5F6A MODS QUESTIONS!

[Travis8]

Hi! Everyone!! I build a 5F6A after build one 5E3 (I love it but I want more headroom and clean).

Now I wanna do this mod on my Tube-Town 5F6A kit!

I´m not sure about the wiring of RobRobinette webs about pentode/triode mode.

Also I wanna ask you guys if anyone try to use the 5F6A throught 1x12, now I´m using it just with a torpedo captor at home, but I wanna run it into a speaker and I´m not sure about it.

THANKS!!!!!!

[Travis8]

Also I wanna add Adjustable Bias.

Could I use this component? https://www.tube-town.net/ttstore/miniatur-potentiometer-50-k-laydown-pt15.html

[sluckey]

I´m not sure about the wiring of RobRobinette webs about pentode/triode mode.

What's your question?

You can use any speaker you want. Just match the speaker impedance to the amp impedance.

Could I use this component?

yes

[Travis8]

I´m not sure about the wiring of RobRobinette webs about pentode/triode mode.

What's your question?

You can use any speaker you want. Just match the speaker impedance to the amp impedance.

Could I use this component?

yes

I wanna know if 1*12 sounds good with 5F6A circuit.

My question is how can I wire this switch on my schematic. Why I have more Resistors than robrobinette schematic ?

Thanks again

[sluckey]

I wanna know if 1*12 sounds good with 5F6A circuit.

Gonna sound way better than that torpedo captor.   

My question is how can I wire this switch on my schematic. Why I have more Resistors than robrobinette schematic ?

Just follow the diagram. Remove the red wire between V4 and V5 pins 6. Remove the other red wire from V4 pin 6 and reconnect to the bottom of the switch, both lugs. Connect a red wire from the middle right switch lug to V4 pin 6. Connect a red wire from the middle left switch lug to V5 pin 6. Connect any color wire from the top right switch lug to V4 pin 3. Connect any color wire from the top left switch lug to V5 pin 3. That's all. DON'T MESS WITH ANY OTHER WIRES ON THE SOCKETS!

The original 5F6A did not have the grid resistors R27 and R28 shown on your layout. Rob followed the original circuit. Your additional resistors makes your amp better than the original.

[Travis8]

Thanks you very much! Now I understand how it´s work the switch.

[Travis8]

Also I wanna add Adjustable Bias.

Could I use this component? https://www.tube-town.net/ttstore/miniatur-potentiometer-50-k-laydown-pt15.html

Could I do this and Bias the circuit for 6L6?

Why I didn´t need bias adjust with 5881?

Thanks again, you make me learn a lot :)

[sluckey]

Could I do this and Bias the circuit for 6L6?

Yes. For practical purposes you may consider 6L6 and 5881 to be identical tubes.

Why I didn´t need bias adjust with 5881?

The original 5F6A did not have adjustable bias. See original schematic...

     https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_5f6a.pdf

[Travis8]

Could I do this and Bias the circuit for 6L6?

Yes. For practical purposes you may consider 6L6 and 5881 to be identical tubes.

Why I didn´t need bias adjust with 5881?

The original 5F6A did not have adjustable bias. See original schematic...

     https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_5f6a.pdf

Ok thanks, so I can changue 5881 with 6L6 without bias adjust?

Thanks!!

[Travis8]

Obviously matched pair

[sluckey]

Ok thanks, so I can changue 5881 with 6L6 without bias adjust?

I recommend to always check bias (and adjust if needed) every time you change power tubes in a fixed bias amp such as your 5F6A. That's the reason you want to install a bias pot, right? Speaking of bias pot, since you seem to prefer the small trim pot that will mount on the board, you will be pulling the chassis every time you adjust bias. With that in mind, I suggest you install a 1Ω 1W 1% resistor between pin 8 and chassis ground for each power tube. This will provide a safe easy monitor point for adjusting the bias. I would remove the black wires from pin 8 of each tube, install a solder lug (see link) under a nut for each tube socket, then connect a 1Ω resistor between pin 8 and the solder lug for each socket. Then simply monitor the voltage on pin 8 while adjusting the bias pot. For details on biasing please read and understand the info in this link...

     http://www.duncanamps.com/technical/lvbias.html

Solder lug link...
     https://www.tubesandmore.com/products/solder-lug-6-or-8-hole-locking

[Travis8]

Ok, I will do this.

Thanks again, I like this method. I will use the Bias trimmer and this 1 ohm resistor.

THANKS

[Travis8]

Now I just read the 1Ohm resistor volt drop .

Then I connect to black ground and red to lug 3 and read plate voltage.

And I use the above readings to calculate the
static dissipation wattage.

I´m right?

Thanks again !!!

[Travis8]

Just another question.

Wich caps are the best?
Jupiter, Yellow or Blue SoZo, Orange drops, Mallory...

Thanks everyone :)

[sluckey]

Xicon is king!

[Travis8]

Xicon is king!

I've never try It. Thanks!!

[pdf64]

The behaviour of electrons in a circuit is determined by the circuit itself and the electrical characteristics of the components that the circuit is compromised of. Not snake oil mojo BS branding / marketing.
It is notable that the marketing of high cost snake oil parts is heavy on hyperbolic text, light on actual data, eg their electrical characteristics.

[Travis8]

I have another question, (I hope I'm not too booring, thanks guys)

If I use the amp with 8 ohms the NFB resistor doens´t work, true?

Could I use the Eminence GA-SC64 ( 40watts) with this amp? I think the amp output is around 45 watts.

[sluckey]

If I use the amp with 8 ohms the NFB resistor doens´t work, true?

Not true. NFB works regardless of which speaker jack you plug in to.

Could I use the Eminence GA-SC64 ( 40watts) with this amp? I think the amp output is around 45 watts.

Already been answered.

[Travis8]

Could I use the Eminence GA-SC64 ( 40watts) with this amp? I think the amp output is around 45 watts.

Already been answered.
[/quote]

Ok, I will suppose I can´t. xd

Thanks again

[sluckey]

You can use any speaker you want. Just match the speaker impedance to the amp impedance.

[Travis8]

You can use any speaker you want. Just match the speaker impedance to the amp impedance.

Impedance, but not wattage xd

Thanks! :)

[pdf64]

...If I use the amp with 8 ohms the NFB resistor doens´t work, true?

The degree of NFB will tend to be significantly higher.

Could I use the Eminence GA-SC64 ( 40watts) with this amp? I think the amp output is around 45 watts.

Sure you can use it but unless you’re running the amp at low power outputs, I suggest you restrict its use to amps with a rated power output of about 20W max.
Because an amp’s overdriven power output can be up to perhaps twice its rated power output.

Note that a SR won’t be able to achieve its rated (ie into 2ohm) power output into an 8ohm load.

[Travis8]

Thanks!!

I'll start looking for another speaker

[Travis8]

...If I use the amp with 8 ohms the NFB resistor doens´t work, true?

The degree of NFB will tend to be significantly higher.

Thanks!

Now I understand this. So If I will use it 8ohms will be better use 56k?

[pdf64]

That info is predicated on the OT having a non standard secondary winding. Hence the reference to ‘taps’.
The standard OT for your amp has a secondary winding with a single 2ohm output. What secondary outputs does your OT have?

[sluckey]

According to the Tube-Town schematic the OT has 4Ω, 8Ω, and 16Ω taps. A 27K NFB resistor is connected to the 16Ω tap.

[Travis8]

According to the Tube-Town schematic the OT has 4Ω, 8Ω, and 16Ω taps. A 27K NFB resistor is connected to the 16Ω tap.

And If I wanna have a switch for 5F6A or JTM45 feedback, how can I do it with my OT ?

I´m trying to learn about this

[Travis8]

Last question I promise

If I changue tone stack could be a good idea?

I also look at just changue tone slope to 100k and A1M with A500K bass pot, to get less bass

Thanks again guys..

[sluckey]

And If I wanna have a switch for 5F6A or JTM45 feedback, how can I do it with my OT ?

See attached pic.

Last question I promise

If I changue tone stack in that way could be a good idea?

Thanks again guys..

Last answer. I promise. 

Just try it. Let your ears tell you if it's a good idea. That resistor is called the slope resistor. Changing it or even replacing with a pot is a popular mod.

[Travis8]

Thanks! here is the schematic

[pdf64]

Bassman treble pots are typically linear taper.
Regarding the ‘log A’ control tapers, are you sure that’s what you’ve got? I could well be wrong but recall that log A is 30% which very few suppliers carry, maybe only Weber, Mojo and some Fender branded spares. Whereas log B is 10%, which is ubiquitous.
As you are taking the frequency plot with the controls set halfway, their particular taper is extremely significant to the resulting plot.