Preamp and inverter on B7G base tubes circuit. - diyAudio
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Old 9th April 2013, 07:15 PM   #1
diyAudio Member
Join Date: Oct 2009
Default Preamp and inverter on B7G base tubes circuit.

Hi All.

Due to a poor quality of the preamp in my Chinese integrated PP amplifier I would like to modify this stage. I have to admit I'm not a valve expert, just starting with this and I have some issues regarding the circuit I "designedĒ. The original input stage with inverter is based on a differential amplifier (attached pre_old.jpg) and itís using two EF95 pentodes tubes. AFAIK this scheme is burdened with some drawbacks and itís far from a perfect one. Main assumption of my modification leaves the existing B7G heptal base, there are two sockets per channel available.

My new preamp stage is described in two section:
1 Ėas an input was chosen a EC92 triode (exactly half of a ECC81) in a circuit of an anode load without a cathode bypass cap
2 Ėthe Long-Tailed Pair phase splitter, this cathode coupled inverter is based on a ECC91 - twin triode with common cathode

This following stage has a quite high gain, if Iím not mistaken 41 and 13, but I am not sure if it have enough gain to swing a EL34 pentode in the PP output stage, regarding that, I donít know how deep a negative feedback is, I only know NFB resistor value (39k). So what do you think my circuit? Can it improve an overall sound quality of this amp? Maybe some other circuit, other tubes (B7G)? Please let me know your opinion,the original preamp and my circuit schemes attached, my load lines of two triodes also Ė resistors and caps values verification would be greatly appreciated.

Thank you, Rafal.
Attached Images
File Type: jpg new_pre.jpg (51.3 KB, 100 views)
File Type: jpg old_pre.jpg (59.2 KB, 98 views)
File Type: jpg ec92.jpg (304.0 KB, 94 views)
File Type: jpg ecc91.jpg (195.0 KB, 86 views)
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Old 9th April 2013, 10:30 PM   #2
diyAudio Member
Join Date: Apr 2004
Location: Monroe Township, NJ
Why not use "classic" Mullard style circuitry, which DC couples the voltage gain tube to the LTP. Put a 10M45S constant current sink in the tail of the LTP and force symmetry between the 2 sides. The 6GK5, an outstanding combination of high μ, high gm, and low RP gets my nod for the voltage amplifier job. The 6J6/ECC91 was "born" to be used in a LTP.
Attached Images
File Type: jpg Mullard 5-20 Schematic.jpg (380.3 KB, 87 views)
Eli D.
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