Plate Voltage mismatch

[ruievan]

Nedd Help,

My Plexi bass 50W gives me 470V on the plates of V4 and 490V on V5.
Can't figured out what's wrong.
The amp worked nice for about a year. Then a filter can cap went away, after I tried to connect the negative feedback to the 16 ohms tap. Worked for about two weeks in this configuration.
Rewired the amp from scratch, same thing.
The tubes are unmatched.
I'm using the ceriatone layout.
Any suggestions?

Rui

[novosibir]

I for myself don't like it much, when people who step on a new forum the first time can't say 'Hello' or something similar
Maybe because I've learned a different social behaviour in my childhood?

And this even is killing my mood, to give any help or advice

Larry

[ruievan]

I am a little rusty on English, so I can take quite a bit time to choose the correct words, but you are rigth Larry, saying "Hello" is not to dificult . So to everyone in this forum my apologies. This forum has been a great help for me, and I thank to everyone that contributs, and provided great information to new builders. I am really shamed.

[shakti]

No need to be embarassed, but novosibir likes to keep this a corteous and friendly place. Nothing wrong with that.

That out of the way; welcome! You'll find this a great and helpful place.

Do you measure those voltages with or without tubes? If you get a spread like that on pin 3 of V4/5 *without* tubes, something is fishy with the wiring, or possibly the OT? If it's only there with the tubes in the circuit, the tubes are at fault. When tubes start to drift, you can find the load the OT differently and can cause a mismatch like that, but it seems pretty extreme with a 20V difference.

[ruievan]

Well, thank's to everyone , its nice to know that I'm welcome.

I've collected some data so here goes.

Wall voltage 240V
PT toroidal 230V Input 345-0-345 HV
B+ without load 547V (Huge!!! need to drop this)
B+ with load 500V
Heathers 3,5V

Pin 3 V4/5 without tubes 530V

Measurements with first set off tubes (JJ EL34L unmatched and used)
Plate V4 - 519V (cathode current 31ma)
Plate V5 - 498V (cathode current 41ma)

After this measurementes I've changed the position off the tubes and goted this

Plate V4 - 527V (cathode current 43 ma)
Plate V5 - 492 (cathode current 30 ma)
_______________________________________________
With another set off tubes (JJ EL34 used)

Plate V4 - 515V (current 24ma)
Plate V5 - 518V (current 25ma)

With tubes position changed

Plate V4 535V (current 26ma)
Plate v5 512V (current 23ma)

________________________________________________

I'm aware that my B+ is high, I'm thinking on a solution (new PT or some zener mod). But this measurements are really weard.
I'd really apreciated your help, because if I'm gotta have to order something, I'd like to order everything at one time, saving on shipping costs.

[darkbluemurder]

Hi,

I would take the set of used EL34s, they are reasonably matched. Take the E34L as spares.

24-25ma is a very low current. When you bias to arrive at an idle current at 70% of the tube's dissipation capability (25 Watts for an EL34) you can go up to 33 ma.

0.7 * 25 Watts / 520 Volts = 33.65 ma

If you adjust the bias to get 33 ma you will find that the plate voltage drops, thereby allowing you to set the idle current even higher.

Good luck
Stephan

[Structo]

Yes, the two tubes that measured 515 and 518 are fine.
Nothing wrong with a few volts apart like that.

But the others are suspect.

[ruievan]

Thank you all for the help, I think I've found the cause for the mismatch in the plate voltage.
It was a bad tube socket. I've noticed that the pins sockets were too expanded.
The first time I soldered the power tubes sockets I guess I've put too much solder . The solder sliped to the sockets lug hole, and the lug got stucked. So, when I installed the power tubes, the pins add to expand because the lugs did not move. This expansion leave to a bad contact now between the tubes and the pins.
I've removed the pins from the sockets, removed the excess of solder squezzed the pins, and it worked. the plate voltages are now similar betwenn the two tubes.
I will replace the tube sockets anyway.

[Structo]

You know, that's not that uncommon of a thing for someone to do.

As you are feeding in the solder wire to the joint, it starts running down the pin if you apply too much.
It's easy to miss that it is collecting down inside the pin.
But the good news is, you now know about this and will probably never do that again.

Another thing I have seen, and this seems to be more prevalent with 6L6 tubes, is different manufacturers use different diameter pins on the tubes.
So, say you used a pair or quad of tubes for quite a while then decide to try a different brand.
The new tubes have smaller pins but you don't notice this and install them.
Sometimes they are intermittent right off the bat or other times it waits until things get heated up before you have problems.
The cure is to re-tension the sockets if possible. I can't remember which tubes have big pins but they are out there.
Another thing you learn as you travel the road as your own amp tech.

[ruievan]

I guess one off them is my Bias-Rite that I use to bias my amps. I think the pins are larger than most valve pins.
Well now is time to go for PPIMV an Effects Loop im my JTM-45. I already order them from Metro.