ac30 cathode resistor question (wattage)

[bakerlite]

Hey - I am pretty much following an old jmi ac30/6 schematic for my ac30 build that i am gathering the last parts for.


instead of the 250uf/25v + 50R/10w  cathode bias setup in the original I am considering running 4x 68uf/25v (as I actually some unused philips here and i dont have a 250uf) with a 240R resistor on each.


Can I lower the wattage of the resistor to say 5W? as each one is only handling one tube??? seems ok in my head but just in case i am missing something basic!!??


cheers

[HotBluePlates]

... I am considering ... a 240R resistor on each. Can I lower the wattage of the resistor to say 5W? as each one is only handling one tube???

The original AC30 schematic shows 12.5v across a 50Ω resistor. What power should that be rated at? Use a variation of the equation for power to find out.

Power = Voltage² / Resistance = 12.5² / 50Ω = 3.125w

To keep the resistor cool and allow a safety margin, double the calculated dissipation; now you get that the 50Ω resistor should be 6.25w, and 10w is probably the closest standard value (there may be a series out there which has power ratings between 6-10w).

Assuming you still get 12.5v of bias with the 240Ω resistor (you'll likely get a little higher, because more resistance biases the tube a little cooler which only happens if more voltage appears across the resistor), the equation above gives us:

12.5² / 240Ω = ~0.65w

I'd round that up to 2-3w, whichever is handy.

[bakerlite]

Thanks very very much for going through that - it makes perfect sense and hopefuly ill be able to apply it myself in future....  :)


Thats great i can use those philips caps - i picked them up in a lot of 200 or so small components not knowing when any of them would be used. its great when you get to use stuff on hand.


cheers

[HotBluePlates]

Thats great i can use those philips caps

Oh yeah, I forgot to mention that with the resistor ~4 times bigger, the cap could be ~4 times smaller to have the same effect. So yeah, it seems the Philips caps should work nicely.

[kagliostro]

But using a single resistor isn't that there is a better distribution of current by the fact that each tube isn't exactly equal to each other ?

About the 4 capacitors why simply don't parallel it (and use only one resistor for all the tubes) ?

Franco

[HotBluePlates]

If you have 4 tubes which vary widely in their matching, individual bias resistors probably work better, because each tube will settle on its own best idle current and bias voltage.

In a production amp, it just makes sense to use 1 resistor for all tubes so you pay for 1 part instead of 4. Same with bypass caps. But if you're building the amp and you want to use up parts on hand, you could choose to use 8 individual parts instead of buying 2 parts you don't have now. If you only needed to buy those 2 items, the shipping cost may be several times bigger than the part-cost.

As long as both setups deliver the same result, it's up to the builder which makes more sense for his situation.

[bakerlite]

That's exactly the reason. I have spent a lot on parts all over recently and had forgotten I needed the cathode cap and resistor. Shipping would be double cost of parts and I seem to have a selection of 240R resistors on hand in various wattages.  Its win win as I don't spend more money on this project and theres a minor cool off on the el84s which us so small it shouldn't hurt the usual/original 50R/220uf setups sound... Hopefully :-)

[PRR]

The single common resistor hardly needs a cap when working clean.

A cap may smooth the bias-shift as you go into over-drive. This has to be by-ear. And it isn't clear what you are listening for.

With separate caps, the cap sets a low-frequency shelf.

On the face of it, 68uFd against 240 Ohms is 10Hz, plenty-plenty low for guitar. Actually you must account for the tube cathode impedance. But you will find this is very nearly equal to a reasonable cathode bias resistor. So dart-toss 20Hz, still plenty-low for an 82Hz (even ~60Hz drop) guitar. Also plenty low for any speaker which will fit in the van yet fill a good lounge. (Even "bass" stage-speakers rarely get below 50Hz.)