Fender Red Knob Twin -> 5F6A Conversion

[Dagan8]

Hello

I currently own a Fender "Red Knob" Twin. I am not crazy about this amp. It does have some cool features like reverb and an fx loop, but it just doesn't do it for me.

My original plan was to gut the amp and replace the trannys, pots, board etc. and transplant a 5F6A circuit. I opened up the amp and I had a change of heart. I am sure I can easily remove the 2 pre-amps and replace them with a single 5F6A type preamp on eyelet board. There is tons of room. I don't like either of the 2 existing channels and I don't care for channel switching. I think this would a cool project. A 5F6A type preamp with reverb ,fx loop and Hi/Low switch.

I just need some help confirming my thoughts.

Here goes:

1. Installing the 5F6A Pre:
- In the schematic I can see where the 2 existing channels come into the reverb select switch. It looks very symmetrical in design and I'm sure I can just tie the unused one to ground. I'm just not sure if I should remove C117(10pF).
- I will keep the existing preamp PCB intact to maintain the reverb control and circuit. I will just relocate it to free up space for the new pots.

2. The existing phase inverter:
- The design is actually very similar to the 5F6A PI. I can almost duplicate the 5F6A with the following small changes
R122 and R123 change to 1M
R120 change to 10K
Jump R125
R126 change to 82K
Add 47pF cap from plate to plate
Add presence control circuit where R121 exists
Not sure about the value of R119 NFB since it is from a 4ohm tap instead of the 5F6A 2ohm tap

3. Power Supply:
- After some help from members on the forum I have decided to add a 30W 150ohm sag resistor to get more of a tube rectifier feel instead of trying to convert to tube rectification.


I know it will never sound the same as a 5F6A but I think it could still be one cool amp.

If I still don't like after this or I blow something up then I can gut it.

[ThermionicScott]

Sounds like a cool project -- I've got an old 4x6L6G power amp from an organ that I want to turn into a 5F8A (high-power Tweed Twin) sometime.

You sure about the sag resistor? Adding a 5V/3A transformer would be easy, and the power transformer has a center tap...

- Scott

[txstrat]

I'm just not sure if I should remove C117(10pF).

C117 is a high pass filter for the clean channel. Try it with or without it and keep what you like more. I'd also change the value of R101 (R103) to 3.3M since this is the common value in BF reverb amps.

Not sure about the value of R119 NFB since it is from a 4ohm tap instead of the 5F6A 2ohm tap.

I'd try it with the given "Red Knob" value, since the OT is not from a 5F6A. If the amp don't behave like you want it to (e.g. too clean or something), you can still change the value.

The 47pF cap from plate to plate is there to shunt high frequencies due to parasitic oscillations. You might don't need it.

I've never thought of changing an amps circuit in such a significant way, since I usually think this would change the "genuine state" of the amp too much.

But all in all a cool idea. Good luck.

[Dagan8]

You sure about the sag resistor? Adding a 5V/3A transformer would be easy, and the power transformer has a center tap...

Unfortunately I would need 3 tube rectifiers to replace the "bridge" type of SS rectifier in this amp.

C117 is a high pass filter for the clean channel. Try it with or without it and keep what you like more. I'd also change the value of R101 (R103) to 3.3M since this is the common value in BF reverb amps.

Thanks. I will look into R101 and R103.

Quote:
Originally Posted by Dagan8 View Post
Not sure about the value of R119 NFB since it is from a 4ohm tap instead of the 5F6A 2ohm tap.
I'd try it with the given "Red Knob" value, since the OT is not from a 5F6A. If the amp don't behave like you want it to (e.g. too clean or something), you can still change the value.

I'll start with the current value of 1.5k. I have a feeling I will end up increasing the value to reduce the amount of NFB to more like 5F6A/5F8A levels.

I've never thought of changing an amps circuit in such a significant way, since I usually think this would change the "genuine state" of the amp too much.

Not too worried about it. I was going to gut it anyway. They sell for 300-400 on ebay quite often.

[ThermionicScott]

Unfortunately I would need 3 tube rectifiers to replace the "bridge" type of SS rectifier in this amp.

Nah. The high-tension secondary has a center-tap, so you can hook up a tube rectifier as normal if you add a filament transformer. The RKT uses a neat trick to accomplish its "low-power" mode -- when you take B+ from the center-tap, rather than the output of the bridge, you get *half* the voltage. (Cover up the right side of the diode bridge, and you can see how you just have a backward full-wave rectifier now.)

That said, it's your project, so I look forward to seeing it develop either way!

- Scott

[Steve Conner]

That'll only give half of the B+ voltage you had before, though. It'll be as if the low-power switch were on all the time. See this thread for a more detailed discussion.

http://music-electronics-forum.com/t16581/

To summarize: Adding a tube rectifier to this amp would need two silicon diodes besides, to complete the bridge and get the full output voltage. It would still have the sag of the tube rectifier.

I don't see any problems turning this into a great sounding amp. I'm sure they sell for a bit more than $300 used, though!

[ThermionicScott]

Good catch, Steve. Guess it would be a "low-power 5F6A" unless you did a hybrid bridge...

- Scott

[Dagan8]

Thanks again Steve.

Ordering the parts to start this project soon.

[Dagan8]

Looking at the NFB Resistor again.

This amp uses a 1.2K NFB resistor off the 4ohm tap.

The 5F6A uses a 27K resistor off a 2ohm tap.

I am changing the values of R120-R123 to 5F6A values, so to have the same NFB voltage would I need to use say something close to 54K??

I just noticed that the 5F8A HP Tweed Twin uses a 56K and is pretty much a high power 5F6A

I understand that there may be more factors than just the OT secondary impedance but I want to order some different values to try and get closer to the 5F6A NFB voltage. I'll only be using 2 6L6's most of the time if it matters.

Makes sense that the current amp would have more NFB for a tighter sound.

[Steve Conner]

The NFB is set by the ratio of R119:R121, not the actual value of either one of them.

You have 1.2k and 120 ohms in your red knob, for a ratio of 10, the 5F6A has 27k and 5k, for a ratio of 5.4.

But it's off the 2 ohm tap so we have to multiply that by the square root of 2, and then the 5F6A is only 50 watts instead of 100, so the OPT will have a different ratio, so that's another square root of 2. That gets us to an effective divider ratio of 10.8.

So all in all the red knob has about the same amount of feedback as the 5F6A did. Probably no accident, Fender have been in the amp business for a few years :-)

Of course if you want to add a presence control, you'll need to change the NFB resistor. The 120 ohm resistor gets replaced by the presence pot, but you don't get 120 ohm pots. If you use a 5k pot, then by the argument above, the NFB resistor should be about 56k. If you used a 10k pot, then it should be 100 or 120k.

[Dagan8]

Steve. You're a very helpful individual.

I will be using the presence control circuit from the '59 Bassman LTD.

It uses a 25K pot wired as a variable resistor. This pot is in series with a 0.1uF to ground. Those 2 components are wired in parallel with a 4.7K resistor. It uses a 27K NFB resistor like the original.

I'm guessing that I will require the 56K resistor in my case.

I attached a pic of the LTD presence control.