Fender Red Knob Tweed Face

[Dagan8]

This could have went in the Mod section, but it is going to be virtually a new amplifier.

I have finished gutting the preamp section from my Fender Red Knob Twin. I was originally going to just put a simple 5F8A/5F6A preamp circuit in right before the reverb section, but I've changed my mind. Again. This amp was built very modular so I can easily put 2 preamps in.

Here is the plan:

1. There will be no more channel switching because the vactrols for the channel switching were on the preamp board which I removed. I wont miss it. I never used it. Instead I will attempt to blend the 2 channels before the PI section.

2. One channel is going to be a 5F6A and it will enter directly before the PI. I want to keep this channel very similar to original 5F6A topology. It will use a DG Hiwatt input scheme so I can "link" the Norm/Bright channels to use both triodes in the 12AY7.

3. The other Channel is a AB763 Black Face type and it will use the existing reverb and FX loop section. It will use a single "high" level input. The reverb will be foot switchable using the existing vactrol.

4. Some of the resistor values in the PI will be changed to Black Face/Tweed values. I am going to try a 15K resistor to find middle ground over the Tweed 10k and Black Face 22K. The 5F6A presence control and NFB circuit will be used and will therefore affect both channels. I will use a 12AX7 for the PI.

5. A 150ohm 25W "SAG" Resistor will be used in the power supply section to simulate tube rectifier sag. A tube rectifier is not an option with the current PT because a bridge rectifier used. Besides. I can retain the Low power switch.

6. I have already removed 2 of the power tubes because I don't and will never need 100W.

7. I have ordered a Hammond 1760K multiTap OT.

I have attached the final schematic. I would really appreciate any feedback, suggestions, and criticism from some of the experts on this board. I am fully aware that it will not sound identical to a black face or tweed when finished, but I think it will be one cool amp.

[Dagan8]

I apologize for the double post.

I just figured I should explain which aspect of the design that I am concerned with the most instead of just a general "Whatcha think?"

My main concern is where the 2 channels enter the PI. Mainly the 5F6A. I'm not too worried about the AB763 because this is how fender does the Normal/Vibrato mix. I'm just not sure about the mixing right after the tone stack of the 5F6A.

Will this make the channels "interactive" when I'm only using the AB763?

Will I get excessive noise from the 5F6A channel even if it's volume is "off" when using the AB763?

[Dagan8]

Ok. Now it's a triple post. I apologize.

I found a thread that is addressing the same issue.

HERE

It looks like I may able to insert the 5F6A circuit right where the AB763 pre enters a recovery stage as well as the reverb signal. The cool thing about this is that I would end up with an FX loop in both channels.

Would I just tie it in with another 3.3M resistor?

Like This?

[hasserl]

As I posted at the end of that other thread you linked to, I don't know why I had the problem with that Bassman, but I've built a couple of other very similar amps without any problems with the two channels, two 250k mixing resistors worked just fine to isolate the channels and all worked well.

Note that the 3M3 reverb mixing resistors in Fender amps causes a huge loss of voltage across it. That is one of the areas you can pick up a lot of gain in a Fender amp, by adjusting the value of that resistor. I think if you run the 5F6A channel across one as you have it drawn you're going to see a large drop there as well. However, you've got a recovery stage right after that, which should boost it right back up again.

As far as interaction between the channels, I don't think there will be any with the 3M3 resistors isolating them.

[Dagan8]

Thank you very much for your input Hasserl. I was hoping for your feedback because I know you did a similar build.

I'm going to try mixing pre-amp sections with 3.3M resistors like discussed. Hopefully the following stage will bring the levels back up. The original doner amp used 8.2M resistors so I'm sure the 3.3M won't cause too much loss. I have attached the modified schematic.

Just building a BOM so I can order the parts.