Re: Eico 2080 Mod / Cathode Follower - Redux

in article 9KMHj.318045$F7.31403@xxxxxxxxxxxxxxxxxxxxxxxxxx, Iain Churches
at IainNG@xxxxxxxxxxx wrote on 3/30/08 9:39 AM:



"Jon Yaeger" <jono_1@xxxxxxxxxxxxx> wrote in message
news:C413522B.B768F%jono_1@xxxxxxxxxxxxxxxx
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.


Hi Jon. That's something I was told years ago, and have
seen in several books. The values used in schematics
seem to bear this out also.


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 @ 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 supported advice would be greatly appreciated. Thanks in advance,


I would have expected to see a much larger value of Rk.

I often use use a generic active load cathode follower
which came from a BBC manual. It uses the other
section of the double triode as the load.
B+ is 285V. The voltage on the lower cathode is
125V with Rk of 62k so 2mA current.

Another simple cathode-bias cathode follower (also from
the BBC) with an E88CC tube, has 1.3k in series with
100k from cathode to ground. 1M is connected from
the junction of the two resistors back to the grid for
bias. The B+ is 285V. This works well also.

Hope this is "supported" advice:-)

Cordially
Iain


Iain,

Thanks for the advice.

Simplicity is a goal here, and so I'll go for the single tube solution.

Just curious . . . what advantages do you see using a E88C (6922) vs. the
6CG7 in this application?

Jon




.

Relevant Pages

  • Re: Eico 2080 Mod / Cathode Follower - Redux
    ... Install Mikkel Simonsen's remote volume control kit ... the various inputs were coupled to a 7247 cathode ... which drove the active tone control stage. ... I have a B+ of 280 VDC available for the plate of the CF. ...
    (rec.audio.tubes)
  • Re: Eico 2080 Mod / Cathode Follower - Redux
    ... Remove tone control section ... use a cathode follower to buffer the balance control from ... I have a B+ of 280 VDC available for the plate of the ... Doug Bannard suggested a 22K cathode resistor with a B+ of 300, ...
    (rec.audio.tubes)
  • Re: Eico 2080 Mod / Cathode Follower - Redux
    ... Remove tone control section ... use a cathode follower to buffer the balance control from ... the level control, ... I have a B+ of 280 VDC available for the plate of the ...
    (rec.audio.tubes)
  • Re: Eico 2080 Mod / Cathode Follower - Redux
    ... Install Mikkel Simonsen's remote volume control kit ... I had originally used Eico's cathode follower circuit, ... I have a B+ of 280 VDC available for the plate of the CF. ... voltage across the tube, and the remaining 1/3 across the cathode ...
    (rec.audio.tubes)
  • Re: NFB Quiz
    ... In both cases, common cathode with anode an grid AC coupled, as well as ... in the cathode follower case, the output AC impedance of a triode is: ... Apparently it will cause dV/S increment in the plate current. ...
    (rec.audio.tubes)