push pull balancing

[nhuwar]

Ok on a push pull amp with 2 tubes I adjust the bias to each side " ie push or pull side " to balance the current through output transformer.

Now if I have 4 tube or even 6 8 or 10 does the bias on every tube have to be individually adjusted or just the 2 side have to be adjusted.

Thanks Nick

[cerrem]

You are refering to DC current balancing...or nulling...
This is to effectivly keep the Push-Pull transformer core from being "off-set" .....
You don't need to individually adjust each tube....Your concern is to adjust each side of the Push-Pull...
When you parallel tubes, such as on each side of Push-Pull.....
The paralleled tubes effectively become ONE single tube.....
ALso...keeo in mind that you are DC balancing, which is good...but also keep in mind that you need to make sure you are AC balanced....

Chris

[kevinkr]

Replacement post, I didn't express things quite as clearly as I'd hoped to and time was running out.

Per the previous post:

The more tubes per side, the more important it becomes to be able to adjust the bias independently for each tube. Matched quartets are easily obtained for common output tubes. (Not for the 813 unfortunately.)

The reason for the above is that one of the tubes may draw sufficient plate current to exceed its ratings while the pair (or more) overall doesn't. This will result in some interesting pyrotechnics after some indeterminate period of time..

The other thing I wanted to mention was just because two tubes in parallel are operating at the same q point with no signal applied does not mean that their transconductance at differing points in the load line will match. Carried to the extreme this may mean side to side ac imbalances and higher thd as a consequence - it might also mean higher distortion for the composite pair as well, but I have not verified this.

My only PPP amplifier designs used tightly matched quads for the above reasons. Bias was adjusted on a per pair basis.

Note that all recent ARC amplifiers using paralleled output tubes have independent bias adjustments for each tube, and that matched sets must be used for lowest distortion. (This I have verified)

[nhuwar]

Well I kind of meant both I just didnt word it very well. All I was getting at was do I need a pot on each tube or 1 for each side even though I was pretty certain that it was per side just neede to hear someone else reassure it.

Nick

[nhuwar]

Thanks Kevin so I just need to figuare out how I will do it because in that brain fart of a schematic I drew before sure hel isn't going to work but I have an Idea.

Nick