5f4 bias not ballanced

[scole]

how do i measure a stock 5f4 to get dissipation? do i trust the bias meter? or is there another way to get the raedings i need? i dont have the 1 ohm resistors at the cathodes.

[tubeswell]

You need to put a 1/2W 1R resistor between each cathode and ground, or have one of those fancy pancy gizmo plugs that connects to your meter, that fits between the tube and the socket to measure the Plate current. I covered my resistors with heat shrink to minimuse unwanted possibility of shorting onto another socket pin. Easiest way is 1R resistors if you don't want to spend the money/can't lay your hands on the other gizmo.

PS Make sure your amp is switched off whenever you put your hands into the amp when you clip and unclip the 'gator clips from the contacts. (I.e.; don't do what I did several weeks back - and definitely don't work on it late into the night!)

[tubeswell]

Damn! I intermittently can't seem to be able to edit my new posts - the edit screen comes up blank - Why is that?

Anyway I forgot to say 1% 1R resistors, or a couple of 1% 2R resstors in parallel for each cathode (to increase the probability of getting an even more accurate mV reading)

[scole]

I alraedy have a bias meter. i was wondering if theres a way to dounle check.

[scole]

i put each of the jj 6l6 in my champ with the bias meter, they are within 2ma of eachother. now back to why are they 8-10ma apart in my 5f4?

[lowell]

You may also put your ammeter b/t the center tap of output transformer and the plate of the output tubes. This called the transformer shunt method and is the most accurate that I've found. I think most people would agree.

Just make sure in 4 tube amps that you divide your reading by 2 because the meter will read what 2 tubes are drawing. So then use ohms law for dissipation calculation:

I.E. if your amp has 413v on plates and using 6L6 which have Wa dissipation of 25-30 depending on tube brand. (5881 I would use 25 Wa max)

25watts/413v=60ma. That's an absolute max. Some peeps use 70% as a standard setting (60ma*.7=42ma). Use your ears to tweek it.

[scole]

so i dont have to unhook the ct? just put the meter from the ct to pin 3?

[tubeswell]

Yep I'd definitely use 'gator clips for the OT Transformer Shunt method.

Its described by RG Keen here. http://www.geofex.com/ in the Tube Amp FAQ under "How to Bias"

[Enzo]

In post #4 I mentioned the shunt method Lowell describes, it bypasses the transformer winding.

But remember your meter when configured for current readings is essentially a short circuit, so ig yuo inadvertantly touch chassis while going for a socket pin, you will ground off the B+, and likely bliw the meter's internal fuse. SO be careful.

To say again, the perfectly matched tubes read different in the amp because of differences in the circuit paths in that amp.

[scole]

ok i think its over, i redid the cathode grounds, that didnt do anything. i took two jj6l6gc's out of my tonemaster wich are actually a new matched quartet, put those in and bada-bing they are both 28ma dead on, i put a new russian 5ar4 in and that brought them up to 35ma each dead on, at 437 volts. so i guess the hole thing was do to the new tubes not being matched, and the thing about the reading not moving with the tubes when i switched them, well i hope i was wrong, i dont think so but i hope. maybe taking the tubes in and out cleaned the sockets a little bit, who knows. anyway the amp is good to go. ill fire it up in the morning, it should scream. thanks guys.