• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Long tailed pair phase inverter issues

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Hi,

most guitar amplifiers with push-pull output stages feature Schmitt type phase inverters showing details I don't really understand. Let's have a look at the one in the following HIWATT schematic:

DR_Pre4Input_v0.gif


There are two resistors (22k&Omega and 2,2 KΩ respectively) in the common cathode path of V4. Bias is provided by the proceeding cathode follower. Other ones are biased by an additional resistor directly between the cathodes and the next resistor, but this doesn't matter so far. Negative feedback, coming form the OPT, is injected via a 10KΩ resistor into the grid of the second PI triode and, what is disturbing me, into the cathode tail. This means common mode feedback to both triodes!

My doubts, or questions, are the following:

1. What is the use of the common mode NFB? Would stability, or sound quality, be affected if NFB would be applied to the second triode grid only?

2. If we'd agree on NFB to the grid only, would it be a good advice to replace the tail resistor by a CCS? Would this affect sound quality?

3. If we'd agree to a CCS, would it be a good advice to place the CCS to the negatice bias voltage source instead of ground?

4. Do I think right that using a CCS instead of a tail resistor does no longer call for different anode resistors? Or could it be possible that different anode resistors were provided to achieve some certain sound effect?

Thanks in advice for your efforts!
 
Feedback is applied to the first triode only, via its cathode. The second triode has the same signal applied to grid and cathode, so no effect (apart from a small amount due to anode resistance).

The unequal anode resistors are because of the low resistance tail. With a CCS you would have equal resistors. However, you can't just add a CCS to this circuit as you would have to find a different way to put the feedback in.
 
Thanks! In my mind I'm tinkering with putting the feedback solely to the grid of the second triode, which *imho* acts as the inverting input of the whole amplifier. The CCS would be placed to the negative voltage rail that also provides bias for the finals. Both LTP grids' DC potential would be equal to ground, and both anode resistors would be 100 kΩ or so. Are these good ideas?
 
The CCS would be placed to the negative voltage rail that also provides bias for the finals. Both LTP grids' DC potential would be equal to ground, and both anode resistors would be 100 kΩ or so. Are these good ideas?

This is somewhat comparable to your idea. I have implemented that in 200W KT120 design and is working quite well.
 

Attachments

  • PI with Pentode.png
    PI with Pentode.png
    24.9 KB · Views: 481
Hi,
1. What is the use of the common mode NFB?
It is a relic of Fender circuits, which were arranged that way purely for layout convenience. Lot's of other manufacturers copied from Fender. The (very small) amount of common mode feedback has no useful effect here.

Would stability, or sound quality, be affected if NFB would be applied to the second triode grid only?
No, it's unlikely you would notice a difference.

2. If we'd agree on NFB to the grid only, would it be a good advice to replace the tail resistor by a CCS? Would this affect sound quality?
No! Guitar amps are often overdriven, and a CCS will NOT respond nicely to overdrive, and will cut in and out, oscillate, or do other strange things. Sounds horrible!

4. Do I think right that using a CCS instead of a tail resistor does no longer call for different anode resistors?
Technically yes.

Or could it be possible that different anode resistors were provided to achieve some certain sound effect?
It was probably not the designer's intention, but it can make a tonal difference sometimes.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.