5f4 bias not ballanced

[scole]

i put new tubes in my 5f4, i get 30ma in v4 and 38ma in v5. if i reverse the tubes, in other words put each tube in the other socket, the ma reading doesnt go with the tube. v4 stays at 30 and v5 is 38. what would cause the amp to be uneven if the tubes are matched?

[Enzo]

Power down and measure DC resistance from CT to each plate in the OT. Are they different? That DC resistance is in series with the tubes, and if it is not balanced, then neither will be the tube currents.

And of course always make sure bias voltage is the same on both sides in any amp.

[scole]

hi enzo
with the tubes out i measured the resistance from the ot ct to the blue and brown wires, i get 53.6 ohms on the blue and 61 ohms on the brown.

[Enzo]

Well, that could be at least part of it.

If you are now using a bias probe type thing, try instead using the shunt method at the power tube plates. That essentially bypasses the transformer winding. Then any differences left would be "real" ones.

Is the bias voltage exactly the same on both? In fact, you could clip a wire from grid to grid, that would necessitate both grids being at the same voltage.

No one has added screen resistors to this amp have they?

[scole]

no screens were added. should i ?

[tubeswell]

hi enzo
with the tubes out i measured the resistance from the ot ct to the blue and brown wires, i get 53.6 ohms on the blue and 61 ohms on the brown.

I had the same thing on my recent 5G9 build. It was due to the different lengths of wire in the primary windings (because one side of primary winding was over the top of the other side of the primary I believe, but was assured that it was the ratio of primary turns to secondary turns that mattered to deliver the appropriate load resistance). So my 5G9 biases up like yours - I get about 28mA on one tube and 29.something on the other. But it sounds okay.

[scole]

should i just let it go? will anything bad happen if i leave it?

[Enzo]

No, nothing will happen, it just is the sort of thing that bugs the more picky among us.

[tubeswell]

You can do separate bias supplies for each tube to be able to get them all the same if you really wanted to I guess. I doubt whether it makes much difference in terms of sound or tube life

[Enzo]

I think the educational thing to do is find out WHY the two sides are different. We can always find ways to FORCE a circuit to act differently.

[scole]

i forgot to mention, i put a 5ar4 rectifier in just to see what that would do and it blows the fuse. seems ok with the 5u4.

[tubeswell]

i forgot to mention, i put a 5ar4 rectifier in just to see what that would do and it blows the fuse. seems ok with the 5u4.

Hmmm.... could that possibly have anything to do with the 8mA or so difference in the output tubes current?

My understanding is that a 5AR4 uses more current than a 5U4 from the PT. Maybe the reason the fuse blew was that the PT recto winding was drawing too much current (i.e.; if it wasn't rated highly enough)? (I'm just guessing because I don't know anything about the VA ratings on your PT.)

But surely that wouldn't cause the OT to load the output tubes differently. Where did you get the OT? - is it a stock item?, or did somebody make it especially for you? Has it ever been damaged, or run without a load, in the past? Are the output tube grid load resistors matched? That could cause bias to be different. (There wouldn't appear to be any damage to either one of the output tube screens because when you swap the tubes, the same socket delivers the same sort of reading.)

[scole]

i cant believe this. i didnt like the way the solder joints look at pins 1 and 6 on v3, i replaced the stranded wire with mil spec silver teflon solid wire. i put the old russian 5881's back in with a new 5u4 (russian), checked voltages,
v4, v5 pin 2, 413v, pin 4, 415v, pin 5, -43.8, v3 pin 1, 213.9, v3-6, 276, v3-8, 67.9. i put the bias meter back on and now i get 29ma on v5 and 30ma v4. i guess those solder joints were not very good to begin with and got werse over time.

[scole]

hold the phone, i put the new jj 6l6's in, now im back where i started, using a bias meter, they are 10ma apart except this time when i reverse the tube position the reading goes with the tube. so it looks like the tubes are not matched. here are voltages,
v4 and v5, pin3 408v, pin 4 410v, pin 5 -43.7v. how do i calculate the disipation?

[Enzo]

current through the tube times voltage across the tube.