Dr504 build el34 flutter

[a_e_a]

Hey guys
I just finished putting together a ceriatone dr504 build.
I was testing section by section and when I finally but the power tubes in and turned it on there was a mechanical whirring noise and then a flutter of light in one of the el34s. I had the bias pot right at the minimum if that makes a difference?

This is only my second build and I am a little lost. Any ideas or help guys?
I can send pictures if needed for further assessment. Thanks again guys for all the help. Such a great forum

Adam

[Hellhammer]

I had the bias pot right at the minimum if that makes a difference?

Adam

By 'at the minimum' you mean max negative voltage at the grids right? Did you measure plate, grid and screen voltages?

[Structo]

You should always start up a new amp with a current limiter to guard against damage from a wiring mistake, such as a short circuit.

That being said, you could have a bad power tube.

Grab some quick voltage readings and post them.

CAREFUL HIGH VOLTAGES!

Clip the black probe to a chassis ground, touch the red probe to pins 3 (plates), 4 (screen) and pin 5 (grid).

Do you have pins 1 & 8 on the EL34's jumpered together?

[a_e_a]

Ok so voltages are
Plate 517
Screen 552 & 562
Grid 38 & 39

This is without power tubes installed.

Now I put the tubes back in and ere was still the whirring noise but no flutter (I swapped the tubes around to see if it would follow the one that fluttered.


Now my big mistake (dumbass)
After a few checks I went to bleed the caps and the power was still on. Just a big spark but no whirring noise from now on. Now have I blown something up? What would have been my problem in the first place? Have I just made it a whole lot worse?

And yes 1 and 8 are jumped together. Thanks for your help.

( and I do take a lot of care whilst working. Always the hand in the pocket trick and other safety measures. Please don't let my mistake think I'm uncautious.)

[Structo]

Mistakes happen, the important thing is that you observed safety rules.

I guess I should have been more clear, measure those voltages with tubes in and amp on.

Best way to do this safely is to clip the leads onto the tube pins before powering up.
Because if your hands shake the least bit, you may have an accident.

You can leave the black probe stuck into the black bias test point for ground.

Then power off, unplug, measure for voltage, when safe clip lead to another pin.

It looks like your amp has balancing resistors on the big caps which should act as voltage bleeders when the amp is powered off.

Always unplug the amp and measure voltage at the filter caps before touching anything inside.

Not sure what you arced but if you happened to touch the plate pin (pin 3) to the filament pin (pin 2) then you might have blown a fuse.

Does your power transformer have a center tap on the heater supply?

If not and you used two 100R resistors as a center tap, it might have blown those.

[Firestorm]

Ok so voltages are
Plate 517
Screen 552 & 562
Grid 38 & 39

This is without power tubes installed.

Something wrong here. With no tubes in, HT1 and HT2 should be nearly identical. The plate should certainly not be higher than the screen.

After a few checks I went to bleed the caps and the power was still on.

That will blow the HT fuse.

[a_e_a]

With tubes in the voltages are pretty much the same.

Plate 517
Screen 550
Grid 40

The fuses haven't blown.

http://www.ceriatone.com/images/layoutP ... hat504.jpg

That's the layout I used. It is wired up exactly to that. I have checked that it's right. I will go triple check that it's right.

Now there is no sound coming through the speakers when it is on. I am at a bit of a loss as to what is wrong.

[a_e_a]

Oh and thanks for all the help so far

[Firestorm]

With tubes in the voltages are pretty much the same.

Plate 517
Screen 550
Grid 40

The fuses haven't blown.

Screens should not be higher than the plate and the absolute difference between them should not be so great in this circuit. EL34s will not like a screen voltage that high.

The grid is actually minus 40, right?

What do you see on the cathodes?

Maybe just post readings by pin#, indicating plus or minus and remember to set your meter for AC when you read the heaters.

[Structo]

Double check that you have your OT center tap to B1+. (right after the diodes)

Double check the screen resistor values. (1K)

1R on the bias resistors right?

Triple check all the connections for errors on the skinny board by the power tubes.