Something shorted and now one output valve draws twice as much current as the other.

[corduroyew]

I've been playing around trying to mod and fix my 1966 bassman ab-165 for a while now. And it looks like I’ve messed something up. A



The bold red wire running from pin 6 of valve 1 ended up shifter under the solder where the big red arrow is pointing so when I was soldering in some new resistors it crated a short. When I powered up the amp (before I knew about the short I had made) there were some sparks and it burnt out my PI tube. The amps volume cut in half and it just sounded bad. I quickly figured out what I had done and fixed the short as well as changing the burnt out tube but there was still a problem. The plate current of the circled power tube has jumped from about 27ma to about 80ma. The other power tube is just fine and still rus at about 27ma. I’ve tried replacing tubes and changing the .1 caps (because I has some) and I’ve also checked all the resistor values and the seem fine. I have also tried different phase inverter tubes.

Any idea what I might have blown up or can you help me get started on figuring it out?

[52 Bill]

How do the voltages on the two output tubes compare with each other? Is there enough bias or drive on the grid of the tube that is pulling more current? Have you switched the position of the two output tubes?

[corduroyew]

I have switched the positions of the tubes and I've replaced the tubes all together.

I'll double check the voltages but If I remember correctly they were around 390 or 400 volts on pin 4 and it was about the same on both tubes.

"Is there enough bias or drive on the grid of the tube that is pulling more current?"

I'm still new at this sort of thing and I'm not exactly up to date with the Jargin. I don't suppose you could reword the question so an noob like me could understand it...

[52 Bill]

Sorry, never know what level someone's at...

The bias voltage sets the idle current flowing through your output tubes. This is the negative voltage read on pin 5. Compare the readings at each output tube socket pin 5 and pin 1. There should be something like -40 to -50 volts at each socket pin 5.

If the bias voltage is set too low, the tube will draw too much current. Even though you said that you replaced the .1mF caps, if one should become "leaky" and allow positive voltage to pass through it, it will change the bias voltage and cause the tube to draw too much current.

If your amp still has the bias balance control as shown in the schematic then I am not surprised that each power tube is idling at a different point, but 80mA is way too much for any single tube. How are you measuring the tube current readings? If you rotate the balance pot do the current draws change? Did you check the resistors in the bias circuit (the 2 -10K and the 200K on the balance pot)?

Hope that this helps.

[Amp Kat]

unless you are using a bias rite to measure the current you may want to make it easier and more safe by inserting a 1 ohm resistor in betwwen pin 8 and ground. Then you can measure voltage across the resistor as a 1 to 1 ratio 1mv=1ma. Don't be fooled by a possible second fried tube drawing to much current going into thermal runnaway. If you measure pin 5 on both your tubes at idle and the bias voltage doesn't drop then it's either a bad connection on the socket or a plate volatge swing but it really sounds like a fried tube.

[corduroyew]

Sorry, never know what level someone's at...

The bias voltage sets the idle current flowing through your output tubes. This is the negative voltage read on pin 5. Compare the readings at each output tube socket pin 5 and pin 1. There should be something like -40 to -50 volts at each socket pin 5.

Pin 5 reads -42 on the side that seems to work fine and it reads -50 on the side that I have circled on the drawing (the side drawing 80ma current).

If the bias voltage is set too low, the tube will draw too much current. Even though you said that you replaced the .1mF caps, if one should become "leaky" and allow positive voltage to pass through it, it will change the bias voltage and cause the tube to draw too much current.

I've replaced them and tested them. I haven't replaced the filter caps in the cap pan since the short. Do you think it could be those?

If your amp still has the bias balance control as shown in the schematic then I am not surprised that each power tube is idling at a different point, but 80mA is way too much for any single tube. How are you measuring the tube current readings? If you rotate the balance pot do the current draws change? Did you check the resistors in the bias circuit (the 2 -10K and the 200K on the balance pot)?

Hope that this helps.

I did get rid of the balance pot and put in a true bias pot but forgot to change that on the layout. I double checked both resistors that make the V and they read exactly 220K. The pot is 10k and the resistor on the pot is 15K.

Don't be fooled by a possible second fried tube drawing to much current going into thermal runnaway. If you measure pin 5 on both your tubes at idle and the bias voltage doesn't drop then it's either a bad connection on the socket or a plate volatge swing but it really sounds like a fried tube.

I've tried swapping the tubes with each other and the readings stay the same. No matter what tube I put in the circled socket it draws 80+ma current so I'm pretty sure it's not the tube.

What do you mean when you say it could be a "plate volatge swing" and how do I fix that?

I'll order 2 new tube sockets and see if that helps anything. The tube sockets are only going to cost me a couple quid but the shipping is going to be around 15 quid ($30) so I want to try and order everything I might need all in one shot. Do you think it could be the big electrolytics in the cap pan? If it could be then I need to order some of those at the same time.


Thank you very much for the help.

[rf7]

Pin 5 reads -42 on the side that seems to work fine and it reads -50 on the side that I have circled on the drawing (the side drawing 80ma current).

I did get rid of the balance pot and put in a true bias pot but forgot to change that on the layout. I double checked both resistors that make the V and they read exactly 220K. The pot is 10k and the resistor on the pot is 15K.

Something is up with your bias pot wiring, pin 5 should be the same voltage on each tube. Those voltages would normally only be possible with the stock bias wiring. And the tube with the lowest bias voltage (-50V) should be pulling the least current, theoretically. The voltages on both 6L6's should be almost identical, pin for pin. Check that out. It's definitely a weird problem you've got going on.

[corduroyew]

The voltages on both 6L6's should be almost identical, pin for pin. Check that out. It's definitely a weird problem you've got going on.

Just to clarify, are saying both 6L6's should have close to matching voltage on both tubes, or are you saying the the voltage should b the same on all the pins of either 6L6 (IE pin 5 should be the same as pin 3)

Something is up with your bias pot wiring, pin 5 should be the same voltage on each tube

That was my 1st thought too, but I can't figure out what it could be. I've checked the values of all my parts multiple times and everything looks correct. I've tried using a balance pot, I've tried an adjustable bias pot, and I've even played with putting in 2 bias pots and none of that seems to help..

I still have not ordered new electrolytic caps for the cap pan because shipping is high on those. Anybody know how I can test to see if they are good other than just replacing them?

[corduroyew]

I figured it out. It was a bad switch. I had set up switch so that I could toggle between a .001 cap and a .022 cap on the PI. I knew both caps were fine (I had checked and replaced both of them) but I didn't think to check the actual switch. As it turns out, the switch burt out when I created the original short. That is why it blew my PI and why 1 tube was pulling too much current. The amp works great now. Thank you for the help.