Re-designing 6DJ8 circuit for the 6CG7 - diyAudio
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Old 13th June 2012, 02:18 PM   #1
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Default Re-designing 6DJ8 circuit for the 6CG7

Pls see attached circuit - I'm planning a build but hoping to use the 6CG7 rather than the 6DJ8 (primarily because I have some 6CG7 tubes, but also because they seem to have a better reputation among audiophiles than the 6DJ8/ECC88).

Looking at the circuit, the 10K plate resistor might be a bit low for the 6CG7, probably 20-33K would be more linear? But using this value might see the plate voltage drop quite a lot due to R11 18K dropper.

The 18k dropper is probably needed to keep a low plate-voltage for a 6DJ8: +/- 100V, but from the datasheet the 6CG7 appears to be good for up to 300V. Would 5K as a dropper be enough to smoothen the B+?

Do we also need to increase R8?

Any suggestions re other changes most welcome.....
Attached Files
File Type: pdf 6DJ8 preamplifier Sch v2.pdf (12.8 KB, 229 views)
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Old 13th June 2012, 03:07 PM   #2
DF96 is offline DF96  England
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R5/R10 should presumably be on the other side of the output capacitor C7/C8, so they act as an output ground reference.

The existing circuit has far too much gain for a line stage. 6CG7 would reduce the gain, but still rather high. A redesign would be needed to cope with the different valve characteristics. Reckon on changing every resistor value apart from R1,R2,R6,R7. C3 might need to be bigger too, as you might have worse PSRR with the 6CG7.
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Old 13th June 2012, 03:41 PM   #3
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Well spotted - I didn't notice the R5/R10 issue...I'll just replace R5/R10 with wire link, then solder resistors from signal wire to GND on the output side of capacitor....

What values shall we use for R3,R4,R5,R11,R12,C3?
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Old 13th June 2012, 03:48 PM   #4
DF96 is offline DF96  England
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Don't replace R5/10 with a wire link as this will simply short the output.

Three ways to find the correct resistor values for 6CG7: copy an existing design, read the datasheet, or learn about bias and load lines and determine them yourself.
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Old 13th June 2012, 03:59 PM   #5
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Presumably the 100k r5/r10 is not doing any harm where it is? I could solder another set of 100k resistors from tip to sleeve at the RCA sockets?
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Old 13th June 2012, 04:14 PM   #6
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R5/R10 are totally out of place here and will not do any good. Probably a drawing error. Remove or move them to the other side of the cap. They should also be made bigger try 3-500k. About the 6DJ8 circuit it is very illdesigned with badly chosen component and voltage values. But as you are redesigning for 6CG7, no problem with that. Note the 6CG7 has much higher Rp giving higher Zout, between 5-10k.
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Old 13th June 2012, 07:04 PM   #7
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Interesting. The circuit will have reduced highs and low frequency response (especially vs 6dj8), which will give a single ended type of sound. Would be good for SS amps that often need help opening up.

Cheers.
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Old 13th June 2012, 07:30 PM   #8
DF96 is offline DF96  England
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Eh? Are you talking about Miller effect - if so unlikely to be a major issue in either case. LF is determined by capacitors, not valves.
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Old 13th June 2012, 08:44 PM   #9
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Quote:
Originally Posted by DF96 View Post
Eh? Are you talking about Miller effect - if so unlikely to be a major issue in either case. LF is determined by capacitors, not valves.
Ah no. The Rp of the 6cg7 is some three times higher than a 6dj8, not counting the plate resistor value. With X (both resistance and capacitive load), whatever the high frequency response is for the 6dj8, it will be reduced by some 3 times with a 6cg7. (Using very general figures.)

Yes, low frequency response is indeed limited by output coupling cap and somewhat by the cathode bypass cap (until say, Xc is some 10 times that of Rk). Sorry for the error.
In fact, we should also check for differences in the current feedback between tubes, as it will impact the bass as well.

As such the highs will be slightly reduced with the 6cg7 in the circuit provided. Bass will depend upon any change in Rk and Ck.

Cheers.

Last edited by Positron; 13th June 2012 at 08:59 PM.
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Old 13th June 2012, 09:24 PM   #10
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Quote:
Originally Posted by Positron View Post
Ah no. The Rp of the 6cg7 is some three times higher than a 6dj8, not counting the plate resistor value. With X (both resistance and capacitive load), whatever the high frequency response is for the 6dj8, it will be reduced by some 3 times with a 6cg7. (Using very general figures.).
The output impedance of a CC stage is the tube Rp in parallel with the plate load (10K in this case). Even if the 6CG7 plate resistance is 3 times that of the 6DJ8 it will increase the output impedance by a smaller ratio. Rp for the 6CG7 is typically 10K giving an output impedance of about 5K. Even with 100pF capacitive load the -3dB point is over 300KHz.

Cheers

Ian
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