Questions on building a modified 5E3 from spare parts

[PuertoRicanDavey]

Hello. I am new around here and just jumping into this. Any help would be much appreciated. So I am wanting to build a slightly modified Fender 5E3 circuit and have modified a schematic to reflect what I will be doing. I’m just hoping to get some approval to make sure that what I will be doing will work, and if not, what I should change before actually building this thing.

Here is a quick summary of the changes I made:

1. I removed the two of inputs of one of the channels along with the coupling capacitor and volume pot.
2. Instead of having a bright and normal channel, I have put a bright switch on the capacitor.
3. I removed the ground switch and plan on installing a 3 prong power cord.
4. I am reusing a power transformer I have laying around that came from an amp using two 7868 tubes and so plan on installing a pair of 6L6 tubes instead of 6V6s.
5. I replaced the first resistor in the power filtering with a choke.
6. I installed a standby switch.
7. I am reusing the solid state rectification setup from the amp I took the power transformer out of.

My main questions are these…

1. I have the amperage to run the 6L6s, but don’t know if there is more to it than just popping them in. I know the tubes are cathode biased. Do any component values need to be changed?
2. Will my rectification scheme work?
3. Does anything else look suspect?

Thanks a ton in advance for anyone able and willing help out.

[eleventeen]

Looks generally OK.


I think most would suggest 470 ohm 2 watt resistors from the second node of the power supply (eg; AFTER the choke) to the screens (pin 4) of the 6L6. Each 6L6 gets its own 470 ohm screen resistor, those resistors junction at the after-choke node. See: Any Fender dual 6L6 amp. Good protection item. Early Fenders using 6V6 for the most part omit these screen resistors. Later AB763 Fenders have them even those using 6V6, so I think you want them.


The 22K resistor which is the last dropping resistor in your power supply chain might need to be adjusted depending upon how high your initial B+ is. If the B+ to the preamp stages is too high, that would likely make the amp very "stiff".


You *may* need to get more drive voltage out of your PI tube to drive 6L6 rather than 6V6. To drive the 6L6 HARD, you may have to adjust things to get some more gain, voltage swing, to drive the 6L6. Someone here will suggest a way you can do this. It should only be 1-2 resistor values to change, no big deal. 

[PuertoRicanDavey]

Thanks eleventeen. I will plan on getting those couple of grid resistors and look into values for that 22k, though I assume I will need to put it all together to actually get voltage readings and troubleshoot from there. Hopefully someone can weigh in on driving the 6L6s harder, but I will look around and see if I can find anything.

[shooter]

driving the 6L6s harder

most of my builds are cathode biased, I use "my" *rule of thumb* for PA tube drive of about 1.5 -2 times bias voltage.  so if you're idling at 11vdc, 17-22 VAC driving the tube.  From there, it's tweak as needed

[HotBluePlates]

... I am wanting to build a slightly modified Fender 5E3 circuit ...
...
4. I am reusing a power transformer I have laying around that came from an amp using two 7868 tubes and so plan on installing a pair of 6L6 tubes instead of 6V6s.
...
1. I have the amperage to run the 6L6s, but don’t know if there is more to it than just popping them in. I know the tubes are cathode biased. Do any component values need to be changed?

Make sure you get a ~4kΩ primary impedance output transformer, or one advertised for two-6L6 amps. If you really want the 6L6's to make extra output power, your power supply is up to the task (because it already powered such an output stage), but you also need an output transformer with a lower primary impedance than what the 5E3 used for a pair of 6V6's.

You *may* need to get more drive voltage out of your PI tube to drive 6L6 rather than 6V6. To drive the 6L6 HARD, you may have to adjust things to get some more gain, voltage swing, to drive the 6L6. Someone here will suggest a way you can do this.

See the 22kΩ resistor in the power supply rail (between the output tube screens and the rest of the preamp)? Make that a 10kΩ. Consider adding another 10kΩ and a filter cap for the preamp-proper.

Why? Both the 5E3 Deluxe and the 5F4 Super use the same phase inverter with the same parts. The only real change is a larger voltage is supplied to the phase inverter because of the smaller dropping resistor (10kΩ instead of 22kΩ), and the Super has an extra stage of filtering for the preamp (because of the extra circuitry involved for the tone controls).

Obviously, they both worked for Fender; I built both amps previously, as well.

Other than that, do make sure to connect the input tube grids together, so they both get the input signal. I'm sure that was just a drawing error, and you would have done it inside the amp...

[PuertoRicanDavey]

OK. Thanks! It looks like my OT is rated at 4k ohm primary impedance, so that should work perfectly. And I have modified the schematic to reflect suggestions. If you can look it over to make sure I am understanding correctly, that would be great. Thanks a bunch!

Once I get it together I can start testing and adjusting voltages as needed.

[Willabe]

Fender tweed Pro 5E5 is the 5E3's big brother;

http://el34world.com/charts/Schematics/files/fender/Fender_PRO_5E5.pdf

Other than that, do make sure to connect the input tube grids together, so they both get the input signal. I'm sure that was just a drawing error, and you would have done it inside the amp...

The 5E3, 5E5 and other Fender tweed amps have interactive volume controls. When you plug into 1 channel the volume control on the other channel will also effect the amps sound and tone. Comes in very handy.

Turn the volume control on the channel your plugged into to 12:00 and the volume control on the channel your not plugged into full up and it scoops the mids and you get a nice clean tone. Keep the controls the same but plug into the other channel and you get full drive/distortion with the most amount of mids the amp can give.

Use an A/B box to act as a channel switcher. 

[PuertoRicanDavey]

Hm. I didn't known about the whole volume control thing. That sounds super useful. Maybe I will actually put the volume knob in for the other channel.

[PRR]

The un-used grid should go somewhere. Strapped to ground is simplest.

As others say, part of the charm of this scheme is the unexpected ways the several knobs interact. Omitting one makes it a different amp.

If you go 6L6 for "Power!", as said you want a lower Z OT and a bigger PT. Essentially steal the big-end from one of the 2*6L6 Fenders.