Tweed Overdrive Special tweaks

[daveyajd]

Hello,
So I built a TOS as my first build about 5 years ago. I have since built 3 more amps and love them all! Since I just completed a scratch build I’m between projects and decided to get my TOS back up on the bench. I’m looking to maybe do some tweaking inside to satisfy my urges. 


 I built straight from the schematic so what tweaks have you all found work well in this amp? I’m liking my clean channel but maybe a gain structure switch to swap cathodes? I also read a post about changing the power tube cathode cap from 150uF to 10uF? Would I need to change other component values as well when I do this?


Just spitballing ideas so thanks in advance for any help.


Aaron

[tubenit]

It would have been useful if you had posted your exact schematic.  AND if you had been more specific in what you want different in tone.

Geezer and I always felt like the TOS was a build and tweak amp.

So, I will offer these tweaks as a consideration.  Not saying you "should" do any of them. 

With this schematic with tweaks,  my goal was to preserve the clean tone you have AND increase the OD channel overdrive and make it smoother.   Note the use of 5751's instead of 12AY7's.  You might want to continue with 12AY7 in V1 and substitute the 5751 into the V2?   I think the clean tone would sound about the same after these tweeks and the OD would sound smoother?

I changed two of the values of the pots in the OD and also noted to use audio instead of linear.

The idea of the cathode follower behind the 5879 pentode is that I found having a cathode follower there increases the smoothness of the OD channel.  The "enhance" cap paralleling the 82k LTPI plate resistor adds smoothness in my experience and eliminates some of the harshness of the treble tone.

Orange Drop PS or the other Orange Drop polyester caps add smoothness.  Using Vishay-Dale RN65 plate resistors helps with smoothness. 

Regarding the 430R/10watt power tube cathode resistor.  You could try a 330R or 360R.   You could also try a 50uf or 60uf or 100uf cathode cap there.  I would be inclined to experiment with the cathode caps there first prior to trying a different cathode resistor.

Regarding eliminating features ……………………….
The "FAT" control is kind of a cool feature but I found it to be a set and forget thing for me & perhaps it really didn't add that much in tone?   Some of the components between the clean/OD switch and the OD trim pot I found to be not needed at all.  I have found that I tend to not use the master volume after the clean channel when I have a PPIMV, so the "tweaked" schematic removes that altogether. 


IF you are more specific in how you want to change the tone, perhaps we can help you out more effectively?

With respect, Tubenit

[JustMike]

Tubenit, I did the mods shown in your TOS tweaks schematic and now I'm getting a squeal when I turn DOWN the OD tone control. The squeal happens when the control gets to about 9 O'clock. Does anyone have any suggestion as to what is causing this?
 A couple of things I did differently are;
1. V2a & V2b are reversed because the original schematic had v2a as the FAT ckt and this was being abandoned. V2a is now the CF after V3. V2b remains unchanged.


2. I left the clean channel Master in place as well as the original switching configuration.

[tubenit]

I'm getting a squeal when I turn DOWN the OD tone control

That is really odd! 

Occasionally, I've had a bad ceramic cap do strange things.  Remove the 500p in the OD tone control so you have the signal going to the wiper of the tone pot going to .0047 to ground without the 500p, any better?  IF not, disconnect the OD tone control altogether ……… is that any better? 

IF neither of those help get the issue resolved, then post a picture of that OD section with tone control and level.

With respect, Tubenit

[tubenit]

You can also move the tone stack from the OD level pot to the OD drive pot prior to the 5879.

However, IF disconnecting your tone pot as is doesn't resolve the issue, then suspect something else as the problem.

With respect, Tubenit

[JustMike]

So I disconnected the 500p and the squeal went away. Soldered it back in and the squeal returned. Scratched my head and replaced v2 with a 12au7. Squeal is gone, but so is the sound (almost). 12au7 is too little gain. I think the squeal is due to too much gain of a 12ax7. I have a 5751 in V1 and a 12ax7 in V2. I think I need another 5751.

[tubenit]

5751 is 70%, 12AT7 is 60%. 12AY7 is 40%

12DW7 and reverse 12DW7 could be options also with one section being a 12AX7 and the other being a 12AU7. 

With respect, Tubenit

[tubenit]

So I disconnected the 500p and the squeal went away. Soldered it back in and the squeal returned.

OK, so I just reread this.  It may be the cap?  As I mentioned earlier in the thread, I've had silver mica and ceramic caps of small values contribute odd noises.

You can try a different cap OR just leave that cap out and use a tone pot like is illustrated in reply #4 using something between .002 to .0047 to ground.

With respect, Tubenit