tone stack placement

[Randall]

I am building a BF Bassman and I want to play with the Bass channel a bit. I want to get close the a 5F6A preamp, which places the stack after the second triode. AB165 is first triode, tone stack, Vol, second triode. I'd like to try changing one side to first triode, Vol, second triode, tone stack. Problem is, I don't how or where to mix the channels. The BF channel is sitting at plate voltage at the 220K mixing resistor, but the 5F6A would have no DC due to the blocking caps. Could I bring the wiper of the Treble pot directly to the grid of the following triode?

Bassman-AB165-schematic.pdf

[loudthud]

Problem is a tone stack wants a high input impedance. Try changing the 220K on the BF channel to 470K and use a 470K between the 5F6A treble and the mixer tube grid. Change the 470K feedback resistor around the mixer tube to 1Meg.

[Randall]

Not sure I follow that loudthud. In the normal BF channel, the stack is fed directly by the first triode plate, and if the volume is turned up full, it outputs directly to the grid of the second triode. What is the reason to change the mixing resistors from 220K to 470K? And do I understand you to leave the BF side connected to the 'hot' left side of the .01uF, and connect the 470K from the treble wiper to the other side of same cap?

And I don't understand what changing the 470K feedback resistor does?

[nosaj]

Not sure I follow that loudthud. In the normal BF channel, the stack is fed directly by the first triode plate, and if the volume is turned up full, it outputs directly to the grid of the second triode. What is the reason to change the mixing resistors from 220K to 470K? And do I understand you to leave the BF side connected to the 'hot' left side of the .01uF, and connect the 470K from the treble wiper to the other side of same cap?

And I don't understand what changing the 470K feedback resistor does?

If you plug that into Duncan's tonestac it might give you a visual understanding of what your asking.

TSC
nosaj

[Chuck H]

Here's how I swapped between BF topography and VOX (top boost)/5f6a topography (with a cathode follower as the third stage driving the tone stack). Note the centralized switching. This required some layout consideration. Also note that I used dual ganged pots to keep the front panel to one set of knobs. Component values were worked out to give the "correct" frequency response with the averaged dual gang pot values. You have to be REALLY into the concept to trouble with this. This was a custom order for a good customer and it works a treat.

[loudthud]

Refer to the schematic below. What we have is a feedback circuit with a single gain stage. R3 is the feedback resistor. R1 and R2 are the input resistors. The gain of the circuit from either input is (roughly) R3/R1 or R3/R2. The advantage of this circuit is that it can handle a huge input of perhaps 75V pk-pk without distorting and there is very little interaction between the two inputs until the output reaches it's limits (clipping). So as built in the AB165, the circuit doesn't have much gain, but it can handle a big signal and mix two signals with low interaction.

Because of the feedback action, the impedance at the grid of the tube is pretty low, probably 15K or lower. The Voltage gain is about what you'd expect from a 12AX7, but the low impedance at the grid would degrade the signal from a tone stack. Another bad effect would be that adjusting the tone stack would alter the signal from the other input.

[Dave H]

What is the reason to change the mixing resistors from 220K to 470K? And do I understand you to leave the BF side connected to the 'hot' left side of the .01uF, and connect the 470K from the treble wiper to the other side of same cap?

And I don't understand what changing the 470K feedback resistor does?

Because of the negative feedback on the mixing stage its input impedance is about equal to its input resistor. LT changed the 220k to 470k to reduce the loading on the tonestack. Doubling the input resistor halves the gain so he also doubled the value of the feedback resistor to make the gain the same as the original circuit.

The AB165 circuit you posted has both mixing resistors and the feedback resistor connected to 'hot' tube plates. This is OK because the three tube plates are all at about the same voltage so little DC current flows in the resistors. If you connect one of the input resistors directly to the treble pot wiper current will flow through the wiper to ground. I'd put a 10n cap in series with the wiper to block the current.

[Dave H]

And do I understand you to leave the BF side connected to the 'hot' left side of the .01uF, and connect the 470K from the treble wiper to the other side of same cap?

Yes, I think that would be OK. I was looking at LT's schematic in post #6 and missed the above.

[loudthud]

If you connect one of the input resistors directly to the treble pot wiper current will flow through the wiper to ground. I'd put a 10n cap in series with the wiper to block the current.

That's probably the easiest thing to do.

The Double Agent schematic shows lots of switching between stages, I thought the idea was to use the mixing stage like the AB165. If you remove the feedback around the mixing stage you will suddenly have an insane amount of gain.