Stereo 5f2a build - TMB stack issue

[cusebassman]

Hey everyone - I've done a lot of searching on the forum here to no avail. I recently built a stereo 5f2a based on a wiring diagram from the following thread over on tdpri:

http://www.tdpri.com/threads/stereo-5f2a-build.476120/



I'd like to replace the tone knobs with two separate TMB tone stacks, but my initial attempts at using Rob Robinette's have failed. I tried wiring in the first-from-last on the following page:

https://robrobinette.com/How_The_TMB_Tone_Stack_Works.htm

I tried wiring the input and output to the volume pots the same as the tone knob, and also tried running the output from the volume pot to the TMB input and running the output straight to the second stage of each 12ax7. In either case, the mid and bass knobs worked fine, but as I rolled the treble down, it would only cause volume fluctuations. In the case of wiring it "in series" after the volume pot, it would eventually totally cut out until I removed the TMB stack all together.

The only thing I omitted from the original design was omitting the "hi" inputs from each side. I also tried the TMB tone stack on a mono 5f2a build that worked fine with a single tone knob, and had the same results (the mono 5f2a had both the hi and low inputs).

How is a TMB stack "supposed" to be added to a 5f2a amp?

Note: the only issue with the original wiring diagram that I saw while building the stereo amp is the 5Y3 wiring which, when corrected, makes the amp work just fine. I used the same recommended power transformer but wired it for 120v mains. During the build, I also switched the filter caps from 16-16-8-8 to 22-16-16-16, as that's how the original builder ended up building his.

[sluckey]

How is a TMB stack "supposed" to be added to a 5f2a amp?

Look at this Champ schematic...

     http://el34world.com/charts/Schematics/files/fender/Fender_Champ-AA764-Schematic.pdf

[kagliostro]

Steve preceded me 

Franco

[cusebassman]

Thanks for the reply! The only difference I see here is that the tone stack is placed before the volume pot, so it's always receiving the full output from the first stage of the 12ax7. It's also a 2-band stack, but looks similar to the TMB in how it splits the signal between treble and everything else. Is placement after the volume pot going to cause issues? Has anyone experienced a miswired tone stack where the treble knob caused volume changes but didn't properly filter the signal?

[sluckey]

I would put the tone stack before the volume pot just as shown in the champ schematic. It's very easy to add a mid control. Just replace the 15K resistor with a 10K pot. Look at the VIB channel of this Super Reverb if you don't understand...

     http://el34world.com/charts/Schematics/files/fender/Fender_Super-Reverb-AB763-schematic.pdf

The Fender TMB controls act like volume controls, but only affect a certain frequency band. The treble pot controls the volume (level) if high frequency. The Mid pot controls the volume (level) of the mid frequencies and the Bass pot controls the volume (level) of the low frequencies. If you turn all three pots to zero, you will have no sound. Now turn up only the treble pot and you will increase the volume of only the high frequencies. Or turn up only the bass pot and you will increase the volume of only the low frequencies. Same for mid pot.

I'm sure you can miswire a tone stack such that it misbehaves like you describe. But if you forget the robinette layout and wire the tone stack according to the champ schematic it will work as it should.

[cusebassman]

Thanks, everybody. I'll give the Champ tone stack a shot, and then if that works, "upgrade" to the TMB tone stack mod mentioned here. I didn't realize the Champ ever had anything more than a single tone knob.

[kagliostro]

To maintain the measure of the chassis limited

can I suggest to mount the TS pots in a triangle like in the Fender PA 100 ?



Franco

[cusebassman]

If I can get the prototype to work with the 3-band tone stack, that's been the plan! I love the look of the triangle-shaped tone sections per side. The prototype is currently built into the chassis of a former Fender Blues Deville that had previously died, but I ultimately want to build a few heads using Hammond 16x8x3 boxes.

[kagliostro]

The PA 100 has TS with only two pot, the third is volume pot

obviously you can arrange a 3 pot TS as a triangle and put separated volume pots

Franco

[cusebassman]

That's the thought - have the two inputs together with the two volume knobs, and then triangles for each TMB stack. I'll give the schematic designs above a try, and also measure the voltage at the various marked points to make sure there isn't a separate problem (e.g. wildly overdriving the tone stack and blowing the cap).

[cusebassman]

Comparing the Champ and Princeton schematics, I see that the Princeton has a .022uf 400V cap just before the volume and tone controls. This cap is not present in the Champ. Could this be causing the issue with a more complex tone stack being shoved into the Princeton circuit, or should it make little difference? It's definitely in my circuit before the volume and tone.

[sluckey]

Comparing the Champ and Princeton schematics, I see that the Princeton has a .022uf 400V cap just before the volume and tone controls. This cap is not present in the Champ. Could this be causing the issue with a more complex tone stack being shoved into the Princeton circuit, or should it make little difference? It's definitely in my circuit before the volume and tone.

Remove that .022µF cap. It was needed by the 5F2A circuit but it's not needed in the blackface champ circuit, or any other Fender TMB circuit. It wasn't even needed in the Robinette circuit.

You need to make up your mind. ♫Stay with the one and let the other one ride.♫ Once you do that, you're only 15 minutes away from happy.

The Robinette circuit is fine too. First remove the old tweed tone control and pots so you have only a volume pot. Test that everything still works without the tone circuit. Then, if you have built your amp using the board layout you posted, just remove the .022µF cap from the board (Talking about the one that connects to V1-1). Now connect the input of the Robinette circuit to V1-1 and connect the output of the Robinette circuit to the right side of the volume pot. Done. Now repeat for the second channel.

[cusebassman]

Thanks for the patience - I'll give that a go this evening and see what happens 

[sluckey]

Let us know how this turns out. If you run into any boogers we got your back. We would love to see some pics.

[cusebassman]

Well, I removed the 0.022uf caps from just before the volume knobs. I added a Robinette TMB to the right side of the amp, and left the left side with just volume. I am still experiencing the issue of bass and mid working, but treble just going between normal sound and very loud, very mid-y. I've made a video.

youtube.com/watch?v=hGeoDOC8tJk

The video goes as follows:

• Playing through the left side, with no tone stack
• Playing through the right side with the TMB tone stack at all full-on
• Playing through the right side with the TMB tone stack mid knob all the waydown (counterclockwise)
• Playing through the right side with the TMB tone stack bass knob all the way down (counterclockwise)
• Playing through the right side with the TMB tone stack treble halfway down and then fully down (counterclockwise) - distortion is from the mic, not the amp

Also, many pics of the build as it currently stands.

http://imgur.com/a/2cwyH

[sluckey]

Remove that orange cap from the treble pot. It's not supposed to be there. Look at the champ schematic I posted. The center lug of the treble pot goes DIRECTLY to the right lug of the volume pot and the center lug of the Volume pot goes to V1 pin 7.

I know Rob has a .02 cap on the output of his circuit but his circuit output is connected to a phase inverter and the cap is necessary in that circuit. Look at the 5F6A schematic. But that cap is not needed for what you are doing.

[cusebassman]

That's interesting - I figured there would need to be some sort of filtering out of the DC voltage running through that part of the circuit (as it was originally in the Princeton with single tone). I'll give it a try tonight.

[sluckey]

That's interesting - I figured there would need to be some sort of filtering out of the DC voltage running through that part of the circuit (as it was originally in the Princeton with single tone). I'll give it a try tonight.

Well the 5F2A used a coupling cap between V1 plate and the volume/tone circuit to block dc voltage. But the TMB stack doesn't need a dedicated coupling cap for that purpose because the three caps in the TMB all block the dc voltage from V1 plate.

The cap on the output of Rob's circuit is needed to block the dc voltage that's present on the grid of the 5F6A PI tube from appearing on the TMB pots. Not so much as to keep the dc off the pots but to prevent the pots from changing the bias voltage on the PI tube.

But the second gain stage in your 5F2A operates with zero volts on the grid. In fact, the grid relies on the volume pot to provide a ground path to ensure it operates at zero volts.

If you make the corrections I mentioned above your circuit will work as expected, that is, if everything else is wired correctly.

Good pics BTW.

[cusebassman]

Well, I'm getting closer! Thanks to the help from everybody here. I removed the .022 cap from the output of the treble knob, and it is starting to act like a treble knob. I replaced the 0.022 cap running into the tone stack and the noise that appeared is gone again. The last issue may be due to the values of the pots / resistor / caps in the tone stack - as I turn the treble knob down, the amp still gets multiple times louder, but the highs do roll off now, so I'm still trying to figure that out...

[cusebassman]

The center lug of the treble pot goes DIRECTLY to the right lug of the volume pot and the center lug of the Volume pot goes to V1 pin 7.

Well. Figured out the issue, and it was a stupid one. Re-read this part of sluckey's reply, went back and checked the volume pot, and sure enough, I had the center lug of the treble pot running to the CENTER lug of the volume pot, not the right lug. Switched the tone stack output to the right lug, and the output wire from the volume pot to the center lug, and now it works a treat. Thanks everyone for helping with this... now I can start voicing the stack