Can I use 1/4 W resistors for an amp PCB?

[powerplayj]

What's the minimum wattage needed for tube amp PCB resistors?

[The Tone God]

What's the minimum wattage needed for tube amp PCB resistors?

Generally recommended 1/2W and in certain areas 1W.

Andrew

[R.G.]

P = I*V
P = I^2 *R
P = V^2 / R

Tsurface @ Prated = 100C

Derate 50% from rated power.

[R.G.]

(removed, posted in error)

[R.G.]

That is, you have asked an insufficiently complete question.

The actual dissipation of a tube's grid resistor is microwatts. The actual dissipation of power supply dropping resistors and screen resistors may be several watts. The power dissipation of a triode plate resistor may be 1/8 to 1/2W.

Any blanket "use all xxx W resistors" will inevitably have to use the biggest ratings and will cost needless money.

There is no subsitute for calculating the power  a resistors dissipates as it works, doubling that calculation, and picking the next biggest size.

Many of them can be 1/4W. But that's not what you need to know. You need to know which ones CAN'T.

[MartyMart]

What amp are you considering ?
There is very often resistor "watt" ratings listed on either schem/layout and
many of these are available through places like AX84 and 18watt forums .
Input r's and bypass r's can almost always be 1/4 watt, like cathode bypass caps
need be rated no larger than 10v , whereas PS filter caps will be rated at perhaps 450v
and plate interstage caps usually 600v ( orange drops/Mallory etc ).
Power limiting r's can be 1 watt to even 50 watt !
Plate resistors are often 1 watt and between 82k to 200k values .......

This all depends on the specific amp, power rating and transformers being used.

MM

[powerplayj]

Classic 30 mods

I think all these mods are on the amp PCB and none of the resistor changes seem to be in conjunction with the transformer (where I assume most of the current is).  I'll check the Peavey Schem and see if the component specs are listed.

- changing the input: removing a cap and changing a few resistor values -> (33k series, 1M to gnd.  Referred to as Fenderizing the input.  Peavey throws in an cap and resistor which I assume lowers the input impedence)

- raising the resistance in the feedback loop to provide a bit more presence (resistor value swap)

- replacing caps in the tone circuit with better quality caps

- changing a few caps in the VII section (gain pathway) to lower the frequency and provide a smoother OD channel.

- treble bleed cap on the volume pot if I can find a quality cap at 25uf (the Peavey requires a much higher value than Marshall and Fender).  So far, all I see is Tant, Cer and Electros.


These mods are detailed at www.blueguitar.org

[MartyMart]

I see, if you're modding an existing amp then it should be a little easier, you'll
see by the size as much as anything, what's 1/2 watt / 1 watt or bigger on the board "as is"

MM.

[d95err]

- treble bleed cap on the volume pot if I can find a quality cap at 25uf (the Peavey requires a much higher value than Marshall and Fender).  So far, all I see is Tant, Cer and Electros.

25uF as a treble bleed cap simply doesn't make sense. It would bypass the whole signal, not just the treble. It must be a misprint somewhere in the documents, or you've misread them. Are you sure it's not 25nF? (which also would be a unusually large value for a treble bleed cap).

EDIT: Looking at the Blueguitar Classic 30 documents, the suggested treble bleed value is in the picofarads (22pF or 47-100pF)