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Thoughts on amp mods

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#21
I suppose I want better bass. At the moment there is significant roll off making significant bass sound like kick drum bass guitar double bass and low organ notes sound extremely distant. I thought that increasing current would allow more power to the speakers that would allow better bass.

Thanks
 
#22
Quick empirical method: connect the 100 ohm resistor to the KT88 anode, instead of to the output transformer. This triode connection lowers output impedance, giving slightly better damping factor. Also, you could increase the 0,1uF capacitor to 0,2uF to see if there's a bottleneck for low frequencies.
 
#23
So I have now put a switch in to connect the screen to either the plate or the ultra linear tap via the 100 ohm resistor. I also added a 3 way switch that connects the feedback direct, via a 1.2k resistor or disconnects it. Currently there is nothing to choose between ultra linear or triode connection, with feedback disconnected it sounds harsh, with the normal feedback it is a bit distant with the extra 1.2k it sounds a bit crisper and has a bit more bass. What should I try next? I have another 0.1uF cap should I fit this in parallel with the existing interstage cap ? Would reducing resistance on the KT88 anode likely make an improvement?

Thanks🙂
 
#24
Increasing current most probably saturated the OPT. Simple to test if this is so; just revert the current back to where it was.

If the first stage input capacitor really is 22nF, and not say, 220nF, it has a -3dB somewhere over 30 Hz. This would make the low end quite a bit distant sounding.

My advice to find out more:

1) revert back to old current
2) remove input capacitor

Listen what happens.
 
#25
As to the larger issue of how to get good overall performance out of that amp.

With that starting point and those ingredients, I'd go this route:

1) remove gNFB

2) Simplify PSU, get a bit higher voltage to the input section. Currently it's below 200V, which is very bad for the 12AX7, and completely horrible for the 12AU7 totem pole. Maybe put a simple 2 FET reg, that outputs 250V or so.

3) Some sort of fixed bias for input stage; LED on the cathode, or preferably fixed grid bias with 250nF input cap.

4) Gyrator plate load for the input stage 12AX7

5) Completely remove the middle 12AU7 totem pole. It is operating very badly at the moment. It's only real function seems to be to provide extra gain for gNFB. And probably a fashion statement as George said. Without gNFB the gyrator loaded 12AX7 input stage provides plenty gain with excellent linearity.

6) Schade KT88 with source follower driving the grid.


These are my suggestions.
 
#26
So I have bypassed the input capacitor. Reducing the current didn't help so I went the other way and increased the current a bit more and now I have some reasonable bass. MrCurwen, I am afraid that you are beginning to lose me. I don't really understand what a Gyrator is, What you mean by totem pole for the 12AU7s nor do I understand Schade. Could you possibly help by giving some reference for these terms that I could read up on?

Thanks
 
#27
A gyrator is a type of plate load. It outputs a constant DC voltage (plate voltage) and provides a high impedance load for the tube. With that kind of a plate load, you can draw the load line almost horizontal. It gives very high fidelity when used with a good tube.

A totem pole is an umbrella term for a topology that has two tubes or tube sections placed in series, one on top of another for example SRPP or a mu follower.

Search this forum for explanation and examples of Schade. It is a way of linearizing a pentode.
 
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