome any comments / criticisms / suggestions on the proposed design.
Thank you.
Jon