what is the purpose of the anode resistor on output tubes

[neskor]

I just bought second hand Fucsh ODS Classis 100W amp.
Open up to see what is inside and notice one strange thing.
33R/3W resistors between anodes of output tubes and primary of output transformer.
Never seen this before in guitar amp.
What is the purpose of that resistor?

[martin manning]

Andy might comment, but my guess is they are there to suppress oscillations.

[nuke]

Quad output tubes?

(looks like).

Maybe to promote current sharing between the pairs sharing the same phase?

[jabguit]

On a tube like the 807 they're to prevent oscillation. I would assume the same of your Fuchs amp.

cheers,

[sluckey]

Andy had this to say back in 2021...

I recently experienced this in a few production amps with 4 power tubes, due to output transformer changes. Mild instability on transients. I added 30-OHM 3-watt resistors on the power tube plate leads and it cleaned right up. You do have the added bonus of it acting as a fuse, but not my intent. Lots of amps had this back in the day. I would stay below 50 ohms and at leas 3-5 watts, depending on value. LIkely avoid wire wound resistors.

[GAStan]

Couldn't it also be used to determine true output power for the tube, unlike the cathode 1 ohm resistor which is combined anode and grid/screen currents?

Care would need to be taken with the measurement as it is at B+.

[sluckey]

Couldn't it also be used to determine true output power for the tube

Absolutely. Just measure the voltage drop across the resistor and perform the simple calculation to solve for current through the resistor, which is the same as the true plate current.

[pdf64]

Consider that the power amp's margin of stability is already minimal, as evidenced by the need for anode stoppers.
Hanging probes and meter off the anode isn't going to help matters, rather it'll temporarily create a chaotic lead dress for this critical node.
The only way to make such a reading and be reasonably sure the amp is idling not oscillating is to remove the valve in the preceding stage, probably the phase splitter.