Filter Cap Reduction

[Dagan8]

I need help yet again.

If you have read some of my other posts recently you will see that I am heavily modifying/rebuilding a Fender red knob twin.

I'll cut to the chase.

I am removing 2 of the 4 6L6's, replacing the 100w OT with a tweed bassman style "50 w" OT, and changing the pre amp scheme to that of vintage 5F6a and AB763.

Since I'm recapping I would like to reduce the filtering stages to reflect more of the original 5F6A/AB763 circuits.

Here is what I was thinking:

C301 & C302 reduce to 100uF/350v caps to give a series value of 50uF.

C306 & C307 keep at stock value of 47uF ea for a series value of 23uF

C110 increase to 47uF to make up for the loss of filtering of 301/302 since 306/307 are not included in the filtering of this stage due to the High/Low scheme.

C111 keep at stock value of 22uF

C112 keep at stock value of 22uF

R128 & R129 will be increased to 15w rated resistors to male up for the increased in rush charging current of C110.



Does this make sense???

[stokes]

Personally I wouldnt reduce any of the filter caps value,but thats just me.I like the sound of an "over filtered" output.But I would definately not advise "upping" the value of the 22uf's.At that point in the B+ rail,there isnt much filtering needed and those caps are more for "decoupling" the stages,it could be dtrimental to your tone if you use too much filtering there.C306 and 307 are in the filtering regardless of the position of the Hi/Lo switch.I would also recommend using F&T caps instead of the Xicon or whatever asian brand is used in that amp.As for the OT,I dont know if the Red Knob Twin has the same OT as other Twins,but the 100 watt Twin OT sounds great with 2 6L6's and an 8 ohm speaker load.I've used 2 of them in a couple of builds I've done and they sound great.The 8 ohm load will correct for the reflected impedance you need with 2 6L6's and that OT.Theres nothing wrong with the 50 watt OT you plan to use,but if you haven already spent the $$ for the new OT,try the one you have,might save you a few.

[Dagan8]

Thank you Stokes.

My plan is to use a combination of F&T and Illinois Caps.

I will leave C110-C112 at 22uF then.

I will just reduce the values of C301&C302 as planned.

I wasn't sure of the impact of C306 & C307 in the filtering at these later stages because of the way the this "Power Amp" filtering is in a sense "Parallel" to the preamp filtering stages unlike the traditional B+ rail but I guess they still add filtering to the later stages since they are connected to the B+ rail.

I'm not sure if the OT in the red knob twin is the same as other 100W Twins. The reason for the Bassman OT was to get a more usable tap range. I used the amp with 2 6L6 tubes pulled most of the time, but I end up with a tap selection of 8,16,32ohm with 2 tubes pulled. I was also hoping the Bassman style OT would get me a little less headroom and a more true to Tweed/Blackface tone.

Thank you Stokes.

[Chuck H]

am removing 2 of the 4 6L6's, replacing the 100w OT with a tweed bassman style "50 w" OT, and changing the pre amp scheme to that of vintage 5F6a and AB763.

Just an observation... The tween Bassman OT is designed for a 2 ohm load. So... Yes the red knob Twin OT would have taps of 8, 16 and 32 ohms with two tubes pulled. But a tweed Bassman OT will want a load of two ohms. In your Twin cabinet that would mean two four ohm speakers in parallel?!? I don't see how this is a more usable range. If the OT your looking at has taps then I digress.

Chuck

[Dagan8]

If the OT your looking at has taps then I digress.

Yes. Sorry. Forgot to mention that I'm using a multi tap Bassman OT. 2,4 & 8. If I end up using both stock speakers I will connect them in Parallel and use the 4ohm. The NFB will come off of the 2ohm tap like a 59 Bassman.

Thanks Chuck.