KAT88 cathode resistors

[Phil Y]

Hello all,

As I have found them beneficial in the past, I was considering changing the o/p valve cathode resistors for Mills wire wounds from the standard green ceramic jobs.
My amp has the bias mod done so I expected it to have the standard 150 ohm cathode resistors. It does not, they are 180 ohm which seems odd. Should I order the standard 150 ohm?
Perhaps 180 ohm was an early attempt to cure the "red plate" problem that some users experienced ?

Thoughts please. Phil.

[Greg.]

I think 180R was a value Richard arrived at. All of the upgrades were simply a follow of what Richard did.

[Phil Y]

OK thanks Greg,
Maybe if I have another look at the notes I will find an explanation, or maybe Richard will chip in.

Phil.

[Richard]

Hi Phil, I was an early builder and found the KT88 plates running 40w and glowing red in the dark. Increased cathode Rs from 150 to 220 which gave 32w and fine. WAD then increased grid bias via the bias divider which gave about 35w on the plates with 150Rs. Going to 180 gave 32w again with that later divider. Should say mains was high back then around here. Best to check yours running, cathode voltage divided by cathode R multiplied by anode voltage gives wattage

[Phil Y]

Thanks for the explanation Richard, I know what I am looking for now.

Regards, Phil.

[Phil Y]

Update.

I opened the amp up Friday and measured 510 on the anodes which worked out to a dissipation of about 29.5 W.
On the strength of this I thought I would go for 150 ohm resistors. This morning I fitted them and re-measured. Anode voltage was 500 (presumably mains variation) and dissipation worked out to 32 W.

It looks like for my mains, the final spec. of 18k in the bias circuit and 150 ohm on the cathodes is the correct choice.

Phil.

[Richard]

Hi Phil,

Yes that sounds a good result! The anode V will have dropped a little due to the increased load.