EDIT: Wrong schematic, right one on the 3rd message, broken IC
These are the first stages of the Hartke Model 2000 preamp. It has active and passive inputs and "preamps" that are tube and solid state.
Can I bypass the first IC stages and go straight to the first tube stage? Like straight from the input jack to the point before C107. Should the cathode or plate resistors be adjusted to increase gain? I would miss the active input and the solid state preamp that way but they are pretty much useless anyway.
These are the first stages of the Hartke Model 2000 preamp. It has active and passive inputs and "preamps" that are tube and solid state.
Can I bypass the first IC stages and go straight to the first tube stage? Like straight from the input jack to the point before C107. Should the cathode or plate resistors be adjusted to increase gain? I would miss the active input and the solid state preamp that way but they are pretty much useless anyway.
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If you move the input to C107, you still have both preamp channels. But why do you want to eliminate the input buffer? The whole amp is solid state except for the little tube stage stuck in the middle of one channel.
The only difference between active and passive inputs is the difference between R105 and R106. The two jacks perform the same duty as the two jacks on a zillion Fender amps over the last 60 years.
The only difference between active and passive inputs is the difference between R105 and R106. The two jacks perform the same duty as the two jacks on a zillion Fender amps over the last 60 years.
Wrong schematic, broken IC
On closer inspection the circuit is totally different from the one shown above. Here is a correct one drawn by someone on the net.
I found that the dual op-amp 2068DD is shot. I can't find it locally. Is NE5532 a good replacement? Or some other one that's easily available? The suplied voltages are +18 and -18. That's exactly the maximum recommended for the 2068.
On closer inspection the circuit is totally different from the one shown above. Here is a correct one drawn by someone on the net.
I found that the dual op-amp 2068DD is shot. I can't find it locally. Is NE5532 a good replacement? Or some other one that's easily available? The suplied voltages are +18 and -18. That's exactly the maximum recommended for the 2068.
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Ok. So the amp is fixed now.
I found that on the tube preamp the IC amp stage and the RC-network between C104 and C112 were quite useless. What they do is they make the "tube"-preamp sound artificially different than the "solid state"-preamp. So I just bypassed everything between C104 and C112.
IC101A was faulty but it doesn't matter now anyway. I got a socket for the IC and tried NE5532 and OPA2134PA in there. With headphones I heard a little difference in sound between them. The expensive OPA2134PA was a little more "hi-fi" sounding.
When the IC101A and the following RC-circuit was bypassed I found that the "tube" and "solid state" sounded very much the same. Anyway the "tube" preamp sounded a little bit more useful with more mids and less extreme bass and treble content.
If anyone ever happens to tinker with these old Hartkes I recommend doing the same.
Thanks to everyone that helped!
I found that on the tube preamp the IC amp stage and the RC-network between C104 and C112 were quite useless. What they do is they make the "tube"-preamp sound artificially different than the "solid state"-preamp. So I just bypassed everything between C104 and C112.
IC101A was faulty but it doesn't matter now anyway. I got a socket for the IC and tried NE5532 and OPA2134PA in there. With headphones I heard a little difference in sound between them. The expensive OPA2134PA was a little more "hi-fi" sounding.
When the IC101A and the following RC-circuit was bypassed I found that the "tube" and "solid state" sounded very much the same. Anyway the "tube" preamp sounded a little bit more useful with more mids and less extreme bass and treble content.
If anyone ever happens to tinker with these old Hartkes I recommend doing the same.
Thanks to everyone that helped!
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