Can I have 2 switchable cathode resistors on 5e3?

Hi all,
I am planing to install 2 cathode resistors (250 and 300ohms) with a switch on the positive side of cathode cap to be able to jump from one the other. What do you think? Any special attention selecting the switch? I have a mini switch laying around rated at 2A 250vac

Thanks

The voltage on the cathode is pretty low so a 250VAC is probably plenty good, but you will not want to flip such a switch with the amp on. The cathode may momentarilly be out of the circuit during the switch flipping.

It's easy to wire. Solder both resistors positive sides in as normal to the 6V6's cathodes (which are jumpered together). Solder either negative end to the outer lugs of a SPST switch with the middle lug of the switch soldered to ground.

The switch selects which resistor is grounded and therefore in the circuit.

Going from 250 to 300 might not be as drastic as you'd think. Also, if you have a beefy enough PT and OT, you could use one resistor value for 6V6's and one for 6L6's. After swapping tubes you could use the switch to select the proper cathode resistor for either 6V6's or 6L6's.

Again, power the amp off before flipping this switch.

The voltage on the cathode is pretty low so a 250VAC is probably plenty good, but you will not want to flip such a switch with the amp on. The cathode may momentarilly be out of the circuit during the switch flipping.

It's easy to wire. Solder both resistors positive sides in as normal to the 6V6's cathodes (which are jumpered together). Solder either negative end to the outer lugs of a SPST switch with the middle lug of the switch soldered to ground.

The switch selects which resistor is grounded and therefore in the circuit.

Going from 250 to 300 might not be as drastic as you'd think. Also, if you have a beefy enough PT and OT, you could use one resistor value for 6V6's and one for 6L6's. After swapping tubes you could use the switch to select the proper cathode resistor for either 6V6's or 6L6's.

Again, power the amp off before flipping this switch.

I had this idea because I noticed when I install a weber copper cap (wy3gt) the cathode current drops from 38Ma to 30Ma, and plate voltage drops as well. This will give room to play between a nos rectifier and the copper cap.

Also, I am moving back to Europe and I decided to get my tweed a Mercury Magnetics set. I know some think they're a little pricey and they put out a huge amount of voltage on the secondary.

Since I am still in the experimenting and learning with my firt build I thought that this might be kinda cool... sure I can tease the folks in Europe with the MM :lol: but I have also 3 different B+ options.

For an instance, I will have 220Vac I was advised that I can connect it to any of the Vac primaries 220v, 230v or 240v this will give different B+ as sown below.

380v/220v=1.72, 380v/230v=1.65, 380v/240v=1.58

1.72 x 220v = ~380v-0-380v
1.65 x 220v = ~363v-0-363v
1.58 x 220v = ~347v-0-347v

To conclud the cathode resistors swtchicing would be just an extra adjustment. Is this a good thinking?
:rolleyes:

Again, power the amp off before flipping this switch.

Sure I do turn it off, I will install ne switch in the under side of chassis, in the line out perforation.

You'll see more of a difference if you go with cathode bias to fixed bias.

Agreed. What you can do, is wire a 100ohm/5W, wirewound pot in series with the cathode resistor for and adjustable cathode bias.

I've done it for a 250 - 350 ohm range. Didn't do a whole lot, though.

If you want to switch between 300 and 250 ohms and have the cathode in the circuit,solder the 300 ohm between the cathode and the ground.Then solder a 1.5 kohm resistor to the ground .the other end of the resistor goes to one pole of the switch. The other pole of the switch goes to the cathode.Now the switch selects whether the 1.5 kohm resistor is in paralell with the 300 ohm (which should give you a resistance of 250 ohm) or with the switch open,300 ohm.The 300 ohm is ALWAYS in the circuit.Good Luck!

If you want to switch between 300 and 250 ohms and have the cathode in the circuit,solder the 300 ohm between the cathode and the ground.Then solder a 1.5 kohm resistor to the ground .the other end of the resistor goes to one pole of the switch. The other pole of the switch goes to the cathode.Now the switch selects whether the 1.5 kohm resistor is in paralell with the 300 ohm (which should give you a resistance of 250 ohm) or with the switch open,300 ohm.The 300 ohm is ALWAYS in the circuit.Good Luck!

This is great! So, a 300ohm resistior in paralell with a 1.5k gives the resistance of 250ohms, correct? How you do this calculation?

It will look clean. I have a on-off-on switch, I can add an extra resistor...

Thanks

This is great! So, a 300ohm resistior in paralell with a 1.5k gives the resistance of 250ohms, correct? How you do this calculation?

It will look clean. I have a on-off-on switch, I can add an extra resistor...

Thanks

Here's the equation, and a little calculator -

http://www.sengpielaudio.com/calculator-paralresist.htm

steven

IFor an instance, I will have 220Vac I was advised that I can connect it to any of the Vac primaries 220v, 230v or 240v this will give different B+ as sown below.

380v/220v=1.72, 380v/230v=1.65, 380v/240v=1.58

1.72 x 220v = ~380v-0-380v
1.65 x 220v = ~363v-0-363v
1.58 x 220v = ~347v-0-347v


Yeah, but:

If you have additional filament windings those change, too.

This is great! So, a 300ohm resistior in paralell with a 1.5k gives the resistance of 250ohms, correct? How you do this calculation?

It will look clean. I have a on-off-on switch, I can add an extra resistor...

Thanks

Is it recommend to use a cement 5w power resistor, or can i use something else? I have a bunch of metal films and carbon comps rated for 1/2W.

Most of your power will go through the lower resistance so the 300 ohm resistor should be 5 watt. I'd use a metal film rated for a watt or so for the 1.5k. I wouldn't use a 1/2 watt carbon comp in that app.

It's no big deal if the 1.5k cooks, though... you still have the 300 ohm resistor in-circuit.

Thanks a lot guys. I did as Limbe recommended:
http://i98.photobucket.com/albums/l269/88nuno88/P1030010.jpg
http://i98.photobucket.com/albums/l269/88nuno88/P1030005.jpg
http://i98.photobucket.com/albums/l269/88nuno88/P1030012.jpg

For an instance, I will have 220Vac I was advised that I can connect it to any of the Vac primaries 220v, 230v or 240v this will give different B+ as sown below.

380v/220v=1.72, 380v/230v=1.65, 380v/240v=1.58

1.72 x 220v = ~380v-0-380v
1.65 x 220v = ~363v-0-363v
1.58 x 220v = ~347v-0-347v

To conclud the cathode resistors swtchicing would be just an extra adjustment. Is this a good thinking?
:rolleyes:


Yeah, but:

If you have additional filament windings those change, too.

I see, if I do this my filaments will run around 5.5V instead 6.3V. Is this still ok?!

If you want to switch between 300 and 250 ohms and have the cathode in the circuit,solder the 300 ohm between the cathode and the ground.Then solder a 1.5 kohm resistor to the ground .the other end of the resistor goes to one pole of the switch. The other pole of the switch goes to the cathode.Now the switch selects whether the 1.5 kohm resistor is in paralell with the 300 ohm (which should give you a resistance of 250 ohm) or with the switch open,300 ohm.The 300 ohm is ALWAYS in the circuit.Good Luck!



I completely forgot about a switch to parallel in or out. I do this on my 5E3's preamp cathodes to go from Marshally 2.7k+0.68uf to stock 820+25uF. Helps with humbuckers and it increases gain.

tube.tone I would NOT use that 1/2 watt carbon comp in parallel. I'd go for something higher wattage, maybe another cement 5/10 watt.

Thanks for the advise will get another cement 5w resistor.

Actually,the current going through the 1.5 kohm resistor will be very small,since it is 5 times bigger than the 300 ohm resistor.If you go for the same margin of safety that Fender did with the cathode resistor a 1 or 2 watt resistor will be fine.Thanks for the nice photos of a wellbuilt amp.Limbe