Re: Eico 2080 Mod / Cathode Follower - Redux



Alex wrote:

ECC88 has a much higher transconductance than 6CG7 and hence the cathode
follower will have lower output impedance. Besides, ECC88 can stand 150V
cathode-to-heater, therefore the cathode resistor (which was 6.8K) can be
increased to perhaps 10K. Note that with ECC88 a grid stopper
(220...1000ohm) is a must.

The ECC88 or 6DJ8 only has marginally more gm than 6CG7 with low Ia.

To get the high gm of the ECC88, you need high Ia, and that means
keeping Ea low for low Pda and long tube life.
The 6CG7 has a higher cathode to filament rating than ECC88/6DJ8.

The main reason ECC88/6DJ8 in CF has lower Rout is because it has higher
µ and lower Ra, so hence higher gm.
But only marginally at say 4mA of Ia.

Using a CCS cathode resistance is a fine way to get the best working
point for the CFF
and to prevent the tube from having to produce output across both the
external load R and the dc carrying R.
So if the external load is 20k, its OK to drive with ECC88.
But if the dc carrying R was say 10k to get high Ia, then RL total
becomes a low 6k6.

Patrick Turner.



Regards,
Alex

"Jon Yaeger" <jono_1@xxxxxxxxxxxxx> wrote in message
news:C415364E.B98EB%jono_1@xxxxxxxxxxxxxxxx
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
    ... ECC88 has a much higher transconductance than 6CG7 and hence the cathode ... follower will have lower output impedance. ... Install Mikkel Simonsen's remote volume control kit ...
    (rec.audio.tubes)
  • 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
    ... Install Mikkel Simonsen's remote volume control kit ... the various inputs were coupled to a 7247 cathode ... I had originally used Eico's cathode follower circuit, ... I have a B+ of 280 VDC available for the plate of the CF. ...
    (rec.audio.tubes)
  • Re: Eico 2080 Mod / Cathode Follower - Redux
    ... Install Mikkel Simonsen's remote volume control kit ... the various inputs were coupled to a 7247 cathode ... I had originally used Eico's cathode follower circuit, ... I have a B+ of 280 VDC available for the plate of the CF. ...
    (rec.audio.tubes)