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Hi
As I got my hands on the circuit diagrams for my CD-player, I got some doubts abouts the topology/working points agt the OP-stage.
My doubts/thoughts are:
1. Any overall sugestions for easy mods (topology) ?
The only mod made, is the orginal 6922EH replaced with Mullard E88CC SQ.
2. What is the optimal working (bias) point for E88CC/ECC88/6922/7308/E188CC ?
IsŽnt 2.1mA for the CC-stage to low ? (the hole working line is at the "bended area" in the characteristic diagram).
3. How about removing the CF-stage ? (IŽm only using very short inteconnects), using the full 126.5V for the CC-stage, and replacing the CF-stage bias resistor (750 Ohm) with 2 x Ni-cd bateries (Bias aprox. 2.4V)
Regards
Pix
Follow Ups:
PixHow about a choke from the 126.5V to the plate.
Then a couple of 1.5V NiCad/NiMH batteries
for the the cathode bias.
You get you ~8.5mA according to TubeCAD.
I have similar with my 12B4 DAC/preamp.
Sounds open and clear.DanL
Thanks Dan, yes, that might be a good idea.
The anode choke will increase the anode voltage to a better level.
I really would like to try battery csthode bias.
Is Ni-mh batteries as good as Ni-cd for this use ? (Ni-cd has a lower internal resistance, but are hard to get these days)
Regards
Pix ;-)
PixIn my circuit the NiCad sounded more articulate
while the MiMh sounded more musical.
I went with the NiMh. YMMV
On the choke I would recommend 100H+
I like over-kill in this position.
Now DCR will determine the current
with 1300DCR being about 10mA.
Lower DCR will give you more current
and higher DCR will give you less current.
1000DCR will give you about 8mA.
Since you're in Sweden try the Lundahl
LL1667 but only use one winding or
the LL1668 and add a 620R in series.
The LL1668 would be less prone to hum pickup
because you are using both windings.DanL
Thanks Dan
This idea of choke loading the tube really looks nice.
IŽll have some gear allready wich might come in handy:1 Pair Lundahl LL1660/18mA (100H primmary)
Several alternative NOS tubes:
ECC88/6922/E188CC/7308/
5751
6H6pi/6N6pi
6C45pi
Also most of the "ususal" octals:
6SN7
6SL7
etc..Would you say that IŽll should stick to the orginally 6922/ECC88 or go for some alternative tube in this configuration ?
Could I use my LL1660/18 as anode load (using the 100H primary), or would you recomend using it as output transformer instead of the LC output ?Pix :-)
PixDepends on how much gain you need.
Did you have too much gain
with the original design?I would use it as a trannie myself.
If you have the right gain then I would wire the
LL1660 in 4.5:4 (Alt S with Pri & Sec reversed).
If too much gain then Alt T reversed (4.5:2).
No output resistor or cap needed.
Just Cathode battery, Grid Stopper and Trannie.
Dan,
The gain is ok, even with the passive stepped attenuator I am using.As a second step I was planing on replacing the recent avctive I/V/buffer (hugh amount of OP-amps) with a resistor, and in that case some extra gain will propably be needed.
My CD-player is built around two parallell B-B PCM1792 Dacs, so I am planing on using aprox. 40Ω Holco resistor to convert the output current to voltage (replacing all those OPs).
But as I was saying, the first step is to improve the OP-stage.
Thanks for your advice Dan, IŽll sent some POVs when the OP is playing.
Regards
Pix :-)
handle and extra 18mA or so without any problems?If your interconnects are short AND low capacitance - then you may LOVE the sound without "Miss Piggy" (a conventional CF) in the signal path. Try it!
Thanks Allen,
"can the PS handle and extra 18mA ?"
Well, my CD-player has initally a ballanced OP-stage, in which I only use the SE option (Two out of four 6922EH is removed), so I am sure it can handle twice what I am using today.
Futher, If rebuilt to be used as a single CC-stage, the 7.3mA running in the "piggy-stage" today could be added, so I think 10mA/ch should not be a problem.Would you say that removing "miss piggy" and increase the bias current to 10mA is the main improvement, or is there other "hi satisfaction per effort" to go for ?
- Add cathode bypass cap to the CC-stage to remove feedback ?
- Replace those Wima MPK output caps to PIOs or PKs ?I am planing to rebuilt a complete new OP-stage for this CD-player, but never seems to have the time for it, so this modefication is just aimed at the highest improvements with the lest effort.
Regards
Pix :-)
Would you say that removing "miss piggy" and increase the bias current to 10mA is the main improvement <Should make a significant improvement - and what I'd do in your situation.
> or is there other "hi satisfaction per effort" to go for ?
- Add cathode bypass cap to the CC-stage to remove feedback ? <
Ugh - that, IMO & IME will worsen things - I hate the sound of bypassed cathodes!
> Replace those Wima MPK output caps to PIOs or PKs ? <
I dislike all PIO I have tried - but you may like them. Caps are up to your personal tastes - but I stopped using those red WIMA MKPs 20 years ago...
Try Mundorf/SCR/Solen Tinfoil&polyprops (same thing)
Regards, Allen.
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