This is part II of the Eico 2080 modification project. The 2080 is very
similar to the ST70 in design but is a bit more rare.
Special thanks to Doug Bannard and "Alex" for their very generous
assistance.
Goals:
1. Remove tone control section
2. Install Mikkel Simonsen's remote (motorized) volume control kit
3. Fabricate a new front panel
4. Figure out a way to use the mute relay
Considerations:
In the original circuit, the various inputs were coupled to a 7247 cathode
follower stage, which drove the active tone control stage. The output
from
the tone section fed a balance pot scheme and then a 500K level control.
Mikkel's motorized pot is an Alps 50K control. I thought it would be best
to
use a cathode follower to buffer the balance control from the level
control,
but most of all to drive the 50K level control pot.
I had originally used Eico's cathode follower circuit, until the folks
noted
above kindly pointed out some problems.
I'd like to run through the assumptions and calculations and invite any
comments and corrections.
* * *
See the illustration at: www.yaegeraudio.com/eico_mod.jpg to follow
along.
1) I have a B+ of 280 VDC available for the plate of the CF (6CG7).
2) According to Alex, a good rule of thumb is to put 2/3 of the plate
voltage across the tube, and the remaining 1/3 across the cathode
resistor.
Using that assumption, the plate voltage would be about 187 volts; and 93
volts across Rk.
3) Looking at a 6CG7 plate characteristic curve, the operating current
with
Ec=0 volts (Ug=0) and 187 volts is about 28 mA.
4) 1/2 of 28 mA, or 14 mA, is selected for a target quiescent current.
5) The grid voltage for 14 mA @[EMAIL PROTECTED]
187 volts is about -3.3 V, looking at the
plate curves.
6) The value of the cathode resistor, Rk, is 93V / 14 mA = 6.8K (approx)
7) The value of the bias resistor, Rb, is 3.3 V / 14 mA = 240 ohms
(approx)
Offhand, 14 mA seems a bit high for a quiescent current.
Any sup****ted advice would be greatly appreciated. Thanks in advance,
Jon Yaeger
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