Krankenstien plus amp thats drawing alot of current

[Andrew58]

Hey guys, ive been working on this krank plus amp. It originally came to me with a 6550 that would short out. So i removed them and replaced. Now im getting alot of current draw when after i turn the amp on and turn the standby over and once the tubes heat up it starts drawing lots of current and humming. Ive checked the big filter caps and bias caps and replaced them. but still it does it. Ive checked all the surface mount diodes, all the ceremic resistors, the bias and i still havent found anything yet.
Was hoping some one could lead me in the right direction. Oh and no i dont have a schematic seeing as its krank.

Thanks Guys

[Jazz P Bass]

How much current is it drawing?
What is your bias voltage at pin #5.
I would pull the output tubes & test each socket.
Then put one tube in at atime, while monitoring mains current.
You just may have a bad output tube.
The hum could be that or a bad phase inverter circuit, or....

[Enzo]

Well, you don;t need a schematic to know what the pins do on the output tubes at least.

Pull the power tubes out and power up the amp. Does any sound come out now when standby is turned on? Does it draw excess current still? If so, then either there are flyback diodes that are shot, or the output transformer is suspect. Also possible arcing somewhere.

If it seems happy without tubes, then...

Go down the row and test EVERY power tube socket. Is there B+ on pin 3 AND pin 4 on each tube? Do all the tubes have full bias voltage? And is that bias voltage smooth - no ripple?

Are the pins 8 all grounded?

I don't know this amp, was it made with 6550s in mind, or is that something someone stuck in there in place of 6L6 or something? If it was not made for 6550s, is ther anything connecting to pin 1 on the tube sockets?

If we have reason to think the 6550s we are using are good, then take one of them and install it in the end socket, just the one power tube. Fire up the amp. Does that one tube hum and draw current? Now move that tube to the next socket. How many power tubes does this have? Try that tube in each power tube socket until you run out of sockets. DO all the sockets act the same, or does the problem really only exist in one or two of them?

With no schematic, here are some things to keep in mind. First, really, tube amps are all pretty much the same, so think of this as some Fender amp just laid out different. What would you do if it were a Fender doing this?

And second, when a tube amp draws a lot of current, and only after standby goes on, then that means something running on the B+ is causing the problem. And USUALLY, when the small tubes fail, their short is drawing current through a high value plate resistor like 100K or more, and that means USUALLY they just have no way to draw a lot of current. After all, if you have 300v and a 100k resistor, and you ground that resistor - as if the 12AX7 was a dead short - the tube would only conduct 3ma. SO that means your problem is almost always in the power tube stage.

[Andrew58]

With no tubes in the output section it doesnt hum or draw any current no noise at all like i would figure. ive tried one tube at a time in each socket and it does the same thing no matter what tube or what socket or how many tubes or what order the tubes are in. ive tried several diffrent brands of tubes and 6l6's and 6550s. same problem.

for pin 3 i get 290 volts dc when i have the tubes in
pin 4 i get 250 volts dc with the tubes in.

pin 5 tubes 1 and 2 i get -277mv and with standby off +80mv then it bows my variac breaker
pin 5 for pins 3 & 4 i get-480 mv and when i swithc the standby off it drops down to -80mv before it blows the breaker on my variac

all pin 8s are grounded

the krank plus was made for 6550 yes.

looks like the plate voltage is loaded down somewhere cause its really low. and i dont know what the pin 5 bias is dffirent for the first two and second two tubes

[Andrew58]

also the cermic resistors connected to pin 4 are getting super hot when ever a tube is plugged into its socket

[52 Bill]

From your posting of pin 5 voltages, your bias supply is dead or not getting to the tubes. This is why the tubes are drawing all that current.

Follow the trail from the tube sockets back to the bias supply and see why the negative voltage is not there.

[Enzo]

SOunds like it.

I think I suggested taking those voltage readings without the power tubes first. If we tried that and discovered the bias voltage was missing, we would then not want to install any tubes until that was corrected.

[Andrew58]

sorry i mis read your posting i thought it said if its not drawing lots of current with out the tubes and not humming the plug in the tubes one at a time and test the pins.
Ill run through the bias circuit. What should the bias be at for the 6550's? is there something that fails alot in the bias circuit?
thanks for the help so far guys.

[Enzo]

Yours seems to be missing completely. That is different from being wrong. I bet if you just find out why it is gone, it will come back and be more or less right.

Bias circuits don't up and fail much, but they are also simple circuits.


Sorry if i was unclear, I wanted you to check the voltage at the pins of the socket, not the tube. No harm done.

[Andrew58]

I feel like an idiot. I went all the way back from pin 5 and all the surface mount resistors the 2 caps again and the one dioide and the all tested good. i even swapped out the caps with other ones. But i allways get .400 volts on the first two tubes and .068 volts on the last two tubes. i cant figure out what the deal is at all. what should the bias be at?

[52 Bill]

You should have somewhere around negative 50 or 60 (-50 to -60) volts available at pin 5 of the output tubes.

Perhaps you should start at the power supply side and first identify the negative power supply. If you know that the supply is working then figure out why it isn't getting to the tubes.