Fender Super Twin Reverb with Dumble-Style Mods...continued

[ic-racer]

This is just a continuation of this thread with a more appropriate title for the search engines, etc. http://ampgarage.com/forum/viewtopic.php?t=5268

Here is the Schematic I wound up with. Some of the values are just "whatever resistor or capacitor" I had lying around.

I am getting a lot of popping from some of the switches. Looks like I need to put in some Megaohm resistors in key places.

The values on the reverb pot and resistor are just what were on the amp originally. They don't seem to work that well with the 'Dumble Style' trimmer setup. The reverb sounds fine, but it is not very adjustable from the front panel. Still need to do some tweaking there.

Also, not shown on the schematic is a 'Dumble Style' presence control. I used a 2k pot, 1.0uf cap and 390r resistor. I am using the amps existing NFB resistor, which is 850R. These values worked very well. The exact middle of the control DOES seem to sound 'normal.'


(See final schematic later in the thread...)

[jaysg]

I think you do need the 5, 10 or 20M resistors for those pull switches. fwiw, have you contemplated getting rid of the 6C10? The economy of Dumble design would seem to help there. You can make an adapter for a noval socket.

[ic-racer]

have you contemplated getting rid of the 6C10? The economy of Dumble design would seem to help there. You can make an adapter for a noval socket.

I should be getting a 12AX7 socket from Antique Radio Supply tomorrow. Not sure yet how to mount it without drilling into the chassis. I was thinking of constructing a washer with two studs to hold the new socket. Then the washer would be drilled with two holes line up with the holes in the chassis for the 6C10 socket.

[ic-racer]

Here are the pictures for the 12AX7 convertion. I used a thick washer from the hardware store. This allowed me to countersink the little screws that hold the new socket in place. The whole assembly mounts right over the existing holes and uses the existing mounting holes and screws.

[ic-racer]

More pictures...

[ic-racer]

So, how does it sound???

Great!

The triodes in the 6C10 are not really the same as those in a 12AX7. It seems to be somewhere between a 12AX7 and a 12AT7.

The bass response is really crummy with the 6C10 and the distortion is grainy.

[ic-racer]

Here is a sound clip demonstrating the 'Deep' switch with a strat: http://soundclick.com/share?songid=6524788

Here is another with the strat, demonstrating the OD channel with the 12AX7 instead of the 6C10. http://soundclick.com/share?songid=6524783

[ic-racer]

Did some more modificaitons.

Finally put the 100k pot in place of the original 10k mid pot (per "Cool Mod For the Clean Channel" layout). This gives that added boost to the signal so that the PAB and Non-PAB are closer together in volume. It also changes the whole character of the amp. Now that I have more gain from V1, now I can think of putting the snubbers back on to quell some of the high end in the OD.

I also put "Input 2" to use by using a 22k screen resistor there. Now I can choose 33k at "Input 1" or 22k at "Input 2"

I put a 22M resistor after the .0022uf cap in the "Mid Boost" circuit to cure the popping. Now I see that the other two 22M resistors that I thought were part of the PAB circuit are probably to keep the "Jazz/Rock" switch from popping. I ran out of 22M resistors, so I will have to wait on adding those.

[ic-racer]

Here are the most recent schematics.

[mat]

Here is a sound clip demonstrating the 'Deep' switch with a strat: http://soundclick.com/share?songid=6524788

Here is another with the strat, demonstrating the OD channel with the 12AX7 instead of the 6C10. http://soundclick.com/share?songid=6524783

Great playing The sound is to me somewhere between fender and dumble. Did You use the fenders own reverb ? If You dont have the NFB yet in this amp, I would suggest You to try it. To me it makes the nonHRM sound more dumbly.

Really nice phrasing

[ic-racer]

Here is a sound clip demonstrating the 'Deep' switch with a strat: http://soundclick.com/share?songid=6524788

Here is another with the strat, demonstrating the OD channel with the 12AX7 instead of the 6C10. http://soundclick.com/share?songid=6524783

Great playing The sound is to me somewhere between fender and dumble. Did You use the fenders own reverb ? If You dont have the NFB yet in this amp, I would suggest You to try it. To me it makes the nonHRM sound more dumbly.

Really nice phrasing

Thanks for the comments. Now I have added the 100k pot (which just came in the main) to the Mid and it really changes the whole character of the amp. Now the clean channel has much more bass and mid gain. Sounds less like a Fender, but it drives the OD section much better. The difference between PAB and Non-PAB is just right.

Now that I have more gain, I thought I would try the grid stoppers (270p) again. However, again they make the amp sound just bad. Reminds me of a transistor distortion with the treble rolled off to try to make it less harsh.

So, I have them off now.

In terms of the NFB, check out that file "Modified SuperTwin Power Supply.pdf" and I show what I have done. The original NFB resistor value got cut off in the file, but it is a 850R. This hooks to a typical Dumble type Presence control.

[ic-racer]

In terms of the reverb, I just re-wired it so that it uses both sections of the same tube (rather than sharing part of the C610). Then I converted it into a "one-tube reverb" by feeding the signal in after the master volume like this schematic. It works Ok but the control knob on the front does not do anything, so I think I need to swap out the "reverb pot" for the 'Dumble' value of 2M per the schematic.

[mat]

In terms of the NFB, check out that file "Modified SuperTwin Power Supply.pdf" and I show what I have done. The original NFB resistor value got cut off in the file, but it is a 850R. This hooks to a typical Dumble type Presence control.

Sorry, I messed with the words. What I meant was the V1b local feedback loop

[ic-racer]

In terms of the NFB, check out that file "Modified SuperTwin Power Supply.pdf" and I show what I have done. The original NFB resistor value got cut off in the file, but it is a 850R. This hooks to a typical Dumble type Presence control.

Sorry, I messed with the words. What I meant was the V1b local feedback loop

Oh. I have run out of switches, but since V1 is right next to the input jacks I was thinking of using a switching jack that will open and close the circuit. That way I can plug into #1 and have the V1b feedback or plug into #2 without the feedback.

[jelle]

Hi,

You might want to increase the value of the 68k resistor on the OD entrance 100k trimmer. I suggest 150-470K maybe 220K. Also the 500K level pot at the end of the OD might be too high. Most schems show a 100k. You could add a resistor across it to lower the impedance. I haven't been able to listen to the clips so maybe it sounds great already.

Very heroic conversion BTW, I love it!

Jelle