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Hi,
please let me show You my current project. It is a preamp for bass which shall replace the (transistorized) preamp of a vintage hybrid amplifier. The target amp is a Dynacord G-2000, 150/220 W from 4 EL34. At its PI (ECC81, Cathodyne with preamp stage) an input voltage of 7 V (i.e., 20 V peak to peak) is required.
A boundary condition and the motivation of my concept i that i want to reuse the old pots (which are custom tandem pots) in order to be able to keep the original design of the amp. This means that the volume pots are 100 k log and the tone pots 500 k linear (from a James stack).
In order to match the relatively heavy load by these pots i decided to use the ECC81 - low plate impedance, still relatively large amplification. Tone stack will be a modified bone ray circuitry, but in a later 2nd channel other topologies are possible, maybe a James with either the original layout or the time constants of old Ampegs.
The attachments show the circuitry and the small signal analysis, the latter mostly in order to see if the tone stack and the highpass by the coupling resistors work as desired.
The ECC81 systems are biased to a quiescent current of 1 mA. This yields a large possible input swing of +- 3V without an onset of grid current (which is quite large for an ECC81). Amplification should be around 50 per stage. The volume pot before the tone stack will actually increase the input resistance the tonestack sees a bit, but it should remain in the range of the plate impedance of an ECC83 - 50 kOhms or lower, dependent on the volume setting.
The configuration of the triodes has actually been taken from the PI of the amp itself. After analysing that i did not see any obvious reason to try something else: the large possible input swing appears to me desirable for a bass guitar, and an expected anode voltage of around 160 V seems to fit well to the possible input swing as well as to the amplification of the stages.
State of the project: i am preparing the target amp (actually two, there is also a "HiFi-Favorit" which actually is the smaller variant of the G-2000 with "just" 80/110 W from 2 EL34).
Comments and suggestions are, of course, welcome.
please let me show You my current project. It is a preamp for bass which shall replace the (transistorized) preamp of a vintage hybrid amplifier. The target amp is a Dynacord G-2000, 150/220 W from 4 EL34. At its PI (ECC81, Cathodyne with preamp stage) an input voltage of 7 V (i.e., 20 V peak to peak) is required.
A boundary condition and the motivation of my concept i that i want to reuse the old pots (which are custom tandem pots) in order to be able to keep the original design of the amp. This means that the volume pots are 100 k log and the tone pots 500 k linear (from a James stack).
In order to match the relatively heavy load by these pots i decided to use the ECC81 - low plate impedance, still relatively large amplification. Tone stack will be a modified bone ray circuitry, but in a later 2nd channel other topologies are possible, maybe a James with either the original layout or the time constants of old Ampegs.
The attachments show the circuitry and the small signal analysis, the latter mostly in order to see if the tone stack and the highpass by the coupling resistors work as desired.
The ECC81 systems are biased to a quiescent current of 1 mA. This yields a large possible input swing of +- 3V without an onset of grid current (which is quite large for an ECC81). Amplification should be around 50 per stage. The volume pot before the tone stack will actually increase the input resistance the tonestack sees a bit, but it should remain in the range of the plate impedance of an ECC83 - 50 kOhms or lower, dependent on the volume setting.
The configuration of the triodes has actually been taken from the PI of the amp itself. After analysing that i did not see any obvious reason to try something else: the large possible input swing appears to me desirable for a bass guitar, and an expected anode voltage of around 160 V seems to fit well to the possible input swing as well as to the amplification of the stages.
State of the project: i am preparing the target amp (actually two, there is also a "HiFi-Favorit" which actually is the smaller variant of the G-2000 with "just" 80/110 W from 2 EL34).
Comments and suggestions are, of course, welcome.