1963 Fender Twin, Tonally Impaired. Assistance appreciated.

[keithb7]

Thanks for all your insight and help folks. I am starting to believe the tonal EQ differences between the two channels of this amp is naturally inherent of the circuit design. What you folks are saying makes sense. Other blonde Fender amp owners in other forums, with the same harmonic vibrato are experiencing the same thing. I was a little taken back by it in the beginning as I have never owned a vintage Fender amp that sounds like this one. This is my only vintage Fender with the harmonic vibrato that this Twin has.
I will proceed to test all that I can to ensure the amp is healthy and functioning properly. I will probably still experiment a little with the final cap in the vibrato circuit to see what effect it may have. Otherwise, it is what it is. Fender dropped it for a reason.
It sounds neat, but it may not be tonally perfect. I have several vintage Fenders, they all do different things. Thanks again for your help. - Keith

Here it is. Before, when I bought it, and today.

[Chuck H]

Here's the final stages of each schematic for the two different harmonic vibrato circuits (6** and 6**-A respectively). Notice that the "EQ" of the chosen frequencies affected is different. And so will the un-vibrato-ed signal be. Clearly it's possible to monkey with the circuit. There may be a better balance between the harmonic effect and replicating the classic BFish Fender tone with the oscillator shut down if one wanted to fuss with it. It should be noted that the split plate load from the preamp channel feeding this stage uses a different ratio for either circuit so the actual drive level may also need to be adjusted there for any changes made. I might just try either circuit and choose the one I liked better. On that note...

I suppose the advantage to having the two channels sound the same would be the ability to use the harmonic vibrato with a more BFish eq'd channel. But that's not the way the amp ever sounded so YMMV. I think there's actually more advantage to having two channels that sound more significantly different. Why not? If you want the normal channel sound for the un-vibrato-ed tone you could use an A/B pedal to either channel input too. This could even offer versatility for, say, a more focused lead tone when a dirt box is kicked in. Punch off the vibrato and punch in the dirt box on the vibrato channel. I guess my point is, use and enjoy the difference rather than lamenting it.

JM2C

[Leo_Gnardo]

There is a 2uf/25V cathode bypass cap in the final stages of the vibrato circuit. Also circled in red. You can increase the lows passed by this circuit by increasing the value of this cap.

There's a good reason to not increase this 2 uF cap: the value was selected to minimize the subsonic LFO wave from causing "thump" in the amp's output. If anything, the value of that cap could be reduced if there's too much thump getting into the output. In some amps with this type of vibrato, there's so much LFO subsonic junk leaking through that it causes speaker voice coils to slap into the bottom of their gaps for an annoying clack noise when vibrato depth is turned all the way up. What a headache. No wonder Fender went to the blinky light AM type vib circuit.

[Chuck H]

Good info to have I had considered that possibility. Ergo my "stability" and "function" concessions above. Unfortunately the 2uf value probably does limit LF a bit for reamplification of the padded signal. Not much in the audible range, but it does make a difference. Some earlier versions of the harmonic vibrato used a 4uf cap. Even that must have been too large since all the later versions use the smaller value.

[g1]

Here it is. Before, when I bought it, and today.

Nice job Keith!

[Dave H]

Dave H that is a pretty cool simulation on the EQ. Not sure how to did that, but is there a chance you could provide the same chart for the normal channel to compare?

I was done with LTSpice. I modified an amplifier circuit I already had to be like the vib channel. The simulation is of the vib channel circuit between its vol pot and the PI input. The normal channel doesn't have that circuit nor the 3n cap across the second triode plate load so the normal channel frequency response should be flat from its vol pot to PI input.

[pdf64]

I think that it may help to reduce trem 'thump' if the cathodyne plate or cathode resistor had a trimmer added; it may then be possible to tweak out the thump, as the opposing polarity modulation signals should cancel out in the mixer, if perfectly balanced.

[Dave H]

I will play a little with the suggestions offered here. The .003 uF already pulled.

Is that the .003 across the second stage plate load (100k +22k in series)? Pull that then follow the output of that stage to the right through the .05 then down through a 220k to where it joins a .01 off to the right. Change the .01 to .001 (1n) and it will flatten the frequency response. I have absolutely no idea how this will affect the vib effect but its worth a try as it is easily reversed.

Edit: below is a sim of the mod. The vertical scale is the same as the previous sim (15dB)