i need help! 6L6 large mismatched

[johngovan]

I have built a vibrolux (non reverb) amp with a pair of 6L6's. But there's a great mismatched in the 2 tubes. The 1st 6L6  draws around 30mA and the other draws 2.8mA.

My output transformer dc resistance in my primary is 119 ohms (1st half winding) and 143 ohms (2nd half winding)

 What's the best way to deal with this problem? Is it the output tube or the output transformer that makes the tube have a large mismatched?

[Latole]

One tube is weak or bias negative voltage are wrong at one 6L6 grid or ground is bad at one 6L6

Remove both 6L6, read negative bias voltage at pin 5 of each 6L6, You should read same voltages ( between -30 to -38 DC) to give you a average.

1-Same voltage;  one tube is weak the one at 2.8 ma or défective ground at pin 8 ( solder ? Socket ? )
2-Not same voltage; check bias wires

[sluckey]

Swap the two tubes. Does the low current reading follow the tube, ie, always low no matter which socket it's in? If so, get a replacement tube. Better yet, get a new matched set.

[johngovan]

the issue follow the tube. When i try to switch the tube, i got the same reading.

Tube A (30ma) on socket 1. Tube B (2.8mA) socket 2.

Tube A (30mA) socket 2. Tube B (2.8mA) socket 1.

This is a new set of tube that has been stock in almost 2 years unused. This is the first time i will ever use it.

[johngovan]

One tube is weak or bias negative voltage are wrong at one 6L6 grid or ground is bad at one 6L6

Remove both 6L6, read negative bias voltage at pin 5 of each 6L6, You should read same voltages ( between -30 to -38 DC) to give you a average.

1-Same voltage;  one tube is weak the one at 2.8 ma or défective ground at pin 8 ( solder ? Socket ? )
2-Not same voltage; check bias wires

The bias voltage are good on both sockets. When i swap the tubes. The problem remains as it follows the tube.

[Latole]

One tube is wrong. it's happened some time.

If I need 2 power tubes for a amp, I order 4, never only 2.

[johngovan]

One tube is wrong. it's happened some time.

If I need 2 power tubes for a amp, I order 4, never only 2.

So it is normal to have this large mismatched from unmatched pair of tubes?

Does a separate bias pot for each power tube can fix it to even the bias voltage for each tube?

[Latole]

No, one tube is bad. Nothing to do with

I never order matched pair and my tubes are always not more the 8 ma between them. These are a very goo match

[johngovan]

No, one tube is bad. Nothing to do with

I never order matched pair and my tubes are always not more the 8 ma between them. These are a very goo match

Could my mistake make one of my tube bad?

When i try to turn it on for the first time, the amp sounds too weak, then when i trace the problem, i saw that I forgot to wire the screen grid to B+2?

[Latole]

I don't know.

[johngovan]

I don't know.

Thanks for the help. I really appreciate it.

[Latole]

I'm building a Vibroverb AB763 from scratch. I'm mostly finish.
I will power ON after I do double, triple check and more with schematic.

[acheld]

Could my mistake make one of my tube bad?   When i try to turn it on for the first time, the amp sounds too weak, then when i trace the problem, i saw that I forgot to wire the screen grid to B+2?

I doubt that your mistake caused the problem with your bad tube.

[pdf64]

With 0 screen grid voltage, there will be 0 anode current.
No potential for damage.

It’s a very different scenario from there being zero bias voltage.

Were the valves advertised as being tested, with a warranty? Any warranty would have expired, but it would indicate that the vendor had confidence in their product.

[johngovan]

Thank you for the confirmation about the screen grid. I think the seller send me a bad tube. The warranty has already expired.

Thanks for the help.

[HotBluePlates]

... Tube B (2.8mA) ...

This is a new set of tube that has been stock in almost 2 years unused. ...

It is possible the low-current tube is useable, but just has very low current when provided the same bias voltage as the other tube.

[valvetones]

Are they 6l6 metal, 6l6g, 6l6gb, 6l6gc?
What’s the screen voltage at?
What’s the plate voltage at?

Output transformer primary’s are never the same ohm reading from one side to the other.
So I doubt that’s the problem.

[tubeswell]

It can be normal to have quite a bit of variation in a batch of new tubes. I buy 'em 20 at a time and 'match the closest pairs', but there will always be some at the outer limits.


One option is making individual bias adjustments for the separate tubes