5F2A With Neg Feedback Mod - Noisy!!

[Morgan]

Hi guys,

First time on the new forum. Seems nice...

Anyway, I just completed a 5F2A Princeton build for a friend. I built it with the negative feeback mod. The mod consists of a 25@25 bypass cap connected straight to the junction of the 12AX7 pin 8, the 1.5K and 22K feedback resistor. The cap goes to a DPDT switch on the volume pot (pull to boost). The other side of the switch goes to ground when engaged (grounded at the same spot as the two pots on the brass plate). Pins 2 and 7 of the 12AX7 are shielded.

The amp works perfectly in "stock" mode. Nice and quiet even when dimed. When the boost is engaged, a lot of noise is entering the circuit (or getting more amplified). The boost does do what it's supposed to, but it does it at the cost of a lot of noise.

Any ideas on how to quiet down the boost?

Thanks,
Morgan

[Arthur B.]

You've stuck a capacitor onto thee cathode of the second stage. That I can understand. But I still have some questions. You're using a DPDT switch, but you're only switching one thing. Is the other pole unused? The capacitor is switched to ground in one direction, what is it switched to in the other direction?

[Bruce / Mission Amps]

With the cap grounded through the switch, you have the normal gain boost of biasing resistor bypassed with a cap but all the NFB from the OT is going to ground too, not to the cathode.
So, it boosts like a big fat boost switch and kills the NFB at the same time.
That can make it noise/gainy sounding.

Bruce

[Morgan]

You've stuck a capacitor onto thee cathode of the second stage. That I can understand. But I still have some questions. You're using a DPDT switch, but you're only switching one thing. Is the other pole unused? The capacitor is switched to ground in one direction, what is it switched to in the other direction?

Arthur - I could only find a push/pull pot in the right value in a DPDT - no SPST's. Yes, the other pole is unused. The cap is an open connection when the switch is pushed in.

Morgan

[Morgan]

Do you have any suggestions on controllong that noise, Bruce? Or should I be trying only the NFB or only switching in the bypass cap for the gain boost that he wants? Will both together always be pretty much too noisy?

Thanks,
Morgan

[Bruce / Mission Amps]

I'd use two switches so I could do both.
Put a 100 to 150 ohm resistor under the 12AX7's 1500 ohm cathode biasing resistor (that of which has the bypass cap).
Insert the negative feedback at the junction of the 100-150 ohm resistor and the biasing resistor.
Now you just play with the value of the resistor that feeds NFB to the stage until you like it, put that on a switch for feedback or no feedback by grounding the junction of the two resistors to get that effect.
You can then switch the NFB in and out.
At the same time, make up a new boost spst switch to bypass the 1500 part of the circuit with a 2.2uF to 22uF cap.
Again, that must only be connected in a way to allow the capacitor to bypass the 1500 part of the circuit!

[Morgan]

Hey Bruce, thanks a lot for your response. I don't have that many builds under my belt yet and thus am not all that clear on what you're saying.

Put a 100 to 150 ohm resistor under the 12AX7's 1500 ohm cathode biasing resistor (that of which has the bypass cap).

By "under" the 1.5K cathode resistor, do you mean between the 12AX7 cathode and the junction of the cathode, 1.5K cathode resistor, and 22K feedback resistor, or do you mean insert the resistor between the feedback resistor and the cathode resistor so that the cathode resistor has the 100-150 ohm resistor between it and the feedback resistor?

Insert the negative feedback at the junction of the 100-150 ohm resistor and the biasing resistor.
Now you just play with the value of the resistor that feeds NFB to the stage until you like it, put that on a switch for feedback or no feedback by grounding the junction of the two resistors to get that effect.
You can then switch the NFB in and out.

This to me sounds like the 100-150 ohm should go from the cathode to the junction of the cathode bias resistor and the 22K feedback resistor (which connects to the tip of the speaker jack and thus the OT). I should play with the value of the 22K resistor to find the value I like. And the feedback resistor should be on a switch to add/remove -fb to the circuit at that cathode junction.

At the same time, make up a new boost spst switch to bypass the 1500 part of the circuit with a 2.2uF to 22uF cap.

I think I already have that. Right now the bypass cap connects to the cathode where the cathode resistor does (as does the feedback resistor). Closing the switch grounds that cap at the same point that cathode resistor is already grounded. Is this not bypassing just the 1.5K part of the circuit? How should I hook it up differently?

Thanks again man!

Morgan

[Morgan]

I removed the bypass cap and ran a 150-ohm resistor from the cathode/feedback junction to the switch. I ran the bypass cap from the other end of the switch to ground. Putting the 150-ohm resistor in series with the bypass cap got rid of the excess noise.

I now have a nice gain boost when I yank on the volume knob.

Thanks a lot for the help!

Morgan

[Bruce / Mission Amps]

The 150 ohm resistor will hold the parallel RC combo of the 1500 and bypass cap up from ground if the cap is grounded.
Your NFB connects to the junction of the 150 ohm resistor and the bottom of the 1500 resistor.
The switch is in parallel with the 150 ohm resistor so that it can be bypassed or shorted to ground.

[Arthur B.]

Arthur - I could only find a push/pull pot in the right value in a DPDT - no SPST's. Yes, the other pole is unused. The cap is an open connection when the switch is pushed in.

Morgan

You mean that end of the capacitor is floating around when it's pushed in? That can't be good.