Help needed - problem hoffman AB763 Build - tubes Arching

[whoops]

Hi,
Im building an Amplifier using the Hoffman AB763 layout.

I have +425V at 6L6 pin 3 both tubes... the Bias voltage is -51V.

All seemed ok, but both the 6L6 are arcing when Power on.. when I remove the phase inverter everything is ok.
I re checked All connections, wiring and component values with a help of a friend and everything is correct, I dont know why is this happening!


Any ideas?

Thanks for your help
Cheers

Tiago

[eleventeen]

It could be a number of things, and I'm sure you'll get some suggestions as to what to do. Without pictures and without personally inspecting your build, I can only throw out some drive-by suggestions, so I hope that is understood.

Got your screen grids hooked up? What voltage do you measure there?
Are you sure the caps leading from the PI plates are good?
Good 6L6's? (they may not be any more, unfortunately)
Pull the power tubes and power on the amp. Can you give it some kind of input (ideally a sine wave of low audio freq) and measure what AC shows up on the output sides of the .1 caps (as on a Super Rev) coming from the PI plates?

You say the bias is -51, which is fine. Are you sure it is truly landing on those tubes, pin 5, each? Not measured on the output of the bias supply, but really measured right on the tube pins.

[Willabe]

All seemed ok, but both the 6L6 are arcing when Power on.. when I remove the phase inverter everything is ok.

What exactly do you mean by "arcing"? Are you seeing sparks inside the 6L6's?

With no PI tube there's no signal coming into the 6L6's so their just running at idol.

What is the screen dcv?


            Brad    

[eleventeen]

Your problem strongly implies "6L6 screen grids" to me. The connection to these is normally a feed only a touch lower than highest B+ to an unused pin (pin 6) on the 6L6 sockets, then a resistor (470 ohm, 1+ watt) over to pin 4 on the same socket, repeat for each 6L6. Pin 4 is the REAL destination of this voltage feed. Pin 6 is only a tie-point. You sure you got this right?

[plexi50]

Got your screen grids hooked up? What voltage do you measure there?

That or just tubes that are not good. I have made the mistake myself of miss wiring before. Some tube sockets dont have numbers on the base. Easiest way to remember is pin #1 is slightly just right of the key guide pin. #8 is just left of the guide pin

[Willabe]

Easiest way to remember is pin #1 is slightly just right of the key guide pin. #8 is just left of the guide pin

When looking at the socket from inside the amp chassis.


            Brad     

[plexi50]

Easiest way to remember is pin #1 is slightly just right of the key guide pin. #8 is just left of the guide pin

When looking at the socket from inside the amp chassis.


            Brad     

Ha!!!!!!! Ya know you have to explain it in visual detail or else it could be from looking through the glass down! 

[plexi50]

Pin #1 is on the Dark Side Of The Moon! 

[whoops]

Hello, thanks so much for your help and support.

By Arcing I mean theres sparks happening inside the tubes,
and theres noise coming from the tubes also.

We changed the two 6L6 and the phase reverter for other set and the problem still persists.

The voltages are:

489v at Pin3

485v at the grids, pin4

-51v at Pin6 with the Bias Pot at minimum position, should We change the bias range resistor so that when the bias Pot at min the voltage is a bit higher?

Thanks

[sluckey]

Easiest way to remember is pin #1 is slightly just right of the key guide pin. #8 is just left of the guide pin

When looking at the socket from inside the amp chassis.


            Brad     

And that's only true when the key is pointing up. Rotate the socket 180º and guess what!

Maybe a better way to put it would be, "as viewed from the bottom, pin 1 is ALWAYS the first pin that is clockwise from the key".

[sluckey]

By Arching I mean theres sparks happening inside the tubes,
and theres noise coming from the tubes also.

Disconnect the NFB wire from the speaker jack. Does the sparking go away?

[punkykatt]

What voltage do you have on pins 5 of the power tube. Pin 6 has no internal connection to the tube.

[sluckey]

What voltage do you have on pins 5 of the power tube. Pin 6 has no internal connection to the tube.

The voltage on pin 5 will be the same as on pin 6 if he followed the Hoffman AB763 layout.

[whoops]

Here are the voltages on the tube pins I meadured without the 6L6 in the sockets

File attached

[whoops]

Here are the voltages on the tube pins I meadured without the 6L6 in the sockets

The voltages in Black are from a know well working amp, the voltages in RED are the voltages we measured in my amp.
The problem seems to be related to the voltages that the Vibrato tube (12AX7) and the Phase Inverter (12AT7) are receiving,
all the other voltages in the other tubes seem spot on.

Any ideia of what could be be the cause of this? Having all the other voltages spot on but having this 2 tubes way off?

Thanks

[Slimtim]

a pic of your work might help the fellows see something that you might have missed.

[sluckey]

Any ideia of what could be be the cause of this? Having all the other voltages spot on but having this 2 tubes way off?

Easy to explain the differences for the vibrator tube. You are comparing to totally different circuits. Hoffman did not follow the AB763 circuit for the trem. He used the 6G16 bias vary trem circuit. Two different circuits. Two different sets of voltage readings.

I don't trust the voltage readings for the PI because you show two very different voltages on pins 3 and 8. Those two pins should be connected together and read exactly the same. Check those again.

Edit... All your positive voltages will be higher than the other amp because you pulled the output tubes.

[eleventeen]

One thing seems clear: You haven't jumpered the two cathodes on V6, the last tube. They should be connected common, on the socket, with a piece of bare wire. (Or however you wish to do it, but that's how it's generally done) You have one plate of that tube at full B+ (500v) which means that it is drawing zero current, and that's sucking up the cathode. Those two cathodes have to be the same voltage: they are hard wired together (or, should be)

Then you have something else going on with your tremolo tube, V5. You understand that a 12a_7 is two identical triodes, so, it is a matter of choice whether you happen to use the triode represented by pins 1-2-3 for one thing and the triode represented by pins 6-7-8 for another or vice versa. As long as you were coherent, it would not matter. What I am getting at is, regardless of that/those choices, the cathodes (pins 3 and 8) of V5 sit atop those paralleled res-cap structures and both of them should be sitting at single-digit volts. 2-3-4 volts, closer to 2, usually. You show 248 volts on pin 8 of that tube. No way. Pin 8 is a cathode. You should have 2-3 volts on it. Either side of that tube should have 2-3 volts on the cathodes It must not be connected to ground in its own private way.

[whoops]

Any ideia of what could be be the cause of this? Having all the other voltages spot on but having this 2 tubes way off?

Easy to explain the differences for the vibrator tube. You are comparing to totally different circuits. Hoffman did not follow the AB763 circuit for the trem. He used the 6G16 bias vary trem circuit. Two different circuits. Two different sets of voltage readings.

I don't trust the voltage readings for the PI because you show two very different voltages on pins 3 and 8. Those two pins should be connected together and read exactly the same. Check those again.

Edit... All your positive voltages will be higher than the other amp because you pulled the output tubes.

Sluckey,
I was not clear enough Im comparing the voltages of two Hoffman AB763 Circuits, an healthy one and mine.
Their are exactely the same circuit but with a diferent power transformer.

I will re-check the voltage readings on the PI, thanks.

I also re-checked all the connections and components and everything seems fine, so I dont know what to do next.
Pin 3 and Pin 8 of the PI socket are connected together, I checked that, so I probably did a mistake while measuring the voltages.

Slimtim,
I will take some pics and post here, thanks

[sluckey]

Then you have something else going on with your tremolo tube, V5. You understand that a 12a_7 is two identical triodes, so, it is a matter of choice whether you happen to use the triode represented by pins 1-2-3 for one thing and the triode represented by pins 6-7-8 for another or vice versa. As long as you were coherent, it would not matter. What I am getting at is, regardless of that/those choices, the cathodes (pins 3 and 8) of V5 sit atop those paralleled res-cap structures and both of them should be sitting at single-digit volts. 2-3-4 volts, closer to 2, usually. You show 248 volts on pin 8 of that tube. No way. Pin 8 is a cathode. You should have 2-3 volts on it. Either side of that tube should have 2-3 volts on the cathodes It must not be connected to ground in its own private way.

He has a Hoffman AB763 and the trem circuit is like the 6G16. One triode is the LFO and the other triode is a direct coupled cathode follower. So, his second triode will have a very high grid voltage and very high cathode voltage.

[sluckey]

Sluckey,
I was not clear enough Im comparing the voltages of two Hoffman AB763 Circuits, an healthy one and mine.
Their are exactely the same circuit but with a diferent power transformer.

They cannot be the same circuit!

The black voltage readings for V5 pins 2 and 7 are -48v. That's a typical grid reading for a real AB763 amp with the footswitch either unplugged or turned off. The real AB763 uses the negative bias supply to ensure a fast trem start when kicking the footswitch.

There is no negative voltage anywhere in the Hoffman AB763 trem circuit.

[sluckey]

Here's a schematic for the Hoffman AB763. Might be helpful.

[Willabe]

Your chart for the tube voltages says 501dcv on 6L6 plates and 498dcv for screens. That's pretty high.

Your 1st post says 425dcv on the plates (pin 3). Where/how/when did you now get 75dcv more? Something's wrong here.

What PT are you using?

It almost seems that you could be getting those high readings because the 6L6's are not pulling any current. That would leave the PT mostly unloaded and the voltages would go up.


               Brad        

[whoops]

Your chart for the tube voltages says 501dcv on 6L6 plates and 498dcv for screens. That's pretty high.

Your 1st post says 425dcv on the plates (pin 3). Where/how/when did you now get 75dcv more? Something's wrong here.

What PT are you using?

It almost seems that you could be getting those high readings because the 6L6's are not pulling any current. That would leave the PT mostly unloaded and the voltages would go up.


               Brad        

Hi Brad,
the voltages are 501v (plates) and 498v (screens) because it was measured without the 6L6 in the sockets. With the 6L6 in place those voltages are lower.

The problem is not in the PT because heaters , Bias tap and HT secondary are all supplying the right voltages at the moment. The problem is somewhere in the circuit itself, Im just having difficulties in finding where.

THanks

[whoops]

Sluckey,
I was not clear enough Im comparing the voltages of two Hoffman AB763 Circuits, an healthy one and mine.
Their are exactely the same circuit but with a diferent power transformer.

They cannot be the same circuit!

The black voltage readings for V5 pins 2 and 7 are -48v. That's a typical grid reading for a real AB763 amp with the footswitch either unplugged or turned off. The real AB763 uses the negative bias supply to ensure a fast trem start when kicking the footswitch.

There is no negative voltage anywhere in the Hoffman AB763 trem circuit.

Hi Sluckey,
thanks so much for your comments.
I know the Hoffman AB763 layout uses the Vibrato section from the 6G16 Vibroverb, thats why I decided to build this one.
I probably was lead into mistake with the other amplifier since I thought was an Hoffman Ab763 also, so as you explained it is not, its a normal AB763 circuit so I shouldnt be using that one to compare with mine.

Do you know what should be the normal range of voltages in the pins  for the PI and Tremolo 12AX7 in a Hoffman AB763 circuit?

Thank you so much

[PRR]

As I said on another forum you are shopping-around at: V6 is wired wrong. Cathode string isn't gone to ground as it should.