ABC's of presence control and phase inverters in A/B amps

[Sorr]

Chris

Star earthing takes the most work and gives the best results.
I use a version where I would connect the input signal earth to the bottom of the first stage cathode resistor, then take that to the star point etc.
I usually make the negative of the first filter cap which connects to chassis as the star point.
If you dont want to do a full on star earth, make sure you seperate the power supply and signal earths.
On the Laney the can filter caps are individually connected to chassis and this makes what I mean by seperate power supply earths.
Good earthing not only reduces hum but also back ground noise, the subjective result being that you hear more of the sound of the instrument.
And not all vintage Marshalls have a thin and glassy sound!
Recently I have had two JMP's in my workshop and they did not sound thin or glassy
Sorr

 

[walter]

I also have an amp that squeals when the treble is turned up. I tried a bunch of things to fix it, but nothing worked until I grabbed the second pre-amp tube. I had used tube sockets without shields to cut costs(the first tube has a shield). Now I have to add tubeshields. Thanks for the links, that helps fill in the gaps. I'm still a little unclear as to the exact action of the power resistors 82k, and 100k. I read both links, and each seem to contradict the other a bit. As it seems, there is higher signal with the feedback side of the tube, so the 100k lowers the gain. The voltage drop across the 100k is greater than the 82k drop, so the plate voltage is lower than that of the 82k, resulting in a lower headroom = less gain. o.k. I think that's right.
On the O-scope, I use a ground lift, so the probes have a floating ground. this works good for devices with multiple grounds. for checking two signals for phase relation, set the trigger to ch.1, so both channels trigger from one source. I use some big wirewound resistors for a dummyload, I think it's four 2ohm 100watt wired in series, and two resistors to drop down the signal 10x with a bnc cable connected to the scope.

 

[Sorr]

Walter,
The circuit is called a cathode coupled phase splitter.
The the second valve can be considered to be a grounded grid amplifier driven by the first valve that acts as a cathode follower.
In short, the gain of the first stage needs to be reduced to match that of the second stage. to get balanced drive to the O/P valves.
This is done by using 82k on the first stage and 100k on the second.
82k however is not the ideal value but it is used as it is the closest "standard value".
I use 91k.
Getting back to you having to use a valve shield to stop the squeal, I would suggest that you have a layout problem.

Sorr

 

[walter]

perhaps there is a layout problem, I tried to do it right, but the tone pots are in front of the P.I. When it squeals, I can probe the wires, and it doesn't change. I suspect there may be issues due to the aluminum chassis. It's basically an L-bracket. I've lined the cab with aluminum foil, but it didn't change. I'll install a tubeshield, and check it out some more.